Blind Keyway in a Bore: Shape It, Mill It or EDM It?
A blind keyway can't be push-broached. How CNC shops single-point broach one with a chip relief, when a right-angle head works, and when sinker EDM fits.
A standard push broach needs to exit the far side of the bore. A blind keyway doesn't have a far side, so it has to be made a different way.
How Do You Cut a Keyway in a Bore That Doesn't Go Through?
Most often by single-point CNC broaching, which is really shaping: an insert tool strokes in and out of the bore on a lathe or mill, taking a light cut each pass. Big bores can be milled with a right-angle head. Sinker EDM is the fallback when neither fits.
An r/Machinists user was quoting four blind keyways in a 6" ID, 1" long bore in 1018 steel, with only 1/8" of clearance before a shoulder. Replies said to lock the spindle and use the lathe as a shaper, with light passes. Several said the keyway needs a relief groove or cross hole at the end. One noted that CNC Broach Tool recommends 1/4" of relief, so 1/8" may be just barely enough. Another suggested pricing it out to an EDM shop.
Chip relief for a blind keyway
Cutting Tool Engineering's article on blind-keyway broaching explains why the bottom matters. On a lathe, gravity helps chips fall away. On a mill it doesn't, so John Gardner of CNC Broach Tool recommends a cross-hole, groove or notch relief to push chips into, big enough that coolant doesn't pack them in.
The cuts are tiny. Gardner talks about roughly 0.001" per stroke, depending on the material.
The article's case study is a lathe job. Muthig Industries cut 1/4" × 2" blind keyways in cold-rolled steel crank adapters, with a cross-hole relief, and had three hard crashes early on. Then they caught a chip welded to the insert. They aimed the coolant at the cut, ran a richer mix, and cut the stop from about 0.1" past the end of the stroke to 0.04". That's one shop's fix, but it shows how small the margins are.
Right-angle heads and sinker EDM
In larger bores, a 90° angle head on a machining center can mill the keyway. CTE's angle-head keyway story describes PMC-Colinet doing this in bores from 6" to 13", replacing a shaper. Their custom head looked too slender to stay rigid, but it held up.
When there's no room for a relief or a head, sinker EDM can burn the slot to shape. It's slow, and it's a different machine from wire EDM.
On the drawing
Show the relief if the part allows one, and give the keyway's start and stop. Like any feature you can't reach from above, it costs more than it looks, as machining undercuts explains. Bore quality matters first, so see chatter on ID bores and live tooling vs a separate mill op.
Our CNC capability page lists milling, turning and a Sodick wire EDM. It doesn't list sinker EDM. If your part has a blind keyway, ask us when you quote.
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