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Chap4 CNC Machining

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  • Section IV

    Computer Numerical Controlled Machining and Computer Aided Manufacturing

    Revised 8/2/05

  • AML Laboratory Manual

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  • CNC MACHINING & CAD

    Table of Contents 1.0 Introduction to CNC CAD/CAM Lab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Instructional Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    1.2 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2.0 Computer Numerical Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Defining Numerical Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Advantages of NC and CNC Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3 Coordinate Systems and Machine Motions . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.0 Machining Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1 Machining Operations and Machining Practice . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Determining Cutting Conditions and Performance . . . . . . . . . . . . . . . . . . . . . . 18 4.0 Manual Part Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1 Introduction to G Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2 Programming Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.0 Computer Assisted Part Programming/Computer Aided Manufacturing Software . . . 27 5.1 Mastercam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.2 Post Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.0 CNC Milling Machine Setup and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.1 Haas VF1 CNC Milling Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 7.0 Abrasive Waterjet Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 7.1 Introduction to Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 7.1 Comparison to Other Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 7.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 7.4 Machine Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 7.5 Creating a DXF File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 7.6 FlowPATH Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7.7 FlowCUT Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Appendices A. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 B. Reference Tables for Standard Drill, Tap, and Screw Sizes . . . . . . . . . . . . . . . . . 48 C. Haas VF1 Control Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 D. G-Code References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

    List of Figures

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    4.1 A simplified schematic of an NC system. . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2 Computer numerical control (CNC) system schematic. . . . . . . . . . . . . . . . . . . . . . 9 4.3 DNC System Configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.4 Right hand coordinates for vertical milling machines. . . . . . . . . . . . . . . . . . . . . . 12 4.5 CNC lathe coordinate system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.6 Absolute versus incremental positioning. . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.7 Comparison of control system paths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.8 Two-dimensional NC contouring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.9 Three-dimensional NC contouring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.10 Peripheral milling; up milling and down milling shown considerably exaggerated to illustrate the principle of operation . . . . . . . . . . 18 4.11 Pocket Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.12 Milling Tool Parameter Illustrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.13 Tool Length Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.14 Water Jet Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.15 Abrasive Water Jet Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.16 Jet Lag in thick part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.17 Corner Blowout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.18 Curve Taper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.19 Yo-Yo Contact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.20 Grid Part Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.21 Nested Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.21 Lead In / Lead Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    List of Tables 4.1 Recommended Surface Speed and Chip Load for Slotting Operations on Different Materials (HSS End Mills with Small Diameters (d

  • CNC MACHINING & CAD

    1.0 Introduction to CNC CAD/CAM Lab The AML CNC CAD/CAM lab provides an introduction to machine tools under computerized numerical control and the use of modern CAD/CAM software packages. Numerical Control (NC) or Computerized Numerical Control (CNC) -we will often use the terms interchangeably, - is the operation of machine tools (and other processes) by a series of coded instructions. Note that numerical control is not a kind of machine tool, but a technique for controlling a wide variety of machines. NC has been applied to assembly machines, inspection equipment, drafting machines, typesetting machines, welding tools, woodworking machines, punch presses and nibblers, wire wrap and insertion machines, as well as the more common metal-cutting machine tools. Since we can program the machine, we can make a variety of parts which are not possible when using specialized or fixed automation. CAD/CAM tools are used to model geometry and create tool paths associated to the geometry.

    In this course the focus will be on machining using a CNC vertical milling machine. You will learn two different ways to create a part on a CNC machine. The first, which we will use with the CNC vertical mill, is the use of Computer Aided Manufacturing software to interface Computer Aided Design (CAD) technology with CNC machine tools. You will use this technology to transform CAD representations of a part into a tangible product. This is one primary application of Computer Aided Manufacturing (CAM) software. The second method is called manual part programming where the user writes the code by hand and directly enters this code into the CNC machine via a keypad on the machine itself. 1.1 Instructional Objectives In the five lab sessions you will: Learn safety procedures. Learn standard machining procedures used for the selection of tools machining operation of

    the milling machine. Learn and or review typical CNC machine tool systems and the differences between point-

    to-point, straight cut, and contouring machine tool control systems. Learn how to write and debug a manual part program. Learn how to use Mastercams machining software to design and machine an injection

    mold insert. 1.2 Requirements For Safe Operation of a CNC Milling Machine PERSONAL SAFETY: A) SAFETY GLASSES MUST BE WORN AT ALL TIMES WHEN IN THE AREA OF THE MILLING MACHINE! Sharp fragments of metal may fly off at high velocity. Protect your eyes at all times. Failure to comply may result in revocation of shop privileges. B) AN ATTENDANT MUST BE AT THE CONTROL PANEL AND A TA OR AML SUPERVISOR MUST BE IN THE IMMEDIATE AREA AT ALL TIMES DURING CNC OPERATION. Furthermore, each student is responsible to know how to emergency stop the

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    milling machine before starting the machine. Software glitches can cause the cutting head to veer off unexpectedly resulting in severe damage to the machine. C) WARNING!! Loose clothing, long hair, personal stereo wires, jewelry, and gloves may become entangled in rotating equipment leading to serious injury or death! Make certain that such articles are removed or securely fastened to avoid entanglement. D) WARNING!! Milling cutters can be extremely sharp. When changing tools, always wrap the cutter in a rag. Do not touch the cutting edges with your bare hands. NEVER touch a rotating tool bit. E) The chips produced in the milling process can also be razor sharp. Always use a brush to clean a machine. Do not use compressed air to blow the chips off of the machine or your clothes. Blown chips may get into eyes or puncture skin. F) Never reach over the machine while the cutter is rotating and never attempt to measure parts or clean the machine while the milling cutter is rotating. G) WARNING!! Make certain that the workpiece is securely fixtured and that all components of the fixture are securely fastened to the table. Because of the enormous forces involved in milling, failure to check security may result in items being flung from the setup causing bodily injury. If you are not sure if your setup is safe, have a TA or staff member check it out before you begin cutting. Pay extra attention to the position and angle of toe clamps. H) Apply all coolants to the tool bit in a safe manner. Use extreme care when adjusting spray nozzles! The magnetic bases holding the spray nozzles may slip. Therefore, hold the base while adjusting the nozzle. If a base falls into the cutter, personal injury or machine damage may result. It is recommended that the spindle be placed on hold before adjusting spray direction. I) REPORT ALL OIL AND GREASE SPILLS IMMEDIATELY! These are an extreme slip hazard!! J) If the workpiece begins to vibrate, or the cutter makes excessive noise, stop cutting immediately. K) Before powering up the spindle, make certain that the milling cutter, its tool holder, and the spindle, are free of the workpiece and will not run into any of the fixturing components. Also, make certain all loose tools, spindle wrenches, chuck keys, and measuring tools have been removed from the machine and put in the proper location. MACHINE SAFETY:

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    A) The spindle must be completely stopped before attempting to change from low gear to high gear or vice versa. Conversely, speed selection within a gear range should only be done with the spindle running. B) Calculate the proper spindle speed and table feed rate before beginning a cut. Do not attempt to take a heavier cut than the cutter or the workpiece setup can handle. Make certain to use a proper safety factor for the rigidity of the set up and the condition of the tooling. If you are not sure about your calculations, ask! C) Make certain that the milling cutter is rotating in the proper direction before beginning a cut, otherwise the milling cutter will burn up. D) Check that table or spindle locks are off before engaging the associated power feed. If you do not know how to operate a machine or do not fully understand the instructions you have been given, ask a supervisor until you are certain about what is required. IF YOU DONT KNOW, ASK. 2.0 Computer Numerical Control Systems This section deals with general background information for Numerical Control (NC) machines, Computerized Numerical Control (CNC) machine tool systems, and Direct Numerical Control (DNC) systems. This section describes CNC machine operation and identifies planning that is necessary before running a CNC machine tool. 2.1 Defining Numerical Control When we think of automation, we tend to think of fixed automation like Detroit-type flow or transfer lines. Here the configuration of the equipment is fixed and optimized to produce one type of part or product in large quantities. This allows high production rates and low cost per unit. However, when the part or product line is changed, the equipment must undergo a major changeover and re-tooling. Under these circumstances we would like to have the flexibility to produce a variety of parts on demand and in smaller quantities. For this type of application the programmable automation available on NC machines is beneficial. Schematically, an NC machine, as illustrated in Figure 4.1, has three basic components: A program of instructions A controller unit A machine tool With this relationship among the components we can define NC as a method of automatic control

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    that uses computer-like instructions to cause the machine to perform a series of operations. In the next unit we will learn the programming instruction set for our Haas CNC so that we can write manual programs to machine a variety of parts.

