Quick Guide to the G40 CNC Code [Cutter Comp Cancel]

a graphic of a cnc machine with text that says learn g code today G40 cutter compensation cancel

G40 is the CNC G code to turn off cutter compensation. 

Cutter compensation is turned on with either the G41, for cutter compensation left or G42, for cutter compensation right.

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What is cutter compensation?

illustration that shows how a CNC will act when there is no cutter compensation mode active

Cutter compensation is a CNC mode that allows the CNC controller to adjust for the size of the cutting tool. Setting the size of the cutting tool allows you to use the same program with multiple different cutting tools.

Turning cutter compensation on tells the machine to shift the cutting tool either left with G41 or right with G42. The shift is half of the diameter of cutting tool, otherwise known as the radius.

Visualizing which way the machine will shift can be a little tricky. The shift happens as if you are looking towards the direction that the cutter is moving.

The two images below show how the CNC will move when the other cutter compensation modes are active.

illustration that shows how a CNC will act when using cutter compensation left with the G41 code
illustration that shows how a CNC will act when using cutter compensation right with the G42 code

Where does G40 show up in a program?

The G40 command can be found at any point within the program, but the most common place to find one is in safety lines of the program.

Safety lines are a set of codes used to put the CNC machine into the correct mode(s) prior to running the program. Sometimes this means turning modes on, sometimes it means turning them off.

G40 is one of the off commands.

Safety lines are also used at the start of a new section of code. For example, you 

Things to know when using the G40 code

The two main things to pay attention to when using G40 to cancel cutter compensation are:

  • Cancel cutter comp when you are off the part more than half the diameter of the cutter
  • Make a move when canceling cutter compensation

Canceling cutter compensation when off the part enough keeps the machine from running back into the part. A good practice is to move the machine in the Z axis so that it is away from the part before canceling cutter compensation.

Making a move forces the machine to move in a consistent way. Some machines can react unexpectedly if no move is made when canceling cutter compensation. The machine basically thinks it has teleported location.

Don’t teleport your CNC. Make a move when canceling cutter compensation.

Other cancel commands

There are multiple G code cancel commands. The table below shows the most common codes for turning modes off:

Code

Description

G40

Cancel cutter compensation

G49

Cancel tool length compensation

G50

Cancel scaling

G67

Cancel custom macro call

G69

Cancel rotation

G80

Cancel canned cycles

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