A keyway that wouldn’t accept a standard key. The shaft was 40 mm diameter, keyway width 12 mm. The keyway was milled with an end mill. The width measured 12.05 mm at one end and 12.15 mm at the other. The key (12 h9) was loose. The customer re-milled the keyway. It was still tapered. The issue: milling a keyway with an end mill produces a tapered slot because the tool deflects. A broached keyway is straight. This is about keyway machining methods and when broaching is required.
Milled vs broached keyways
A keyway can be cut by two methods. Milling uses an end mill or slot drill. The tool rotates and feeds along the shaft. The tool deflects under cutting force. At the start of the cut, the tool is fully engaged and deflects less. At the end, the tool exits and deflects more. The result: the keyway is wider at the exit end by 0.05-0.15 mm. For a 12 mm key, that’s 0.05 mm of taper. The key fits at one end but is loose at the other.
Broaching uses a multi-tooth broach that pulls through the keyway. Each tooth cuts a small chip. The broach is rigid — it doesn’t deflect. The keyway is straight and parallel within 0.01 mm. The surface finish is Ra 1.6. Broaching is the standard for production keyways. Milling is acceptable for prototype or one-off parts where the taper is acceptable.
What was changed
1. Broached the keyway instead of milling. The shaft was sent to a broaching shop. The 12 mm keyway was broached to H12 tolerance (12 +0.18/0 mm). The standard key (12 h9: 12 -0.043/0 mm) had a transition fit. No rocking. No fretting. The keyway cost $20 to broach (vs $5 to mill). The $15 difference prevents a $500 shaft replacement.
2. Specified the keyway tolerance on the drawing. The drawing now calls out “12H12 broached, surface finish Ra 3.2”. The machinist knows to broach, not mill. The tolerance is specified. No ambiguity. A keyway that isn’t toleranced will be milled to whatever tolerance the cutter happens to produce.
3. Added a keyway chamfer. The keyway ends were chamfered. A sharp end on a milled keyway creates a stress concentration. The chamfer (0.5 x 45°) removes the sharp corner. The key slides in without burrs. The stress concentration drops. The shaft doesn’t crack at the keyway end.
The keyway tolerance table
| Keyway width | H7 tolerance | h9 key | Fit |
|---|---|---|---|
| 6 mm | 6 +0.012/0 | 6 -0.030/0 | Clearance 0.030-0.042 |
| 10 mm | 10 +0.015/0 | 10 -0.036/0 | Clearance 0.036-0.051 |
| 12 mm | 12 +0.018/0 | 12 -0.043/0 | Clearance 0.043-0.061 |
| 16 mm | 16 +0.018/0 | 16 -0.043/0 | Clearance 0.043-0.061 |
For a transition fit (key that doesn’t rock), the keyway is H7 and the key is h9. The average clearance is 0.03 mm. Under torque, the key seats on one side. For heavy loads, the key is ground to k8 (interference) and pressed in. For general power transmission, H7/h9 is standard. A milled keyway that’s 0.1 mm tapered makes the fit uncertain.
The keyway rule: broach for production, mill only for prototypes. The loose key wasn’t a wrong size — the milled keyway tapered 0.1 mm from tool deflection. Specify H7 tolerance and Ra 3.2 on the drawing. Chamfer the ends. For 12 mm and larger keyways, broaching is the only way to get a straight, parallel slot.
