1_Metal cutting theory

transcript

No. Topics

1Theory of Metal Cutting: Basic principles of cutting tools, mechanics of chip formation, cutting tool material and fluids, single point cutting tool geometry and tool life and wear.

2Material Removal Processes & Machines: Material removing machines. Lathe, milling, shapers and planers, drilling machines and their operations. Tool and work holding devices. Safety measures.

3Abrasive Machining & Finishing Operations: Abrasives, grinding wheels, grinding processes, grinding fluids, types of grinders, finishing operations, honing, lapping, and coated abrasive.

4Non-conventional Machining Processes: Electro chemical machining, chemical milling, electrical discharge machining.

Powder Metallurgy: Production of metal powders, compaction and pressing, sintering, finishing operations, design considerations.

6Surface Treatment & Coating: Mechanical surface treatment and coating, electroplating, electro-forming, thermal spraying, anodizing, hot dipping, surfaces cleaning.

7 Control of Machine Tools & CIM: Automation, NC, adaptive control, computer aided design, computer aided manufacturing, computer numerically controlled machines, artificial intelligence, flexible manufacturing system, JIT concept, computer integrated manufacturing systems, group technology, cellular manufacturing, factory of the future.

Course Outline (Manufacturing Process II)

Manufacturing?

Manufacturing is the application of physical and chemical processes to alter the geometry, properties, and or appearance of a given starting material to make parts or products; manufacturing also includes assembly of multiple parts to make products.

Manufacturing Vs Production

Manufacturing Process

Starting material Processed part

Scrap and waste

Toolin

Manufacturing is the transformation of materials into items of greater value by means of one or more processing and/or assembly operations.

Property enhancing process

Surface processing operations

Manufacturing processes

Processing operations

Assembly operations

Processing operations

Permanent joining processes

Mechanical fastening

Solidification processes

Particulate processing

Deformation processes

Metal Removal

Heat Treatment

Cleaning and surface treatment

Coating and deposition processes

Welding

Brazing and soldering

Adhesive bonding

Threaded fasteners

Permanent fastening methods

Manufacturing Processes

Material Removal Processes (Machining)

Machining refers to cutting operations that are based on the removal of material from an originally rough-shaped workpiece, for example via casting or forging through power driven machines.

Machining is the process of removing unwanted material from a workpiece in the form of chips. If the workpiece is a metal, the process is often called metal cutting or metal removal.

Machining is a manufacturing process in which a sharp cutting tool is used to cut away material to leave the desired part shape.

Importance of Machining

• Variety of work materials

• Variety of part shapes and geometric features

• Dimensional accuracy

• Good surface finishes

Disadvantages

Wasteful of materials

Time consuming

A complex process and difficult to analyze. Why?

• Different materials behave differently

•The process is asymmetrical and unconstrained, bounded only by the cutting tool.

•The level of strain is very large.

•The strain rate is very high.

• The process is sensitive to variations in tool geometry, tool material, temperature, environment (cutting fluids) and process dynamics (chatter and vibration).

There are seven basic chip formation processes: shaping, turning, milling, drilling, sawing, broaching and grinding.

For all metal cutting processes, it is necessary to distinguish between speed, feed and depth of cut

Basic Machining Processes

Cutting parameters

V=πDN

Speed (V) is the primary cutting motion, which relates the velocity of the rotating workpiece with respect to the stationary cutting tool.

Units: sfpm, in/min, m/min, m/s

Cutting parameters

Feed (f) is the amount of material removed per revolution or per pass of the tool over the workpiece.

The tool feeds parallel to the rotational axis of the workpiece.

Units: in/rev, mm/rev

Cutting parameters

The depth of cut (DOC), d, represents the third dimension. In turning, it is the distance the tool is plunged into the surface.

Units: in, mm

DOC=(D-d)/2

Material Removal Rate (MRR)

It is the amount of material removed by the cutting tool in unit time.

