A spiral bevel gear pair carries the entire torque flow through a commercial-vehicle drive axle, and the meshing quality of the driving and driven tooth surfaces sets both the efficiency of the drivetrain and the noise perceived in the cab. When the flank of a driven gear leaves the cutting machine covered in light-and-dark stripes that can be felt with a fingernail, the part is not a cosmetic reject: the stripes are a periodic geometric error, and the gear will run loud and wear unevenly. The case discussed here concerns a production line that had to solve such a problem without any support from the machine tool builder, because order volumes were climbing steeply while pandemic travel restrictions made an on-site service visit impossible. The work was carried out by the plant’s own manufacturing maintenance team.
The part and the cutting kinematics
The component is a 596 driven spiral bevel gear whose tip-circle diameter is more than four times its overall thickness and whose web is a continuous disc. This geometry — a stiff-looking but axially thin disc with a large central bore — is what makes the part sensitive to any variation in clamping force and to any periodicity in the cutting force.
Tooth profiles are generated on a German-built Oerlikon C50 spiral bevel gear cutting machine. A SPIRON 175-L/R carbide form cutter head, hardened and ground, is driven by the tool spindle; the head carries 34 carbide blades set at an inclination of 8.5°. Cutting is dry and continuous-indexing, with six CNC axes interpolating: the tool spindle (A), the workpiece spindle (B), the base-angle axis (C), the depth axis (X), the tool positioning axis (Y) and the workpiece positioning axis (Z). The workpiece spindle is fitted with a built-in 1FW6 torque motor.
Upstream of cutting, the blank is finish-turned on the tip cone and back cone (operation 20). At operation 25 the gear is turned over 180° so that the tip cone and inner cone face upwards, and is located on the Ø295 mm bore together with an unmarked plane; a locating disc, pull rod and disc-spring stack then clamp it automatically. The tooth profile is formed in a single dry pass under continuous numerical control of all six axes, after which chamfering tools deburr both flanks.
Part requirements and cutting data
The defining geometry and the acceptance criterion for the flank are summarised below. The accuracy requirement is the decisive figure for the whole case: DIN 3965 grade 6, with the flank checked on a P65 gear measuring machine.
| Parameter | Value |
|---|---|
| Normal module at mid-face, mmn | 8.9429 mm |
| Normal pressure angle, α | 22°30′ |
| Number of teeth, z1 | 37 |
| Pitch cone angle, δ | 70°54′49″ |
| Mid-face pitch diameter, dm | Ø399.13 mm |
| Bore, B | Ø295 H7 mm |
| Spiral angle at mid-face, βm | 34° |
| Mounting distance, tB | 110 ± 0.1 mm |
| Crown-to-back distance, tE | 41.9433 mm |
| Hand of spiral | Right |
| Ratio, u | 3.7 |
| Tooth accuracy | DIN 3965 grade 6 |
Symptom: a squealing spindle and a striped flank
During dry cutting of the driven gear the machine developed a sharp squeal at roughly 80 dB. The noise came from the workpiece spindle B: it appeared both while the spindle was rotating and while it stood still with the drive enabled and the operator door closed. That second observation is worth stressing, because a spindle that squeals at zero speed is being excited electrically rather than by a rotating mechanical element.
The cut flanks showed the marks seen in production rejects: stripes of alternating brightness under oblique light, uneven to the touch. Measured on the P65 machine, the flank reached grade 9 of DIN 3965 against the grade 6 required, so the parts were scrapped. With a unit value of 815.22 yuan per driven gear, the line was losing parts continuously while the machine stood waiting for repair.
Electrical screening: current balance, drive swap, insulation
The team worked from the simple to the complex. A clamp meter on the output of the B-axis servo drive showed phase currents of 32 A, 76 A and 19 A in U, V and W — a severe imbalance on a spindle that was supposed to be at standstill. The consequence is not limited to heating: the clamping force Fz is not measured directly but derived from a percentage of B-axis motor current. In the calibration routine the motor turns clockwise for a few revolutions, the current is then reduced until the thread end position A of the brass nut is detected, and the motor reverses to the position that corresponds to the commanded Fz. With one phase carrying four times the current of another, that search can terminate at the wrong position, and the clamping force actually applied no longer matches the setpoint.
Swapping the servo drives of the tool spindle A and the workpiece spindle B — both 611D units, whereas two comparable machines in the plant use S120 drives — produced no change in either the squeal or the flank pattern. A megohmmeter check showed good insulation resistance from each of U, V and W to protective earth, with equal resistance between phases. Successive reductions of the speed-loop gain MD1407 from 15000 to 12000, 10000 and finally 8000 silenced the squeal, but test cuts still produced striped flanks. The electrical symptom had been masked, not the cause removed.
Frequency-domain work: Bode plot and speed-loop filters
The next step was to look at the loop in the frequency domain using the drive optimisation software embedded in the machine’s PCU50.3 control. The B-axis speed loop was measured with an amplitude of 0.3 r/min, a bandwidth of 4000 Hz, averaging over 30 runs, a settling time of 100 ms and an offset of 4 r/min; monitoring was active with limits of +360° and −360°, and the measurement type was the reference frequency response. Marking the highest point of the resulting Bode plot gave the frequency and width of the resonant peak.
Acting on that peak, the current-loop filters were reconfigured: the number of active filters MD1200 was raised from 3 to 4, the blocking frequency MD1219 lowered from 1180 to 510, the band-stop width MD1220 narrowed from 600 to 150, and the speed-loop gain MD1407 set to 12000. The amplitude response then fell smoothly through 20.0 with no peak exceeding 3 dB on the descending branch.
