the system created another highly skilled job--a part programer. This new job requires the expertise of both a skilled machinist and a mathematician's understanding of computer languages. It is his job to extract from the engineering drawings all the data necessary to program the numerical calculations to control the machine movements.*

Numerical control by computer

Although tape is the most common input device in use today, in the latter 1960s software was developed to enable the computer to directly input the numerical data to the machine control unit and to bypass the tape. There are several ways to accomplish this. A large central computer can be used to directly control any number of machines. This system is referred to as direct numerical control or DNC. However, what may be a more important technological advance to computer-integrated manufacturing is the minicomputer and more recently the microcomputer.

These computers can be used to operate a single machine or several machines and can be linked to larger computers in a hierarchal system. This is referred to as computer numerical control, or CNC. Before the development of the minicomputer and microcomputer, only companies with large computer complexes could consider DNC. However, the minicomputer and microcomputer have been reduced in costs so that CNC units are competitive with the older, tape-driven control units.

The use of computers to replace tape controllers adds an element of time saving in reprograming, on a real time basis, the numerical sequences in part programs to correct errors or to implement design changes and enables the feedback of data to management.

As a flexible automation method NC can be programed to perform different functions on a variety of different parts, using the general purpose machine tool in a more efficient manner than possible with manual operation. A major benefit to be gained from using NC equipment is the quality of the part. For example, sophisticated jet aircraft parts require

*Since the same mathematical data base is needed to create the design as well as to program the machine tool to make the part, the technology has advanced to the point where, in some companies, no engineering drawing need be made. The design is captured in the data base and the computer programs the cutting path.

high precision cuts to meet tolerance requirements. Such parts can be made only with NC. Also with NC each part will be identical to the preceeding one since the judgment element is removed from the cutting procedure. This provides tremendous benefits to the follow on assembly operations, because with closer tolerances, assembly becomes simpler and consequently less costly. Some users feel the major cost advantages come from easier assembly.

COMPUTER-AIDED MANUFACTURING

It has been estimated that a part is on a machine less than 5 percent of the time it is in the shop and for only a portion of that time is the tool cutting metal. (See chart 3.) The remaining time is spent on waiting, moving, setup, downtime, assembly, inspection, etc. The improvements being developed in these operations, not just additional improvement of the metal cutting operation, hold the key to future quantum increases in productivity of batch manufacturing operations.

Early NC machines controlled the operation of only a single type of cutter, such as a drill. As they evolved, these machines were equipped with automatic tool changers that hold various types of cutting tools. These are known as machining centers (see fig. 6) which, as one single machine, perform the functions of several NC machine tools, i.e., drilling, milling, boring, etc. For work in these centers the parts can be fixed onto moveable pallets which eliminates setup time at the machine.

Additionally, there are systems which integrate multiple machining centers and automatically control the movement of work pieces from one NC center to the next on computercontrolled material handling systems. Not only can several different machining functions be performed without removing the parts being machined from this system, but the machine tool operator is relieved of various manual duties in that a single operator can apply his skills to several machines rather than to one. (See fig. 7.) Unlike mass production transfer lines, these computer-controlled flexible systems can randomly handle different types of parts and select and load the part onto the machine that can most economically perform the required operations.

Systems are also in use or are under development for automatically selecting work pieces from storage and moving them to machining centers; moving semifinished parts to intermediate storage and return for final processing, and moving finished work pieces to storage pending shipping and distribution--all under computer control.