Broaching Services: Keyways, Splines, and Internal Forms in One Pass

Broaching services cut a finished internal or external form in a single linear pass, using a tapered tool whose teeth each remove a few hundredths of a millimetre more than the one before it. Push or pull the broach through a pilot hole and a round bore becomes a hexagon, a square, a keyway, or a 20-tooth involute spline in a few seconds. No indexing, no multiple setups, no tool changes. For internal forms produced in quantity, broaching is the fastest and most economical process in the machine shop, and for some geometries it is effectively the only practical one.

What Broaching Is and Why It Is So Fast

A broach is a bar carrying a graduated series of cutting teeth. Roughing teeth at the leading end remove the bulk of material, semi-finishing teeth refine the form, and finishing teeth at the trailing end, all cut to identical size, produce the final dimension and surface. Because the entire cutting sequence is built into one tool, a single stroke completes the operation.

Compare this with the alternative. Cutting a keyway by conventional means requires a slotter or a vertical shaper making dozens of reciprocating passes, or a wire EDM burn taking many minutes. A broach does it in under ten seconds. The trade-off is that the broach is a dedicated, expensive, single-purpose tool. Broaching is therefore a volume process: the tool cost has to be spread across enough parts to matter.

Cutting speeds are modest, typically 3 to 15 metres per minute, but material removal rate is high because so many teeth engage at once. Machines are usually hydraulic, rated by pull force, commonly 5 to 50 tonnes.

Internal, Surface, and Rotary Broaching

Internal broaching pulls or pushes the tool through an existing hole. The hole must be pre-machined accurately, since it pilots the broach and determines concentricity of the finished form. That pre-machining is typically done by CNC boring when concentricity is critical, or by drilling and reaming for less demanding work. Internal broaching produces keyways, splines, square and hex holes, serrations, and gun-rifling forms.

Surface broaching moves the tool across an external face, or moves the part across a fixed broach. It generates flats, slots, contoured profiles, and multiple parallel surfaces in one stroke. Automotive engine plants use it heavily for cylinder head faces and connecting rod parting surfaces because it beats milling on both cycle time and flatness.

Rotary broaching, sometimes called wobble broaching, is the outlier and the one most useful to job shops. The tool is mounted at a one-degree offset and free to rotate, so it cuts a hexagon or square with a shaving action while mounted directly in a lathe turret or mill spindle. It needs no dedicated broaching machine, tooling is comparatively cheap, and it forms hex sockets and square drives on turned parts as a simple secondary operation. For low and medium volumes this is often the pragmatic answer.

Broaching vs Wire EDM vs Shaping

These three processes compete for internal form work, and the right choice hinges on volume and hardness.

Wire EDM cuts any conductive material regardless of hardness, holds tolerances to ±0.005 mm, and needs no dedicated tooling, so it is unbeatable for prototypes, one-offs, and forms in fully hardened steel. It is slow and priced by the hour. Our wire EDM and EDM machining capabilities cover this work, and the process detail is set out in our EDM machining services guide.

Shaping and slotting need only inexpensive single-point tools and suit very low volumes, but they are slow and surface finish is mediocre.

Broaching dominates once volume climbs. The crossover typically falls somewhere between 200 and 1,000 pieces depending on form complexity and broach cost. A production keyway broach might cost 500 to 3,000 US dollars; a complex spline broach can exceed 15,000. Divide that across your annual quantity and the decision usually makes itself.

Materials and Hardness Limits

Broaching is a cutting process, so it is bounded by tool hardness. Free-machining steels, low and medium carbon steels, alloy steels in the annealed condition, cast iron, aluminium, brass, and bronze all broach well. Stainless steels broach but work harden and require sharper geometry, slower speeds, and generous flood coolant. Titanium and nickel superalloys are broachable but tool life drops sharply.

The practical hardness ceiling for HSS broaches is around 30 to 35 HRC. Carbide-tipped broaches extend this to roughly 45 HRC at considerably higher tool cost. Above that, wire EDM is the answer.

This dictates a firm sequencing rule: broach before hardening. Cut the form soft, then send parts for heat treatment. Be aware that hardening distorts splines and keyways slightly, so where post-heat-treat precision is essential the form is either finished by grinding or cut by EDM after hardening.

Tolerances, Surface Finish, and Design Guidance

Broaching produces excellent finish and repeatability. Surface roughness of 0.8 to 1.6 µm Ra is routine and 0.4 µm is achievable with sharp finishing teeth. Dimensional tolerance on the broached form is typically ±0.025 mm, with keyway width often held to ±0.013 mm. Because every part passes over the same finishing teeth, part-to-part consistency is outstanding, which is exactly the repeatability discussed in our CNC machining tolerances guide.

Several design rules will save you money. Provide a through hole wherever possible; broaching a blind hole requires relief at the bottom and is often impossible. Keep the form concentric and parallel to the pilot bore, as the broach follows the hole. Avoid interrupted cuts such as a cross-hole intersecting the broach path, because teeth chip on re-entry. Specify the largest root radius the design allows, since sharp corners raise both tool cost and stress concentration. Where a broached spline mates with a ground shaft, finishing the shaft by surface grinding or cylindrical grinding controls the fit better than machining alone.

Broached features almost always sit on parts that also require milled faces and drilled holes, produced by CNC milling, and the integration of these operations into one controlled routing is what our precision machined parts guide addresses.

Frequently Asked Questions

What is the minimum quantity that justifies broaching? Roughly 200 to 500 pieces for a simple keyway with standard tooling, higher for custom spline forms. Below that, wire EDM or rotary broaching usually costs less overall.

Can you broach a blind hole? Rarely, and only with a relief groove machined at the bottom for chip clearance and tool runout. Design a through hole if at all possible.

Do you have standard broaches available? Standard keyway sizes to ISO and ANSI, and common involute spline forms, are held as stock tooling, which removes tool cost entirely. Send your form specification and we will confirm before quoting custom tooling.

How long does custom broach tooling take? Typically four to eight weeks for a custom internal spline broach, less for simple keyway tools.

Can broaching hold position tolerance to the outside diameter? Concentricity depends almost entirely on the pilot bore. Bore and broach in a controlled sequence and 0.05 mm total runout is realistic; loose pre-machining will not be corrected by the broach.

How to Get a Broaching Quote

Send a 2D drawing showing the form specification in full: keyway width, depth and tolerance, or the spline designation including number of teeth, module or diametral pitch, pressure angle, and fit class. Include the pilot bore diameter and tolerance, part length and wall thickness, material grade and delivered hardness, required hardness after heat treatment, and annual volume with batch size.

If you are currently paying for wire EDM on a part that has grown in volume, that is the clearest opportunity for a cost reduction. Send the drawing and your current annual quantity, and we will calculate the break-even point including tooling so you can see exactly where broaching starts paying for itself.