Mode of operation

Running a CNC machine involves different modes or procedures, collectively known as its Mode of Operation. CNC machines can operate in various modes, and the following are some standard modes relevant to CNC machining processes:

• Manual or Jog mode
• Manual Data Input or MDI mode
• Automatic mode
• Reference mode
• Program Edit mode

The picture below displays two different mode selector switches found in CNC machining centers.

Manual or Jog mode:

In this mode, an axis can be manually moved in a positive or negative direction by pressing the corresponding switch buttons (e.g., X+, X-, Y+, Y-, Z+, Z-). The selection of the axis is done using an Axis selector switch, and the feed rate remains constant for axis movement. Manual or Jog mode can be further classified into Continuous, Incremental, and Hand-wheel modes.

Continuous Mode: In Continuous mode, the machine axis will continue to travel in a positive or negative direction as long as the "Jog button" switch is pressed. The CNC operator does not have complete control over the axis positioning, and it is impractical to stop the axis precisely at a predefined distance. This mode is typically used for random driving of an axis in a positive or negative direction.

 

Incremental Mode: In Incremental mode, an axis can be positioned at a specific distance with each command input. In a CNC machine, the axis movement is usually made in increments of 1, 10, 100, 1000, and 10000 microns (1 mm = 1000 microns). This means that each input command moves the axis by the defined distance intervals. For example, to move the X-axis in a positive direction by 10 microns, the operator selects the X-axis using the axis selector switch, sets the incremental mode switch to position '10', and presses the 'Jog+' button. Continuous pressing of the 'Jog+' button indicates a single-step movement. To change the direction of movement to negative, the operator can press the 'Jog-' switch after shifting the axis.

Hand-Wheel Mode: In Hand-Wheel mode, the axis is moved using a specific device called a Hand-wheel. This mode is usually used when an axis needs to be moved away from the operator panel, such as during a new work-offset setting. To move an axis, the operator selects the axis to be moved and rotates the hand-wheel knob clockwise or counterclockwise to move the axis in a positive or negative direction.

Manual Data Input Mode:

In this mode, the operator can enter commands to the controller in written form. Most programming-related instructions can be executed individually using this mode. Instructions such as spindle rotation, automatic tool change, and cutting coolant on or off can be performed separately. Axis movement commands are also possible, and sometimes small machining operations can be performed using this mode.

Automatic Mode:

In Automatic mode, the CNC machine runs continuously, following a selected program. The program is executed in a block-by-block sequence, where the subsequent block is not executed until the preceding block has finished successfully. The program execution can be continuous or step-by-step, using a control panel. It is also possible to manage the machine from any single block, following a part program, through the "Block search" option. The program execution can be interrupted and restarted with the "Optional stop" selection.

Reference Mode:

In Reference mode, all the axes are moved to the "Home" position, and the CNC machine calculates the measurements of the axis moves from the home position. Before proceeding with the referencing, there is no valid position information of the axes in the CNC controller, and it is not permissible to run a program in automatic mode until all the axes are referenced. Referencing the axes is necessary when using an Incremental encoder for position feedback. However, when using an Absolute Encoder, referencing the axes is not mandatory every time the CNC machine is switched on. A detailed discussion of the working principle of Absolute and Incremental encoders is provided in another chapter.

Program Edit Mode:

Program Edit mode is usually not considered a mode of operation for the CNC machine operating group. This mode is used to write a new program or edit an existing one. The CNC controller is set to this mode for such purposes. External devices like programming units, punch tape, and computers can interface with the CNC controller through a cable to load/unload programs, subprograms, machine data, and tool offsets. The loading/unloading process is performed through a RS 232 port on the CNC controller using a serial communication system. Currently, devices such as 'Compact Flashcards' or 'USB' support devices like 'Pen drives' are used to upload/download all necessary machine data with many CNC controllers.