Broaching

Why broaching matters

Most machining processes build a shape by making the same cut many times — a mill takes pass after pass, a shaper strokes back and forth, a lathe indexes in. Broaching flips that logic: instead of one cutting edge repeating a light cut, it stacks dozens of edges into a single rigid tool and takes the entire depth of cut in one continuous stroke. The tool is the process plan. Roughing teeth at the front hog out most of the stock, semi-finishing teeth clean it up, and the last few finishing and burnishing teeth generate the final surface and size — all in the two or three seconds it takes the broach to travel through the part.

That is why broaching dominates a very specific niche: internal profiles that are impossible or slow to make any other way. A keyway inside a gear bore, an internal involute spline, a square hole, a hex socket, rifling in a gun barrel — a rotating tool cannot reach or generate these easily, but a broach pulled straight through the bore produces them in one shot. On the outside, surface broaching cuts the fir-tree slots in turbine disks, the flats on connecting rods, and rack teeth, where the profile is complex but the motion is simple and linear.

• Keyways and splines. The signature internal broaching jobs, held to spline-fit tolerances.
• Turbine and compressor disks. Fir-tree blade-root slots surface-broached in nickel superalloys.
• Automotive. Connecting rods, differential gears, steering racks, ratchet teeth — high volume.
• Firearms. Rifling and chamber flats cut by internal broaching.
• Non-round holes. Square, hex, and custom cross-sections from a round starting hole.
• Complex profiles. Any form that is constant along the stroke direction and needs high repeatability.

How broaching works, step by step

A broach is a graded staircase of cutting edges. Follow the tool from front to back and the geometry tells the whole story.

1. Pilot and pull end. The broach starts with a pilot that fits the workpiece's starting hole to align the tool, and a pull (or push) end that the machine grips.
2. Roughing teeth. The first cutting teeth carry the largest rise per tooth (0.05–0.10 mm in steel). Each removes a chip equal to its step and does most of the work.
3. Semi-finishing teeth. A middle zone with smaller rise (0.02–0.05 mm) refines the form and reduces cutting force per tooth.
4. Finishing teeth. The last cutting teeth have near-zero rise; they are all the same height and generate the final size and surface.
5. Burnishing teeth (optional). Round, non-cutting teeth a few microns oversize that iron the surface smooth and improve size control on internal broaches.
6. One stroke, no return cut. The machine pulls the broach through in a single linear motion at 2–15 m/min. On the return, the broach clears the part without cutting — there is no reverse cut, which is why the process is so fast.

Each tooth has a face (rake), a land, and a gullet — the chip-storage pocket in front of it. Because a broach cuts a fixed depth per tooth, the chip is a curl of predictable thickness, and it must fit entirely inside the gullet until the tooth exits the part. If the gullet is too small for the chip, the chip packs, force spikes, and the tooth can chip or the broach can jam — one of the two hard limits on how aggressively you can cut.

Rise per tooth and the pull-force equation

The geometry of a broach is governed by two simple relations. The first ties the tool length to the job:

d = n_c × RPT

where d is the total depth of material to remove (mm), n_c is the number of cutting teeth, and RPT is the rise per tooth (mm), equal to the chip thickness each tooth cuts. Smaller RPT means more teeth, a longer broach, and a longer, costlier tool — but lower force and a better finish. The tooth pitch p (mm) is chosen so at least two or three teeth engage the workpiece at once for smooth cutting: p ≈ 1.25…1.75 √L, with L the length of cut (mm).

The second relation is the peak pull force, which sizes the machine and checks the broach against breakage:

F = k_c × A × n_e

where F is the pull force (N), k_c is the specific cutting force of the workpiece material (N/mm², e.g. ~2500–3500 for mild steel at these chip thicknesses), A is the chip cross-section removed by one tooth — width of cut times RPT (mm²) — and n_e is the number of teeth engaged simultaneously. The force must stay below the tensile strength of the broach's weakest section (the root behind the first tooth), or the tool snaps. This is why internal broaches are pulled in tension, where a slender tool is strong, rather than pushed in compression, where it would buckle.

Worked example: broaching a keyway

Suppose you must broach a 6 mm wide keyway to a total depth of 3.0 mm in a mild-steel gear bore 25 mm long. Choose a roughing rise per tooth of RPT = 0.06 mm and finishing teeth at zero rise. The number of cutting teeth needed to remove the depth is d / RPT = 3.0 / 0.06 = 50 cutting teeth (plus a handful of finishing and burnishing teeth). Set the pitch so two to three teeth engage the 25 mm length: p ≈ 1.5√25 = 7.5 mm, giving n_e ≈ 25 / 7.5 ≈ 3 teeth cutting at once.

The chip cross-section per tooth is A = width × RPT = 6 × 0.06 = 0.36 mm². Taking k_c ≈ 3000 N/mm² for mild steel at this chip thickness, the peak pull force is F = k_c × A × n_e = 3000 × 0.36 × 3 ≈ 3.2 kN per engaged set — modest for a small keyway, which is why light-duty keyway broaches run on a simple arbor press or a 20–50 kN hydraulic slotter. Scale the same math to a wide internal spline with A of several mm² and a dozen teeth engaged and the force jumps into the hundreds of kilonewtons, demanding a large horizontal or vertical broaching machine. The finishing teeth then generate a 0.8 µm Ra surface and the keyway is complete in a single stroke of a few seconds.

Common misconceptions and failure modes

• "Broaching is slow because the speed is low." Cutting speed is low (2–15 m/min) but the whole form is made in one pass — cycle time is seconds, and total productivity is very high.
• "Bigger rise per tooth is faster." Too large an RPT overfills the gullet, packs the chip, spikes the force, and breaks teeth. The gullet space factor must stay above ~3.
• "You can push any broach through." Long internal broaches must be pulled in tension; pushing a slender broach buckles it. Only short surface broaches are pushed.
• "One broach fits many jobs." Each profile needs its own dedicated broach; the tools are expensive, so broaching only pays off at medium-to-high volume.
• "Broaching works on hardened parts." HSS broaching is limited to roughly 30–35 HRC. Harder parts are broached soft then heat-treated, or finished by grinding/EDM.
• "Chips take care of themselves." Chip control and coolant are critical; a packed gullet or a re-cut chip ruins the surface and can shatter a tooth on the next part.