Most controllers auto switch to degrees per minute when a 4th axis move is made. A simple formula to calculate IPM into DPM is,
DPM = 360 x IPM / circumference
this works pretty well in most cases. just be carefull if you are making small index moves and large X or Y moves. Imagine making a 1 degree index with a 10" X move, at a feed of F360, thats one RPM. the index will take about a second, but at the same time the X axis will try to move the 10" in that one second. youch.
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