Cutting Keyways and Slots: Methods and Tolerance
Short answer: keyways and slots are cut by end milling, side milling, broaching, shaping, or EDM, and the method you choose sets the tolerance you get. Milling holds slot width to about ±0.05 mm routinely and ±0.02 mm with care; broaching holds a keyway width in the ±0.015 mm range and produces the straight sides and flat bottom that a key needs. The critical dimension is rarely the depth — it is the width and its parallelism, because that is what transmits torque without looseness. At BQUQ we machine ±0.005 mm, so send the drawing and get a quote within 12 working hours.
A keyway is a small feature that carries a lot of function: it locates a hub or gear on a shaft and transmits torque. A slot is the general case. Both are typically cut after the main form of the part is done, which makes them a good place for a second-operation mistake. Understanding the methods and their limits prevents both.
The Methods, Ranked
End Milling (Slotting)
A milling cutter is plunged or fed along the slot to create the width and depth. This is the most common method for open slots and for keyways on parts that can be reached. It is fast and flexible, but the cutter deflects, so width tolerance and side finish suffer on long slots or hard materials.
Side Milling
For open slots, a side-and-face cutter or a thin slitting saw cuts the width in a single pass, giving better width control than plunging. Good for slots that run off the end of the part.
Broaching
A broach is a multi-tooth tool pushed or pulled through a hole or along a slot, cutting progressively. For internal keyways in hubs — the classic case — broaching gives straight sides, a flat bottom, and a width tolerance in the ±0.015 mm range. It needs a broach for each width, so it suits repeat work.
Shaping / Slotting
A single-point tool reciprocates to cut a keyway or slot. Slower than broaching but economical for one-off work and for hard-to-reach features.
EDM
Wire or sinker EDM cuts slots and keyways in hardened material or in awkward geometry, with excellent tolerance but higher cost and slower cycle.
| Method | Width tolerance (typical) | Best for | Relative cost |
|---|---|---|---|
| End milling | ±0.05 mm | Open slots, reachable keyways | Low |
| Side milling | ±0.02 mm | Through/open slots | Low–medium |
| Broaching | ±0.015 mm | Internal hub keyways, repeat work | Medium (tooling) |
| Shaping/slotting | ±0.02 mm | One-off, awkward shapes | Medium |
| Wire/sinker EDM | ±0.005–0.01 mm | Hard material, complex | High |
Keyway Width Tolerance Is the Critical Call
A key is a precision fit in both the shaft keyway and the hub keyway. The standard practice is to make the shaft keyway a slightly tighter width than the hub keyway, so the key presses into the shaft and slides in the hub. Under ISO and DIN practice, the shaft keyway width is held to roughly N9 and the hub to JS9, with the key fitting accordingly. The exact numbers depend on key size and standard, but the principle is constant: the two keyways do not use the same tolerance class.
| Feature | Typical width tolerance class | Result |
|---|---|---|
| Shaft keyway width | N9 (tight) | Key pressed in shaft |
| Hub keyway width | JS9 (looser) | Key slides in hub |
| Slot width (general) | H9–H11 | Clearance feature |
Depth matters too, but it is usually less critical than width. The depth of the keyway affects how much of the key engages; the width controls the torque path and the looseness that turns into wear and rattle.
Standard Key Sizes by Shaft Diameter
Parallel key sizes broadly follow shaft diameter. Indicative values:
| Shaft diameter (mm) | Key width × height (mm) | Hub keyway depth (mm) | Shaft keyway depth (mm) |
|---|---|---|---|
| 10–12 | 4 × 4 | 1.8 | 2.5 |
| 12–17 | 5 × 5 | 2.3 | 3.0 |
| 17–22 | 6 × 6 | 2.8 | 3.5 |
| 22–30 | 8 × 7 | 3.3 | 4.0 |
| 30–38 | 10 × 8 | 3.3 | 5.0 |
| 38–44 | 12 × 8 | 3.3 | 5.0 |
| 44–50 | 14 × 9 | 3.8 | 5.5 |
These are industry-standard pairings. Deviating from them means a non-standard key, which complicates spares and assembly.
Practical Rules for Specifying Keyways and Slots
First, state the width tolerance class explicitly; do not leave it to a general note unless you intend the general default. Second, specify whether the keyway is a through keyway or a blind one — blind keyways need a way for the tool to exit or a specific end geometry. Third, give the datum: keyways should be located relative to the functional axis, not to an arbitrary face. Fourth, runout between the keyway and the shaft axis matters more than most drawings show; a keyway cut parallel to the axis but offset from it will make the hub wobble. Our guide to hole making covers related location practice, and the tolerance guide shows how to set width and depth calls your shop can hold.
Frequently Asked Questions
Q: Milling or broaching — which is better for a keyway?
A: Broaching gives the best keyway: straight sides, flat bottom, and a tight width in the ±0.015 mm range, which is what a key needs. Milling is cheaper and more flexible but produces a rounded slot bottom and a looser width unless you spend time on a finishing pass.
Q: What tolerance should I put on a keyway width?
A: Follow the standard key practice: a tighter width class on the shaft keyway and a slightly looser one on the hub, so the key presses into the shaft and slides in the hub. State the class explicitly rather than relying on a general tolerance note.
Q: Why does my keyed assembly rattle or wear?
A: Usually a loose keyway width or a runout error between the keyway and the shaft axis. Width controls looseness; runout makes the hub wobble. Check both, and inspect the key fit in both the shaft and hub.
Q: Can you cut a keyway in hardened material?
A: Yes, by EDM or by grinding after heat treatment. Cutting before hardening is cheaper but distorts. The right sequence depends on the hardness, geometry, and quantity.
Q: Does BQUQ cut keyways and slots?
A: Yes, by milling, broaching, and EDM, on both turned and milled parts. Send the drawing with width and depth calls to sc@bquq.com or WhatsApp +86 13713157787 for a quote in 12 working hours.
Related Resources
- Hole making guide: location, tolerance, and practice for drilled and bored features.
- CNC milling parts: slots and keyways cut on our milled components.
- About BQUQ: an ISO9001-certified source factory in Dongguan running CNC, stamping, springs, and heat sinks under one roof.
- Contact us: keyway and slot quotes within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
