Basic information about CNC milling machine

CNC milling machine with three axis X, Y, Z is similar to vertical milling machine. All the movement of axis is controlled by CNC system. Programming is same as CNC lathe machine. Only Y-axis is added to the program.

Comparison between conventional and CNC milling machine are as follows:

(a) Conventional milling machine tool holding devices are different from CNC milling.

(b) Separate power supply system unit is provided with CNC milling machine.

(c) Movement of axis in CNC milling is done by CNC system. Where as in conventional milling, axis is controlled manually/automatically one by one.

(d) CNC milling axis X and Z can be combined with one system, but in conventional milling automatic combination of axes is not possible.

(e) Complicated countering can be done easily in CNC milling but in conventional it is very difficult.

(f) Work holding devices are equipped with hydraulic or pneumatic system in CNC but in conventional one this does not exist.

(g) Axis movement feedback is available with the CNC milling but it is not present in conventional one.

(h) Warning safety devices are available with CNC milling but it is not present in conventional one.

(j) Accuracy and high finished surfaces are obtained in CNC  system which is remote in conventional milling machine.

(k) Mass production with greater accuracy is the main advantage of CNC machine which is absent in conventional milling machine.

Some of the additional features available in CNC milling machine tools are as follows:-

(a) The part program can be input to the controller unit through key-board or the paper tape can be read by the tape reader in the control unit.

(b) The part program once entered into the computer memory can be used again and again.

(c) The part program can be edited and optimised at the machine tool itself. If there is any change in the design of the component, the part program can be changed according to the requirements.

(d) The input information can be reduced to a great extent with the use of special sub-programs developed for repetitive machining sequences. For common operations such as drilling holes on a pitch circle, special cycle programs can be built and stored in the computer memory. These sub- programs or sub-routines can be retrieved and used any number of times within a part program; only certain parameters have to be specified and the computer control carries out the necessary calculations and subsequent actions.

(e) The CNC machines have the facility for proving the part program without actually running it on the machine tool. The control system processes the part program and the movement of the cutting tool in each operation is shown on the monitor screen (video display unit). The shape of the component which will be produced after machining is also shown on the screen without actual machining taking place.

(f) CNC control unit allows compensation for any changes in the dimensions of the cutting tool. When a part program is written, part programmer has a particular type and size of cutting tool in mind. But while actually using the part program on the machine tool that particular cutter may not be available. CNC control system allows the compensations to be made for difference between the programmed cutter and the actual cutter used.

(g) With the CNC control systems, it is possible to obtain information on machine utilization which is useful to the management. The control system can provide the information such as number of components produced, time per component, time for setting up a job, time for which a particular tool has been  in  use,  time  for  which  machine  has  not  been  working  and  fault diagnosis, etc.

Principle of CNC Milling Operation

The following are the principles of CNC milling operation:

(a) Movement of X, Y, Z axis are controlled by motor supplied either with AC or DC

(b) Movement of machine part is done by giving commands

(c) All the operations are carried out by codes like speed, feed, depth of cut etc.

(d) For each operation separate code is available.

(e) Warning system is available to safe guard the various operations and components.