Al,
I thought it might be done with the C-axis too, however, in the latest version of Mach3, there's a place to specify the axis of rotation (X, Y, or Z) for the A-axis. It's under Config->Toolpath->Axis of Rotation. I don't think it is extra work for the Mach team, since they would simply plug in different axis DROs to the same feedrate formula, as follows:
X-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(Z^2 + Y^2) - T))
Y-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(Z^2 + X^2) - T))
Z-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(X^2 + Y^2) - T))
OK, the following g-code is a example of "turning" the outer diameter of my part. It will "plunge" sideways at 5 IPM, then rotate 360 degrees at 5 IPM, and repeat. However, I must specify DPM (degrees per minute) for A-axis moves, instead of IPM (inches per minute). The conversion is done with the formula (above). It assumes Mach3 is zero'd to the center of rotary table. I used a MS Excel app to generate the g-code. However, Mach3 could very easily do this math, and probably should be doing it.
;TURN OUTER DIAMETER OF PART
;TOOL RADIUS IS 0.15625
N14 X0.469 F5
N15 A-360. F915.9997
N16 X0.4665 F5
N17 A-720. F923.5297
N18 X0.4639 F5
N19 A-1080. F931.1845
N20 X0.4614 F5
N21 A-1440. F938.9673
N22 X0.4588 F5
N23 A-1800. F946.8812
N24 X0.4563 F5
N25 A-2160. F954.9297
;FINISH
N27 A-2520. F954.9297
Rick