    Figure 4.1: A simplified schematic of an NC system. While the terms and abbreviations Numerical Control (NC) and Computer Numerical Control (CNC), will for the most part be used interchangeably, their definitions and, in particular, the control technology behind them are different. The Electronics Industries Association (EIA) definition of numerical control is,

    "A system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some portion of this data."

    The technology to implement this definition was primarily the hard wired logic control available in the decades of the 1950's and '60's. With the advent of integrated circuits and the microprocessor technology since the late 1960's, the controllers themselves became programmable, leading to the definition of computerized numerical control:

    A numerical control system wherein a dedicated, stored program computer is used to perform some or all of the basic NC functions according to control programs stored in the read-write memory of the computer.

    These definitions indicate that the concepts of NC are handled in a more flexible and reliable way with computer technology on CNC machines. Since the 1970's all machine tools that have been built are CNC, including the Haas used in the AML Lab. A typical CNC schematic is shown in Figure 4.2. The basic difference between NC and CNC is that many of the hardware functions done by the NC machine are performed by software in the CNC system.

    Figure 4.2: Computer numerical control (CNC) system schematic. The last type of NC system is Direct Numerical Control (DNC). This is really a method for managing many CNC machines by a large computer. We can define DNC as:

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    A system connecting a set of numerically controlled machines to a common memory for part program or machine program storage with provisions for on-demand distribution of data to the individual machine memories.

    The DNC approach embraces the Computer Integrated Manufacturing (CIM) philosophy by integrating many functions such as scheduling or inventory and can directly aid in the manufacturing process planning and NC program preparation. Figure 4.3 shows some possible DNC system configurations.

    Figure 4.3: DNC System Configurations. In all three cases, the three basic components of a numerical control system are the same:

    a program of instructions entered by: tape, manually through a keypad or CRT, or downloaded from another system into resident memory,

    a controller unit using different technologies with different features, and

    a machine tool with servos and the tools and fixtures needed to produce a part.

    The controller unit interprets the program and converts the commands to electrical signals to control various machine functions. For example, it could cause the servo or stepper motors to move the machine tool spindle up or down or to move the machine tool table in its plane. In this course we are not concerned with the controller unit design, but the controller is what makes a CNC machine tool different from a conventional machine tool.

    The last basic component we have seen is the milling machine itself. It consists of a worktable that moves in the X-Y plane and a spindle that holds the tools and can move up and down (along the Z-axis). Notice that the tool itself cannot move in the X and Y directions. However, when

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    we program the machine, the instructions we use assume that the tool is doing all of the moving. Thus, the instruction set is designed for conceptual ease and we could say that the way in which XY motion occurs is transparent to the user.

    Before using a CNC machine tool, it is usually necessary to:

    Start with a sketch, dimensioned drawing, or solid model of the part. Select tools, plan a fixturing and cutting strategy, and determine cutting conditions

    which will meet the accuracy and finish requirements for the part and not exceed the capabilities of the machine tool.

    Put together a sequence of instructions that will cause the machine tool to generate the desired part geometry.

    Verify the program to avoid damage or harm to people, the machine tool, and the part you are trying to produce.

    Setup the machine for operation and execute the program. Note: In section 4 of this manual you will learn how CAD/CAM software can be used to fulfill these requirements

    Before we begin the unit on programming it will be helpful to know something about machine tool motions and coordinate systems, and the tools needed to machine a part. 2.2 Advantage of NC and CNC Machining NC and CNC machining provide several advantages over conventional manual machining. With NC and CNC machining, parts can be reproduced with improved accuracy. NC and CNC technology automates the machining process, therefore requiring fewer machine operators and avoiding operator error. This automation, combined with the improved repeatability, yield improved quality control over the machining process. Furthermore, when linked to computer aided design software, NC and CNC technology provide the foundation for agile manufacturing.

    While NC and CNC technology can provide substantial improvements over manual machining, there are applications where manual machining is more effective and practical. For small batches or single parts where accuracy and repeatability are not critical, manual machining may provide adequate accuracy, save time, and cut cost. Manual machining typically requires less set up time, and no programming time. Therefore, there are many applications where manual machining will be more cost effective. In general, we should consider using a CNC system when our production situation has: Similar workpieces in terms of raw material, Workpieces produced in various sizes and with complex geometries, and in small-to

    medium batch sizes, Sequence of similar machining steps required to complete the operation on each

    workpiece, Design changes are frequent.

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    2.3 Coordinate Systems and Machine Motions To program a sequence of tool motions, we need a coordinate system and must remember that programs are written as if the tool moves around the workpiece (even though in most cases it is the workpiece that moves around the tool). All vertical CNC milling machines use the same right hand coordinate system shown in Figure 4.4. The z-axis is always along the axis of spindle rotation, with the positive direction away from the workpiece. The primary or longest travel direction of the table determines the x-axis and the third or y-axis can be determined by the right hand rule. This means that for the Haas VF1 CNC the axes are:

    X axis (table motion, +X to the right) Y axis (saddle motion, +Y away from you) Z axis (quill motion, +Z is up away from the workpiece table)

    For the CNC lathe this means that the axes are (Figure 4.5):

    X axis (motion perpendicular to the spindle, +X towards the operator) Z axis (along the axis of the spindle, +Z away from the quill) (Note that there are only two axis on a lathe so there is no Y)

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    Figure 4.4: Right hand coordinates for vertical milling machines.

    Figure 4.5: CNC lathe coordinate system.

    The coordinate system is very important during machine setup. Should you make an axis sign error you could easily send the tool into (and through!) the machine table when actually you thought you were retracting it. (One reason for +Z being away from the workpiece is that if a minus sign is lost during data transfer, the tool will move away from the table.) We must have an origin to locate the tool in relation to the workpiece. One possibility is to use the fixed zero provided by some machines. The tool will be located above this point with the quill (Z) in the home position (fully retracted). With this origin, all tool locations are now defined by positive X and Y coordinates.

    A more convenient feature is the floating zero. It allows you to specify the origin so that you can take advantage of workpiece symmetry or the origin used in your engineering drawing or CAD model. Dimensions shown on a drawing are often with reference to some datum (origin) on the part, perhaps the center of a hole or both sides of a corner. In this case, the programmer can save calculation time by using the coordinates directly from the drawing. The operator establishes the floating zero manually when the workpiece is setup (initialized).

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    Once weve selected an origin we have the option of defining other further points in space by either absolute or incremental positioning. Absolute positioning is what we are used to seeing; all tool locations are defined in relation to the origin. By contrast, incremental positioning means that the next tool location is defined with reference to the previous tool location as shown in Figure 4.6. This is of particular use when drawings specify locations from particular workpiece features and not from a common datum location. While CNC technology can handle either type of coordinate, if the part will be checked on, for example, a coordinate measuring machine, it is good practice to have a common or absolute coordinate system or datum for the engineering drawing, NC part program, and the measurement of the part.

