Gear hobbing accuracy is how closely the profile, lead, pitch and runout of a hobbed gear stay inside a declared ISO 1328 or DIN 3962 grade. Machine geometry, hob runout, workholding and heat distortion set it; a measuring centre or a roll test proves it. Hobbing alone holds ISO 6 to 8, and hobbing plus grinding reaches ISO 3 to 5.
Main points
- Errors come from four sources: machine, hob, workholding and heat distortion
- The main metrics are profile deviation Fa, lead deviation Fβ, total cumulative pitch deviation Fp and radial runout Fr
- Inspection splits into single-element, double-flank roll test and gear measuring centre, chosen by module and batch
- Carburising shifts radial runout by 0.02 to 0.05 mm, so parts are re-checked and compensated after heat treatment
- Acceptance runs on a closed loop: first-article FAI, in-process SPC, sampling and a report
The same hobber can hold two grades apart from one batch to the next. The difference is whether each error source is controlled.
A gear shop once shipped two batches off one hobber in the same week. The first held ISO 6 and the second slipped to ISO 8; a worn fixture, not the machine, caused the drift. The fix was a new datum face and a tighter clamp, and the grade returned.
Machine geometry is the base. Spindle runout, guideway straightness and index-chain error are copied straight onto pitch, so spindle runout is usually held below 2 μm.
Hob runout and alignment are the second source. Radial runout and axial wobble after mounting are amplified once per tooth, and hob alignment is held to about 2 μm. Gear hobbing tolerances tighten quickly when either value drifts.
Workholding and heat distortion decide batch consistency. Worn fixtures drift radial runout between batches, and a shop held at 20±1 °C limits spindle growth, which can reach 0.01 mm after two hours of cutting. Want the wider process picture? The six core precision processes cover hobbing, grinding and inspection together.
Profile deviation Fa measures how far the flank departs from a true involute, and it drives mesh smoothness and noise. Lead deviation Fβ captures tilt across the face width, and a large value pushes the contact pattern to one end.
Total cumulative pitch deviation Fp and single pitch deviation fpt reflect index accuracy and are a main source of noise and vibration. Radial runout Fr comes from workholding and heat treatment, and it upsets how evenly the ratio is transmitted.
| Method | What it measures | Where it fits |
|---|---|---|
| Single-element | Fa, Fβ, Fp, Fr item by item | First article, arbitration, reports |
| Double-flank roll test | Composite radial deviation | Fast 100% batch screening |
| Gear measuring centre | Full profile and topography | High grades, complex flanks |
A roll test is fast but returns only a composite value, while a gear measuring centre maps the whole flank and gives traceable numbers. A gear profile inspection service pairs the two: a roll test screens the batch and a measuring centre confirms it. Capability and fixturing are described under quality and inspection control.
The gear hobbing accuracy grade ISO 1328 is the baseline, and DIN 3962 and AGMA 2015 run in parallel, so the grade is agreed before the first part. ISO 1328 splits Fa, Fβ, Fp and Fr into grades 0 to 12, where a lower number is tighter. DIN 3962 sits close to it, and AGMA 2015 uses A and Q numbers, where a higher Q is tighter. Gear material and heat-treatment background is covered by ASM International.
Automotive gearbox gears usually sit at ISO 5 to 7, while robot joints and aerospace parts sit at ISO 3 to 5. Acceptance also checks the contact pattern: 60% to 80% across the face width and 45% to 60% up the tooth height.
Finish is quoted next to the grade. Hobbed flanks usually sit at Ra 1.6 to 3.2 μm, while ground flanks reach Ra 0.2 to 0.8 μm, which is why a grade callout alone is not enough on a sealing or bearing face.
Closed-loop gear hobbing accuracy control starts with the first article. The first part is measured against every element on the drawing, and the batch starts only once the machine and fixture settings pass.
During the run, SPC tracks drift in Fp and Fr, and a trend on the control chart triggers a hob shift or a compensation before parts leave tolerance. Carburised parts shift radial runout by 0.02 to 0.05 mm, so the grinding allowance is corrected from measured data.
When you vet a custom gear hobbing supplier in China, ask four questions. First, what grade can the supplier hold in steady production, not on the best single part? Second, is there a gear measuring centre and a roll test, and can a full report be issued?
Third, is the process run on SPC, with hob-shift and compensation records? Fourth, for build-to-print work, are first-article reports and material certificates provided?
One EV customer had a batch reworked because the supplier quoted only a single-part value, and the gears exceeded the noise limit once in production; a supplier with full-element inspection solved it. Ready to check achievable grades? The Yujiaxin Tech precision gear line lists them per part, and the cross-process view sits in CNC machining tolerance control.
Small-module gears have narrow tooth spaces and a low-stiffness hob, so even a small cutting force pushes the profile out of tolerance. These parts go into robot joints and medical devices and often call for ISO 4 to 6.
Control means limiting hob runout, taking smaller cuts per tooth and using a rigid fixture. Inspection leans on a measuring centre rather than a roll test, because a composite value hides single-element drift. Related material is covered in low-alloy steel PM gears, whose standard base is set out by MPIF.
Yujiaxin Tech runs a group of high-precision automatic hobbing machines that reach DIN 3, backed by a gear measuring centre, double-flank testing and CMM inspection, with hobbing, grinding and heat treatment under one roof. Each shipment carries a full inspection report and supports build-to-print work with EN 10204 3.1 certificates.
Gear hobbing accuracy is not a single trick but a combination of error sources, inspection methods and an acceptance loop. Hold the four sources, match the method to the grade, and protect the batch with first-article checks and SPC, and the grade repeats.
If your drawing carries a specific grade or a small-module custom gear, gather the profile, lead, pitch and runout callouts. Then send the drawing and accuracy requirements to our engineers, and Yujiaxin Tech returns the achievable grade, an inspection plan and a quote.
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