What actually drives the cost of a machined gear
I've quoted a lot of gear parts over the years, and the pattern never changes: the drawing either over-specifies things the shop can't measure, or leaves out the one number that decides whether the gear works. A gear is not a shaft with teeth. The cost lives in tooth accuracy, and tooth accuracy lives in the process — hobbing, milling, wire EDM, or grinding — not in the tolerance block.
Before you send an RFQ, decide which gear quality class you actually need. AGMA 8 covers most general industrial drives and automation: it gives you reliable, quiet-enough running at a reasonable price. AGMA 10-12 is what you ask for when the gear feeds a servo axis or a high-speed spindle, and it usually forces grinding after heat treatment. If a drawing says 'AGMA 8' but the teeth are also called out at ±0.005 mm on the profile, something is wrong — those two statements don't fit on the same process.
Backlash: what to call out (and what not to)
Backlash is the most misunderstood callout on gear drawings. Buyers ask for 'zero backlash' as if it's a standard option. It isn't. Every gear pair needs clearance between the teeth or it will bind, generate heat, and wear out fast. What you can control is how tight the band is.
- For general automation and packaging: 0.08–0.15 mm (0.003–0.006 in) is normal and works fine
- For servo and positioning drives: 0.03–0.08 mm with matched pairs — but plan on selective assembly or lapping
- For truly zero-backlash systems: don't spec it on the gear. Use a split/spring-loaded gear or a preloaded reducer. Machining can't buy you zero backlash
- If the drawing calls backlash on the pitch circle without a working depth or center distance, the number is meaningless — the shop needs the whole pair geometry to hold it
Runout and tooth-to-tooth accuracy
Runout is where gears actually fail in production. A gear can have perfect tooth profiles and still drive badly if the pitch circle runs out relative to the bore. On a hobbing machine, bore runout is the first thing the operator fights — if the blank runs out 0.03 mm at the bore, that error shows up directly in the gear's radial runout.
| Quality class | Typical use | Process | Radial runout (typical) |
|---|---|---|---|
| AGMA 8 | General industrial, automation | Hobbing / shaping + shaving | 0.05–0.08 mm |
| AGMA 9-10 | Servo, moderate speed | Hobbing + grinding | 0.02–0.04 mm |
| AGMA 11-12 | High-speed spindles, precision instruments | Grinding, lapping | 0.005–0.015 mm |
If you only add one tolerance to a gear drawing, make it radial runout relative to the bore — not a blanket profile tolerance. And remember: whatever class you spec, the shop has to be able to measure it. A CMM with a rotary axis can check runout and pitch deviation, but profile and lead deviation really need a gear measuring machine or a form tester. Ask in the RFQ what inspection equipment the shop has before you demand AGMA 11.
Heat treatment changes everything
The moment a gear is hardened, the game changes. Induction-hardened teeth move, carburized gears move, and the tooth flank finish you got from hobbing is gone. For hardened gears above AGMA 9, plan the drawing around a two-stage process: rough machine in the soft state, heat treat, then grind the teeth and the bore from the hardened state. That grinding step is the expensive one — it's usually 30-50% of the total gear cost. If the gear doesn't need that class, save the money and skip the grind.
What to send with the RFQ
A gear RFQ without the mate gear is a guessing game. Send the full pair: module or DP, pressure angle, helix angle, center distance, number of teeth on both gears, and the actual working depth. With those, a good shop can hold backlash consistently and tell you honestly what class is achievable. Without them, every quote you get is a guess with your money on the line.
Frequently Asked Questions
What is a realistic gear tolerance for a machined gear?
For hobbed or shaped gears, plan on AGMA 8 as a solid default: radial runout 0.05-0.08 mm relative to the bore. If you need AGMA 10+, expect grinding after heat treatment and a 30-50% cost increase on the gear.
Can you machine a gear to zero backlash?
No. Machining always leaves some clearance. True zero-backlash systems use split gears, spring-loaded anti-backlash gears, or preloaded reducers. On the drawing, spec a backlash range that matches your drive, typically 0.03-0.15 mm depending on application.
Is it cheaper to cut gears or use wire EDM?
For one-off or low-volume gears, wire EDM is often cheaper because there is no hob or shaper tooling cost. Above a few hundred pieces, hobbing wins. Wire EDM also handles hardened blanks cleanly and holds excellent tooth profiles.
Do I need a gear measuring machine for inspection?
Only if you spec profile or lead deviation (AGMA 9+). For AGMA 8, runout and pitch deviation checked on a CMM with a rotary axis are enough. Ask your supplier which they have before over-specifying.