Guide
Keyway broaching, slotting or wire EDM: how should an internal keyway be made and drawn?
Keyway broaching is the usual way to cut an internal keyway in a hub or gear bore: a toothed broach, guided by a slotted bushing, is pushed or pulled through the bore and cuts the keyway to width in one stroke and to depth in one or more passes. It suits through bores in unhardened material; blind keyways are better slotted (shaped) on a CNC machine, and keyways in hardened hubs or with very tight positional requirements are often cut by wire EDM.
By Saola Manufacturing engineering team · Published · Updated
The keyway width and depth come from the key standard; those values and their tolerances are covered in the keyway dimensions and tolerances guide. This guide is about the step before that: which process will cut the keyway, what each process needs from the part design, and what the drawing should say so a supplier can choose a method without asking you to change the part later.
On this page
What are the three ways to cut an internal keyway?
Broaching with a keyway broach and bushing, slotting with a single-point tool that strokes back and forth (on a slotting machine or a CNC lathe or mill with a slotting head), and wire EDM.
A keyway broach set is a family of tools rather than one cutter. A broach maker’s catalog describes sets that come with the broaches, slotted bushings and the shims needed to reach depth, and one-pass keyway broaches that cut to full width and depth with the matching bushing and no shimming (Pilot Precision, keyway broaches). The bushing locates the broach in the bore; each shim behind the broach takes the next pass deeper.
Slotting uses a single tooth instead of many. On CNC lathes and mills, a slotting or broaching head strokes an insert along the bore while the machine feeds it deeper; the same maker offers indexable broaching tools and a motorized slotter that cut internal and external keyways and splines on CNC lathes (Pilot Precision, CNC indexable broaching). Wire EDM cuts the keyway by spark erosion with a traveling wire threaded through the bore, independent of material hardness.
| Question | Broaching | CNC slotting | Wire EDM |
|---|---|---|---|
| Through bore needed? | Yes, or a relief groove for the broach to exit | No; blind keyways with a short runout | Yes; the wire must pass through |
| Hardened hub | Poor; cut before hardening | Limited | Good; hardness does not matter |
| Tooling | Broach per width, bushing per bore, shims | One holder, inserts per width | None special; programmed path |
| Volume fit | Batches of identical keyways | Low to medium, done in the turning setup | Low volume, hard or precise parts |
| Keyway position to other features | Set by fixturing in the press | Set by the machine axis in the same setup | Set by the program and part setup |
| Bottom corners | As ground on the broach | As ground on the insert | Wire radius in the corners |
| Special forms (splines, hex) | Special broach | Indexed strokes | Any profile the wire can follow |
When is broaching the right way to cut a keyway?
When the bore goes through the part, the material is in a machinable condition, and there are enough parts with the same keyway to justify a broach and bushing.
Broaching is fast once the tooling is on hand, and a standard keyway width usually is. Its constraints are physical. The broach has to exit the far side, so a blind bore needs a relief groove deep and long enough to collect the chip and let the broach clear. The part has to be clamped square to the press ram, because a broach pushed at an angle drifts and the keyway walks off the bore axis. And the hub length relative to the broach tooth pitch matters: a very short hub may be cut by too few teeth at once, a very long one may need more stroke than the press has.
Depth is built up in passes with shims, so the depth tolerance depends on the shim sequence and on how the bore diameter compares with the bushing. A bore at the large end of its tolerance gives a shallower keyway for the same shims. That is why keyway depth is best dimensioned from the opposite side of the bore, as the keyway dimensions guide explains, and checked after the bore is finished.
When should a keyway be slotted on a CNC machine instead?
When the keyway is blind, must be positioned accurately to other features of a turned part, or the volume is too low to justify a broach.
Slotting in the turning or milling setup cuts the keyway without re-chucking, so its angular position to other features, such as a set-screw hole or a cam lobe, comes from the machine axis rather than from a press fixture. It can stop short of a shoulder: the tool needs only a small runout at the end of the stroke, not a through bore. The trade-off is cycle time, because the single tooth removes material one stroke at a time.
