Workshop Log: Hobbing and Grinding of Gears

Workshop Log — Gear Production: Hobbing and Grinding

Date: 2026-07-08 | Topic: Gear Manufacturing Processes | Session: 3

I. Hobbing as the Primary Roughing Route

Across the shop, hobbing accounts for over 70% of all gear roughing work, which makes it the cutting method used most often.

Principle of operation: A generating motion links the rotating hob and the rotating workpiece. Think of the hob as a virtual rack that indexes without interruption, so every tooth space is formed within one pass. Output is high and the method suits a wide range of parts.

Parameters to start from:

• Cutting speed v: 25–60 m/min (HSS); 80–200 m/min (Carbide)
  • Notes: Determined by tool material
• Feed rate f: 0.5–3.0 mm/r
  • Notes: Higher for roughing, lower for finishing
• Depth of cut aₚ: Full tooth depth or 2–3 passes
  • Notes: For module > 6, use multiple passes
• Coolant: Oil (recommended) or emulsion
  • Notes: Oil extends tool life by 30–50%

Hobbing normally lands in GB/T 10095 grade 7–9; grade 7–8 is where the economics work best.

Troubleshooting hobbing

• Torn or rough flanks — the feed is too aggressive, the hob has dulled, or speed is set too low. Bring feed down to ≤1.0 mm/r and check the hob edges.
• Tooth thickness drifting out of tolerance — the cutter was set incorrectly or the hob O.D. has worn. Cut one trial piece and check span length before touching the setting.
• Chatter — backlash in the machine drive, spindle runout, or a workpiece that is not held firmly. Verify machine backlash and the pressure at the tailstock center.

II. Where Accuracy Is Won and Lost

Blank → Forging → Normalizing → Rough turning → Semi-finish turning → Hobbing (7–9)
 ↓
 Carburizing & quenching (0.05–0.15mm distortion)
 ↓
 Grinding (4–6) → Finished gear

One fact drives the whole chain: grinding cannot undo cumulative pitch error (Fₚ) that hobbing has already built into the part. Only profile and lead errors respond to grinding. The ceiling on final accuracy is therefore fixed at the hobbing stage.

III. Grinding as the Precision Finishing Step

Grinding, performed after heat treatment, is the operation that produces high-precision tooth flanks. Transmission gears are mostly ground.

• Method Worm wheel grinding: Continuous generating, like hobbing
  • Accuracy: ISO 4–6
  • Best For: Small-medium modules, high volume
• Method Form grinding: Wheel shape copies tooth profile
  • Accuracy: ISO 5–7
  • Best For: Large modules, special profiles
• Method Disc wheel grinding: Two disc wheels grind both flanks
  • Accuracy: ISO 3–5
  • Best For: High-precision tools, master gears

The main hazard: grinding burn

When grinding heat is excessive, the tooth surface can re-harden or temper. Microcracks follow, and those cracks seed pitting and spalling early in service. To counter this, use CBN wheels, apply coolant at high pressure, and keep feed slow.

IV. Notes from the Shop Floor

1. Split roughing and finishing into two operations. Roughing takes deep cuts and fast feed; leave 0.3–0.5 mm for the finishing pass. During finishing, reduce feed to 0.5–0.8 mm/r and mount a sharp, fresh hob — surface roughness Ra can improve from 3.2 to 1.6.
2. Allow for distortion during heat treatment. After 20CrMnTi is carburized and quenched, the tip circle grows by roughly 0.3–0.5% of the module. Set aside no less than 0.15–0.25 mm of grinding allowance per flank, otherwise the complete involute tooth surface cannot be generated.
3. CBN wheels earn back their cost. An alumina wheel is 5–10× cheaper than a CBN wheel, yet the CBN wheel lasts 100× longer, requires dressing far less often, delivers a lower total cost of ownership, and resists heat better — every one of which lowers the chance of grinding burn.
4. Coolant is mandatory, not optional. Grinding burn sends many parts to scrap. Pair a CBN wheel with high-pressure filtered cooling (≥10 bar, flow ≥40 L/min); no parameter adjustment matters more.
5. Resharpen hobs when wear triggers it. Past 0.2 mm of flank wear, resharpening can no longer be postponed — roughness falls off quickly after that threshold. Check cutting edges every 50–80 workpieces.

V. Terminology Reference