G41 and G42 cutter compensation

We can use two different G-Codes to apply cutter compensation depending on the direction of the cut.

G41 is left compensation and G42 is right compensation.

On a CNC machine, it is usually recommended to use climb milling, this needs to be taken into consideration when writing our programs and deciding which direction to apply cutter compensation.

When we wish to cancel the compensation, G40 is used to turn it off. It is a good working practice is to end each section of the program with G40 so if we need to jump back to re-run a tool we won't have G41 or G42 applied unless it is called upon after the tool change.

There are three different ways that cutter compensation can be applied

G42 P5.0;

By using a 'P' value we can tell the machine that we wish to offset the cutter by a defined amount, in this case, 5mms.

This technique is usually used on desktop routers and small CNC machines without a built-in tool table

G42 X5.0;

Another way we can write this is by using an 'X' value, it means the same thing.

G42;

On machines with a built-in tool table within their control system, we normally see cutter compensation just applied using a simple G42 (or G41) command

Larger industrial machinery normally uses this style of defining the radius of the tool as the tooling information is entered into the tool table during the set up of the machine. During the tool call line of our program, the machine pulls all the tooling information into its memory and G42 is used to let the machine know that we wish to use cutter compensation. No other information is needed.

When we talk about cutter compensation on a Lathe or machining centre, we refer to it as tool nose radius compensation.

When turning, the machine has to calculate the small radius on the nose of the tool when applying our tool paths to ensure we cut the same size as we have programmed.

The direction that the tool cuts will determine if we use G41 or G42 for the correct direction to offset with compensation. This graphic shows the correct direction when applying tool nose radius compensation on a lathe.

When setting up the tooling information in the tool table of the machine controls, we add the size of the radius and the orientation of the tool. This enables the machine to select the correct direction to offset the tool when it reads the G41 or G42 in our program.

The graphic above shows what we set the tool quadrants to in relationship to the tool. For example, an external roughing tool would be set to 3 and an internal boring bar would be set to 2. This is assuming the tool turret is above the spindle, if it is below these values would be reversed.

For a left-handed external roughing tool we would select 4 if the turret is above the spindle at its tool change position or 1 if it is a lower turret design.