Computer Numerical Control (CNC) and CNC M-Codes
Computer numerical control (CNC) is a fundamental part of modern manufacturing. The majority of machines operate using instructions and guidelines that have been downloaded using a CNC program controller. For a machine to interpret the commands from CNC, the commands have to be entered using G and M codes. CNC operators are required to know the appropriate codes and instructions as well as how to use them. Both types of coding are necessary for the system of a CNC device to perform correctly.
M-code is a part of the language that AutoCAD and CAM, computer aided manufacturing, use to input instructions into CNC machines. G-codes and M-codes work in unison for positioning a workpiece and guiding the machine’s actions. M-codes, miscellaneous or machine codes, control the operations of the equipment telling it when to operate or cease operation. While the G-code can direct a machine to move in a line or arc, once the tool is positioned, it won’t know to stop, change tools, add coolant, or complete any other actions, which are provided by M-codes. Instructions for a tool to turn on or off is part of the M-code language.
The use of M-codes varies depending on the machine. During programming, one M-code is required per code block giving the commands for a tool to turn on or off and activate other operations. Having more than one M-code in a code block can cause problems. The definition of M-code functions and their uses is spelled out by the machine’s manufacturer.
Operators use M-codes to tell a machine to change tools, turn on the spindle, load coolant, or open and close a door. There are several M-codes that operators need to know for a machine to perform properly. Also, each machine has a different method for downloading the M-codes. One controller may require a zero between the M and the number while others don’t need the zero. The particular method for a machine is clearly laid out in the instructions from the manufacturer.
The types of commands programmed into a CNC machine
M-codes are an important component of the operation of a CNC machine. While G-codes describe the positioning for an operation, M-codes provide data for a machine’s actions. For the proper functioning of a CNC machine, G and M codes have to be entered. They work in tandem and together to instruct, guide, and program the responses of a CNC device. As with any computer, CNC machines have a controller for data input. Though most computer languages are built on C or C++, there are variations for each type of controller.
Fanuc manufactures robotic controllers that use M-codes for commands for CNC machines. Their controllers use the M zero number form of M-codes. Below are several of the Fanuc controller M-codes.
- M00 Program stop
- M01 Optional program stop
- M02 End of program
- M03 Spindle start forward CW
- M04 Spindle start backward CCW
- M05 Spindle stop
- M08 Coolant on
- M09 Coolant off
- M29 Rigid tap mode
- M99 End of program
M commands are part of an information group that determines how and when a machine should start or stop an action. Beginning with M00 they continue in an arithmetic progression to M99, which ends the program. How an M-code is used differs between vendors and producers. In many cases, not every M-code is programmed into the machine. Knowing the codes and how they make the machine function is critical. In some cases, when a code is not used or programmed, the definition of the code is left to the discretion of the user.
Examples of the programmable codes for a lathe and milling operation are listed below. Table 1 has codes for a lathe while table 2 has the M-codes for a milling operation. Both tables are examples of M-codes for Fanuc controllers.
| M code | Description |
|---|---|
| M00 | Program stop |
| M01 | Optional program stop |
| M02 | End of program |
| M03 | Spindle start forward CW |
| M04 | Spindle start reverse CCW |
| M05 | Spindle stop |
| M08 | Coolant on |
| M09 | Coolant off |
| M29 | Rigid tap mode |
| M30 | End of program reset |
| M40 | Spindle gear at middle |
| M41 | Low Gear Select |
| M42 | High Gear Select |
| M68 | Hydraulic chuck close |
| M69 | Hydraulic chuck open |
| M78 | Tailstock advancing |
| M79 | Tailstock reversing |
| M94 | Mirrorimage cancel |
| M95 | Mirrorimage of X axis |
| M98 | Subprogram call |
| M99 | End of subprogram |
| M code | Description |
|---|---|
| M00 | Program stop |
| M01 | Optional program stop |
| M02 | End of program |
| M03 | Spindle start forward CW |
| M04 | Spindle start reverse CCW |
| M05 | Spindle stop |
| M06 | Tool change |
| M07 | Coolant ON – Mist coolant/Coolant thru spindle |
| M08 | Coolant ON – Flood coolant |
| M09 | Coolant OFF |
| M19 | Spindle orientation |
| M28 | Return to origin |
| M29 | Rigid tap |
| M30 | End of program (Reset) |
| M41 | Low gear select |
| M42 | High gear select |
| M94 | Cancel mirrorimage |
| M95 | Mirrorimage of X axis |
| M96 | Mirrorimage of Y axis |
| M98 | Subprogram call |
| M99 | End of subprogram |
There may be some confusion regarding the codes for CNC machines since some operators refer to all codes as being G-codes even though they input both G and M codes. To avoid misinformation and misunderstandings, it is important to know that every code block has to have one M-code to begin and end a function. The G-code tells the machine where and when to do a job. M-codes stop an operation, end a programmed task, or begin a movement after the tool has been positioned.
Most parts and products produced by CNC machines are programmed using CAD or CAM software that give directions for CNC machines using alphanumeric programming. Even though engineers are fluent in those two forms of software, it is still important for them to have an understanding of how G and M codes direct a CNC machine.