Form Grinding vs Generating Grinding: How Gear Grinding Achieves ISO Grade 5

What Grinding Does After Hardening

Grinding is the abrasive operation applied to tooth flanks that have already been heat treated. Two jobs fall to it: erasing the shape change left behind by carburizing and quenching, and returning profile, lead and pitch errors to within the drawing limits.

A carburized gear sits at HRC 58–62. Neither hobbing nor shaving will cut at that level. Quenching typically displaces lead error by 0.02–0.08 mm. Install such a part without a grinding pass and both noise and load sharing degrade. Among all gear machining steps, this is the most expensive one, and it is the step that decides how accurate the finished part can be.

Summary: Grinding follows heat treatment. Its task is to undo hardening distortion and to lift hardened flanks to ISO 1328-1 Grade 5, with fα ≤ 5 µm and Ra 0.2–0.8 µm. Two process routes cover most work. Generating grinding uses a threaded (worm) wheel in continuous cut and handles modules 0.5–8 mm, with per-piece cost in batch production around 20%–30% under form grinding. Form grinding uses a dressed profile wheel, cutting one tooth space at a time, and fits large modules (mn ≥ 6 mm), low tooth counts and unusual profiles. Three inputs drive the choice: module, batch size, profile complexity.

Form Grinding Versus Generating Grinding

Form grinding dresses the wheel to the shape of the tooth space. The wheel feeds radially into one space, then the work indexes to the next.

Generating grinding meshes a threaded (worm) wheel with the workpiece. Flanks are produced in a continuous rolling motion, so there are no index stops.

• Wheel type: Dressed profile wheel
  • Generating route: Threaded (worm) or dish wheel
• Cutting motion: Plunge into each tooth space, then index
  • Generating route: Continuous generation, no indexing
• Module range: mn ≥ 6 mm, low tooth counts, internal gears
  • Generating route: mn 0.5–8 mm, standard external gears
• Typical grade: ISO 5–6
  • Generating route: ISO 4–6
• Productivity: Low to medium
  • Generating route: High, suited to batch work
• Cost per piece: Reference level
  • Generating route: 20%–30% lower in batches
• Special profiles: Stepped, modified and internal geometries handled easily
  • Generating route: Constrained by worm wheel geometry

The selection rule is short. A large module, a small tooth count, or a stepped or modified profile argues for form grinding. A medium module in lots above one thousand pieces argues for generating grinding. Accuracy ranges overlap; the real separation lies in throughput and profile flexibility.

Parameters Used on the Shop Floor

• Wheel speed: 25–35 m/s conventional; 45–80 m/s high-speed
  • Note: Higher speed reduces burn risk
• Roughing depth: 0.02–0.05 mm/pass
  • Note: Clears quench distortion
• Finishing depth: 0.005–0.01 mm/pass
  • Note: Determines final profile accuracy
• Coolant pressure: 8–15 bar (oil)
  • Note: Flushes and cools the contact zone
• Surface roughness: Ra 0.2–0.8 µm
  • Note: Reached directly, no polishing step required
• Grinding allowance (per side): 0.15–0.25 mm
  • Note: Typical for carburized gears

Allowance is where problems usually surface first. Below 0.1 mm, the wheel cannot remove the distortion and the profile is out of tolerance. Above 0.3 mm, cycle time grows and burn risk rises.

Burn: A Defect You Cannot See

Burn is a metallurgical alteration of the flank caused when grinding heat pushes past the tempering temperature of the material. Two varieties exist: temper burn and re-hardening burn.

• Temper burn: Hardness loss of HRC 1–5, dark etch response
  • Consequence: Accelerated wear, pitting
• Re-hardening burn: Brittle re-hardened layer, bright white etch response
  • Consequence: Grinding cracks, flank spalling

On the floor, nital etching serves as the standard first-piece check. Barkhausen noise testing covers full inspection without harming parts. Four practices hold burn in check:

• Lower the finishing depth
• Dress the wheel on time so it stays sharp
• Deliver enough coolant
• Do not grind dry

Where Grinding Sits in the Finishing Chain

• Shaving: Applied before heat treatment (≤ HRC 45)
  • Grade achievable: ISO 6–7
  • Roughness: Ra 0.4–1.0 µm
  • Cost: Low
• Grinding: Applied after heat treatment (HRC 58–62)
  • Grade achievable: ISO 4–6
  • Roughness: Ra 0.2–0.8 µm
  • Cost: High
• Honing: Applied after heat treatment, frequently after grinding
  • Grade achievable: Improves surface texture and noise behaviour
  • Roughness: Ra 0.1–0.4 µm
  • Cost: Medium

For production gears the usual route runs hobbing, then carburizing and quenching, then grinding. Where acoustic targets are strict or texture direction matters, a honing pass is added downstream of grinding.

Observations from Production

Observation 1: Measure allowance, do not assume it. Pull 3–5 parts per lot after quenching and record lead and profile deviation. Set the per-side allowance at the largest measured value plus 0.05 mm. A blanket 0.2 mm figure either leaves distortion behind or spends time cutting air.

Observation 2: Dress earlier than instinct suggests. A threaded wheel calls for dressing every 50–100 pieces. Acoustic emission (AE) monitoring to time dressing can pull burn scrap from 2%–3% down below 0.5%, while oil usage drops by about 10%.

Observation 3: Filtration quality shows up in the surface. Moving filtration from 20 µm to 5 µm gains roughly 0.1–0.2 µm in Ra. Cutting corners on filtration costs more than it saves.

Observation 4: Etch the first parts. Nital-etch 2–3 pieces before a lot is released. Finding a dark zone at the etch station is far cheaper than finding pitting after assembly.

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