    Figure 4.6: Absolute versus incremental positioning.

    Consider the problem of drilling a line of 100, 0.100 inch holes spaced 0.500 inches apart. We could use absolute coordinates and command the tool to move to each hole location with respect to the origin and drill a hole. That would take about 100 instructions. Using incremental coordinates we could define this repetitive operation with fewer instructions using a loop. The pseudo-code commands would look something like:

    Move the tool to X = 3, Y = 3 Drill a hole Repeat this Move the tool in the X direction 0.5 inches operation Drill a hole 99 times Stop drilling after 99 holes have been drilled Note that these are not actual commands from the Haas vocabulary. However, the technique of looping is, and so we can see the value of incremental coordinates.

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    So far we have not concerned ourselves with how the tool moves from one point to another. As it turns out, the path the tool follows depends upon the type of control system used. There are two types of numerical control systems: point-to-point, and contouring systems.

    A point-to-point system moves the tool from one position to another and disregards the path along the way. Each axis of motion is controlled independently so that the path steps from the start position to the next position as shown in Figure 4.7. The path shown is not unique as some point-to-point systems first satisfy the X command and then the Y, while others reverse the order. Because the traverse path is not controlled, point to point systems are primarily used in applications for positioning, e.g., drilling or part insertion.

    Figure 4.7: Comparison of control system paths.

    Contouring is the most versatile and sophisticated type of control system. The controller can simultaneously coordinate the motion and control the tangential feed rate of more than one axis. The path of the cutter is continuously controlled to generate the desired geometry of the workpiece. In this system the controller generates a path between points by interpolating intermediate coordinates. All contouring systems have a linear interpolation capability - they can generate a straight line between two points - but some, like the Haas, can perform circular interpolation as well. Figure 4.8 and 4.9 show examples of contouring. In two dimensional contouring two axes move simultaneously with one axis fixed. During three dimensional contouring, three axis move at the same time.

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    Figure 4.8: Two-dimensional NC contouring

    Figure 4.9: Three-dimensional NC contouring

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    3.0 Machining Fundamentals 3.1 Machining Operations and Machining Practice The machining operations done on a CNC machine are no different than those done on a conventional machine tool. They are done faster and more repeatable but still require planning to make a quality part at reasonable cost. Some of the many operations we will be concerned with are:

    slot milling boring peripheral end milling counter boring face milling spot facing face grooves deburring recesses chamfering pockets threading and tapping drilling center drilling While you will not become an expert machinist in a few lab periods, knowledge of basic functions, standards in selecting tools or specifying operations when you are designing will help you in the future. The future means when you design your product and tools in AML II, but also when you leave Rensselaer.

    The drill is a tool for making holes of moderate accuracy and is often the first operation when making a more accurate hole by reaming or producing a threaded hole by tapping. There are two main drill types: twist and the spade drills. Twist drills are common for holes under 1.000" and there are standard sizes that should be specified in design and used in manufacture to save cost and tooling inventory. Standard drills are categorized by three systems: fractional drill sizes range from 1/64" upward by 64ths of an inch, letter drill sizes range from "A" (0.234 inch) to "Z" (0.413 inch), and number drill sizes run from No. 80 (0.0135 inch) to No. 1 (0.228). These drills are summarized in Table 4.B.1 in Appendix B. You should understand the number and letter convention and use table 4.B.1 as a reference.

    It is common for twist drills with a diameter larger than 1/2 to have a large web for strength. As a result, they often center cut poorly. Therefore, to accurately drill a hole with a diameter greater than 1/2, one must first drill a smaller hole and use a second operation to drill or ream the hole to the desired size. In addition, since small drills lack rigidity, small holes are often centerdrilled or spotfaced. The tools used for these operations make a shallow indentation in the surface of the part, so that the drill point locates correctly. Therefore, drilling both large and small holes will often require multiple operations to obtain the final size.

    When drilling deep holes, holes whose depth exceeds the drill diameter, the drill must be retracted to clear the chips from the hole. The drill repeatedly plunges into the material, and retracts out of the workpiece to clear any built up chips. This is called pecking.

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    When a tapped hole is required, a hole is needed for a dowel pin to align mold sections, or a bolt hole with a standard clearance is needed, Table 4.B.2 can be used to correctly size the hole. This table lists tap drill sizes and clearance drill sizes for standard screws.

    For example given a 7/16" bolt and told to tap a hole, one needs first to determine the thread size. The threads are specified by the diameter of the bolt and the number of threads per inch. Although different standards exist, the one we shall deal with, and the most common is the Unified National system. Bolts of a given diameter (under 1.500 inch) are specified in this system under three different threads: A coarse series (UNC), a fine series (UNF) and an extra fine series (UNEF). The tap drill size for the 7/16" hole can be found from the corresponding entry in Table A2. Thus if 7/16"-UNC is specified, the recommended tap drill is a "U" letter drill (0.368").

    Since almost no additional holding strength is gained past five threads, often a clearance hole is desired to reach the threaded section of the hole.

    The tap is one tool used to thread holes and is available in all the Unified National sizes. Taps may be used by hand with a tap wrench or on the milling machine with a device called a tapping head. Except for cast iron, tapping fluid is usually used when tapping to lubricate the tap and aid in cutting.

    The reamer is used to produce a hole that is round and has an accurate diameter. Reamers are used because twist drills often will not provide a hole which is uniform enough for parts requiring a precise hole such as dowel pins. The reamer is typically used by first drilling a hole about 1/64" under the desired size, and finishing the hole with the reamer. Reamers with a square at the end of the shank (like a tap) are designed for hand use with a tap wrench. Reamers for machine use have either a straight or tapered shank.

    The counterbore is a feature of a drilled hole that provides a recess for a cap screw below the surface of the part. The counterbore consists of a shank, flutes and pilot. The pilot of the counterbore does not cut, and serves to keep the counterbore aligned with the previously drilled hole. Counterbores with fixed pilots are made to bore holes for the standard capscrew sizes. Counterbores with changeable pilots are made in many of the same sizes that twist drills are available. Countersinks are used to make a cone shaped depression to accept the head of the flat head screw so it will be flush with the part surface. Large countersinks often have pilots similar to the counterbore. Table 4.B.3 shows the geometry and sizes of machine and cap screws that may need to be counterbored or countersunk.

    The most useful and versatile tool for milling is the endmill. Two and four flute endmills are common and are readily available in 1/16" size increments. Other sizes in 1/32" and 1/64" increments are made, but because they are less common are also more expensive. The main difference between two and four flute endmills is that the four flute endmill is generally not center cutting, so, like large diameter drills it cannot be simply plunged into the work. However, since the four flute endmill has an additional two flutes, it can remove material faster than the corresponding two flute endmill. Peripheral milling, where teeth located on the periphery of the cutter body generate the milled surface, produces a plane parallel to the cutter axis (Figure 4.10).

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    Note that milling can be either up (conventional) or down (climb). Pocket milling refers to the operation of removing material so that a desired cavity is formed as shown in Figure 4.11.

    UP or CONVENTIONAL DOWN or CLIMB

    Figure 4.10: Peripheral milling: up milling and down milling shown considerably exaggerated to illustrate the principle of operation.

    Figure 4.11: Pocket Milling 3.2 Determining Cutting Conditions and Performance Before initiating a machining operation it is necessary to program appropriate cutting speeds and feeds. The speed refers to the rotational spindle speed of the cutting tool. This is measured in rev/min and it is the angular velocity of the tool. The feed rate is the rate at which the tool is fed into the workpiece; in the English system the units are inches per minute. While these are the quantities programmed, there are other more fundamental quantities related to metal cutting and materials that have to be determined before programming the spindle speed and feedrate. These quantities are the surface speed and the chip load.

