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How to Use a Slotting Attachment on a Mill

Guide · Updated October 28, 2026

A slotting attachment bolts to a knee mill head and drives a reciprocating ram, turning the mill into a small vertical shaper. It cuts internal keyways, splines, square holes and blind slots that a broach cannot reach, one light stroke at a time.

Check the keyway width

Set your caliper or mic on gage blocks at the key width before checking the slot. The free Gage Block Calculator builds the stack.

Step by step

Mount and tram the ram: Square the ram stroke to the table front-to-back and side-to-side with an indicator on a square, so the slot walls come out straight.
Set stroke length: Stroke = slot length plus about 1/8 in overtravel at each end for chip clearance.
Set stroke position: Position the stroke so the tool clears the top of the part and runs just past the bottom of the bore.
Grind the tool: Grind a form tool to the slot width with side clearance and a small top rake; the cutting edge must be relieved on the return stroke.
Locate on center: Indicate the bore true and center the tool on the bore centerline with the table dials.
Feed per stroke: Feed the table about 0.002–0.005 in per stroke in steel; less near final depth.
Measure and finish: Measure width and depth, then take a spring pass at final depth.

Worked example: 1/4 in hub keyway in a 1 in bore

For a 1.000 in shaft, ANSI B17.1 calls for a 1/4 in square key (shafts over 7/8 to 1-1/4 in). Hub keyway depth is half the key height, 0.125 in, measured at the side wall. Because the bore is round, the tool first touches at the center, so add the chordal height: c = R − √(R² − (w/2)²) = 0.500 − √(0.250 − 0.015625) = 0.500 − 0.48412 = 0.0159 in. Total depth from first touch at the bore centerline = 0.125 + 0.0159 = 0.1409 in. To check, measure from the opposite bore wall to the keyway bottom at the side wall: 1.000 + 0.125 = 1.125 in. Hold width 0.250–0.252 in for a standard class 2 fit.

Stroke rate

Slotting heads run roughly 50–200 strokes per minute. Use the slow end in steel and on long strokes; faster in aluminum and brass. Fewer strokes with a sharper tool beats fast strokes that rub.

Tool geometry

Cutting edge square to the stroke, 1–3° side clearance on each side, 5–8° end clearance, and a small back-off so the tool lifts or drags lightly on the return. Check width on a mic before cutting.

Common problems

Tapered slot: ram not trammed. Wide slot: tool not centered or worn side clearance. Chatter: too much tool overhang or too heavy a feed. Tool chipping on the return stroke: the clapper or relief is not lifting the tool.

Tips

Keyways: cut a keyway on the shaft side.
Broach: keyway broach for through holes.
Bore: indicate the bore true first.

FAQ

What is a slotting attachment?

A reciprocating ram head for a knee mill that works like a small vertical shaper.

When use it instead of a broach?

Blind keyways, odd sizes, splines and square holes.

How much feed per stroke?

About 0.002–0.005 in in steel, less near size.

How do I set stroke length?

Slot length plus about 1/8 in at each end.

Why is my slot tapered?

The ram is not square to the table.

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