CNC Control Systems

A machine unit equipped with a computerized numerical control system is commonly known as a CNC machine. In an analogy of the machine tool being the body of a CNC machine system, the control unit is its brain, its nerve center. There are no levers, no knobs and no handles on a CNC machine the way they function on conventional milling machines and lathes. All machine speeds, feeds, axes motions and hundreds of other tasks are programmed by a CNC programmer and controlled by a computer that is major part of the CNC unit. To make a program for a CNC machine tool means to make a program for the control system. True, the machine tool is a major consideration as well, but it is the control unit that determines the program format, its structure and its syntax.

In order to fully understand CNC programming process, it is important to understand not only the intricacies of how to machine a part, what tools to select, what speeds and feeds to use, how to setup the job and many other features. It is equally important to know how the computer, the CNC unit, actually works without the need to be an expert in electronics or a computer scientist. Figure 5-1 shows an actual Fanuc control panel.

Machine manufacturers add their own operation panel, with all switches and button needed to operate their CNC machine and all its features. A typical operation panel is illustrated in Figure 5-2. Another item required for the system, the handle, will be described as well.

Figure 5-1 – A typical example of a Fanuc control panel – actual layout and features will vary on different models (Fanuc 16M)

General Description

Even a brief look at any control unit reveals that there are two basic components – one is the operation panel, full of rotary switches, toggle switches and push buttons. The other component is the display screen with a keyboard or a keypad. A programmer who does not normally work on the CNC machine will seldom, if ever, have a reason to use either the operation panel or the display screen. They are available at the machine to the CNC machine operator, and used for machine setup as well as to control the activities of the machine.

Should the CNC programmer be at least interested in the machine operation? Is it necessary for the programmer to know and understand all functions of the control system? There is only one answer to both questions – definitely yes.
The control unit – the CNC system – contains features that only work in conjunction with the program, it does not do anything useful on its own. Some features can be used only if the program itself supports them. All switches and buttons and keys are used by the machine operator, to exercise control over program execution and machining process.

Operation Panel

Depending on CNC machine type, the following table covers the most typical and common features found on modern operation panel. There are some small differences for operation of a machining center and a lathe, but both operation panels are similar. As with any general reference book, it is always a good idea to double check with the manufacturer specifications and recommendations. It is common that many machines used in the shop have some special features.

Figure 5-2 – A typical operation panel of a CNC machining center – actual layout and features will vary on different models

ON / OFF switch : Power and control switch for the main power and control unit
Cycle Start : Starts program execution or MDI command
Emergency Stop : Stops all machine activity and turns off power to the control unit
Feedhold : Temporarily stops motion of all axes
Single Block : Allows program run one block at a time
Optional Stop : Temporarily stops program execution (M01 required in program)
Block Skip : Ignores blocks preceded with a forward slash ( / ) in the program
Dry Run : Enables program testing at fast feedrates (without a mounted part)
Spindle Override : Overrides programmed spindle speed, usually within 50-120% range
Feedrate Override : Overrides programmed feedrate, usually within 0-200% range
Chuck Clamp : Shows current status of the chuck clamping (Outside / Inside clamping)
Table Clamp : Shows current status of table clamping
Coolant Switch : Coolant control ON / OFF / AUTO
Gear Selection : Shows current status of working gear range selection
Spindle Rotation : Indicates spindle rotation direction (clockwise or counterclockwise)
Spindle Orientation : Manual orientation of the spindle
Tool Change : Switch allowing a manual tool change
Reference Position : Switches and lights relating to setup of machine, from reference position
Handle (MPG) : Manual Pulse Generator (MPG), used for Axis Select and Handle Increment switches
Tailstock Switch : Tailstock and/or quill switch to manually position the tailstock
Indexing Table Switch : Manually indexes machine table during setup
MDI Mode : Manual Data Input mode
AUTO Mode : Allows automatic operations
MEMORY mode : Allows program execution from memory of the CNC unit
TAPE / EXT or DNC mode : Allows program execution from an external device, such as a desktop computer (DNC) or a punched tape
EDIT mode : Allows changes to be made to a program stored in CNC memory
MANUAL Mode : Allows manual operations during setup
JOG Mode : Selects jog mode for setup
RAPID Mode : Selects rapid mode for setup
Memory Access Key (switch) : to allow program editing
Error Lights : Red light indicating an error
Even if some features may not be listed, virtually all of those in the table are somewhat related to CNC program. Many control systems have unique features of their own. These features must be known to the CNC operator. The program supplied to the machine should be flexible, not rigid – it should be ‘user friendly’.

Screen Display and Keyboard

Screen display is the ‘window’ to control operation. Any active program can be viewed, including the control status, current tool position, various offsets, parameters, even a graphic representation of the toolpath. On all CNC units, individual monochrome or color screens can be selected to have the desired display at any time, using the input keys (keyboard pads and soft keys). Setting for international languages is also possible.
Keyboard pads and soft keys are used to input instructions to the control. Existing programs can be modified or deleted, new programs can be added. Using keyboard input, not only the machine axes motion can be controlled, but the spindle speed and feedrate as well. Changing internal parameters and evaluating various diagnostics are more specific means of control, often restricted to service people. Keyboard and screen are used to set program origin and to hook up to external devices, such as a connection with another computer. There are many other options, particularly for multi axis machines. Every keyboard allows the use of letters, digits and symbols for data entry. Not every keyboard allows the use of all alphabet letters or all available symbols. Some control panel keys have a description of an operation, rather than a letter, digit or symbol, for example, Read and Punch keys or the Offset key.

Handle (MPG – Manual Pulse Generator)

For setup purposes, each CNC machine has a rotary handle that can move one selected axis by as little as the least increment of the control system. The official Fanuc name for the handle is Manual Pulse Generator. Associated with the handle is Axis Select switch (often duplicated on the operation panel as well as on the handle) and the range of increment (that is the least increment X1, X10 and X100). The letter X in this case is the multiplier and stands for ‘X times’. One handle division will move the selected axis by X times the minimum increment of active units of measurement. Handles with digital display on a small screen are also available. In Figure 5-3 and the following table are details of a typical traditional handle.

Figure 5-3 – An example of a detached handle, called the Manual Pulse Generator (MPG), with a typical layout and features. Layout and features may vary on different machine models.