Developments in CNC Systems

The machine control unit (MCU) in the early NC machine tools was hardwired with all the control logic to operate the machine tool embedded in the hardware circuitry. This makes the controller really bulky, with a large amount of power consumed that used to generate a large amount of heat. In addition because of a large number of wires running through the controller, the reliability also suffered. The availability of the microprocessors from mid 1970s helped in changing that situation. The microprocessor allowed for the availability of large processing power inside the controller thereby allowing for a large number of innovations.

All the control logic of the machine tool is converted into software, which allowed for a reduction in a large amount of electronic hardware used in the controller. As a result the controller is a small unit unlike the earlier NC controllers. In addition the computer is available in the controller, which can be utilized for other associated functions. Consequently the controller is now called as Computer Numerical Control or CNC to recognize the computer inside the controller. All of the controllers that are being manufactured in the present day are CNC incorporating the latest microprocessors. Consequently the word NC that represented the earlier day controllers is strictly speaking obsolete. However, people tend to use NC now-a-days as synonymous to CNC.

As mentioned earlier, the computing power built into the controller allows for providing a number of advances, which were not possible with the older NC systems. Some of these developments are detailed below:

1. It is possible to have large part program storage compared to single blocks in the previous controllers. This would allow a number of part programs to

2. To be stored in the controller for easy job changeover. It is also possible to store large part programs, which are generally required for complex 3D surface machining.

3. he computer present in the control can allow for part program graphical proving and editing to be carried out on the controller. This would allow for carrying out necessary changes in the part programs at the machine as well as allowing for a graphical proving of tool path directly on the controller.

4. It is also possible to generate a part program using computer aided part programming such as workshop oriented conversational part programming methods such as FAPT TURN. This would make the development of part programs on the controller without a separate part programming station and downloading from that to the controller.

5. It is also possible to have background part programming to be carried out when the machine tool is actually doing the cutting operation. This is possible since most of the current day microprocessors are more powerful, and as a result, spare processing power is available in addition to the control of the machine tool in real time.

6. The computer and a large processing capability of the MCU helps with a range of enhanced part programming facilities which are otherwise not possible in the NC controls.

Some examples are:

1. Complex interpolations such as parabolic, elliptical and helical  

2. Additional canned cycles (other than the drilling series G 80 to 89)

3. Circular pocket milling

4. Rectangular pocket milling

5. Pitch circle diameter hole making

6. Rough turning stock removal

7. Rough facing stock removal

8. Thread cutting stock removal

9. Grooving (ID/OD)

10. Face grooving

11. Reduction of the bulk of part program can be done by using repetitive part programming facilities such as IF and DO loops. This will also help to make the part programs more intelligent.

12. Many a times, it is necessary to reuse a part of the program at a number of locations in a part program, because of the symmetry of the geometry. This will be made easier with the facilities such as subroutines and macros that will be made possible in the CNC.

13. Another function that is made possible with the CNC is the use of touch trigger probes discussed later. These probes can be used for dimensional and accuracy measurement.

14. It is also possible to use variables or parameters in writing part programs. The use of variables helps in customizing the part programs for specific applications to take care of the possible variations in design or manufacturing methods.

15. The controller can have the tool life management function that may include a large number of tool offset registers as well as the ability to monitor the life of the individual tools used.

16. Better communications with the outside world can be developed, basically for part program downloading and uploading using a CNC.

17. A number of diagnostic functions can be built into the controller, which will improve the maintainability of the machine tool. It may also provide the possibility of direct linking with the service centres using modems for reducing the machine downtime.

18. Use of standard operating systems such as Windows 2000 is possible now in CNC. This also helps in better integration of these controllers with other factory networks.

19. It is possible to provide special machine control functions such as adaptive control, lead screw pitch error compensation, thermal compensation, etc.