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Multi-Axis Guides

G93 Inverse-Time Feed Rate Explained

Understand how a G93 F-word represents the reciprocal of block time and why multi-axis feed output must match the control and post.

Published September 16, 2026Updated September 16, 2026Editorial method

In G93 inverse-time mode, the feed value represents the reciprocal of the time allowed for a motion block, not a linear distance per minute. It is useful when coordinated rotary and linear axes make a single conventional feed distance ambiguous.

Quick answer

For inverse minutes: F = 1 ÷ block time in minutes.

For inverse seconds: F = 1 ÷ block time in seconds. Controller and postprocessor conventions must match.

In this guideBlock-time formulaWorked exampleDegrees per minutePost and control checks

Convert a target path feed to block time

If the CAM system supplies an equivalent path length L and target path feed v, the desired block time is:

Block timetmin = L / vL and v must use matching distance units
Inverse-minute feedF = 1 / tminF describes how many identical blocks would fit in one minute

Worked inverse-time example

A coordinated block has an equivalent path length of 25 mm and target path feed of 500 mm/min.

  1. Block time: 25 ÷ 500 = 0.05 min.
  2. Seconds: 0.05 × 60 = 3.0 s.
  3. Inverse-minute value: 1 ÷ 0.05 = 20.0000 min⁻¹.
  4. Inverse-second value: 1 ÷ 3.0 = 0.3333 s⁻¹.

These two F values represent the same time only when the control interprets their units accordingly. Sending the wrong convention can produce a 60× timing error.

Degrees-per-minute comparison

If a rotary axis changes 30° during the three-second block, its average rotary rate is 30 ÷ 0.05 = 600 deg/min. Degrees per minute is not automatically interchangeable with inverse time; CAM posts can use different combination methods for multiple linear and rotary axes.

The postprocessor owns more than one equation

Equivalent path length may depend on tool-center-point control, machine kinematics, rotary radius, and the CAM post’s calculation. Autodesk’s postprocessor documentation exposes feed-per-minute, inverse-time, and degrees-per-minute modes and distinguishes inverse-minute from inverse-second units.

  • Verify the machine-control definition of G93 and required F on every motion block.
  • Confirm whether inverse units are per minute or per second.
  • Use the machine configuration and approved post for the exact kinematics.
  • Check maximum inverse feed, rotary speed, acceleration, and axis limits.
  • Simulate, single-block, and prove the program safely before production.

Technical references

Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.