Spiral bevel gears are used in commercial-vehicle drive axles to transmit torque between intersecting shafts. The mesh quality of the pinion and the driven gear controls transmission efficiency, noise, and running comfort. In a 2021 production case at a commercial-vehicle axle plant, a 596 driven gear was cut on a German Oerlikon C50 spiral bevel gear cutting machine. The cutter head was a SPIRON 175-L/R carburized-hardened forming head with 34 carbide blades set at an 8.5° inclination. The tool spindle drove the head while six CNC axes—tool spindle A, workpiece spindle B, base-angle C, milling depth X, tool positioning Y, and workpiece positioning Z—interpolated for dry, continuous-indexing cutting. The specified flank accuracy was DIN 3965 grade 6.
The problem appeared during dry cutting of the driven gear. The machine produced a sharp squeal at 80 dB. The B-axis 1FW6 torque motor squealed both at standstill, with the operating door closed and the drive enabled, and while running. The cut flanks showed alternating light and dark bands under reflected light and felt uneven. A P65 measuring machine graded the flank accuracy as grade 9, outside the grade 6 limit, and the part was scrapped. Each scrapped driven gear represented CNY 815.22 in lost value, and the machine was stopped.
Cutting process and quality window
The 596 driven gear has an addendum-circle diameter to overall thickness ratio above 4 and a continuous disc-shaped web. Its main process parameters are listed in the table below. Before tooth cutting, the top and back cones of the blank are finish-turned to drawing dimensions. In operation 25, the part is turned 180°, placed with its top and inner cones upward on the fixture, located on the φ295H7 mm bore and an unmarked plane, and clamped automatically through a locating disc, pull rod, and disc spring. The cutter head and workpiece spindle then rotate at high speed while the C, X, Y, and Z axes follow the programmed motion. The spiral tooth profile is formed in one dry cut, and chamfering cutters remove edge burrs on both flanks.
| 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″ |
| Midpoint pitch diameter, dm | φ399.13 mm |
| Bore, B | φ295H7 mm |
| Spiral angle at midpoint, βm | 34° |
| Mounting distance, tB | 110 ±0.1 mm |
| Crown distance, tE | 41.9433 mm |
| Spiral direction | Right-hand |
| Transmission ratio, u | 3.7 |
| Tooth-flank accuracy | DIN 3965 grade 6 |
The cutting cycle is a plunge method based on depth T, divided into roughing, finishing, and spark-out stages. Because the dynamic load on the fixture and the B-axis drive changes with chip thickness and feed, the cutting schedule was treated as part of the process window rather than a fixed machine setting.
Fault isolation: servo resonance versus clamp-path damage
The fault-isolation sequence began with the B-axis electrical chain. With the 1FW6 motor squealing, a clamp ammeter at the servo drive output read U, V, and W phase currents of 32 A, 76 A, and 19 A—a severe imbalance. This mattered beyond the acoustic symptom: the fixture disc-spring clamping force Fz = 4.5 kN was derived from a percentage of B-axis motor current. During Fz calibration, the motor rotated clockwise for several turns, the current was reduced to find the thread-end position A of a brass nut, and the motor then reversed to the position corresponding to the commanded force. A current imbalance can therefore make the Fz search fail.
To separate drive electronics from the motor and load, the A- and B-axis 611D servo drives were exchanged. The squeal and the flank marks remained. A megohmmeter showed good insulation from each phase to protective earth, and the phase-to-phase resistances were equal. The drive swap and insulation test therefore did not support a simple drive-module or gross winding-insulation fault. They did not, however, clear the B-axis electromechanical chain.
Next, the B-axis speed-loop gain MD1407 was reduced in steps from 15000 to 12000, 10000, and 8000. The audible squeal stopped, but trial cuts still produced chatter marks. This was a critical separation: the 80 dB squeal and the tooth-flank chatter were not the same defect. Quieting the servo did not restore flank quality.
A Bode plot of the B-axis speed loop was then taken in the PCU50.3 drive optimization software. The excitation was set to 0.3 r/min amplitude, 4000 Hz bandwidth, 30 averages, 100 ms settling time, and 4 r/min offset; the reference frequency response was displayed with monitoring active over ±360°. The cursor was moved to the highest peak to record its frequency and width. The first loop optimization changed MD1200 (active current-loop filters) from 3 to 4, MD1219 (blocking frequency) from 1180 to 510, MD1220 (band-stop width) from 600 to 150, and MD1407 from 15000 to 12000. The resulting amplitude response was smooth, with no peak above 3 dB on the falling side.
| Parameter | Initial value | After first optimization |
|---|---|---|
| MD1200 active current-loop filters | 3 | 4 |
| MD1219 blocking frequency | 1180 | 510 |
| MD1220 band-stop width | 600 | 150 |
| MD1407 speed-loop gain | 15000 | 12000 |
The improved loop did not solve the production problem. In a 60-piece trial, six parts—numbers 10, 20, 30, 31, 40, and 52—showed intermittent chatter. The defect remained intermittent and present. The remaining cause had to be sought in the mechanical clamp path.
