The problem with feed per minute
In a move with X and A together, the controller must combine millimetres and degrees into one feed. Depending on the controller, the actual speed of the tool across the part can be much faster or slower than programmed, especially far from the rotary centre.
How inverse time works
In G93 mode, F is not a speed; it is the reciprocal of the time the move should take, in minutes. The post calculates it from the intended tool tip feed and the move length.
Intended feed at tool tip: 1000 mm/min
Move length at tool tip: 5 mm
Time = 5 / 1000 = 0.005 min
F = 1 / 0.005 = 200
G93
G01 X12.5 A15. F200.
G94Controller rules
- On many controllers, F must be programmed in every G93 block; missing F causes an alarm.
- Rapid moves are not affected.
G94must be restored before normal 3-axis cutting resumes.- Very short moves produce very large F values; the post may cap them or combine moves.
Alternatives
| Method | When used |
|---|---|
Inverse time (G93) | 4/5-axis simultaneous without TCP feed control |
| TCP feed (controller applies F at tool tip) | Many modern TCP implementations |
| Degrees per minute calculation | Controllers that interpret F as deg/min for rotary-only moves |
Which method gives correct results depends on the controller and its settings; we confirm it on the machine during commissioning.
With inverse time, the post decides how long each move takes, and the tool gets the feed you intended.
Multi-axis feed handling
See our CNC post processor development service and 4-axis milling posts.