Units: in3/min, mm3/min

MRR= speed x feed x DOC

MRR= (volume of cut)/(cutting time)

CT= (L+Allowance)/(fN)

Supporting websites

1. http://www.menet.umn.edu/degarmo/

Chapters 21,22,23,27

2.http://www.me.gatech.edu/jonathan.colton/me4210/mfgvideos.html

(Manufacturing process videos including machining processes)

Feed (in Milling)

fm=ftnN

Milling, a multiple tooth process has two feeds: the amount of metal an individual tooth removes, called the feed per tooth, ft, and the rate at which the table translates past the rotating tool, called the table feed, fm, feed per minute.

Where n= no. of teeth

Orthogonal Cutting ModelBy definition, orthogonal cutting uses a wedge-shaped tool in which the cutting edge is perpendicular to the direction of cutting speed.

As the tool is forced into the material, the chip is formed by shear deformation along a plane called shear plane, which is oriented at an angle Φ with the surface of the work.

Along the shear plane where the bulk of the mechanical energy is consumed in machining, the material is plastically deformed.

Orthogonal Plate Model

The workpiece is a flat plate. The workpiece is moving past the tool at velocity V. The feed of the tool now is called uncut chip thickness t. The DOC is the width of the plate, w or width of cut. The Cutting edge of the tool is perpendicular to the direction of motion V.

2D Model

Clearance angle

Chip formation

Merchant’s Model

Merchant’s force circle diagram

The Merchant Equation

Assumption: The shear strength of the work material is a constant, unaffected by strain rate, temperature and other factors. Therefore the equation for Φ is an approximate relation rather than an accurate mathematical equation.

The shear plane angle can be increased by :

(1) Increasing the rake angle

(2) Decreasing the friction angle (coefficient of friction)

If all other factors remains constant, a higher shear plane angle results in smaller shear plane area.

Since the shear strength is applied across this area, the shear force required to form the chip will decrease when the shear plane area is reduced.

A greater shear plane angle results in lower cutting energy , lower power requirements, and lower cutting temperature.

Types of Chips in Machining

• Chip formation affects the surface finish, cutting forces, temperature, tool life and dimensional tolerance.

• Understanding the chip formation during the machining process for the specific materials is useful to determine the machining speeds, feed rates and depth of cuts for efficient machining and increase tool life.

During the machining process, four basic types of chips are formed:

Discontinuous Continuous Continuous with Built-up Edge Serrated chip formation

Discontinuous chip formation normally occurs during the machining of brittle work material such as glass and silicon. This type of chips also occurs when machining using cutting tools with small rake angles, coarse machining feeds (large depth of cut), low cutting speeds and lack of lubricant or cutting fluid. Discontinuous chip formation leads to continuously changing forces, resultant vibration and chattering in the machine tools and thus results in a final workpiece with poor surface finish and loose tolerance.

Continuous chip formation is normally considered to be the ideal condition for efficient cutting action as it gives excellent finish and occurs usually for ductile metals. The chip consists of a continuous "ribbon" of metal which flows up the chip-tool zone. It normally occurs at high cutting speed and rake angle, and a narrow shear zone. Chip breakers are used during the machining to prevent the chips from entangling with the tool holder.

Continuous chip with built-up edge is basically the same as continuous chips. However, during the former chip formation, as the metal flows up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool, it affects the cutting action of the tool. This type of chip formation is common in machining of softer non-ferrous metals and low carbon steels. Common problems are the built-up edges breaking off and being embedded in the workpiece during machining, decrease in tool-life and final poor surface finish of the workpiece.

Machining of difficult to cut materials at high speed often results in instabilities in the cutting process due to thermo-mechanical response of the work material. The consequence is shear localized or serrated chips. The serrated chips are usually formed in the pattern of shear localization and the chip deformation is concentrated in the narrow shear band while most part of the chip segment is formed under relatively low strain. Once the shear is localized it can not be shifted to a continuous chip again with the increase of cutting speed

1_Metal cutting theory

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