Mechanical root cause in the fixture
The fixture was disassembled along its mechanical coupling path. A clearance-fit bushing was found seized in the fixture body. This removed the 10 mm free travel of the pull rod during clamping and unclamping. The failure was easy to miss: after the fixture was released, the driven gear could still be rotated by hand, so the clamp appeared to be functioning. In reality, the force path was no longer the intended one. A hydraulic puller was required to remove the seized fixture, and the fixture was replaced as a complete assembly.
The replacement fixture cut 120 driven gears without abnormality. When the count reached 150 and 180 pieces, one part in each group showed chatter; another 50 pieces produced three parts with chatter. The fixture seizure was therefore a real root cause, but not the only one. The clamp circuit and the servo loop still had to be made more robust.
The pneumatic clamp/unclamp circuit used an auxiliary clamping air path P120.60 on the clamping cylinder. This branch was removed, leaving a pure disc-spring clamp; valve port B of the directional valve was plugged. The clamping force was raised from 4.2 to 4.7 kN. The original setup had been calibrated at 4.5 kN; the revised circuit made the force purely spring-generated and increased the preload. The change was intended to remove the auxiliary pneumatic force branch and make clamp-force application more repeatable under production cycling.
Process parameters and production verification
The B-axis speed loop was optimized again after the fixture change: MD1200 was set to 4, MD1219 was moved from 510 to 1080, MD1220 from 150 to 500, and MD1407 from 12000 to 15000. The Bode plot was used to confirm the loop response. The cutting cycle was then set to the three-stage plunge schedule below, with roughing at T1 = 19.113 mm, finishing at T2 = 5 mm, and spark-out at T3 = 1 mm. The feed and chip thickness were reduced from roughing to spark-out, while cutting speed was held at 170 m/min.
| Stage | Depth T (mm) | Feed V (mm/min) | Chip thickness S (mm) | Cutting speed (m/min) |
|---|---|---|---|---|
| Roughing (T1) | 19.113 | 6.5719 | 0.095 | 170 |
| Finishing (T2) | 5 | 5.1883 | 0.075 | 170 |
| Spark-out (T3) | 1 | 3.113 | 0.045 | 170 |
Trial cuts after these changes showed no abnormality. The machine ran continuously for 10 days with stable flank quality. The P65 measurement met DIN 3965 grade 6. During this period, the first part of each shift was inspected fully on the P65, and one part was sampled every 10 pieces thereafter. Correction data were fed back in real time through the plant’s industrial internet connection to maintain tooth-profile consistency.
Implications for spiral bevel gear manufacturing
This case shows that tooth-flank chatter in spiral bevel gear cutting is a system-level failure, not a single servo or tool problem. The evidence supports several concurrent causes: a seized fixture bushing that eliminated the intended pull-rod travel, an unbalanced B-axis current path that could disturb current-based clamp-force calibration, a speed-loop resonance that produced an audible squeal, and a clamp circuit whose auxiliary air path and preload were less repeatable than a pure disc-spring arrangement. The final process combined fixture replacement, 4.7 kN disc-spring clamping, revised speed-loop filtering, and the three-stage cutting schedule.
The diagnostic order also carries a practical lesson. An audible servo squeal can be removed by lowering speed-loop gain while flank chatter remains, so acceptance must be based on P65 flank measurement and trial cuts, not on the machine becoming quiet. Conversely, a part that still rotates freely after unclamping does not prove that the clamp force path is intact. Current measurement, insulation testing, Bode analysis, and physical stroke checks address different failure modes and need to be combined. Maintenance teams should also distinguish mechanical elements—ball screws, guide sliders, timing belts, and bearings—from electrical elements such as the speed loop, current loop, grating scale, and read head, and should be able to use belt tension meters, spectrum analyzers, PWT18, and drive optimization software. Single-axis run-in programs in MDA/MDI mode help isolate axis-specific faults.
For dry cutting of spiral bevel gears, the stable window depends on fixture stiffness, servo-loop dynamics, and the roughing-to-spark-out feed schedule; changing only one of these can leave an intermittent defect.