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    Surface Speed: The velocity of the tool with respect to the workpiece is more important than the spindle speed because it determines the temperatures and life of the cutting edge. This relative velocity is referred to as the surface speed and is measured in surface feed per minute (ft/min) in the common English units. The required surface speed is a function of both the cutter and workpiece materials and the type of cutting operation. For a tool of given diameter D (inches), the relationship between surface speed v (ft/min) and spindle speed N (rev/min) is:

    v = N circumference of tool / (12 in/ft) ( or v = N D / 4 )

    where v is the surface speed in feet/min., D is the diameter of the tool in inches and N is the spindle speed.

    Feed Rates: It is very important to feed the workpiece into the tool at the correct rate. Improper feed can result in poor finish, a damaged piece, damage to the tool, the machine, or the operator and ultimately a loss of time and poor quality. There is a tendency to make the feed rate too high, so beware of this as you go through this lab. To obtain the proper feed you must consider the material being cut and the power of the machine tool or the required finishes of the part. There are several ways to specify feed: inches/min, inches/tooth, or inches/rev. are all acceptable units. Inches/min. (F) refers to the rate at which the tool is fed into the piece. Inches/rev. is the distance the tool travels during one revolution of the spindle. Inches/tooth (sz) is a measure of how much each cutting tooth removes as it passes over the piece. This amount is referred to as the chip load. For a given material and type of cut the recommended chip load is constant. However, because tools can have different numbers of teeth, the feed rate must be varied. We shall define feed rate as the rate, or linear velocity, at which the tool is fed into the workpiece, be it either by the spindle descending or the table traversing. Chip load is related to spindle speed and feed as follows:

    sz = F/ (Nt N). Where sz is the chip load, Nt is the number of cutting edges (or flutes) and N is the spindle speed. For a given chip load and surface speed, the correct feed rate (F) which should to be programmed with an F command is:

    F = sz Nt N

    Calculating Cutting Speed and Feed: With all of these equations and variables, it is not necessarily clear how to calculate an appropriate cutting speed and feed rate. Typically, cutting speed will determine tool life, and feedrate will affect the chip load and surface finish. Therefore, these parameters should be set to obtain the desired tool life and chip load.

    For applications in AML, the material and its Brinell hardness are usually known, the cutting tool (diameter and number of flutes) have been selected, and type of cut is also known. Under these circumstances the following procedure can be followed to calculate an appropriate cutting speed and feed rate.

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    1. First, look up the ranges for surface speed (cutting speed, ft/min) and the feed per tooth (chip load, inches/tooth) for the material being cut. (Table 4.1) The higher values of both v and sz are for the lower hardness values.

    2. Calculate the spindle speed range as follows:

    N=12v/(D)

    3. Calculate the recommended feed rate range from the spindle speed, number of teeth, and chip load, according to the following formula:

    F=NNtsz

    (F is feed rate in in/min, N is the spindle speed in RPM, Nt is the number of teeth, and sz is the chip load in inches/tooth)

    Table 4.1: Recommended Surface Speed and Chip Load for Slotting Operations on Different Materials (HSS End Mills with Small Diameters (d

  • CNC MACHINING & CAD

    Tool Bending Load (deflection): When the tool is being fed into the work piece there is a bending load applied to the tool. If you are using small diameter tools, 1/2 inch and under, reduce the feedrate. Too high a feed rate will cause the tool to shatter, or not produce a dimensionally correct path. The use of small diameter tools requires EXTREME CARE. When a small tool is being used, you should be ready to stop the machine. Tool Type and Selection: Two common tool materials for milling cutters are high speed steel (HSS) and carbide inserts (Carbide). HSS tools are relatively inexpensive and are able to resist the intermittent loading experience in milling. Carbide tools, although more expensive, can be operated at higher surface cutting speeds and still maintain their edge much longer (tool life). When choosing a tool for a particular operation there are several things to take into account. For example, if you are cutting a specific radius in a slotting operation you have to make sure that the radius of the tools is at most the same size as that of the radius to be cut. Preferably the tool radius should be about 75-90% of the desired radius. If it is any larger, then the radius cut will be that of the tool. If there are several operations to be done, try to see if one tool could be used in more than one of the operations. This will save you the time of changing the tools during the run cycle. The number of flutes affects the finish. When used at the same speeds and feeds, a four fluted cutter will provide a better finish. This is to be expected since the chip load on the four fluted cutter is half of that of the two fluted cutter, so the finish would be about four times better. When you specify tools in the lab, check with the instructor to make sure that you have the right tool for the job.

    Along with surface speeds and chip loads, it is also necessary for the part programmer to specify the tools, the order and types of cuts that will be used for the various operations, and how much the cutter projects below the workpiece surface. Taken together, this is part of what is termed process planning: the detailed, step by step instructions and programs to convert engineering or CAD drawings to a finished part.

    4.0 Manual Part Programming The preliminary steps in part programming always involves planning and specifying the sequence of steps to be performed by the CNC machine. The geometry, either in the form of an engineering drawing or a CAD model, is the starting point. Then the cutting speeds and feed rates need to be determined, based on surface finish, forces, or past experience. Next, the programmer specifies the path that the tool follows, taking into account the fact that some features like, for example, a tapped hole require drilling and tapping operations be programmed. This programming can be done either manually or through computer assistance. In the computer assisted mode, which will be covered in Section 5, the part programmer specifies the geometry of the part through an interactive program and can define material to be removed. The computer then automatically generates the program instructions for the CNC machine. In the manual method the programmer writes all the lines of code from a detailed drawing.

    Most of the material in this unit is covered in more detail in the Appendix. Either the Appendix or the manuals available in the lab should be the final arbitrator of how to program and operate

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    the Haas CNC milling machine. These notes will try to cover what we have found over the years to be some of the basics to get you started.

    4.1 Introduction to G-Code G-Code is the computer language used by NC and CNC machine tools. The individual instructions consist of simple letter and number combinations. The program is just a long list of these instructions, given in the order they are to be performed.

    There are two types of codes, G and M. G codes control movement of the machine, tool and coordinate setup, canned cycles. Canned cycles are built in routines for automating common tasks, like drilling a hole. M, or miscellaneous, codes control the spindle, coolant, tool changer, and other non-machining tasks.

    All machine controllers understand G-code, but each has a slightly different format and usage. Some machines may use decimal places in distance coordinates, while others may assume all coordinates are given in ten thousandths of an inch. For example, the command to move to an X location one inch from the origin would look like X1.0 on the AMLs Haas VF1 milling machine, and X10000 on the Science Centers Cincinnati 5VC mill. Different machine may also like the options in a canned cycle presented slightly differently. While this can make it difficult to program machines by hand, most CAM systems can be programmed with the specifics of each CNC machine. Once the user sets up the desired machining steps in the CAM software, a post-processor formats the code for the chosen machine. The precise format is often transparent to the programmer, but can be very useful to the advanced user for optimizing part programs.

    4.2 Programming Examples

    A major part of NC programming is skill and experience. You will not have enough time to develop a great deal of either. In place of experience, we will cover some examples. The details of the instructions and commands you will have to find out from the Appendices taken from the Cincinnati programming manual. The following is a brief description of the most common codes used on the Haas CNC mill.

    G Codes for Cincinnati CNC milling machine

    G00 This is the Rapid Traverse mode. The slides are moved from one position to another with the highest possible speed.

    G01 This is the Linear Interpolation mode. The slides will interpolate between two points and travel in a straight line from one point to the other.

    G02 This is a Circular Interpolation mode. The spindle will travel in a clockwise motion around an arc.

