Gear Cutting
Also: gear hobbing, gear shaping, gear grinding, gear manufacturing, custom gears, spline cutting, gear machining, hobbing and shaping, precision gears
Gear cutting makes spur, helical, internal and worm gears by hobbing, shaping and grinding: hobbed gears reach AGMA Q8-Q10, ground gears AGMA Q12-Q14 after hardening.
318 gear manufacturing companies in the Noramark directory
Three processes cover most gears. Hobbing rotates a worm-shaped cutter (the hob) in mesh with the blank so the tooth flanks are generated by a continuous cut; it makes external spur and helical gears, splines, sprockets and worm wheels quickly and accurately. Shaping reciprocates a pinion-shaped cutter while cutter and blank rotate together; it makes internal gears, gears next to a shoulder, cluster gears and racks that a hob cannot reach. Gear grinding, generating or form, finishes hardened teeth and corrects heat-treat distortion. Broaching handles internal splines at volume, shaving improves soft gears before hardening, skiving and power skiving make internal gears fast, and bevel gears need dedicated machines at specialist shops.
Gears are toleranced by quality grade rather than by plus and minus. On the AGMA 2000-A88 scale (Q numbers, 3 to 15, higher is better) hobbing and shaping produce Q8-Q10 as cut and Q11 with a good machine and a fresh hob; gear grinding after hardening reaches Q12-Q14. The newer AGMA 2015 and ISO 1328 scales run the other way (lower is better), and most U.S. drawings still quote Q numbers. A typical routing is: turn the blank, hob or shape, deburr, heat treat (carburize 8620, nitride 4140, through-harden 4340), grind or hone, and inspect with a double-flank composite roll test or an analytical gear checker for lead, profile and pitch.
Tooling is by size: a hob or shaper cutter serves one module or diametral pitch and one pressure angle, so standard 20° gears in stock pitches are cheap to start and special forms are not. Noise, load capacity and life come from the quality grade, the material and heat treatment, and the bore and face runout to the pitch circle, so a gear drawing has to carry all of them.
At a glance
| Typical tolerances | By grade: hobbing and shaping hold AGMA Q8-Q10 (roughly ISO 1328 grade 8-7) as cut, Q11 with care; gear grinding after hardening reaches AGMA Q12-Q14 (ISO 5-3). In numbers, for a mid-size gear: tooth-to-tooth composite error of about 0.02-0.05 mm (0.001-0.002") at Q9 and 0.005-0.01 mm (0.0002-0.0004") at Q13; lead and profile within 0.013-0.025 mm (0.0005-0.001") at Q9 and 0.004-0.008 mm (0.00015-0.0003") at Q13. Bore and OD come from turning and grinding at ±0.013 mm (±0.0005") with care; tooth thickness is verified by measurement over pins or span. |
|---|---|
| Size limits | Job-shop hobbers cut external gears from about 6 mm (1/4") to 500-600 mm (20-24") diameter, module 0.5-10 (48-2.5 DP), face width to 250 mm (10"); large-gear shops go to 2-3 m (80-120") and module 20 and up. Shapers cut internal gears from about 15 mm (5/8") bore to 500 mm (20"). Gear grinders cover roughly the hobbing range, with fewer shops above 400 mm (16"). Fine-pitch instrument gears below module 0.5 (above 48 DP) are a specialty. |
| Surface finish | Hobbed and shaped flanks 1.6-3.2 µm Ra (63-125 µin) with visible generating facets; shaved or skived 0.8 µm Ra (32 µin); ground 0.4-0.8 µm Ra (16-32 µin); honed or superfinished 0.1-0.2 µm Ra (4-8 µin) for quiet, high-speed gearing. Tooth-edge chamfering and deburring is a separate operation and should be specified. |
| Lead time | Prototype gears in 2-4 weeks when a hob or cutter of the right module and pressure angle is on the shelf, and 6-10 weeks when a special cutter must be made. Production with heat treatment and grinding runs 8-12 weeks; catalog stock gears ship in days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carburizing steel 8620 (and 9310 for aerospace) for the highest loads, ground after case hardening; 4140 and 4340 through-hardened or nitrided for medium loads and low distortion; 1045 induction hardened and 1018 soft for light duty. 17-4 PH, 303 and 416 stainless for corrosion resistance in food, marine and medical drives. Ductile iron and gray cast iron for large, slow gears and housings; aluminum bronze (C954) and phosphor bronze for worm wheels. C360 brass and 6061 aluminum for instrument and light-duty gears; Delrin and nylon for quiet, unlubricated gears that are cut or molded.
