Broaching
Also: keyway broaching, spline broaching, internal broaching, surface broaching, rotary broaching, wobble broaching, keyseating, keyway cutting, broach, production broaching
Broaching cuts keyways, splines, hexagons and other internal or external forms in one pass of a multi-tooth tool: seconds per part, with form-specific tooling.
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A broach is a long tool with a row of teeth, each standing slightly taller than the one before it: roughing teeth, then semi-finishing teeth, then finishing teeth at the final size. One pass through a hole or across a surface removes all the stock, so the feed is built into the tool and the machine only supplies a straight stroke. The rise per tooth is small, roughly 0.02-0.15 mm (0.001-0.006") on roughing teeth in steel and less on finishing teeth, and at least two or three teeth should be in the cut at once to keep it stable. The shape of the last teeth is the shape of the feature.
Internal broaching starts from a drilled, bored or cored hole and produces keyways, involute and straight-sided splines, serrations, squares, hexagons, D-shaped holes and other non-round forms. Pull broaching on horizontal or vertical machines handles production work. Push broaching with a short keyway broach and a slotted bushing in an arbor press or hydraulic press is the usual job-shop method for a single keyway. Surface broaching cuts external flats, slots and contours, such as the fir-tree slots in turbine disks. A keyseater cuts keyways with a reciprocating cutter fed into the bore wall, so it needs no dedicated broach and can cut long bores and, with a relief groove, blind keyways. Rotary (wobble) broaching drives a form tool tilted about 1° to the part axis in a lathe, mill or Swiss machine to cut small hexagons, squares and six-lobe sockets in blind holes.
Broaching is the fastest way to make these features and one of the most repeatable, because size and form are ground into the tool: a keyway or a spline takes seconds. The cost is the tooling. Standard keyway broaches, bushings and shims for inch and metric key sizes are inexpensive and sit on the shelf at most machine shops. A spline or special-form broach is made for one form and one size, costs thousands of dollars and takes weeks to deliver, so it pays only when volume spreads the cost. For one or a few parts with a special form, wire EDM, gear shaping or slotting on a CNC machine is the usual substitute.
A broach needs a through path, or a relief groove at the end of a blind feature for tool runout and chips. The part must be rigid enough to take the cutting force, which is why thin-walled hubs distort and close in afterwards. Most broaching is done before hardening, on material up to about 30-35 HRC; harder parts are broached soft and then heat treated with an allowance for distortion, or finished by wire EDM or grinding.
At a glance
| Typical tolerances | Size comes from the tool. A standard keyway broach typically cuts the width within +0.05/-0 mm (+0.002/-0") of nominal, in line with common hub keyseat tolerances (ASME B17.1 in inch sizes); ±0.013 mm (±0.0005") with care or with a broach ground to size. Keyway depth ±0.05-0.13 mm (±0.002-0.005"), set by shims. Splines and other special forms hold about ±0.013-0.025 mm (±0.0005-0.001") on the form diameters, with involute splines made to the fit classes of ANSI B92.1. Position depends on the pilot and the fixturing: keyway centrality to the bore within about 0.05 mm (0.002") is typical. Thin walls spring back and close the feature, so ask for a gauge check after broaching. |
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| Size limits | Standard keyway broach sets cover key widths from about 1.5 mm (1/16") to 25 mm (1") in bores from about 6 mm (1/4") up; push broaching is practical for lengths of cut up to roughly 150-300 mm (6-12"), depending on the broach and the press. Production pull-broaching machines are rated by pulling force and stroke, commonly 5-50 tons and 0.9-1.8 m (36-72"), and cut splines up to roughly 150-200 mm (6-8") in diameter; larger internal splines and gears are usually shaped. Keyseaters cut keyways from about 3 mm (1/8") wide to 50 mm (2") and wider, in bores several hundred millimeters long. Rotary broaching suits forms up to about 12-20 mm (1/2-3/4") across flats, to a depth of about 1-1.5 times the form size. Capacity varies widely between a general machine shop and a broaching house. |
| Surface finish | 0.8-3.2 µm Ra (32-125 µin) as broached; a sharp tool in free-cutting material gives the fine end of that range, and burnishing teeth reach about 0.4-0.8 µm Ra (16-32 µin) in production. Finish degrades as the tool dulls, and gummy or work-hardening materials tear. A burr forms at the exit edge and is removed by chamfering or deburring. The lay runs along the stroke. |
| Lead time | A standard keyway adds hours to a day or two to a machined part, or 2-5 business days as a stand-alone service. Common spline and hexagon broaches may be on the shelf at a broaching house; a special broach takes about 6-12 weeks to design and grind, after which parts run in days. Production broaching orders typically take 2-4 weeks once the tooling exists. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Steels in the soft or pre-hardened condition broach well: 1018, 1045, 12L14, 4140 and 4340 annealed or pre-hard at about 28-32 HRC, and 8620 before carburizing. Free-machining grades give the best finish and tool life. Stainless 303 and 416 broach easily; 304, 316 and 17-4 PH work harden and tear, so they need sharp tools and good lubricant. Gray and ductile cast iron broach cleanly. Aluminum, brass and bronze broach fast with the right tooth geometry, though soft, gummy alloys can load the teeth. Plastics such as acetal and nylon broach but spring back, so the feature comes out slightly small. Titanium and nickel alloys are surface broached in aerospace work with special tooling at low speeds. Hardened steel above about 35 HRC is not broached with standard tools.
