Modes of operation in cnc machine

MODES OF OPERATION IN CNC MACHINE

Generally, the CNC system can be operated in the following modes:

(a) Manual mode

(b) Manual data input (MDI) mode

(c) Automatic mode

(d) Reference mode

(e) Input mode and Output mode, etc.

(a) Manual mode:

In this mode, the movement of a machine slide can be carried out manually by pressing the particular jog button (+ or -). The slide (axis) is selected through an axis selector switch or through individual switches (e.g., X+, X−, Y+, Y—, Z+, Z-, etc.). The feed rate of the slide movement is prefixed. CNC system allows the axis to be jogged at high feed rate also. The axis movement can also be achieved manually using a hand wheel interface instead of jog buttons. 

In this mode, slides can be moved in two ways:

1) Continuous mode

2) Incremental mode.

In continuous mode, the slide will move as long as the jog button is pressed.

In incremental mode, the slide will move through a fixed distance which is selectable. Normally, the system allows jogging of axes in 1, 10, 100, 1000, 10000, increments. Axis movement is at a prefixed feed rate. It is initiated by pressing the proper jog+ or jog- key and it will be limited to the no of increments selected even if the jog button is continuously pressed. For subsequent movement, the jog button has to be released and once again pressed.

(b) Manual data input (MDI) mode:

In this mode the following operation can be performed:

• Building a new part program

• Editing or deleting of part program stored in the system memory

• Entering or editing or deleting of:

✓ Tool offsets (TO)

✓Zero offsets (ZO)

✓Test data, etc.

1) Teach-In:

Some system allows the direct manual input of a program block and execution of the same. The blocks thus executed can be checked for correctness of dimensions and consequently transferred into the program memory as a part program.

2) Playback:

In setting up modes similar to jog or incremental, the axis can be traversed either through the direction keys or via the hand wheel, and the end position can be transferred into the system memory as command values. But the required feed rates, switching functions and other auxiliary functions have to be added to the part program in program editing mode. Thus, teach- in and playback operating methods allow a program to be created during the first component prove out.

(c). Automatic mode (auto and single block):

In this mode, the system allows the execution of a part program continuously. The part program is executed block by block. While one block is being executed, the next block is read by the system, analyzed and kept ready for execution. Execution of the program can be one block after another automatically or the system will execute a block, stop the execution of the next block till it is initiated to do so (by pressing the start button). Selection of part program execution continuously (Auto) or one block at a time (Single Block) is done through the machine control panel.

Many systems allow blocks (single or multiple) to be retraced in the opposite direction. Block retrace is allowed only when a cycle stop state is established. Part program execution can resume and its execution begins with the retraced block. It is useful for tool inspection or in case of tool breakage.

(d) Reference mode:

Under this mode, the machine can be referenced to its home position so that all the compensations (e.g., pitch error compensation) can be properly applied. Part programs are generally prepared in the absolute mode with respect to machine zero. Many CNC systems make it compulsory to reference the slides of the machine to their home positions before a program is executed while others make it optional.

(e) Input mode and output mode (I/O mode):

In this mode, the part programs, machine setup data, tool offsets, etc. can be loaded/unloaded into/from the memory of the system from external devices such as programming units, magnetic cassettes or floppy discs, etc. During data input, some systems check for simple errors such as parity, tape format, block length, unknown characters, the program already present in the memory, etc. Transfer of data is done through a RS232C or RS422C port.