BIZOMAT MAKEOS · Manufacturing
Inverse Time Feed Rate (G93)
What inverse-time F value programs this move at the feed I want?
Inverse time feed command (G93 F-word) for a linear, rotary or combined multi-axis move: the reciprocal of the move time, from the feed rate and the length or angle of the move, with the time per block and the surface speed of a rotary move at a radius.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Move | choice | Kind of move. |
| Linear length of the move | mm | Straight-line distance of the linear axes in the block. |
| Rotary angle of the move | ° | Angle turned by the rotary axis in the block. |
| Cutting radius on the rotary axis | mm | Radius from the rotary axis to the cutting point. |
| Wanted feed at the cutter | mm/min | Feed you want along the cutting path. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Move
- What it is
- The kind of move in the block: linear axes only, a rotary axis turning with the cutter at a radius, or both together.
- Units and format
- Choose one of 3 options from the list: Linear move; Rotary move at a radius; Combined linear and rotary.
- Example
- The form opens with “Rotary move at a radius” selected as a worked example. Choose the option that matches your case.
- Get it right
- Linear uses the length only; rotary uses the angle and radius; combined uses all three.
What this means: Linear length of the move
- What it is
- The straight-line distance travelled by the linear axes in this one block, for linear and combined moves.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer path gives a longer block time and a smaller F value.
- Get it right
- Use the resultant of all linear axes moving in the block, not just one axis.
What this means: Rotary angle of the move
- What it is
- The angle the rotary axis turns through in this one block, for rotary and combined moves.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 1000000°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More rotation lengthens the arc at the cutter and lowers the F value.
- Get it right
- Use the incremental angle for the block, not the absolute position it ends at.
What this means: Cutting radius on the rotary axis
- What it is
- The distance from the rotary axis centreline to the point where the tool cuts, which turns the rotation into a path length at the cutter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The arc length at the cutter is proportional to the radius, so a larger radius lowers the F value.
- Get it right
- Measure to the actual cutting point on the work surface, not to the part’s outside diameter if the tool is cutting deeper.
What this means: Wanted feed at the cutter
- What it is
- The feed rate you want along the cutting path at the tool, as you would program it in normal feed per minute mode.
- Units and format
- Enter a number in mm/min: millimetres per minute. The form accepts 0.000001 mm/min to 1000000 mm/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 mm/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The F value is directly proportional to the wanted feed.
- Get it right
- This is the feed at the cutter, not degrees per minute of the rotary axis.
Method and formulas
- Inverse time feed = feed rate ÷ distance moved = 1 ÷ time (in minutes) for the block.
- Rotary path at the cutter = π r × angle ÷ 180°; combined path ≈ √(linear² + arc²).
- Program G93 with F = 1 ÷ time on every block; return to G94 afterwards.
- Example: 90° at 40 mm radius at 300 mm/min needs F4.7746.
Limitations
- Combined path is approximated as straight in the developed plane; long blocks with large rotation should be split.
- Controls differ in G93 limits and decimal resolution.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Program feed for 4th- and 5th-axis moves
- Convert a target surface feed to an inverse-time F value
- Check block times in a multi-axis program
Who it is for
- Machinists
- CNC programmers
- Manufacturing engineers
What you get
- Inverse-time F value
- Time for the block
- Path length at the cutter
Interpreting the result
- Every G93 block needs its own F word; a missing F is an alarm on most controls.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1288–1289. Topic reference; no source tables or recommended settings are reproduced.
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