Gear Hobbing to Class 6 Accuracy: How Dual Hydrostatic Spindle Hobbing Machines Hold Precision
Class 6 gear accuracy sits in the sweet spot for most industrial drives: tight enough for smooth, quiet operation under load, yet achievable in volume without the cost of finish grinding. The practical question for a buyer is not whether class 6 is possible — it is whether a hobbing machine can hold it shift after shift. A dual hydrostatic spindle hobbing machine is built to do exactly that.
What class 6 gear accuracy actually means
Under ISO 1328-1, gear accuracy is graded from 0 (finest) to 12 (coarsest). Class 6 demands single-flank composite deviation and tooth-to-tooth pitch variation held within a few micrometres on a medium module gear. AGMA 2000-A88 places an equivalent requirement near AGMA 6–7. For a hobbed gear this means the cutting spindle must stay stable while the hob wears and the blank heats.
Why the dual hydrostatic spindle matters
A hydrostatic spindle floats the rotor on a pressurized oil film with no metal-to-metal contact. That removes the runout and thermal drift you get from rolling bearings at speed. Two independent hydrostatic spindles — one for the hob, one for the worktable — let the machine keep concentricity between cutter and blank through long runs. The result is consistent tooth geometry, not just a good first part.
- Oil film damps vibration, so tooth surface finish stays clean even at higher feed.
- No contact wear means spindle geometry does not drift over the production batch.
- Constant oil temperature holds the thermal envelope, protecting pitch accuracy.
Typical parameters for class 6 hobbing
| Parameter | Typical value | Effect on accuracy |
|---|---|---|
| Radial runout of hob spindle | ≤ 0.5 µm | Directly sets tooth pitch variation |
| Oil film pressure | 2–4 MPa | Stiffness and damping of the cut |
| Oil temperature control | ±0.5–1 °C | Prevents thermal pitch drift |
| Achievable accuracy | Stable class 6 (ISO) | Within AGMA 6–7 band |
| Module range | ~1–10 mm (machine dependent) | Covers most drive gears |
Hobbing vs grinding for class 6 work
For gears in the class 6 band, hobbing is usually the faster, cheaper route when volumes are real. Grinding earns its place above class 5 or on hardened surfaces. We covered the cycle-time and cost trade-off in hobbing vs grinding for gear finishing.
Where these machines earn their keep
- Automotive and commercial-vehicle transmissions
- Industrial reducers and gearboxes
- Machine-tool feed and indexing drives
- Wind-turbine and agricultural drivetrains
Choosing and specifying
If you are scoping a hobbing cell, start from our CNC gear hobbing machine selection guide, then match spindle type and accuracy class to your module and volume. Browse the gear machining equipment range for hydrostatic hobbing options.
Work with us
Marusaku Seiko builds gear hobbing machines around dual hydrostatic spindles for stable class 6 output. Send your module, accuracy class, and annual volume and we will propose a configuration and lead time.