    G03 This is a Circular Interpolation mode. The spindle will travel in a counter clockwise motion around an arc.

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    G17 This allows for circular interpolation in the XY Plane.

    G18 This allows for circular interpolation in the XZ Plane.

    G19 This allows for circular interpolation in the YZ Plane.

    G40 Cancels cutter compensation.

    G41 This selects cutter compensation left; that is the tool is moved to the left of the programmed path to compensate for the size of the tool.

    G42 This selects cutter compensation right.

    G80 This cancels any fixed cycle previously entered.

    G81 Drill cycle. The spindle will rapid move to the axis location in X and Y, rapid move in the Z-axis to the retract plane, feed in the Z-axis to the proper depth, and then rapid retract to the retract plane.

    G82 Drill cycle with dwell. It is similar to the drill cycle, but will pause at the Z-depth for controlled amount of time.

    G84 Tap cycle. It is similar to the drill cycle, but the spindle reverses rotation direction before retracting out of the hole. The spindle returns to its normal rotation once it has retracted to the retract plane.

    G85 Bore cycle. It is similar to the drill cycle, but the spindle retracts out of the hole at the feed rate.

    G86 Ream cycle. It is similar to the drill cycle, but the tool is stopped while retracting out of the hole.

    G89 Bore and dwell cycle. It is similar to the bore cycle, but adds a pause at the full Z-depth.

    G90 This designates the use of global or absolute coordinates. All moves are relative to some origin previously defined.

    G91 This designates the use of incremental coordinates. All moves are relative to the last position of the tool.

    F This designates the feedrate to be used.

    M2 This stops the program execution, and shuts off the spindle and coolant.

    M3 This starts the spindle spinning in the CW direction.

    M4 This starts the spindle spinning in the CCW direction.

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    M6 This tells the milling machine to stop the spindle, return the quill to the home position and wait for a tool change

    M8 This turns on the coolant.

    S This designates the spindle speed to be used.

    T This designates the tool number to be used.

    For the Lathe examples following, these G-Code conventions are used:

    G00 Rapid Traverse mode

    G01 Linear Interpolation mode

    G02 Circular Interpolation in CW direction

    G03 Circular Interpolation in CCW direction

    G22 End of program

    In each of these examples that follow we will emphasize a new programming command or concept. These will be pointed out through the instructional objectives listed before each program.

    When the program is loaded with paper or mylar tape, the first line of every program should start with a "%" character. This is a tape reader control command. (All programs in AML will be down loaded from a computer file and as a result do not need the "%" character.) Each line is written in the same format:

    Nxxx Gxxx Xxxx Zxxx Fxxx

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    Example Program #1

    In this example we will demonstrate the roughing of a part. We want to reduce stock material to the dimensions shown above. Our program will use an absolute coordinate system.

    N G X Z F 00 00 .740 01 01 .750 25 02 01 .750 0 25 03 01 .540 25 04 01 .550 25 05 22

    Example Program #2 This example will show you how to use G01 correctly. In this example, the workpiece from Example 1 has to be finished in one pass. The depth of the cut is 0.01. The tool bits position is indicated on the drawing by the dotted line.

    N G X Z F 00 00 520 0 01 01 -.560 20 02 01 .720 20 03 01 -.760 20 04 01 1.0 20 05 22

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    Example Program #3 In this example we will concentrate on machining a curve. This will be done in two steps. First we have to rough the part with a series of G01 commands then we can profile it with a G02 or G03 command depending on whether the arc is clockwise or counter clockwise.

    Table 4.2: G code using G02 N G X Z F R 01 00 1.0 .125 02 00 .9 03 01 2 25 04 00 .95 .125 05 00 .8 06 01 .650 25 07 00 .850 .125 08 00 .700 09 01 .600 25 10 00 .750 .125 11 00 .600 12 01 .550 25 13 00 .650 .125 14 00 .500 15 01 .500 25 16 02 1.0 .750 .25 17 00 1.2 .200

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    The following diagram should help understand the previous table:

    5.0 Computer Assisted Part Programming/Computer Aided Manufacturing Software Rather than writing a part program manually, computer aided manufacturing/ machining systems can be used to go directly from art to part. A computer aided design (CAD) software package is used to model the part geometry. This geometry information is then used by the computer aided manufacturing (CAM) software to define the manufacturing operations. More specifically, CAM tools can be used to define manufacturing sequences, create tool paths, set federates and spindle speeds, and generate machine control code. Furthermore, most CAM packages also provide on screen verification and machining simulations that allow the user to verify the safety and machining plan that has been defined. By automating the part programming process and providing program verification tools, using CAM software can save valuable time and money. Mastercam is a CAD/CAM package that can perform all of these functions.

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    5.1 Mastercam 5.1.1 About Mastercam Mastercam is an independent software package that provides the tools to create and simulate manufacturing processes. Although Mastercam can be used as a CAD system to create simple geometry from scratch, we will typically be starting with a CAD model from another software package (SolidWorks, Pro/Engineer, Auto CAD, etc). When using Mastercam, NC codes can be created and quickly updated whenever the engineering design model changes. Then, these codes can be output to files and post-processed to drive NC machines. The following section describes how Mastercam can be used to complete exercise number 2. The descriptions that follow are written for Mastercam version 9.1. To begin CAM programming, we must have a manufacturing model in mind. A manufacturing model consists of a workpiece and a design model. The workpiece represents the raw stock that is going to be machined by the manufacturing operations, while the design model, representing the finished product, is used to create and define all of the manufacturing operations. NC sequences are created as workpiece features and represent material removed from the workpiece. Therefore, the workpiece must be larger than the design model so that there is material to remove. Mastercam can produce machining operations for milling, routing, turning, and wire EMD. In this lab, however, we will only use Mastercams milling and drilling capabilities. 5.1.2 Mastercam Process Overview The Mastercam process may consist of the following steps:

    1. Assemble the design model and workpiece together to create a manufacturing model. 2. Setup the process environment by naming the operation an/or cycles, defining a workcell, and specifying other tool and process parameters. 3. Define NC sequences for the operation. At this step, material removal instructions are created. A simulation can be performed to verify the instructions. 4. Create a manufacturing route sheet. 5. Post-process Mastercam file to NC file for machine tool.

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    W O R K P IE C E

    M A N U FA C T U R IN G M O D E L

    SE T U P P R O C E SS E N V IR O N M E N T

    D E FIN E N C SE Q U E N C E S

    FIX T U R E S T O O LS

    D E SIG N M O D E L

    P O ST P R O C E SS

    Manufacturing Parameters Manufacturing parameters specify and control the way in which the tool path and control code is created. Some of these parameters vary depending on the type on NC sequences. These parameters are described in the following section.

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    Common Parameters

    Name

    Clearance The plane at or above which the tool may move at a rapid rate with no danger

    of colliding with the work piece. Specify an ABSOLUTE VALUE.

    Retract The plane at which the tool begins it's cycle. For or milling around islands, the tool will rise to this height while moving to the next pocket/hole. Specify an INCREMENTAL VALUE, as measured from the "Top of Stock."

    Feed plane Height that the tool moves to before changing form the rapid rate to the

    plunge rate to enter the part.

    Top of Stock

    The location of the top of the material to be cut relative to the origin.

    Depth This is the final depth reached by the operation. Specify a ABSOLUTE

    VALUE relative to the origin.

    Depth Cuts Specify the maximum depth the cutter should take in one pass.

    % Stepover The Maximum XY spacing between each machining pass as a percentage of tool diameter.

    Feed Rate Speed of the tool during cutting motion in the workplane

    Plunge Feed Vertical feed rate of the tool (on milling machine) or workpiece (on a lathe).

