# Thread: coverting ipm to dpm macro question?

1. ## coverting ipm to dpm macro question?

When converting inches per minute to degrees per minute, what is the term "rotary axis departure amount". Is that your helix angle of cut or total degrees milled? I dont understand this term. Below is the macro by Mike L.

N055 G65 P1000 F4.5 D2.25 B30.0 R101.0 (Invoke custom macro to calculate dpm feed rate)
N060 G91 G01B30.0 F#101 (Make rotary axis motion at calculated feed rate.)
In line N055, we're calling program O1000 and passing the desired ipm feed rate with F, the distance from the tool tip to center of rotation with D, the incremental angular departure of the rotary axis with B, and the return variable number with R. In line N060, we're using the feed rate calculated by the custom macro (variable #10l). Note that this example uses the incremental mode to rotate the axes, but you could program this motion in the absolute mode as well. Just remember that in line N055, B must specify an incremental rotary axis departure amount.
Now here's the short custom macro:
O1000 (Calculate degrees per minute feed rate)
#[#18] = #2/[[3.1416 * 2 * #7] * #2 /360] / #9 (Store dpm feed rate in variable)
M99 (End of custom macro)
The calculation being done in this custom macro is based upon the formula: DPM = Angular departure distance / time required for motion.
Time is equal to motion distance divided by the desired inches per minute feed rate. We calculate motion distance by determining the circumference of the tool tip circle (pi times 2 times the radius) and multiplying it times the portion of a full circle being machined (angular departure divided by 360).

2. If I'm not mistaken, it means how many degrees you want to rotate the axis.