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Direct Numerical Control (DNC)
There are number of problems inherent in conventional machining which are motivated machine tool builders to seek improvements in the basic NC system. Among the difficulties encountered in using conventional numerical control machine are the following Part programming mistakes, Punched tape, Tape reader, Controller, Management information.
DNC defined
It can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC thus relieving the system of its least reliable component. Instead of using tape reader the part program is transmitted to the machine tool directly from the computer memory. In principle, one computer can be used to control more than 100 separate machines. Figure below illustrates the general DNC configuration
The system consists of four components.
1. Central computer
2. Bulk memory which stores the NC part programs.
3. Telecommunication lines.
4. Machine Tools.
The computer calls the part program instructions from bulk storage and sends data back from the machines. This two way information flow occurs in real time, which means that machine's requests for instructions must be satisfied almost instantaneously. Similarly, the computer must always be ready to receive information from the machines and respond accordingly. The remarkable feature of the DNC system is that the computer is servicing a large number of separate machine tools, all in real time. Depending upon the number of machines and computational requirements that are imposed on the computer it is sometimes necessary to make use of satellite computers as shown in figure.
Two Types of DNC
1) Behind The Tape Reader (BTR)
In this system, the computer is linked directly to the regular NCF controller unit. Except for the source of command instructions, the operation of the system is very similar to conventional NC. The controller units have two temporary storage buffers. One buffer for receiving block of instruction from the central computer and convert them into machine action while the other buffer provides commands instructions to the machine tool.
2) Special Machine Control Unit (SMCU)
In this system, the conventional NC unit is replaced by special machine control unit. The special MCU is a device that is specifically designed to facilitate communication between the machine tool and the computer. One area where the communication link is important is in circular interpolation of the cutter path. The special MCN configuration achieves a superior balance between accuracy of the interpolation and the fast metal removal rates than in generally possible with the BTR system.
Advantages of DNC
- Time Share - the control of more than one machine by the computer.
- Greater computational capability
- Remote computer location - the computer is located in a computer type environment.
- Elimination of tapes and tape reader at the machine, thus increasing reliability.
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