    Units are revolutions per minute on lathe.

    Speed Rotational speed of the tool (on a milling machine or worpiece (on a lathe). Units are in revolutions per minute (RPM).

    Position Location of the tool in the tool holder.

    Offset Minimum distance between the edge of the tool and the model geometry at

    any point. Often used to leave material during a roughing operation.

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    5.2 Post Processing After the manufacturing plan has been defined with Mastercam and the motions of the center of teach cutter have been specified, we must translate this into something the CNC machine can understand. Mastercams .MC9file contains all this information along with the part geometry, tool definitions, view aspects, and other settings. The tool motions are stored in a relatively common format for all CNC machines however it must be tailored to the specific machine being used. This conversion process is performed by a post processor. There is no universal machine tool language so the post processor used must be machine specific. Mastercam has a post processor built in. Mastercam will generate G-code for the machine on the settings defined along with that machine tool. (Note: If more then one CNC machine is to be used on one part, each machine requires a separate sequence. This way, each operation or motion will be output to the appropriate file in the appropriate format for that machine.) To post process the machining sequence G-code: Go to: Main Menu Operations Post Make sure that the active post is the appropriate post for the machine you plan to use

    (usually MPHAAS.PST)

    Save as an NC file Click OK Mastercam will then prompt you to name the file. Once name and location have been

    specified, click SAVE

    A new window will appear with the G- Code.

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    6.0 CNC Milling Machine Setup and Operation

    In this section we will explain the setup and operating procedures for the Haas VF1 CNC milling machine. While these procedures are intended for use with the Haas Mill in the AML, many of these procedures can be applied to other CNC machines.

    6.1 Haas VF1 CNC milling machine 6.1.1 Operating Panel Controls This section explains the operating panel controls for the Haas VF1 CNC. All safety controls and basic setup and operation controls are identified. The teaching assistant and the Haas Programming and Operating Manual will provide details on actual operation of the equipment. Most of the important controls referred to are shown in Figure 4.D.1.

    Main control panel: is located on the right side of the machine. This is where you will manually control the machine and run your programs. The buttons below to the CRT screen allow you to move through the menus available in the Haas operating system, as well as manually control the machine itself (table, spindle, tool changer, coolant). The buttons to the left of the CRT are used to turn the machine on and off, start and pause programs, and manually move the three axes. Stopping the Machine Emergency Stop: The red button which sticks out from the panel will stop everything. Try to use it only in real emergencies like someone or something falling into the rotating spindle. It will shut down the whole machine and reset to the beginning of the loaded program. You will have to exercise some effort to restart the system and continue your machining. Feed Hold: The red button at the bottom left of the control panel will stop all table movement but leave the spindle rotating. Use this button when something unexpected but not life threatening happens. If the tool breaks, is going to hit the part or vise, or some other operator error, this is the best button to hit. It will halt your program, allow you to fix the problem, and then easily continue to execute your program. See Appendix D for a full list of button functions. 6.1.2 Setup Procedure for Producing a Part This section explains the preparation of the machine for operation. These steps in setup are necessary and time consuming, and it doesn't matter whether you will be using manual or computer assisted part programming. Following these steps will make your setup procedure go faster and in a foolproof manner.

    Workpiece Layout

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    Powering Up Securing Workpiece to Table and Setting the Absolute Coordinate System Tool Preparation and Setting the Tool Length Offsets

    6.1.2.1 Workpiece Layout: Determine an accurate geometric feature of the workpiece stock, usually a square corner, and set this point to the origin in your CAD drawing of the part. All features in your CAD drawing should be with respect to this origin. Remember that the workspace of the machine is limited, so make sure the reference chosen will allow the part to be machined easily

    Common sense is a good guide in laying out your part. For example, the length of an 18" bar to be machined should be positioned along the X-axis, NOT the Y-axis. This is because positioning along the Y-axis would cause the ends of the bar to be unsupported and extend off the table. The limits of travel for the axes are 20" in the X, 16" in the Y, and 20" in the Z-axes.

    If the piece to be cut does not have any accurate geometric features, then mark the piece to facilitate mounting it to the machine. This means:

    Mark the origin using either a scribe or a centerpunch. This origin should coincide with the origin used in specifying the part program.

    Draw two perpendicular lines through the origin, parallel to the sides of the workpiece (as close as possible).

    You should be ready to fixture the workpiece. 6.1.2.2 Tool Preparation: Collect the tools you specified when planning the process and developing your NC

    program. Tools are mounted in tool holders, which in turn are mounted in the tool changer carousel. If your program requires any nonstandard tools which are not already in the carousel, you

    will have to mount them in collets and load them into the tool changer. Put the tool holder in the jig (a big, black, hex nut). Place the tool and the collet in the tool holder, screw the retaining screw onto the tool holder and tighten with the wrench.

    Place all tools in the appropriate position in the tool changer carousel. 6.1.2.3 Powering Up and Aliognment: Flip the main power disconnect switch into the up position. This switch is on the fence

    behind the machine. Turn on the main air valve to the machine. It is also located behind the machine. Flip the power switch for the control box to the on position. It is located on the back of the machine. Press the green POWER ON button to the left of the control panel.

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    Press POWER UP / RESET on the control panel. This will warm up the spindle, and move the table and tool changer to their home positions.

    6.1.2.4 Fixturing the Workpiece: If the workpiece is small you can secure it to the table with a vise. The part for the lab exercise can be held this way. The steps are:

    The vise must be bolted to the table and properly aligned. Place a set of parallels in the vise. Put the workpiece in the vise so that the longest sides are held in contact with the vise jaws. Tighten the vise so that the piece is firmly in place. If the workpiece is too big or of irregular shape so that it cannot be fixtured in a vise, it will have to be mounted directly to the table with special clamps called dogs. Dogs are steel bars with steps cut into one end and a slot cut into the middle. This is more complicated than using a vise and requires at least two clamping points. After locating the workpiece stock on the table, usually in the center of the table, the steps to mount a dog are:

    Take a T-shaped nut and slide it into one of the T slots in the table to within 1/2 to 1 of the workpiece stock.

    Thread a rod of appropriate length into the T-nut. Mount a dog of sufficient length on the threaded rod through the slot, and with the non-

    stepped end touching the workpiece. The threaded rod should stick out at least 1 when the dog is held horizontally.

    A step block should be mounted under the stepped end of the dog. The end of the dog on the step block should be just a bit higher than the end on the workpiece.

    Thread a nut onto the threaded rod and use it to tighten against the dog. Repeat this process for the other three corners. If the piece is small you can get away with

    only two dogs. However, there should always be at least two dogs securing the piece to the table.

    6.1.2.5 Setting the Absolute Coordinate System and Tool Length Offsets: The purpose of this step is to define the origin used when developing the NC code. This means setting an absolute coordinate system in terms of the machine X, Y and Z-axes. If this step is not done, the machine will assume that the position of the table when the power was turned on is the origin for the program. These procedures are for workpieces with square corners that serve as a natural reference point for locating the coordinate axes.

    The steps in establishing the absolute X-Y coordinates are:

    Select the 0.200 inch edge finder or "wiggler tool and load it into the spindle. With the spindle running, use the axis motion controls to move so that the edge finder is

    just below the surface of the stock. Then move one of the axes in the step mode until the

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    edge finder aligns and then just starts to wiggle. Then move the quill up and increment the axis 0.100 inches (the radius of the edge finder).

