CNC gear hobbing basics: what buyers need to know before quoting

• 1
  CNC gear hobbing basics: how the hob generates a tooth
• 2
  Hobbing versus shaping, milling and grinding
• 3
  Tolerances, surface finish and inspection
• 4
  Materials, coatings and cutting parameters
• 5
  Cost drivers and lead time expectations
• 6
  What to put on your RFQ, and common buyer mistakes
• 7
  Which process fits your gear

A gear hob is a cylindrical cutter with gashes and helically fluted teeth. Look at one flute in isolation and the cutting edge is a rack tooth. Mount the hob on a spindle, tilt it to the workpiece helix angle, and rotate both in a timed ratio, and the rack effectively walks around the blank. The tooth space is not cut in one pass. It is generated by many overlapping cuts as the hob feeds axially across the face width.

That continuous generation is the reason hobbing holds cycle time down on external gears. One hob covers a range of tooth counts at a fixed module, so a shop does not need a dedicated cutter per part number. Spur gears, helical gears and worm wheels all come off the same machine with different settings and different hobs.

The method has hard limits. The hob must reach the tooth space, so internal gears and teeth close to a shoulder are out. Hobbing also cuts the whole face width at once, so a thin-walled blank deflects under the cutting force. Those two constraints decide more quotes than any tolerance number.

Shaping uses a reciprocating cutter that copies the tooth form. It reaches internal gears, racks and teeth next to a shoulder that a hob cannot enter. The trade is speed: shaping cuts one tooth space at a time, so cycle time per tooth runs longer on the same module.

Form milling with a disc cutter suits one-offs and coarse pitches where a hob is uneconomical. The flank comes out approximate, so treat it as a roughing route unless the drawing tolerance is loose. For very small batches this is often the cheapest path to a working part.

Grinding is the answer above roughly 45–50 HRC and for DIN 5 or better. Hardened teeth cannot be hobbed, so the usual sequence is hob soft, heat treat, then grind the flanks. That adds a second setup and a second inspection, and the price reflects it.

A practical rule for buyers: external spur or helical, soft material, DIN 7 or looser — hob it. Internal, shoulder-adjacent or coarse pitch one-off — shape or mill it. Hardened or DIN 6 or tighter — plan for grinding after hobbing.

Hobbing alone realistically holds DIN 7–8 (AGMA 9–10) on a well-controlled machine with good workholding. DIN 6 is achievable, but it needs careful hob selection, a rigid fixture and usually a separate finishing pass. Promising DIN 6 from a single roughing cut is a warning sign, not a bargain.

Flank finish after hobbing typically lands at Ra 1.6–3.2 μm. If the drawing calls for Ra 0.8 μm or better, plan on grinding or honing afterward. Surface finish and grade move together here; you rarely get one without paying for the other.

Inspection is where buyers get surprised. A hobbed gear is normally verified on a gear measuring center that reports profile, lead, pitch and runout against a stated standard (DIN 3960/3962, AGMA 2015, ISO 1328). Pin or over-ball measurement checks tooth thickness only. It says nothing about flank form, so a part can pass a pin check and still fail on the measuring center.

Put the standard and the grade on the drawing, not just a tolerance band. Ask which measuring center produced the report. A number without a standard attached cannot be compared to anything.

Material choice drives tool life more than most buyers expect. Free-machining and low-carbon alloy steels such as 4140 at 250–300 HB hob well. Case-carburizing grades are common, but they are cut soft before heat treatment, so hobbing happens in the annealed or normalized condition.

Stainless grades work-harden under the cut. Expect roughly half the cutting speed and shorter hob life compared with alloy steel. Cast irons and bronzes hob readily but throw abrasive dust, which wears the flanks fast.

Spindle speed follows the hob material. Carbide hobs on alloy steel commonly run 150–400 m/min. HSS or powder-metallurgy HSS hobs run 40–120 m/min. Feed per workpiece revolution is typically 1–3 mm for roughing and 0.5–1.5 mm for finishing, with the finishing cut removing 0.1–0.3 mm of stock.

Solid HSS hobs are the default for small and medium batches. Indexable carbide hobs pay off in high volume. Coatings such as TiN, TiAlN and AlCrN extend life in abrasive or gummy materials. Class A and AA hobs cost more, and fine grades need them.

Hob wear shows on the flank. A worn hob drifts on lead and profile before it fails outright, so a shop that tracks flank wear by piece count avoids a whole batch of scrap.

Cost is driven by cycle time, hob cost per piece, fixturing and inspection burden. Hobbing is cheap per tooth at volume, but setup is real work: hob mounting, centering and a first-article check. Small lots pay a setup penalty of roughly 1–3 hours per job.

Tight lead or profile tolerances, unusual helix angles and thin-walled blanks that need low cutting force all raise price. So does any requirement that forces a second operation, because the part is handled and inspected twice.

On our side, quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours and parts ship in 3–5 days. No minimum order quantity applies, so a single prototype and a 10,000-part run go through the same first-article route.

If the grade is open, say so. A shop that can choose DIN 8 instead of DIN 6 will quote a shorter route and a lower price, and the gear will still work in most drive trains.

Give the shop module, tooth count, pressure angle, helix angle and hand, face width, outside diameter and bore. Add the quality standard with grade, the material and heat-treatment state, and the flank finish in Ra. A gear drawing without a standard is the single most common reason a quote comes back with questions.

State the inspection you expect. If you need a measuring center report per part, say it up front. If a first-article report is enough, say that too, because it changes the price.

The recurring mistakes are predictable. Buyers ask for DIN 6 from a single hobbing cut. They specify a finish that needs grinding but no grinding step. They design a tooth too close to a shoulder and then wonder why the route changed to shaping. They also forget that heat treatment moves the tooth, so the soft-cut size must be set with allowance.

One more: send the mating gear or the center distance. Tooth thickness alone does not fix backlash, and a gear that measures in spec can still run tight or loose in the assembly.