The improvement was partial. Of 60 driven gears cut in a trial, 6 showed intermittent chatter marks, at pieces 10, 20, 30, 31, 40 and 52 — irregular spacing that pointed away from a purely electrical periodicity and towards a mechanical condition that changed between clamping cycles.
The mechanical root cause: a seized bushing in the workholding
The fixture was therefore dismantled along the mechanical coupling chain. The clearance-fit bushing was found seized inside the fixture body. The practical effect was that the 10 mm of free travel normally available to the pull rod during clamping and unclamping had disappeared. The failure hid itself well: once the fixture was released, the gear still sat free and could be turned by hand, so the fixture appeared to be working, and the seizure went unnoticed in routine checks. The bushing had to be extracted with a hydraulic puller before the fixture could be rebuilt.
With the fixture fully renewed and refitted, 120 gears were cut without a single abnormal flank. The problem then returned sporadically — one marked part at piece 150 and one at piece 180, and three marked parts in a further batch of 50 — which indicated that the clamping force itself, not just the seized bushing, was marginal for this thin-disc part.
| Step | Action | Observation |
|---|---|---|
| 1 | Clamp-meter reading at B-axis drive output | U/V/W = 32/76/19 A, severely unbalanced |
| 2 | Swap A and B servo drives (611D) | Squeal and chatter unchanged |
| 3 | Megohmmeter, phases to PE and phase to phase | Insulation sound, resistances equal |
| 4 | Speed-loop gain MD1407 15000 → 8000 | Squeal gone, chatter persists |
| 5 | Bode plot; MD1200 3→4, MD1219 1180→510, MD1220 600→150, MD1407 15000→12000 | Response smooth, no peak above 3 dB; 6 of 60 gears still marked |
| 6 | Teardown of workholding | Bushing seized in fixture body; 10 mm pull-rod travel lost |
| 7 | Complete fixture renewal | 120 gears clean, then 1 in 150, 1 in 180, 3 in 50 marked |
| 8 | Second speed-loop optimisation: MD1200 3→4, MD1219 510→1080, MD1220 150→500, MD1407 12000→15000 | Verified on Bode plot |
| 9 | Pneumatic circuit changed to pure disc-spring clamping, Fz 4.2 kN → 4.7 kN, valve port B plugged | Clamping force now set by the spring stack alone |
| 10 | Cutting parameters revised in all three plunge stages | No abnormality in test cut |
Clamping force and the pneumatic circuit
Analysis of the clamp/unclamp pneumatics showed that the clamping cylinder had an auxiliary clamping air path, P120.60, working in parallel with the disc springs. That path was removed and the valve port B blanked, so that clamping is now provided by the disc-spring stack alone, with the force increased from 4.2 kN to 4.7 kN. For reference, the force in effect when the fault first appeared had been set at 4.5 kN through the motor-current routine described above. A single, purely spring-driven clamping force removes one variable from the process: the force no longer depends on a pneumatic branch that can drift, and it no longer depends on a current-percentage calibration carried out on a drive with unbalanced phases.
Cutting parameters in the plunge-cut sequence
The tooth profile is produced by a plunge method governed by the total plunge depth T, split into roughing, finishing and spark-out stages, with T = T1 + T2 + T3 = 19.113 + 5 + 1 = 25.113 mm. Cutting speed is held constant at 170 m/min throughout; feed rate and nominal chip thickness decrease from stage to stage, so that the load on the thin disc and on the workholding falls as the flank approaches its final geometry.
| Stage | Plunge depth (mm) | Feed rate (mm/min) | Chip thickness (mm) | Cutting speed (m/min) |
|---|---|---|---|---|
| Roughing, T1 | 19.113 | 6.5719 | 0.095 | 170 |
| Finishing, T2 | 5.000 | 5.1883 | 0.075 | 170 |
| Spark-out, T3 | 1.000 | 3.113 | 0.045 | 170 |
Verification and process control
After the fixture renewal, the pneumatic change and the parameter revision, test cuts were clean. The machine then ran continuously for ten days without recurrence, and the flanks met DIN 3965 grade 6 on the P65 machine. Inspection discipline was tightened to match: the first part of every shift is measured in full, and thereafter one part in ten is sampled. The resulting correction data are fed back to the cutting machine in real time over the plant’s industrial network, which keeps the tooth profile consistent from part to part rather than allowing a slow drift that would only be noticed at the next full inspection.
What the case shows about chatter on spiral bevel gears
Chatter marks on a spiral bevel gear flank are not attributable to a single class of fault. The excitation can come from the cutting parameters, from the cutter head, from the workholding, or from the machine tool itself, and several of these can act at the same time. In this case two independent defects had to be removed: an unbalanced three-phase current on the workpiece spindle drive, which both produced the audible squeal and corrupted the current-based clamping-force calibration, and a seized bushing that silently removed 10 mm of pull-rod stroke while leaving the fixture apparently functional. Fixing either one alone left marked parts in the flow — the pattern of 6 rejects in 60 parts after the first loop optimisation is the signature of a second, intermittently acting cause.
The practical lesson for maintenance of a bevel gear cutting machine group is to separate the mechanical side from the electrical side before changing anything. Ball screws, guide slides, timing belts and bearings belong to the mechanical side; speed loops, current loops, grating scales and readheads belong to the electrical side. Distinguishing them requires familiarity with the cutting mechanism itself, with belt tension gauges, spectrum analysers and the drive optimisation software, and with the ability to write single-axis run-in programs in MDA/MDI mode to excite one element at a time. The same discipline applies to the process window: a disc-shaped gear with a diameter-to-thickness ratio above 4 will only hold grade 6 accuracy if the clamping force is defined by a single, stable source and the roughing, finishing and spark-out stages are matched to the stiffness of the part.