Basic Features of Numerical Control

Numerical control is a form of digital control that is employed on machine tools such as milling machines and lathes to automatically cut and shape the work-piece without a human operator. Each machine has its own set of axes or parameters that must be controlled; case in point is a milling machine as illustrated in the figure below:

With reference to the example of numerical control milling machine above, the work-piece that is being formed is fastened to a movable table. The table can be moved (with electric motors) in three directions i.e. X, Y, and Z. The cutting-tool speed is automatically controlled as well. To generate a part, the table moves the work-piece past the cutting tool at a specified velocity and cutting depth. In this illustration, four parameters (X, Y, Z and revolutions per minute [rpm]) are continuously and independently controlled by the controller. The controller takes as its input a series of numbers that completely describe how the part is to be generated. These numbers include the physical dimensions and such details like cutting speeds and feed rates.

Traditionally, in legacy systems, the data from the part drawing was entered manually into a computer program. This program then converted the input data into a series of numbers and instructions that the NC controller could comprehend, and either stored them on a storage device or sent the data directly to the machine tool. This data was read by the machine tool controller as the part was being created. With the introduction of computer-aided design (CAD), the task of manually programming the manufacturing instructions has been done away with. Now it is possible for a special computer program (referred to as a postprocessor) to read the CAD generated drawing and then produce the required instructions for the NC machine to make the part in what is referred to as Computer Numerical Control (CNC). This whole process from CAD to finished component is known as computer-aided manufacturing (CAM).

One major benefit of computer-aided manufacturing is that one machine tool can efficiently generate many different components, one after the other. This system tends to minimize the requirement for a large parts inventory. If the input software is available, any required part can be made in a very short period of time. This illustrates the use of computer-integrated manufacturing (CIM), which involves using the computer in every step of manufacturing operation, from the customer order, to ordering the raw materials to machining the part, to routing it to its final destination.

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