What drives the cost
- Quality grade: every step above AGMA Q10 adds grinding, inspection and cost, and Q12 and up means a hardened-and-ground routing
- Cutter availability: a standard 20° hob in a stock module or DP is on the shelf; a special pressure angle, profile shift or tooth form means a custom cutter
- Heat treatment and its distortion: carburized gears almost always need grinding afterwards
- Size and tooth count: cutting time scales with face width, number of teeth and module
- Internal gears, gears next to shoulders and cluster gears, which need shaping instead of hobbing
- Inspection: a composite roll check is quick; analytical charts of lead, profile and pitch add time and paper
- Quantity: blank turning, setup and cutter cost amortize, and broaching, powder metal or molding take over at volume
When to use it
- Spur, helical, internal and worm gears, sprockets and splines from prototype to production
- Power transmission gears that need AGMA Q8-Q10 as cut or Q12-Q14 ground
- Replacement and custom gears for machinery where a catalog gear does not fit
- Splined shafts and hubs, which hob on the same machines
- Hardened, ground gears for gearboxes, robots, aerospace actuators and racing
When not to
- Low-load, low-speed gears that catalog stock gears or molded plastic gears cover
- Very high volumes of small gears: powder metallurgy, cold forming or injection molding are cheaper
- Bevel and hypoid gears, which need dedicated machines at specialist shops
- Mechanism prototypes where a wire-EDM or milled gear at Q6-Q8 is good enough to test
Design tips
- Use a standard module or diametral pitch and a 20° pressure angle; 14.5° and odd pitches need custom cutters.
- Specify the gear with a data block: teeth, module or DP, pressure angle, helix angle and hand, pitch and base diameters, tooth thickness or measurement over pins, and the AGMA or ISO quality grade.
- Leave cutter runout room: a hob needs clearance past the face width, and a gear next to a shoulder needs a relief groove or a shaper.
- Choose material and heat treatment together: 8620 carburized and ground for high load, 4140 nitrided when distortion must be small, 4140 or 4340 through-hardened for medium loads.
- Leave grind stock of 0.1-0.2 mm (0.004-0.008") per flank and on the bore when the gear will be ground after hardening.
- Tolerance the bore, OD and face runout to the pitch circle datum; gear quality means little if the bore runs out.
- Call out tooth-edge chamfers, deburring and any tip relief or crowning explicitly, and ask for inspection data at the grade you need.
Frequently asked questions
- What is AGMA gear quality?
- A grade that bounds the allowable errors in pitch, profile, lead and runout for a gear of a given size. On the AGMA 2000-A88 scale Q numbers run from 3 to 15 and higher is better: Q8-Q10 is commercial hobbed quality, Q12-Q14 is ground precision. The newer AGMA 2015 and ISO 1328 scales run the other way, with lower numbers better.
- What is the difference between gear hobbing and gear shaping?
- Hobbing generates the teeth with a rotating worm-shaped cutter in a continuous cut; it is fast and accurate for external spur and helical gears and splines. Shaping uses a reciprocating pinion-shaped cutter and is slower, but it cuts internal gears, gears close to a shoulder and cluster gears that a hob cannot reach.
- Do gears need to be ground?
- Only when the application needs better than AGMA Q10 or when hardening distorts the teeth. Soft or induction-hardened gears at Q8-Q10 run fine in most industrial drives. Carburized gears for high load, high speed or low noise are ground to Q12-Q14 after heat treatment.
- What materials are gears made from?
- Carburizing steel 8620 for high loads, 4140 and 4340 through-hardened or nitrided for medium loads, 1045 and 1018 for light duty, stainless 17-4 PH, 303 and 416 for corrosion resistance, bronze for worm wheels, cast and ductile iron for large slow gears, and Delrin or nylon for quiet light-duty gears.
- How much does a custom gear cost?
- A hobbed steel spur gear of 50-150 mm (2-6") in ones and twos usually lands in the low hundreds of dollars each, with cutter availability the main variable. Hardening and grinding to Q12 or better can double it. Volume brings hobbed gears down to a few dollars each at thousands.
