FANUC grinding chopping: stroke, feedrate and complete cycles per minute
Using the 15/150-B chopping function, distinguish commanded feedrate from reciprocation frequency. Ideal acceleration models for long and short strokes calculate cycle time, while retaining this edition’s boundary that feed hold does not stop chopping.
Contents
At the same grinding-axis F value, shortening the stroke usually increases the number of reciprocations, but not indefinitely in inverse proportion. Each reversal changes velocity. If the stroke is too short to reach the target speed, acceleration and deceleration dominate cycle time.
Chopping and ordinary contour feed have different roles
The chopping function described in the FANUC 15/150-MODEL B machining-centre descriptions manual B-62082E/04, §20.13, page 182 (public mirror of the original manual) reciprocates the grinding axis between upper and lower dead centres while other axes execute the contour program. F in the G81.1 format is chopping feedrate, not cycles per minute. The distance between dead centres and point R also have different meanings. This article covers the configured function in this edition only, not G81.1 as a universal grinding cycle across all series.
Ignoring acceleration gives an initial lower bound on time
In this original example, one-way distance is H=20 mm and commanded speed v=100 mm/s, equivalent to 6000 mm/min. An outward and return stroke cover 40 mm. If that speed could be reached instantaneously throughout, the period would be T=2H/v=0.4 s, or 150 complete reciprocations per minute. One cycle here explicitly includes both outward and return strokes.
This result ignores reversal time and is only an ideal lower bound. Dividing F by just one stroke length does not give the number of complete reciprocations: it omits the return and produces twice the value.
Finite acceleration separates long and short strokes
Now assume acceleration magnitude a=1000 mm/s² and symmetric trapezoidal or triangular velocity profiles, with zero velocity at each endpoint and no dwell. This is a mechanics example, not the velocity profile actually used by the machine's CNC.
| One-way stroke H | Can it reach 100 mm/s? | Complete round-trip period | Complete cycles/minute |
|---|---|---|---|
| 20 mm | Yes, trapezoidal velocity | 0.600 s | 100 |
| 4 mm | No, triangular velocity | Approximately 0.252982 s | Approximately 237.171 |
The combined acceleration and deceleration distance needed to reach v is v²/a=10 mm. For the 20 mm stroke, one-way time is H/v+v/a=0.3 s. For the 4 mm stroke, peak speed is only √(aH)=63.246 mm/s, and one-way time is 2√(H/a), making the complete period 4√(H/a). The shorter stroke still increases cycle frequency, but not to the 750 cycles/minute predicted without acceleration and deceleration.
When measuring cycle time, identify what has actually paused
The manufacturer explicitly states for this edition that chopping can continue in manual mode and during feed hold. Pausing the main contour program therefore does not establish that the reciprocating axis has stopped. This is an important motion-state boundary; watching whether the program line changes is not sufficient.
Actual cycle time also depends on the active override, control profile and mechanism capability. Compare times using the same dead centres and the same definition of a complete reciprocation, then explain differences from the ideal model. This article does not turn example values into grinding-axis parameters or replace the machine's prescribed stopping procedure. More motion-function resources are in the OPTION section.