Components of the NC System

There are three important components of the numerical control or NC system. These are:

1) Program of instructions

2) Controller unit, also called as the machine control unit (MCU) and

3) Machine tool

All these have been shown in the figure below and also described in the subsequent sections.

The typical desktop program gives the instructions to the computers to perform certain functions. The program of instructions of the NC machine is the step-by-step set of instructions that tells the machines what it has to do. These instructions can tell the machine to turn the piece of metal to certain diameter, drill the hole of certain diameter up to certain length, form certain shape etc. The set of instructions are coded in numerical or symbolic form and written on certain medium that can be interpreted by the controller unit of the NC machine. The mediums commonly used earlier for writing the instructions were punched cards, magnetic tapes and 35mm motion picture film, but now 1 inch wide punched tape is used more commonly.

The program instructions are written by the expert who has programming knowledge as well the machining knowledge. The person should know the various steps of the machining required to manufacture a particular product and should be able to write these steps in the form of the program that can be understood by the control unit of the NC machine, which would eventually direct the machine tool to perform the required machining operations.

One can also input the instructions directly into the controller unit manually, this method is called as manual data input (MDI), which is used for very simple jobs. Then there is direct numerical control method (DNC) in which the machines are controlled by the computers by direct link omitting the tape reader.

The controller unit is most vital parts part of the NC and CNC machines. The controller unit is made of the electronics components. It reads and interprets the program of instructions and converts them in the mechanical actions of the machine tool. Thus the controller unit forms an important link between the program and the machine tool. The control unit operates the machines as per the set of instructions given to it.

The typical control unit comprises of tape reader, a date buffer, signal output channels to the machine tools, feedback channel from the machine tool, and the sequence control to coordinate the overall machining operation.

Initially, the set of instructions from the punched tape are read by the tape reader, which is sort of the electromechanical devise. The data from the tape is stored into the data buffer in form of logical blocks of instructions with each block resulting in certain sequence of operations.

The controller sends the instructions to the machine tool via signal output channels that are connected to the servomotors and other controls of the machines. The feedback channels ensure that the instructions have been executed by the machine correctly. The sequence control part of the controller unit ensures that all the operations are executed in the proper sequence.

One important thing to note about the controller unit here is that all the modern NC machines are equipped with the microcomputer that acts as the controller unit. The program is fed into the computer directly and the computer controls the working the machine tool. Such machines are called as Computer Controller Machines (CNC) machines.

It is the machine tool that performs the actual machining operations. The machine tool can be any machine like lathe, drilling machine, milling machine etc. The machine tool is the controlled part of the NC system. In case of the CNC machines, the microcomputer operates the machine as per the set of instructions or the program.

The NC machine also have the control panel or control console that contains the dials and switches using which the operator runs the NC machine. There are also displays to display information to the user. Most of the modern NC machines are now called as the CNC machines.

1) Book: CAD/CAM: Computer-Aided Design and Manufacturing, by Mikell P. Groover and Emory W. Zimmers Jr., Printice Hall of India Private Limited, page 135 to 137