Keyway machining: broaching vs slotting vs milling

An internal keyway looks like a simple slot on a drawing and behaves like a tolerance stack problem in production. The print shows a rectangular pocket 6 mm wide and 3.5 mm deep, and that is all most designers specify. What the print does not say is whether the pocket will be cut with a broach, a slotting tool on a lathe, or an end mill on a machining center — and that single decision moves the price per part by a factor of three.

Start from the function, not the cutter

A key transmits torque through its side walls. The top of the key should never touch the bottom of the hub keyway, because a key loaded on its top face rocks and frets. That means width and side-wall parallelism carry the tolerance budget, while keyway depth is comparatively forgiving — typically minus-zero, plus 0.10 mm is fine for a standard parallel key.

Two numbers govern how the joint feels in service. The first is keyway width tolerance, which sets backlash. The second is symmetry of the keyway to the bore axis, which sets how much the hub rocks under reversing load. A keyway that is 0.05 mm off centre will still assemble, but it will run with one sidewall loaded and the other idle, and that is where fretting corrosion starts. Length matters too: the keyway length plus tool overrun decides whether a broach can pass through and whether an end mill has room to ramp in without leaving a witness step.

Broaching: cheapest per part once volume arrives

Broaching cuts the whole keyway profile in one linear stroke with a tool whose teeth grow progressively deeper. Cycle time is 20 to 60 seconds, the width tolerance is the tightest of any method at roughly ±0.010 mm, and side-wall finish comes off the tool at 0.8 to 1.6 µm Ra.

The trade-off is tooling. A standard push broach for a 6 mm keyway runs $250 to $900 depending on width and length of cut, and it only fits one width. A blind keyway needs a custom broach with a reduced final tooth or a relief groove at the end, which pushes tooling to $700 to $1,800 and adds two to three weeks. Setup is a single shimming exercise, so once the tool is in, the per-part cost drops to a few dollars above material.

Slotting and milling: where low volume lives

A slotting attachment on a CNC lathe cuts the keyway in the same setup as the bore. That is the single biggest quality advantage available: because the bore and the keyway are generated without rechucking, symmetry error drops to 0.02 mm TIR without any extra effort. Cycle time is 2 to 6 minutes per keyway because the tool is a single point that strokes in and out. At 50 pieces the cost lands around $6 to $18 per part, tooling included.

Milling on a vertical machining center is the default for prisms and for parts with a bore too large for the lathe chuck. A 4 mm or 5 mm carbide end mill plunges and ramps through the slot in 3 to 8 minutes. Width tolerance is ±0.020 mm, and the corners carry the cutter radius, which is why so many drawings ask for a full-radius keyway end that the mill cannot produce without a relief. Expect a small step at the closed end where the tool decelerates; if the key is a slip fit over the full length, make the keyway 2 to 3 mm longer than the key and let the step sit outside the load zone.

Method	Typical tooling cost	Cycle time per keyway	Width tolerance	Sensible quantity band
Push broaching, standard width	$250 – $900 per broach	20 – 60 s	±0.010 mm	300 – 20,000
Blind keyway, custom broach	$700 – $1,800	30 – 90 s	±0.015 mm	500+
Slotting on lathe, single point	$0 – $120 insert	2 – 6 min	±0.025 mm	1 – 100
End mill on 3-axis VMC	$30 – $80 cutter	3 – 8 min	±0.020 mm	1 – 300
Involute spline hobbing	$900 – $2,500 cutter plus fixture	40 – 120 s	Class 7 per ANSI B92.1	500+
Straight-sided spline, milled with index	$150 – $500 fixture	5 – 15 min	±0.030 mm	5 – 500

Runout, symmetry and the order of operations

Ask for the keyway to be cut after the bore is finished, in the same setup if the geometry allows. Cutting the keyway first and boring second means the bore is indicated off a slot, and the runout you get is whatever the chuck repeatability allows — usually 0.04 to 0.08 mm TIR rather than 0.02.

Woodruff keys, double keys and splines

A Woodruff key sits in a semicircular seat cut with a Woodruff cutter, typically $40 to $120 for a standard size. The seat is shallow and the joint is cheap, but depth control is poor because the cutter deflects on small diameters, and the seat weakens the shaft more than a parallel keyway of the same width.

Two keys at 180 degrees double the torque capacity of a single key and cost 30 to 50 percent more, because the part needs a second index and a second cut. Use them when the hub material is soft or when a single key would have to be longer than the hub allows.

Splines are a different calculation. An involute spline carries load on all teeth, self-centres under torque, and machines by hobbing at 40 to 120 seconds per part once the cutter exists. Cutter and fixture run $900 to $2,500, so involute splines make sense from roughly 500 pieces upward. Straight-sided splines mill with an indexer for $150 to $500 of fixture and stay practical at 5 to 500 pieces, with a looser fit.

What to put on the drawing

Give the width with a tolerance class rather than a nominal, the depth with a minus-zero allowance, the end condition, the symmetry callout to the datum axis, and a note on whether the keyway is cut before or after heat treatment. Reference ANSI B17.1 for parallel keys and ANSI B92.1 for splines, and state the fit, because that decides whether the shop aims at the low or high side of the width band.

If you want the shop to choose the method, give the annual quantity. A shop that knows 4,000 pieces are coming will quote a broach and a fixture. Our capabilities page lists the equipment behind both routes, and the contact form is the place to send a keyed hub for a DFM note.

Frequently asked questions

What keyway width tolerance should I specify for a 6 mm key?

For a slip fit, specify 6 mm plus 0.020 / plus 0.060 on the hub and 6 mm minus 0.030 / zero on the shaft, following ANSI B17.1. Tighter than that is only needed for reversing loads, and it raises cost because the shop must broach rather than mill.

Can a keyway be cut without a broach at 50 pieces?

Yes. Slotting on the lathe or milling on a machining center both handle 50 pieces at $6 to $18 each, and the lathe version gives better symmetry because bore and keyway share a setup.

Why does my hub rock on the shaft even though the key fits?

Symmetry to the bore axis is the usual cause, not key width. A keyway cut in a second operation can sit 0.05 mm off centre, which loads one sidewall only. Specify 0.03 mm symmetry to the datum axis and require the keyway to be cut after boring.

Is an involute spline worth it for 200 parts?

Usually not. Hobbing needs $900 to $2,500 of cutter and fixture, which adds $5 to $12 per part at that volume. A straight-sided spline milled with an indexer costs less overall and is adequate unless the joint sees high reversing torque or needs self-centring.

What keyway width tolerance should I specify for a 6 mm key?

Can a keyway be cut without a broach at 50 pieces?

Yes. Slotting on the lathe or milling on a machining center both handle 50 pieces at $6 to $18 each, and the lathe version gives better symmetry because bore and keyway share a setup.

Why does my hub rock on the shaft even though the key fits?

Is an involute spline worth it for 200 parts?

Written by the Hengsheng engineering team. Questions about a part like this? Send the drawing and the reply comes from the engineer who would machine it. For holding those tolerances in production, the capability page has the numbers.

Keyway Machining: Broaching, Slotting or Milling?