INTRODUCTIONIn order to meet the increasing demand to manufacture complicated components of

high accuracy in large quantities, sophisticated technological equipment and machinery have been developed. Production of these components calls for machine tools which can be set up fairly rapidly without much attention.

The design and construction of Computer Numerically Controlled (CNC) machines differs greatly from that of conventional machine tools. This difference arises from the requirements of higher performance levels. The CNC machines often employ the various mechatronics elements that have been developed over the years. However, the quality and reliability of these machines depends on the various machine elements and subsystems of the machines. There are some of the important constituents parts and aspects of CNC machines to be considered in their designing, for example Machine structure, Guideways, Feed drives, Spindle and Spindle bearings, Measuring systems, Controls, Software and Operator interface, Gauging, Tool monitoring. The control of a machine tool by means of stored information through the computer is known as Computer Numerically Controlled. The information stored in the computer can be read by automatic means and converted into electrical signals, which operate the electrically controlled servo systems. Electrically controlled servo systems permits the slides of a machine tool to be driven simultaneously and at the apporopriate feeds and direction so that complex shapes can be cut, often with a single operation and without the need to reorient the workpiece. Computer Numerically Control can be applied to milling machines, Lathe machines, Grinding machines, Boring machines, Flame cutters, Drilling machines etc.HISTORICAL DEVELOPMENTS IN CNC MACHINES

A relatively short time ago, machines were operated by craftsmen who determined the many variables such as speeds, feeds, depth of cut etc. Now much of this work is being assigned to computer and machines that are numerically controlled.

Numerical control as applied to machine tools, had it’s beginning in 1949, when the United States Air Force combined with Parsons Corporation to produce contoured surface from instructions in the form of punched tape.

Numerical control equipment was substituted for the tracer controls on a three axis Cininnati Hydrotel Vertical Planer Mill.

A successful feasibility demonstration of continuous contour milling was held in March 1952 followed by a final report; both the Air Force and Private Industry began further work to extend the Numerical Control technology developed at Massachusetts, Institute of Technology, U.S.A.

Between 1955 and 1960 only 500 Numerically Controlled Machine were installed in U.S.A.

During 1960 and 1964 approximately 4000 more were added. Till the middle of 60’s Russia, West Germany, Japan, and U.K. were for behind

U.S.A., in the production of Numerical Control Machine Tools. Japan entered the Numerical Control commercial scene during mid 60’s and by 1971

surpassed the U.S.A. in NC ratio. NC ratio = Production of NC machines tools (units) Total production of machine tools

U.S.S.R. is also one of the largest producers of nc machine tools mainly for industrial use.

FEATURES OF CNC MACHINES Rigid machine structure is provided, in order to bear the static load, Dynamic load ,

Thermal load , Vibration. Guideways are used in machine tools to control the direction or line of action of the

carriage or the table on which a tool or workpiece is held , To absorb all the static and dynamic forces.

On a CNC machine the function of feed rate drive is to provide motion to the slide as per the motion commands. Since the degree of accuracy requirements are high, the feed drive should having high efficiency and response.

Hydrodynamic bearings, Hydrostatic Bearings, Antifriction Bearings are provided to the CNC machine. In order to achieve the accuracy and the quality of the work produced depends directly on the geometrical accuracy, running accuracy and the stiffness of the spindle assembly.

On all CNC machines, an electronic measuring system is employed on each controlled axis to monitor the movement and to compare the position of the slide and the spindle with the desired position.

CNC controls are the heart of the CNC machines. The early CNC controls were developed for simple applications in turning, machining centres and grinding. But with increased capabilities on modern machine tools such as higher spindle speeds, higher rapid traverses and more number of axes, CNC systems have been developed to meet these needs.

Better work piece quality is one of the most important advantages in using a hi-tech CNC machine. To maintain quality the effect of parameters like tool wear and thermal growth can be eliminated by automatic gauging system.

Presently, established tool monitoring sensors and systems are available commercially for integrating and systems are available commercially for integrating with CNC machines. Tool monitoring systems enable the introduction of adaptive controls on machines for optimizing the cutting parameters.

Feedback is taken from various devices, e.g. Ecoders, Transducers.MACHINE STRUCTURE The machine structure is the load carrying and supporting member of the machine tool. All the motors, drive mechanisms and other functional assemblies of machine tools are aligned to each other and rigidity fixed to the machine structure. The machine structure is subjected to static and dynamic forces and it is, therefore , essential that the structure does not deform or vibrate beyond the permissible limits under the action of these forces. All components of the machine must remain in correct relative positions to maintain the geometric accuracy, regardless of the magnitude and direction of these forces. The machine structure configuration is also influenced by the consideration of manufacture, assembly and operation. The basic design factors involved in the design of machine structure are as follows,1. Static load2. Dynamic load 3. Thermal load4. Guideways 5. Feed drive: - 1)Servo motor, 2)Mechanical Transmission System6. Spindle / spindle bearings 1)Hydrodynamic 2)Hydrostatic 3)Antifriction 7. Measuring Systems: - 1)Direct 2)Indirect

8. Controls, Software and user interface9. Gauging10. Tool monitoring systems

CONFIGURATION OF CNC SYSTEMA CNC system basically consists of the following:-

Central processing unit (CPU). Servo control unit Operator control panel Machine control panel Other peripheral devices Programmable logic controller

Figure gives the typical control configuration of Hinumerik 3100 CNC system.

1. Central Processing Unit (CPU) :-The CPU is the heart of a CNC system. It accepts the information stored in the memory as part program. This data is decoded and transformed into specific position control and velocity signals. It also oversees the movement of the control axis or spindle and whenever this does not match with the programmed values, a corrective action as taken. All the compensation required for machine accuries (like lead screw pitch error, tool wear out, backlashes.) are calculated by CPU depending upon the corresponding inputs made available to the system. The same will be taken care of during the generation of control signals for the axis movement. Also, some basic safety checks are built into the system through this unit and continuous necessary corrective actions will be provided by CPU unit. Whenever the situation goes beyond control of the CPU, it takes the final action of shutting down the system and in turn the machine.

2. Servo control unit :-

The decoded position and velocity control signals, generated by the CPU for the axis movement forms the input to the servo control unit. This unit in turn generates suitable signals as command values. The command values are converted by the servo drive units which are interfaced with the axes and the spindle motors. The servo control unit receives the position feedback signals for the actual movement of the machine tool axes from the feedback devices (like linear scales, rotary encoders, revolvers, etc.)3. Operator Control Panel :- The Operator Control Panel provides control panel provides the user interface to facilitate a two way communication between the user, CNC system and the machine tool. This consists of two parts;

Video display unit Keyboard

4. Machine Control Panel :- It is the direct interface between the operator and the NC system, enabling the operation of the machine through the CNC system.During program execution, the CNC controls the axis the motion, spindle function or tool function on a machine tool, depending upon the part program stored in the memory. Prior to the starting of the machining process, machine should first be prepared with some specific takes like,

Establishing a correct reference point Loading the system memory with the required part program Loading and checking of tool offsets, zero offsets, etc.

5. Other peripheral devices :- These include sensor interface, provision for communication equipment, programming units, printer, tape reader interface, etc.6. Programmable logic Controller:-A PLC matches the NC to the machine. PLC’s were basically as replacement for hard wired relay control panels. They were basically introduced as replacement for hard wired relay panels. They developed to be re-programmed without hardware changes when requirements were altered and thus are re-usable. PLC’s are now available with increased functions, more memory and larger input/output capabilities.In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU receives input data, performs logical decisions based upon stored programs and drives the outputs. connection to a computer for hierarchical control are done via the CPU.