Broaching (metalworking)

Typical workpieces include small to medium-sized castings, forgings, screw machine parts, and stampings.

[1] Broaches are shaped similar to a saw, except the height of the teeth increases over the length of the tool.

Broaching is an unusual machining process because it has the feed built into the tool.

Because all of the features are built into the broach, no complex motion or skilled labor is required to use it.

The concept of broaching can be traced back to the early 1850s, with the first applications used for cutting keyways in pulleys and gears.

In the 1920s and 30s the tolerances were tightened and the cost reduced thanks to advances in form grinding and broaching machines.

The process begins by clamping the workpiece into a special holding fixture, called a workholder, which mounts in the broaching machine.

The elevator then releases the top of the follower and the puller pulls the broach through the workpiece completely.

The workpiece is then removed from the machine and the broach is raised back up to reengage with the elevator.

[4] The broach usually only moves linearly, but sometimes it is also rotated to create a spiral spline or gun-barrel rifling.

However, heavy-duty water-soluble cutting fluids are being used because of their superior cooling, cleanliness, and non-flammability.

However, it was soon discovered that broaching is very useful for machining other surfaces and shapes for high volume workpieces.

Most of the time is consumed by the return stroke, broach handling, and workpiece loading and unloading.

[8] The only limitations on broaching are that there are no obstructions over the length of the surface to be machined, the geometry to be cut does not have curves in multiple planes,[9] and that the workpiece is strong enough to withstand the forces involved.

[8] Broaching works best on softer materials, such as brass, bronze, copper alloys, aluminium, graphite, hard rubbers, wood, composites, and plastic.

When broaching, the machinability rating is closely related to the hardness of the material.

Broaching is more difficult on harder materials, stainless steel and titanium,[12] but is still possible.

Except when broaching cast iron, tungsten carbide is rarely used as a tooth material because the cutting edge will crack on the first pass.

The pot is designed to hold multiple broaching tools concentrically over its entire length.

[15] This has replaced hobbing for some involute gears and cutting external splines and slots.

They are notches in the teeth designed to break the chip and decrease the overall amount of material being removed by any given tooth (see the drawing above).

[5] For broaching to be effective, the workpiece should have 0.020 to 0.025 in (0.51 to 0.64 mm) more material than the final dimension of the cut.

The back-off (γ) provides clearance for the teeth so that they don't rub on the workpiece; it is usually between 1 and 3°.

This allows for a deep cut while keeping stresses, forces, and power requirements low.

[8] The pitch defines the tooth construction, strength, and number of teeth in contact with the workpiece.

Push broaching machines are similar to an arbor press with a guided ram; typical capacities are 5 to 50 tons.

Surface style machines hold the broach stationary while the workpieces are clamped into fixtures that are mounted on a conveyor system.

The leading (cutting) edge of the broach has a contour matching the desired final shape.

Spiraling may be undesirable because it binds the body of the tool and prevents it from cutting sharply.

One solution to this is to reverse the rotation in mid cut, causing the tool to spiral in the opposite direction.