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A more advanced version of the NC system is direct numerical control (DNC) machines where one computer controls multiple machines in real-time. The DNC system is not equipped with the tape reader and therefore increases the reliability of the machining system.
The prepared part program is transmitted from the computer’s memory directly to the machine tool. More than a hundred machines can be controlled at the same time by a single large computer system. The instructions needed for a given machine are provided on demand by the main computer.
For example, a machine that requires control commands will be immediately communicated to it by the computer. DNC systems are also equipped to collect the necessary data from machine tools.
DNC Systems Components:
There are four basic components to a DNC system: a central computer, mass memory, machine tools, and telecommunications lines. Part programs stored in mass memory are called up by the central computer and fed into the machine tools. Different machine tools are connected by telecommunication lines. The central computer also receives real-time data from different machine tools.
Thus, the bidirectional information flow must be properly synchronized by the central computer for the effective functioning of the DNC systems. Serving a large number of computers in real-time is one of the outstanding features of the DNC system.
Satellite computers may be required depending on the number of machining systems and computational requirements. These satellite computers serve to take the computational load off the central computer.
A group of machines is served by a satellite minicomputer. Depending on the type of communication link between the control computer and the machine tools, DNC systems can be categorized into two types of variants. One of the variants makes use of a specialized machine control unit and the other is known as a behind-the-tape reader system.
DNC Functions:
DNC systems serve to perform a variety of functions. The functions performed by DNC systems are difficult to perform with both conventional NC and CNC systems. The main functions of the DNC system are NC without punched tape, storage for the parts program, data collection, processing and reporting, and communications to and from the machine tools.
NC Parts Program Storage:
Storing part program is another important function of DNC systems. Subsystems for storing programs must be properly structured. Framework subsystems must be able to readily provide programs to machine tools. Subsystems must have the capability to accommodate new part programs, deleting old programs and modifying existing part programs, if any.
The post-processing function of converting the CLIFILE format from the stored part program to the downloadable instructions for the machine tool must be accurately performed by the DNC system. The other functions that must be performed by the subsystems include processing and managing data, displaying programs, and manipulating data.
Data Collection, Processing and Reporting:
One of the other important functions of the DNC system is to collect machine data and transfer it to the control unit. This helps in real-time monitoring of production in the factory. Data relating to the number of components produced, tool usage, machine tool utilization and other data relating to the measuring characteristics of the machine shop are collected and processed by the DNC system. The processed data is then used by management to take any corrective actions.
Communication:
To perform the above-mentioned functions, a communication network is required. The following components of a DNC system are linked together via the communication network: machine tool and the host computer, NC part program terminals and the host computer, and connection between a host computer and mass storage.
Advantages of DNC:
DNC systems have advantages over conventional NC systems. Here is a list of some advantages of the DNC system:
• Elimination of perforated tapes: The least reliable component of an NC system, ie the tape reader and perforated tape were eliminated in the event of a
DNC systems. The wired control unit has also been replaced in some cases of DNC systems. These were replaced by specialized control systems.
• Greater flexibility and computational power: Large computers have increased computational power and provided greater data flexibility compared to traditional NC systems. Flexibility resulted from the presence of software rather than wired devices.
• Part program storage: DNC system storage is relatively larger than conventional NC systems that use tapes to store part program instructions. This led to data storage for processing large numbers of machine tools simultaneously.
• Store performance reporting: one of the important functions of the DNC system is to collect data regarding store performance measurement. The processed data can then be analyzed by the managers and hence the store performance monitoring can be kept under control.