    Press the OFSET key and PAGE UP until the Work Coordinate page appears. Use the cursor arrows to get to G54 X (or any of the other available work offsets). Push the PART ZERO SET key and the X axis value will be stored as this offset. The cursor will automatically move to the G54 Y location. Repeat the steps above to set

    the G54 Y. Usually the Z axis value will not have to be set and should be zero. The tool length offset (TLO) establishes the Z-axis origin (Z=0) for each tool. This procedure is used to relate the numbered tools called out in a "T## command to the workpiece top surface. Refer to Figure 4.14. This only needs to be done for tools that you added to the carousel. All the tools currently loaded should be set already. The steps in this procedure are:

    Select the new tool and load it into the spindle. Place the 2 gage block on the table surface. Press the OFSET key and PAGE DOWN to the Tool Offset page. Move the cursor to the

    appropriate tool. This must match the pocket number used in defining your machining program!

    CAREFULLY lower the tool to the top surface of the gage block. Use the 0.1, .01 and 0.001 settings when you are within 0.500 inches of the workpiece. Lower the tool until you feel a slight resistance when you pass the gauge between the tool and the piece. The tool is now 2 above the workpiece.

    Press the TOOL OFSET MESUR key and the Z value will be stored in the tool offset. Close the machine doors. Press the NEXT TOOL key and the Z-axis will retract to tool

    change position and the next tool will be loaded into the spindle. Repeat the above steps to set the next tool.

    Figure 4.14: Tool Length Offset

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    7.0 Abrasive Water Jet Machining 7.1 Introduction to Technology Waterjet cutting uses a high pressure stream of water to cut through material. Abrasive waterjet cutting adds small abrasive particles to the stream of water to speed up the process. Both processes force the water (typically 20,000 to 55,000 psi.) through a tiny hole in a jewel orifice (usually 0.010 0.015 in a ruby, sapphire, or diamond). The abrasive is then added to the stream and goes through a mixing tube to produce a stream of wet abrasive moving upwards of Mach 2. The abrasive allows the process to cut metals and other hard materials. The jewel and mixing tube are mounted on the cutting head of the machine, analogous to the spindle of a milling machine. This head is servo positioned above a cutting table. The cutting table is simply a row of thin slats above a large tank of water. The abrasive stream will cut through everything in its way, workpiece, and table. The depth of the water tank slows down the stream and dissipates the power. The tank also serves to contain the water, abrasive, and small particles of the workpiece. The slats are eaten away over time, but are expendable. Robotic arms are also being equipped with water jet heads to provide added flexibility and cutting of complex 3D geometries.

    Figure 4.15: Water Jet Nozzle Figure 4.16: Abrasive Water Jet Nozzle

    Water jet machines are used to cut diapers, candy bars, chicken, fish, foam, rubber, paper, fabric, carpet, and other soft materials. Food is often cut with waterjet machines because it is a very

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    clean process. In the cases of tissue paper and disposable diapers the waterjet process creates less moisture on the material than touching or breathing on it.

    Abrasive waterjet uses small particles of abrasive grit (basically sandpaper without the paper) to cut harder materials. These machines can cut aluminum, steel, copper, marble, wood, glass, plastic, almost anything. Garnet is the most commonly used abrasive because it is inexpensive, hard, and capable of cutting a wide variety of materials. Special abrasives are made for cutting soft materials, or other specific applications. Abrasives come in different hardness, mesh size (particle diameter), sharpness, and purity.

    As the water passes through the jewel, it is about 0.010. Once the abrasive is added and the stream leaves the mixing tube, it is 0.020 0.030. This stream expands slightly as it flows through air, much like water coming out of a fire hose. If setup correctly, the material being cut helps to restrict the stream from fanning out, the cut will be relatively straight. Typically, the stream produces a tapered cut which expands ~0.001/inch of depth of cut. The abrasive water jet is described as a floppy tool. This is because it is not rigid and cannot maintain its shape as it cuts through the material. Rather than staying vertical as it passes through the workpiece, it slowly eats through the material and only hits the upper surface at full speed. As the nozzle moves forwards, the water stream slows down and curves away from the direction of travel (fig. 4.17). The jet lags between where it enters the workpiece and where it exits.

    Figure 4.17: Jet lag in thick part

    7.1.1 Tolerances & Part Accuracy

    There are a number of problems with waterjet technology that limit the accuracy of the process. The cutting nozzle is controlled via servo motors and lead screws. This arrangement is typically capable of 0.0002 positional accuracy when used in milling machines. The nature of waterjet cutting adds a number of additional limitations and control challenges to making perfect parts. The radius of the abrasive jet, stream taper, water pressure regulation, and speed control affect kerf width. These factors typically limit part accuracy to 0.010, however making a few test parts and tweaking the cut parameters may allow accuracies up to 0.005.

    The geometry of a part can also affect tolerances. Jet lag causes the abrasive stream to exit the

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    bottom of the workpiece behind where it enters. When cutting a straight line, this is often not a problem, but when cutting a curve or inside corner it can produce unwanted results. Most controllers automatically adjust the nozzle speed to correct for geometry issues. Speeds are generally reduced around curves, and the jet will pause for a moment when cutting inside corners. Acceleration and deceleration can be controlled to further improve cutting accuracy. All of these changes are generally achieved in the control software and transparent to the operator. Fig 4.18 shows a square hole cut at high speed. The jet cut into the part as it turned each corner. Slowing down as the jet enters the corner, pausing to cut all the way through the material, and then slowly leaving the corner can solve this problem. When cutting a curve, the jet will tend to sweep out an arc and give a conical taper. This is shown (exaggerated) in Fig 4.19.

    Figure 4.18: Corner Blowout Figure 4.19: Curve Taper

    7.1.2 Surface Finish

    Abrasive Waterjet cutting is basically a grinding process. Nozzle speed affects how much material the stream can eat away as the cutting head traverses over the desired geometry. A high federate will produce parts quickly and cheaply, but surface finish will be affected. Going just fast enough to cut all the way through the workpiece may leave an undesirable burr on the bottom of the part. Jet marks will also be visible along the cut edge of the part. 7.1.3 Cost

    An abrasive waterjet machine uses water and abrasive as consumable tooling. In addition to these obvious costs, many parts of the machine have a limited life. High water pressure, and the rough abrasive cause many components to wear out. The mixing tube focuses the stream of water and abrasive. It is made of tungsten carbide for high wear resistance. As the inner diameter of the mixing tube is worn away by the abrasive stream, the stream will grow larger and accuracy will suffer. The jeweled orifice controls the size and shape of the water stream. Minerals deposits can obstruct the opening and degrade the circularity of the water stream. The gaskets and seals of the high pressure pump wear over time and must be replaced to ensure even continuous pressure. All these factors combine to an operating cost of approximately $25/hour depending on machine and nozzle configuration. This does not include workpiece material, labor, or time to generate computer code.

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    Component $/hour of run timeHigh pressure pump $4.42 Orifice $0.17 Mixing tube $1.83 Filters $1.00 Abrasive $.75 / lb.

    These cost parameters are used for the AWJ on campus. Note they do not include the cost of water, electricity, or routine maintenance. Abrasive use varies from .5-2 lbs per hour depending on mesh size, water pressure, and hopper setup.

    This example part is a spoon blank manufactured by AML students. It was cut

    from 0.030 sheets of 304 Stainless Steel.

    Cost $0.38 Cut time 54 seconds Abrasive used .342 lbs Length of cut 22.1

    7.2 Comparisons to Other Technologies 7.2.1 Stamping Stamping dies are used to blank many parts from sheet material. An AWJ machine can be used to cut these same parts from sheet or strip stock. The sheets can often be stacked (to 1-2) for faster production. For short production runs (less than 2000), the initial cost of manufacturing the die is probably higher than cutting parts on a water jet. The waterjet is also more flexible. Part geometry can be changed easily by modifying the computer code. Changes made to a die are prohibitively expensive, so production of the die is usually held off until final part design is finalized. Even if not used in production, AWJ is an excellent way to prototype stamped parts. 7.2.2 Conventional Milling

    Setup and cleanup are much faster for an AWJ than a mill. Chip cleanup and degreasing or cleaning of finished parts is not necessary. Tool changes and calibration are also eliminated on an AWJ. Nesting allows more parts to be produced out of the same stock material, and the scrap

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    generated is still whole, not in chips. This material may be more valuable as a sheet than a pile of chips.

