Broaching

A broach divides a shaped cut among a row of teeth

In conventional broaching, a tool moves in a straight line relative to the work. Successive cutting teeth reach farther into the material. Rise per tooth is that increase in cutting height between successive teeth; it is not the tooth spacing along the tool. The spaces between teeth hold chips.

Roughing teeth remove most of the allowance. Semi-finishing teeth approach the final profile with smaller cuts. Finishing teeth establish size and finish; several may have the same height. The exact arrangement depends on the tool.

Why the finishing teeth need not keep getting taller

Once the intended profile is reached, further rise would make the feature larger. Repeated finishing teeth can maintain the final size as earlier teeth wear. See Nidec's precision cutting tools reference for broach terminology.

Push or pull describes the drive; internal or external describes the surface

A push broach is driven from behind; a pull broach is drawn through by its leading end. An internal broach works inside a prepared opening. An external broach works on an outside surface, such as a flat or an external spline. These are separate distinctions: internal does not mean pull-only.

A suitable hole broach can produce a square, hexagon or internal spline; a keyway broach cuts a slot in an existing bore. Conventional through broaching needs an exit path. Choose the tool from the required finished profile and starting-hole specification, not just a nominal shape name.

Repeated parts can justify dedicated tooling because many cutting edges complete the profile in a short cycle. A small keyway set can also suit repair or short-run work. Compare Material-Removal Process Families and Selecting a Material-Removal Process; Vertical Band Machining covers cutting contours with a continuous band instead.

Production advantages depend on the job

Tool cost, material, cut length, machine capacity and required finish determine the economics. Broaching is not automatically the cheapest process. Pilot Precision's broach overview includes both short-run sets and one-pass production tools. Cutting Fluids and Application Methods explains lubrication choices.

A bushing guides the keyway broach; shims move its cutting edge outward

The hub is the part surrounding the shaft. A slotted bushing fits its bore and guides the broach. A shim is a thin spacer behind the broach that increases the depth reached on a later pass. Broach, bushing and shims must be a compatible set.

Before operation, use Equipment Operating Procedure and press and mill hazard guidance in the safety collection. Follow the specific press and broach instructions for capacity, alignment, support, stroke, cut length and lubrication.

1. Check the drawing, prepared bore and required keyway. Select the matching broach, bushing and specified shim schedule.
2. Seat the bushing in the hub. Insert the broach in its slot for the first pass, normally without a shim on a multi-pass set.
3. Support the hub squarely on the press bed with clearance below for the entire broach to pass through. Align the ram, tool and guide.
4. Apply the specified lubricant and press the broach through according to its instructions.
5. After the completed pass, remove the broach and clean its teeth. If another pass is specified, insert the prescribed shim behind it, reinsert, lubricate and check alignment again.
6. Repeat with the specified shim stack for each remaining pass. Stop at the prescribed final setup; deburr and inspect as described in Machining Keyseats and Keyways.

Do not add a shim after the final pass. One-pass tools and some small broaches require none. Adding arbitrary shims is not a way to correct an uncertain dimension.

Manufacturer reference for the pass sequence

Pilot Precision, Engineering Section: How to Use Broaches, printed page 35 describes guided keyway broaching and the successive shim stack. The tool's own instructions govern the actual setup.

Count the first pass separately from the added shim passes

Rise per tooth and depth per pass are different quantities. For a simple planning exercise, let d₁ be the depth reached by the unshimmed pass and s the extra depth from each added shim increment. After N passes, depth is d₁ + (N − 1)s.

For a target depth D greater than d₁, the minimum count to reach at least the target is 1 + ceiling((D − d₁)/s). Ceiling means round upward to a whole number. If the first pass already reaches the target, the count is one. All depths here are measured radially from the original bore surface.

Example: an invented first-pass depth of 1.0 mm and two 0.5 mm increments give 1.0, 1.5 and 2.0 mm: three passes. This is counting practice, not a real broach specification. If rounding upward exceeds the allowed finished depth, that shim schedule cannot make the part to size.

Practice counting passes

Rotary broaching uses an angled end-cutting tool

Rotary broaching can make polygon forms such as a hex socket using a special holder on a suitable lathe or mill. A small angular offset makes successive portions of the tool's end engage as the tool and work move relative to one another. It does not use the long row of progressively taller teeth shown above.

It needs a prepared opening for internal work. Tool form, available thrust, material, depth and chip clearance limit the application; a blind socket needs a planned place for the chips. This is a process overview, not a setup procedure.

How the rotary holder produces the cutting motion

Slater Tools' rotary broaching explanation describes the holder's angled spindle and the different tool/work motions on a lathe and mill. Its application guide covers preparation and operating requirements.