What drives the cost
- Tooling: whether a standard broach exists; a special spline or form broach costs thousands of dollars and fits one size
- Quantity: the cut takes seconds once set up, so tooling and setup are the cost at low volume
- Material and hardness: stainless, alloy steel above about 30 HRC and nickel alloys wear broaches and shorten the time between sharpenings
- Length of cut and stock removal, which set the broach length, the machine tonnage and the number of passes
- Fixturing: timing a keyway or spline to another feature needs a locating fixture
- Blind features, which need relief grooves, keyseating or rotary broaching in place of a through pass
- Inspection: plug and spline gauges, measurement over pins and position checks
When to use it
- Keyways in gears, pulleys, sprockets, couplings and hubs, from one piece to production
- Internal involute and straight-sided splines and serrations at volume
- Square, hexagonal, D-shaped and other non-round through holes
- External slots, flats and forms on production parts, by surface broaching
- Small blind hexagons, squares and six-lobe sockets cut in the lathe by rotary broaching
When not to
- One or a few parts with a special spline or form: wire EDM, shaping or keyseating avoids a dedicated broach
- Hardened parts above about 35 HRC: broach before heat treatment, or use wire EDM or grinding
- Blind features with no room for a relief groove or tool runout
- Thin-walled or interrupted bores that cannot support the cutting force
- Large internal gears and splines beyond the broaching machine: gear shaping or skiving
Design tips
- Use the standard key size for the shaft diameter (ASME B17.1 inch keys or ISO metric keys) so that a stock broach and gauge fit.
- Make the feature a through cut; if it must be blind, add a relief groove at least as deep as the feature and wide enough for tool runout and chips.
- Specify splines by standard: ANSI B92.1 involute splines with pitch, pressure angle, number of teeth and fit class, or the ISO or DIN equivalent.
- Keep two or three broach teeth in the cut: very short parts are stacked, and very long cuts need special tooling.
- Leave enough wall outside the keyway or spline, and expect thin hubs to close in; tolerance the bore after broaching if it matters.
- Broach before hardening and allow for heat-treat distortion, or plan a finishing operation afterwards.
- Dimension keyway depth from the opposite side of the bore, and call out symmetry to the bore axis.
- Chamfer the bore entry and call out deburring at the exit.
Tools and guides
Frequently asked questions
- What is broaching used for?
- Cutting features that a rotating tool cannot make in one pass: keyways in bores, internal splines and serrations, square and hexagonal holes, and external slots and forms. A toothed tool is pulled or pushed through the part once, and each tooth removes a little more until the last teeth leave the finished shape.
- How much does a broach cost?
- A standard keyway broach with its bushing and shims costs from tens to a few hundred dollars, and most machine shops own a set. A special broach for a spline or custom form is ground for that one shape and typically costs several thousand dollars, more for large or complex forms, with about 6-12 weeks of lead time. That is why special forms in small quantities are usually cut by wire EDM.
- Can you broach a blind keyway?
- Not with a conventional pull or push broach, which must pass through the part. The options are a relief groove at the end of the keyway so that a keyseater or a CNC slotting tool can run out, rotary broaching for small polygon forms, or EDM. Designing the keyway as a through feature costs less.
- Broaching or wire EDM for keyways and splines?
- Broaching when a standard tool exists or the volume justifies a special one: it takes seconds per part. Wire EDM when the quantity is small, the form is special or the part is already hardened: there is no dedicated tooling, but each part takes minutes to hours and the profile must be a through cut.
- What is rotary broaching?
- A way to cut small polygon forms, usually hexagons, squares and six-lobe sockets, on a lathe, mill or Swiss machine. The form tool sits in a holder that tilts it about 1° to the part axis, so the cutting edge wobbles around the form and shears it a little at a time as the tool feeds in. It works in blind holes and takes seconds, which is how many socket screws and fittings get their drive forms.