    7.2.3 Laser Cutting

    Abrasive Water Jet can cut many materials that lasers cannot (aluminum, copper, stainless steel).

    Because there is no heat produced during cutting, there is no warping or heat-affected zone. This eliminates the need for heat treating and stress reducing in post-processing. AWJ are safer, more environmentally friendly, and cheaper than laser cutting systems. Thicker parts are possible with AWJ (2-3 vs.

  • CNC MACHINING & CAD

    this part was four weeks. Cutting all 2400 parts and the jig on the waterjet cost $610. For a larger production run, the die would have been a more economical solution, but because of the low runs required in the AML, the students chose to use the AWJ. An added bonus of the AWJ is design flexibility. After cutting the first set of contacts, the geometry was changed slightly to enhance the assembly process. A progressive die would have been extremely hard to modify once the prints were sent to the vendor. 7.4 Machine Setup 7.4.1 Clamping The forces placed on the workpiece during waterjet cutting are almost exclusively straight down onto the table. The weight of your part is often enough to resist motion from any side forces induced by the cutting action. For thin parts, placing weights onto the workpiece is usually sufficient. Be certain that the weights are not within the path of the nozzle or skirt. A collision may move your workpiece, or damage the cutting head. On larger parts (greater than thick), the part may shift along the cut direction. This is due to the soft cutting tool aspect of the water stream. Along the perimeter of the tank, there are bolt holes which can be used to bolt down dog ear clamps. Hook, or quick clamps can also be used to provide clamping force in the middle of the tank. These clamp the part to the slats in the cutting table. Be sure that the workpiece extends over the slat being clamped to, otherwise, the clamp will simply life the slat out of the tank, using the workpiece as a lever. Again be certain that all clamps and weights are not in the path of the nozzle or skirt.

    7.4.2 Orientation

    The workpiece should be orientated on the cutting table, just as it is in the FlowCUT software. Alignment is difficult on the AWJ machine. Typically since the exterior of the part is being machined, the exact location of the workpiece is not important. Simply running the nozzle back and forth above the edge of the workpiece, and aligning it to the axes of the machine is usually sufficient. For more accurate machining, using a jig to maintain position is suggested. It is possible to mount a dial indicator to the nozzle, but this can be difficult.

    7.5 Creating a .DXF file

    To create a program to cut a part on the Abrasive Water Jet Machine, you need a .dxf file. This standard file format is a 2D line drawing of your part and can be created in ProEngineer, Solidworks, or any other CAD package. Create a drawing of the part you wish to cut. The drawing should be 1:1 scale and have a single view of your part, with the edges you want to cut being perpendicular to the view plane. This view will correspond to looking down at the cutting table of the machine from above, matching the view of the cutting head.

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    ProEngineer Open the drawing you wish to cut. File - export - model Choose dxf format SolidWorks Open the drawing you wish to cut. File - Save As Choose dxf format. Nesting is a method of arranging parts on your workpiece to minimize scrap material. FlowPATH has some simple tools for arranging parts in a grid, which works OK for square parts, but for more complicated geometry, you may want to nest the parts in your original CAD drawing. Fig 4.21 illustrates the patterning capabilities of FlowPATH, while Fig 4.22 shows a more ideal layout for the same part.

    Figure 4.21: Grid Part Layout Figure 4.22: Nested Parts

    7.6 FlowPATH Software FlowPATH is similar to ProManufacture. Based on a CAD drawing, it can generate tool paths for the abrasive waterjet machine to cut. Parts can be imported as .dxf files, or simple parts can be drawn within the program. Programming the parts is relatively simple, compared to milling or turning a part. Tool changes, depth of cut, and retract plane are not a concern. The only things to define is which lines to follow and in what order, which side of the line the tool should stay on (inside for holes, outside for perimeter of parts), and what speed to move at. It is generally a good idea to cut interior features first, and then cut the perimeter of the part. If done in reverse, the part might shift slightly while holes or slots were being cut in the interior.

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    The speed of the nozzle is determined based on the material and thickness, as well as geometry. Cutting 2 steel will obviously take longer to get through the 1/8 Plexiglas. Geometry is also a factor because of the floppy tool characteristics of the abrasive waterjet. A straight line cut can be done faster than a circle or corner. The FlowPATH software automatically adjusts the speed and acceleration rates based on the geometry of your part. As a programmer you have further control of the overall speed used. The material and thickness provide an ideal speed at which the jet would just finish breaking through the material. This provides a fast, but unattractive and wavy cut. For a better finish and improved tolerances, the speed of each line can be set to a percentage of this ideal speed (20%, 40%, 60%, 80%, and 100%). To begin a cut, the abrasive waterjet must first pierce a hole in the material, and then begin to move and cut the part. The pierce time, how long the jet must stay in one place to create a starting hole, is set automatically by the software. This hole is generally slightly larger than the kerf created when cutting, and may leave an undesirable mark on the part. The solution to this problem is to use a lead-in and lead-out for cuts. These are approach and exit paths for the tool to follow so that the kerf is through the part and at a steady state size before the nozzle begins to cut on the desired line. FlowPATH can generate arc, corner, and straight lean-in/outs.

    Figure 4.23: Lead In / Lead Outs 7.6.1 Ordering the Cut Path The cut path can be generated automatically, or manually. If done manually, you will have to draw all necessary lead in/outs and rapid traverse lines. Autopath creates these automatically. It is strongly suggested that you save the part before trying Autopath. This will preserve a clean copy of your .dxf file, since Autopath can produce interesting results for complicated parts. 7.6.1.1 Manual Path Ordering Confirm lead-in/lead-out properties Draw - Lead-in/lead-out properties Assign lead in/outs where necessary Draw - Lead-in/lead-out Link leads with traverse lines

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    Draw - Link leads with traverse Select all entities to be pathed Begin manual path ordering Preprocess - Manual order path Click on the first lead-in Select direction of tool offset As prompted select next line to follow (choices highlighted in red) at each intersection and choose offset direction Click OK when path is complete 7.6.1.2 Automatic Path Ordering Select all entities to be pathed Begin automatic path ordering Preprocess - Auto order path Once the path is defined, export the path information to be used in FlowCUT, and save the .dxf file for later changes. File - Export path as.. generate ordered path file File - Save update .dxf file 7.7 FlowCUT Software This software package runs the abrasive water jet machine. Based on an .ord file created in FlowPATH, and a number of settings (tool radius, material, thickness, pierce time, etc) it controls the servos, valves, and sensors of the machine itself. Although there are a number of variables involved in running the machine, most of these have been set at nominal values, and we will focus on only a few to make operation simpler. 7.7.1 Duplicating Parts FlowCUT is capable of duplicating parts into a rectangular array, as shown in Fig 7.26. This is helpful when cutting several of the same parts from the same material. The software will duplicate the cut motions and connect each part with rapid traverse lines automatically. Edit - Duplicate the part Enter the desired number of rows and columns (ex: 2 rows and 3 columns = 6 parts). Set the distance between rows and columns (0.10 default is usually fine). Preview part again. Record new time and cost estimates. 7.7.2 Pausing and Stopping a Program

    There are 4 ways to stop the machine during a cutting cycle:

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of 67/67
Section IV Computer Numerical Controlled Machining and Computer Aided Manufacturing Revised 8/2/05
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