Changing your IJK’s to R’s and vice versa in Mastercam.

On a weekly basis, I get a lot of calls of people’s machines gouging their parts even though everything on Mastercam looks to be working fine and Verify gives the desired output.

More times than one, the problem is that your specific machine doesn’t like or understand the Circular interpolation method Mastercam is posting, meaning IJK’s or R. So, what is the difference between these two methods and what should we do to change the Mastercam output.

Difference between IJK's and R's.

Circular Interpolation using IJK’s.

In this method the I, J and K are the offsets or distances from the current location (arc start point) to the center point of the arc. The arc start point is determined by the location of the cutter before the command is implemented (before the G02 or G03). Only two of the 3 parameters (I, J and K) will be used and is dependent on what arc plane has been selected (G17, G18 and G19). An example is shown below:

On the other hand, with the Radius method, the R is the distance from the arc start point to the arc center point. The arc start point is determined by the location of the cutter before the command is implemented (before the G02 or G03). The R distance is independent of what arc plane is selected. An example is shown below:

The sign of the R-value is dependent on the angular difference of the arc. An angular difference of 180 or less measured between the start and end point, uses an R positive value. When the angular difference is more than 180, an R negative value is used. 

To change your machine, follow the following steps:

1.      Under the Machine tab, click on the Machine definition icon.

2.      Once you are in your machine definition dialog box, click on the control definition icon.

3.      Then look on the left-hand side of the dialog box for the purple question mark with “arc” next to it and change the XY Plane, XZ Plane and YZ Plane from “Delta start to center” to “Radius”.

4.      Click on the green check buttons to exit the control and machine definitions and this will do it!