An Article Help Master 7 Gear Processing Methods

Every gear shop has a preferred process. That preference is a poor starting point. The right question is what the drawing demands: module, tooth count, helix angle, accuracy grade, material hardness, and batch size. Answer those five and the method usually picks itself.

A 12-tooth pinion with a 20 mm face is not a comfortable hobbing job, because the hob has to reach into a tight tooth space. Run the numbers first. A 48-tooth spur gear at module 2 in 1045 steel is a straightforward hobbing part, and grinding is unnecessary unless the drawing calls for it.

Batch size decides more than most people expect. Cutting one prototype gear on a 5-axis mill is often faster than setting up a dedicated hobbing machine, because setup time dominates. At 5,000 pieces the same gear belongs on a hobber with a fixture that never comes off the table.

Hardness is the hard stop. Any cutting method works on 6061 or 1045. Once the tooth flank reaches 58 HRC, only grinding and hard finishing remain. Choose the soft process with the finishing allowance already written into the drawing.

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  Soft cutting firstHobbing, shaping, and milling shape the teeth before heat treatment.
• 2
  Hard finishing lastGrinding corrects distortion after hardening; leave 0.15–0.3 mm per flank.
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  Batch drives setupBelow roughly 50 pieces, general-purpose machines usually win.

The workpiece sits on a vertical spindle and the hob turns on a horizontal axis. The two rotations are synchronized so the hob's helical motion matches the feed along the tooth width. Each hob revolution removes material from several tooth spaces at once.

Hobbing handles spur and helical external gears, sprockets, and splines. Module 0.5 up to 12 is routine. Because the hob is a continuous cutter, cycle time per tooth is low and the process holds pitch well across a long face width.

It cannot cut a shoulder that blocks the hob path, and it cannot produce an internal gear. A stepped gear with a close shoulder needs shaping instead, or a relieved hob that may not reach full depth.

Tool cost is the trade-off. A single hob covers one module and one pressure angle across a range of tooth counts, so the same tool runs many part numbers. That is why hobbing stays the first choice for volume external gears.

Gear shaping uses a pinion-shaped cutter that reciprocates axially while the cutter and workpiece rotate in mesh. The cutter strokes up and down, feeding radially until the tooth depth is reached. It reaches internal gears, cluster gears, and gears with shoulders that block a hob.

The limit is clearance. A shaping cutter needs room to run past the tooth at each end of the stroke. Blind internal gears with a shallow relief pocket can still be unreachable, and a rack or a very long shaft may not fit the machine at all.

Form milling cuts one tooth space at a time with a disc or end mill shaped to the tooth profile. It is slow and the profile is only correct for one tooth count per cutter. Its value is geometry: a large gear that will not fit a hobber, or a repair cut on a shaft already assembled.

For a one-off 400 mm diameter gear, milling on a 5-axis center with a ball nose tool and a trochoidal path is often the practical answer. We rough the tooth space, semi-finish, and finish with a small stepover, then verify the profile on a CMM.

Gear grinding removes the distortion that heat treatment leaves behind. A case-hardened gear that measured DIN 8 as cut can come back to DIN 5 after grinding, provided enough stock remains on the flanks. Leave 0.15–0.3 mm per flank and specify the allowance on the drawing.

Form grinding uses a profiled wheel that matches the tooth space. Generating grinding uses a threaded wheel and a synchronized rolling motion. Generating is faster for large batches; form grinding is more flexible for small lots and unusual profiles.

Broaching pulls or pushes a multi-tooth cutter through a bore in one stroke. Every tooth on the broach cuts a little deeper than the last, so the complete internal spline or gear form is finished in a single pass. Cycle time per part drops to seconds.

The catch is the tool. A broach is made for one profile, one tooth count, and one face width. Change any of them and the tool is scrap. Broaching only pays when the same internal feature repeats thousands of times. For low volume, EDM or shaping is cheaper.

Wire EDM cuts a through profile with a traveling wire, while sinker EDM burns a shaped electrode into a blind cavity. Both work on hardened steel, which is their main advantage. An internal gear at 60 HRC that cannot be shaped after hardening can be wire cut to DIN 7.

Wire EDM is limited to through features. Sinker EDM handles blind pockets but the electrode wears, so a single electrode produces a taper that grows with depth. For a gear form, plan on multiple electrodes and a finishing pass at low power.

Rolling forms teeth by pressing a hardened die into a rotating blank. No material is removed. The grain flows along the tooth profile instead of being cut across it, which raises fatigue strength on parts like starter ring gears.

Rolling needs a ductile material and a tooth that is not too deep. It struggles with high-carbon and hardened stock, and it does not produce the sharp corner at a tooth root that a drawing sometimes specifies. For thin-walled gears where chip loading would distort the part, rolling is often the only clean route.

Start with the accuracy grade. If the drawing says DIN 6 or tighter, plan on grinding after heat treatment and leave stock for it. If it says DIN 9, hobbing as-cut is enough and grinding only adds cost.

Next, check access. Internal teeth rule out hobbing. A shoulder close to the tooth rules out hobbing too. Both push you toward shaping, broaching, or EDM depending on volume and hardness.

Then confirm the blank. Rolling and hobbing need a rigid setup and a blank that will not deflect. A thin ring gear at 2 mm wall thickness will move under cutting forces. Rolling or a fixture that supports the bore changes the outcome.

Finally, state the reference. Put the datum, the inspection method, and the tooth thickness tolerance on the drawing. A gear drawing without a defined over-pin or span measurement invites two shops to produce two different parts that both pass their own inspection.

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  Accuracy gradeDIN 6 or tighter means grinding after hardening, with stock left.
• 2
  Internal or externalInternal teeth exclude hobbing; choose shaping, broaching, or EDM.
• 3
  Wall stiffnessThin rings deflect; rolling or bore support helps.
• 4
  Inspection methodDefine over-pin or span measurement so the part is unambiguous.