How to Machine an Internal Keyway: Broaching Units vs Six Other Methods

14 hours ago
4 min read

Internal keyways, internal gear teeth, external teeth — these jobs all tend to stall at the same point: the part is already turned on the lathe or machining centre, and now it has to come off and go to a slotter or a wire EDM. Unclamp and re-clamp, and concentricity starts over. So does the lead time.

There are six or seven ways to cut these forms, and the differences are large. Understand them first, then decide whether to finish the part in one setup.

Why internal keyways are awkward

An external slot is easy — put an end mill on it. An internal bore is different: the tool has to reach inside, it can only push in one direction, and chips have nowhere to go.

The real problem is the blind hole. A conventional broach is pulled or pushed all the way through the bore. In a blind hole there is nowhere for it to come out.

Six common methods, side by side

Dedicated broaching machine — progressively larger teeth cut in sequence. Best accuracy and finish, cheapest per part at volume. But the machine and the broach are expensive, blind holes are out, and the part needs a second setup
Static inserted broach bar — the bar sits in the lathe turret and the Z axis strokes it, taking a small cut each pass. Lowest cost, works in blind holes, but slow
Driven broaching unit — the unit itself converts rotary drive into a reciprocating stroke. Far faster than a static bar, still works in blind holes, and the part never leaves the machine
Slotter / keyseater — cheap equipment, handles large parts and deep slots, but accuracy and finish are poorer, it is slow, and it leans on operator skill
Angle head milling — an angle head turns an end mill 90° into the bore. Width and depth are adjustable, but bore diameter limits you and it is slow
Wire EDM — highest accuracy, cuts hardened material, no cutting force. But slow, expensive to run, and the material must be conductive

How a driven broaching unit works

The unit mounts in a lathe turret or a machining centre spindle. Inside, a mechanism converts rotation into a linear reciprocating stroke, driving a form tool that cuts the bore one stroke at a time.

Here is the key difference: a static bar depends on a machine axis stroking back and forth, and every pass is a programmed move. A driven unit strokes itself — the machine only has to feed. On the same keyway, the difference is several times faster.

And because the part never leaves the machine, concentricity is fixed by a single setup — which matters when the bore later has to take a bearing or a gear.

Stroke length decides how deep you can go

The most direct limit on a broaching unit is stroke. The stroke sets how deep the slot can be; past that, you need a different method.

Mongtec broaching units come in 25, 35, 50 and 65 mm strokes, covering most internal keyway and tooth-form depths. When specifying, measure your deepest part — not the average one.

When a broaching unit is the wrong tool

Slot deeper than the unit's stroke — consider a slotter, or cut it in stages
Bore too small — the bar will not reach in, or it will lack the rigidity to hold the slot on position
Material already hardened — not a cutting job; go to wire EDM
Very high volumes — past a certain quantity, a dedicated broaching machine still wins on cost per part
Machine not rigid enough — reciprocating cuts load the turret and spindle; assess an older machine first

Internal teeth, external teeth and special forms

A broaching unit is not limited to keyways. If the form tool can be made, internal teeth, external teeth, splines, polygons and non-standard profiles are all on the table.

This is the part most often overlooked when specifying: the unit is only the power source. What decides the finished form is the bar length, its rigidity, and the geometry of the form tool. Plan all three together — buying them separately is how interference problems start.

What to have ready before quoting

The part drawing (most important — form, tolerance and depth are all on it)
Machine make, model and turret spec (VDI / BMT / spindle interface)
Bore diameter and slot depth (for a blind hole, note the relief space at the bottom)
Workpiece material and hardness
Batch size and cycle time target
Internal teeth, external teeth or keyway

Mongtec broaching units

Mongtec broaching units cover 25–65 mm of stroke and handle internal teeth, external teeth, keyways and special tooth forms. Our approach is to plan the whole job from the drawing: unit, bar and custom form tool specified together, so nothing interferes and rigidity is not left to chance.

For the turret side, see our driven tool holders and static tool holders. Send us the drawing and we can help assess whether it will run on the machine you already have.