Direct numerical control (dnc) system

DIRECT NUMERICAL CONTROL (DNC) SYSTEM

Direct Numerical Control (DNC) can be defined as a manufacturing system in which a number of CNC machines are controlled by a single computer through direct connection and in real time.

All machines are linked to a mainframe computer which feeds the information to the individual machines as required. Also, there may be different types of machines linked to the mainframe computer. Therefore, DNC is also termed as Distributed Numerical Control. The computer is programmed in such way to select the order of manufacture of the components. DNC has the compatibility to integrate with the running of a flexible manufacturing system (FMS) or even an automated factory.

DNC system is expensive, thus, it mainly used in large organizations. The combination of DNC/CNC makes possible to eliminate the use of the program as the input media for CNC machines. A DNC computer is used to download the program directly to a CNC computer memory device. It reduces the amount of communication required between central computer and each machine tool.

The objectives of implementing DNC in the industry are as follows:

• To eliminate the need for manual entry of the part program to the NC machine using keyboard

• To eliminate the use of punched tape in the old NC machines

• To facilitate uploading of part programs from CNC system to the central computer

• To download or transfer the part programs directly from central computer to the individual control units of the machines

• To facilitate easy editing of the existing part programs

• To provide software for processing the shop floor data for management purposes • To establish two-way communication among the various subsystems

• To post process part programs which are written in higher languages similar to APT

• To facilitate transferring of tool length offsets from the tool pre-setters directly to machine tool controls

• To enable the possibility of connecting coordinate measuring machines (CMM) to DNC networks.

Figure 4.40 illustrates a general configuration of DNC system. The components of DNC are given below.

(i) Central computer

(ii) Bulk memory which stores the NC part programs

(iii) Telecommunication lines / computer networks

(iv) NC / CNC Machine tools. *

A central computer is connected to a number of machine tools and controls them. Part program of all machine tools is stored in the memory of the central computer and transmitted on direct transmission lines on demand. The computer calls the part program instructions from bulk memory and feeds them to individual machines. Programs in full or segment can be transferred to NC machines. The computer can be used for program editing. No tape readers are used. It also receives the data back from machines.

Various machine tools can communicate with the computer in real time. The two-way information flow occurs in real time between the following components:

• Central computer and individual machine tools

• Central computer and NC part programmer terminal

• Central computer and bulk memory.

The communication network can be done either through connecting the remotely located computer, with lengthy cables to the individual machine control directly or connecting the mainframe computer with a small computer at individual operator's station known as satellite computer. These satellite computers take some of the workloads off the central computer. Each satellite computer controls several machine tools. The use of satellite computers depends on the number of machines and the computational requirements that are imposed on the computer. Figure 4.41 shows the configuration of DNC system with satellite computers. Sometimes it is necessary to make use of the satellite computers

1. Comparison between CNC and DNC

2. Advantages of DNC

1. It eliminates the use of punched tapes and tape readers.

2. The computer can be remotely located even a thousand miles away.

3. The computer can simultaneously program for many NC machines. 

4. It provides a convenient storage of NC part programs in computer files.

5. It ensures greater computational capability and flexibility

6. It is more convenient to incorporate editing and diagnostic features.