FANUC servo learning oscillation: why sufficient amplitude may still produce no overlap between successive revolutions
An original sinusoidal model based on the 2018 single-axis servo learning oscillation specification compares integer and half-integer frequency multipliers. Peak-to-peak displacement, inter-revolution path spacing and actual chip breaking are distinguished.
Contents
Oscillation cutting cannot be assessed solely by how far the tool reciprocates. If exactly the same oscillation repeats at the same spindle angle on every revolution, spacing between successive paths may remain unchanged. The phase relationship between oscillation frequency and spindle rotation therefore matters too.
Which oscillation function is discussed?
The source is FANUC “Servo Learning Oscillation (1 axis),” A-42148E-046, edition dated 2018-12-17, pages 2, 5 and 10–13 (public mirror of the original specification). This edition covers specified software for 30i/31i/32i-B STEP2 and 0i-F Plus: listed CNC software starts at version 17.0 for the former and D6G3/D4G3 version 1.0 for the latter, with accompanying servo requirements. The panel model alone does not establish eligibility. This article concerns the single-axis servo learning oscillation associated with G8.5 P2 and provides no enabling or commissioning program.
Build an ideal model from phase per revolution
In the specification, I is the frequency multiplier and K the amplitude multiplier. Expressing displacement in terms of spindle revolutions n and feed per revolution f gives the oscillation:
o(n)=(Kf/2)[cos(2πIn)−1]；z(n)=fn+o(n).
The one-sided sinusoidal amplitude relative to the oscillation mean is Kf/2, and peak-to-peak displacement is Kf. Relative to ordinary feed position, the oscillation ranges from −Kf to 0, with mean −Kf/2. The model describes commanded geometry only, assuming steady spindle speed, constant feed and ideal following; it excludes elastic cutting deformation.
Compare positions one revolution apart at the same spindle angle
Original derivation: adjacent-revolution spacing is D(n)=z(n+1)−z(n). For integer I, the two cosines are identical, the oscillation terms cancel, and D remains f. Even if the tool retreats at some instants, paths in the same phase on every revolution do not thereby overlap.
For half-integer I, the cosines have opposite signs, giving D(n)=f−Kf·cos(2πIn), with minimum f(1−K). Use original values f=0.18 mm/rev, K=1.4 and spindle speed 900 r/min:
| I | Oscillation frequency | Peak-to-peak oscillation | Adjacent-revolution spacing D at the same angle |
|---|---|---|---|
| 1 | 15 Hz | 0.252 mm | Constant 0.180 mm |
| 1.5 | 22.5 Hz | 0.252 mm | −0.072 to 0.432 mm |
The second row's negative spacing means commanded paths overlap during part of the phase cycle, providing the geometric condition for intervals without cutting. This does not guarantee chip breaking for a material, tool-tip shape or actual following behavior. Nor are the table values recommended machining settings.
Higher frequency also raises velocity demands
Differentiating the model with respect to time gives peak forward velocity v_max=(1+πIK)fS/60, where S is in r/min. In the second row, mean feed is only 2.7 mm/s, but peak velocity is about 20.513 mm/s, about 7.597 times the mean. Increasing oscillation frequency or multiplier therefore also changes the drive's required motion capability.
Separate three questions: whether commanded phase produces overlap, whether the axis can follow the required dynamics, and whether the material forms the intended chips. Observing that the axis oscillates does not replace actual verification of the latter two. Further advanced-function mechanisms are in the OPTION section.