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Comparison · Gear manufacturing

Ground vs. hobbed gears: accuracy, surface, cost — and when grinding pays

"Do we need to grind it?" is the most common fork in a gear RFQ. Grinding is slower and more expensive, but it reaches an accuracy and noise level that hobbing cannot. This guide shows at which step the two routes diverge, what class each holds reliably, and gives a checklist for deciding by application — Wolfer's defaults are ISO 1328 class 6 for hobbed gears and class 5 for ground gears.

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Comparison · Published 2026-09-12 · Wolfer Transmission

The two routes diverge after heat treatment

The hobbed route: turn the blank → hob (or mill) the teeth → heat-treat (carburise and quench, induction harden, nitride or quench-and-temper) → finish bore and faces → inspect. The tooth form is complete before heat treatment, so the distortion from quenching — helix wobble, pitch change, ovality — stays in the finished part, and the class that can be held reliably is about ISO 1328 class 6; quench-and-tempered or soft gears distort less and can do somewhat better.

The ground route adds flank grinding after heat treatment: the hob leaves 0.1–0.3 mm of stock, and after hardening the profile and helix are re-cut on a gear grinder, removing the distortion without touching the hardness. Wolfer grinds on Qinchuan / Xinghe gear grinders, holds class 5 reliably and verifies it on the PREC60 gear measuring center; the price is 5–7 days more lead time per batch and one more operation.

What grinding buys

First, accuracy: profile and helix are corrected in the hardened state, so class 5 and even class 4 become possible, and modifications such as lead crowning and tip relief can be ground in so the gear still contacts evenly once it deflects under load. Second, the surface: ground flanks typically measure Ra 0.4–0.8 against Ra 1.6–3.2 for hobbed ones, and a smoother flank means less noise, a better oil film and higher resistance to scuffing and pitting. Third, hardness and accuracy together: a case-carburised flank at 58–62 HRC keeps its form after hardening only if it is ground, and the same applies to induction-hardened 45–55 HRC gears such as Wolfer's ground 40Cr series.

Hobbed and ground gears compared
CriterionHobbed (or milled) gearGround gear
Process sequenceCut teeth → heat-treat → finishCut teeth (with stock) → heat-treat → grind → inspect
Wolfer default accuracyISO 1328 class 6ISO 1328 class 5 (class 4 evaluated per project)
Heat-treatment distortionRemains in the finished partRemoved by grinding
Flank roughness (typical)Ra 1.6–3.2Ra 0.4–0.8
Usable flank hardnessMainly quench-and-tempered / soft; accuracy drops on hardened flanksCarburised 58–62 HRC and induction-hardened 45–55 HRC alike
Modifications (crowning, tip relief)LimitedYes
NoiseFairLow
Lead timeStandard lead time+ 5–7 days
CostLowHigher (one more operation)
Typical useGeneral automation, conveying, agricultural, standard reducersServo reducers, machine-tool spindles, robot joints, high-speed or low-noise drives

Roughness values are typical; the drawing governs. Accuracy is evaluated to ISO 1328 — see the accuracy-standards crosswalk for DIN / AGMA / JIS.

Where hobbing is enough

Low-to-medium speed, ordinary noise requirements, class 6–7, quench-and-tempered or soft flanks and a cost-driven brief — hobbing is fully adequate, which covers most general automation, conveying, agricultural machinery and standard reducers. Wolfer's milled spur series (C45 steel, tooth-thickness allowance DIN 3967 8d25) and its hobbed gears follow this route: short lead time, low unit price. Four signals say grinding is really needed: high pitch-line velocity (as a rule of thumb above about 10–15 m/s), an explicit dB target, a required flank hardness above 50 HRC, or a place in a servo positioning chain.

The decision checklist

Run through five questions: (1) Is the pitch-line velocity high, or is there a noise target? (2) Is a flank hardness above 50 HRC required (carburised or induction hardened)? (3) Does the gear sit in a servo or positioning chain that is sensitive to transmission error? (4) Does the drawing call for class 5 or finer, or for profile / lead modification? (5) Is it a single piece or prototype where lead time and cost matter more? A "yes" to any of the first four means grind; a "yes" only to the fifth means hob. When in doubt, send the drawing and the duty conditions — you get a process recommendation within one business day.

FAQ

Common questions on this topic

Does every hardened gear have to be ground?

Not necessarily. After induction hardening or carburising, a gear that only needs class 6–7 and ordinary noise levels can be left unground; but to reach class 5 or keep a modification, only grinding corrects the distortion on a hardened flank. Nitriding distorts little, so hobbed gears are often nitrided without grinding.

Can an existing hobbed gear be ground afterwards?

Only if grinding stock (usually 0.1–0.3 mm) was left at hobbing and the flank hardness allows it. Grinding a finished gear with no stock thins the teeth and opens the backlash, so we generally advise against it; for new parts, simply mark "ground" on the drawing.

How much quieter is class 5 than class 6?

There is no fixed decibel figure — it depends on speed, load and housing. As a rule, tightening cumulative pitch and helix deviation from class 6 to 5 and the flank from Ra 1.6 to Ra 0.8 gives a clearly audible difference on high-speed drives; on slow drives the difference is small and the grinding may not be worth paying for.

How much longer does a ground gear take?

Wolfer's experience is 5–7 days more per batch: stock left at hobbing, heat treatment, the gear grinder, then re-inspection on the gear measuring center. The commitment to ship a first article within 30 days of drawing approval applies to ground gears too.

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