On the drawing, a blind keyway needs its runout shown. Either dimension a relief groove at the end of the keyway, or allow a radius or tapered runout at the closed end and state its maximum length. Without this, a supplier has to assume a square end, which neither broaching nor slotting can make.
When does wire EDM make sense for a keyway?
When the hub is already hardened, the keyway has an unusual form, or only a few parts are needed and broach tooling would cost more than the cutting.
Wire EDM does not depend on hardness, so it can cut a keyway after heat treatment without the distortion and scale that come with hardening a part that was keyed soft. It follows any programmed profile, which suits non-standard key forms and multiple keyways at set angles. Its limits are that the wire must pass through the part, the bottom corners take the wire radius rather than a sharp or ground corner, and the cut surface has a recast layer that may need to be considered on fatigue-critical hubs.
If a keyway is cut by EDM, say on the drawing what corner radius is acceptable at the bottom of the keyway, and whether the recast layer needs removing. The key itself usually has chamfered edges, so a small corner radius in the keyway rarely prevents assembly.
How should the drawing describe a hub keyway so any method can cut it?
Dimension the result, not the process: width with its tolerance, depth from the far side of the bore, length or runout, corner condition, and position relative to a datum, and add process restrictions only where they matter.
- Size: keyway width and depth per the key standard, for example ASME B17.1 for inch keys, which ASME lists as the 1967 edition, stabilized in 2023 (ASME B17.1).
- Through or blind: for a blind keyway, the relief groove or runout.
- Bottom corners: sharp, a radius maximum, or a radius range, especially on fatigue-loaded hubs.
- Position: position of the keyway to the bore axis, and angular position to any other feature that must line up.
- Sequence: if the hub is hardened, say whether the keyway may be cut before heat treatment or must be cut after.
- Inspection: a keyway width plug gauge or a gauge key for fit, and depth measured over a pin or gauge in the bore.
Leave the method to the supplier unless you have a reason to restrict it. A note such as “keyway may be broached, slotted or wire EDM cut” makes it explicit, and still lets you specify corner and surface requirements that rule out one of them.
How are internal keyways handled on a Saola order?
The keyed part is machined at our lead CNC plant, and the keyway method is confirmed at drawing review against your drawing, material and volume.
Hubs, gears and bushings are turned at our lead CNC plant in Vietnam on CNC turning, with second operations on CNC milling; the plant also has EDM. Which method cuts a given keyway, and the type of EDM used where EDM applies, are confirmed at drawing review. The plant’s inspection equipment includes a 2D optical (vision) measuring instrument and a height gauge; there is no CMM. New parts get a first-article inspection report and every lot a dimensional inspection record. Send the drawing for a quote.
Frequently asked
Can a keyway be broached in a blind hole?
Only with a relief groove or a cross hole at the bottom for the broach to clear and the chip to break off. Without one, slot the keyway on a CNC machine or redesign the bore to pass through.
Is it better to cut the keyway before or after hardening?
Before, if the part can tolerate heat-treat distortion of the keyway; after, by wire EDM, if the keyway must hold its size and position on the hardened part. Write the sequence on the drawing when it matters.
Why do broached keyways sometimes come out off-center?
Usually the part was not held square to the ram, the bushing was loose in the bore, or the broach drifted in a long hub. A position callout on the drawing gives incoming inspection a limit to check against.
Is a keyseat the same as a keyway?
Usage varies. Many engineers say keyseat for the slot in the shaft and keyway for the slot in the hub, while ASME B17.1 is titled Keys and Keyseats and many shops say keyway for both. On the drawing, the view makes it clear.
Sources
- Keyway Broaching, Pilot Precision Products (duMONT), product page, accessed November 2026
- CNC Indexable Broaching, Pilot Precision Products, product page, accessed November 2026
- ASME B17.1, Keys and Keyseats, ASME, 1967 edition, stabilized 2023