CNC Machines

Unit - 4

CNC MACHINES

Computer Numerical Control (CNC) machine tools, constructional details, special features - Drives, Recirculating ball screws, tool changers; CNC Control systems Open/closed, point-to- point/continuous Turning and machining centres - Work holding methods in Turning and machining centres, Coolant systems, Safety features.

INTRODUCTION

In the late 1940's, T. Parsons formulated a method of using punched cards containing coordinate position data to control a machine tool. The machine was directed to move in small increments to generate the desired surface of an airfoil. In 1948, Parsons demonstrated his concept to the US Airforce. Then it was taken as a series of research projects at Servo Mechanisms Laboratory of the Massachusetts Institute of Technology (MIT).

The initial stage involved in the development of a prototype NC milling machine by retrofitting (back fit) a conventional tracer mill i position. Servo mechanisms were used for the movement of three machine tool axes. The First demo of an NC prototype was held in 1952. By 1953, the usefulness of NC concept had been proven.

Shortly thereafter, the machine tool builders began the initiation of their development projects to introduce commercial NC units. Also, certain user industries such as aircraft and Space vehicles builders worked to implement the numerical control machines to satisfy their needs. Additional research were going on at MIT. They developed part programming language which could be used for NC machines. This part programming language leads to the development of Automatically Programmed Tools (APT) language. APT is utilized in many industries and most other languages used in NC and CNC machines are based on APT language.

There are numbers of problems in conventional NC machines as given in Chapter 5.1.1 which have motivated the machine tool builders to seek improvements in basic NC systems. To rectify these problems, digital computers were integrated to NC system. The integration of computers with NC machine tools resulted in the following advantages:

(i) The memory for part program processing is increased.

(ii) The capacity for storing large part programs is increased. (iii) Editing of part programs on the control console made easy.

(iv) Substantial improvements made in controls for NC systems. (v) Realization of control logic through software.

(vi) The reliability of operation of NC machines is improved.

(vii) In the manufacturing system, the integration of NC machines is made easy. 

Direct Numerical Control (DNC) was the first of the computer control systems to be introduced around 1968. In this evolution, the computer of that era was quite large and expensive, and the only feasible approach was to use a large computer to control a number of machine tools on a time-shared basis. The advantage of DNC was that a direct control link between computer and machine tool eliminates the punched tape input.

The next stage after getting less expensive computers was introduced a single small computer to one machine tool. This concept is called Computer Numerical Control (CNC). CNC systems were commercially first introduced around 1970, and they applied the soft-wired controller approach to have a better advantage. One standard computer control could be adapted to various machine tools by programming control functions into the computer memory for a particular machine. Being less expensive, the machine tool builders design CNC control panel as an integral part of the machine tool rather than a separate stand-alone cabinet. It reduces the floor space requirements for the machine. Very Large Scale Integrated (VLSI) circuits used in these controllers are advantageous to the machine tool designer and to the machine user.

1. Problems with Conventional NC Machines

1. Wear and tear of punched tape:

A paper tape is especially fragile and its susceptibility to wear and tear. So, the punched tape is unreliable in producing NC components.

2. Least reliable tape reader:

Tape reader is the least reliable hardware components of the machine while sudden breakdown occurs.

3. Flexibility of controller:

The hard-wired controller cannot easily be altered to incorporate further improvements.

4. Less management information:

The machine tool manufacturers have continually been improving NC technology. Therefore, the prepared punched cards cannot further be reused in NC machining process.

5. Part programming mistakes:

When preparing the punched tape, the part programming mistakes may commonly occur. To achieve the best sequence of processing steps, many cards should be punched in order to get the correct punched cards.

6. Non-optimal speed and feed:

During cutting operation, the speed and feed cannot be altered.