Broaching is a machining operation which uses a toothed tool called a broach to remove material. Broaching can remove material from either the inside or outside of the workpiece depending on the position of the machine. Broaching machines usually push or pull the broaches and are either horizontal or vertical. Broaching can produce parts with both a good surface finish and dimensional accuracy. Even though broaches can be expensive, broaching is usually favorable to other processes when used for high-quantity production runs.
Broaching machines have four major operation types. These operations are:
Major design considerations for broaching are:
Another typical use of a broach is to cut rotating splines or a square keyway (see image) on objects such as gears, driveshafts, pulleys etc. The amount of material removed by each broach tooth (or chisel) varies with the material being cut. A broach tooth designed to cut steel might remove only 0.05 mm (0.0025 inch), while a broach tooth designed to cut brass might remove as much as 0.10 mm (0.004 inch). The succession of teeth (chisels) removes the total amount of material required. A broach may also be designed to be pushed or pulled through an existing hole; broaching machines are therefore designed accordingly.
Rotary broaching requires two tooling components: a tool holder and a broach. The leading (cutting) edge of the broach has a contour matching the desired final shape, and this leading edge of the tool is wider than the body. The broach is free to rotate within the tool holder, but the axis of rotation is inclined slightly to the axis of rotation of the work. A typical value for this misalignment is 1 degree. If the work piece rotates, the broach is pressed against it, is driven by it, and rotates synchronously with it. If the tool holder rotates, the broach is pressed against the work piece, but is driven by tool holder rotation. Since the axis of rotation is different, the tool holder appears to "wobble" with respect to the work. This illusion is the reason for the original term 'wobble broach', now commonly referred to as 'rotary broach'.
If the tool is inclined at an angle of 1 degree to the work, the sides of the tool must have a 1 degree or greater draft.
Ideally the tool advances at the same rate that it cuts. So a 1/2" diameter tool should advance at 0.009" per revolution.
If it advances any faster than that, then the tool becomes choked; if it advances any more slowly, then you get an interrupted or zig-zag cut. Since all work material is elastic, you would actually cut a little less than the ideal rate, just to release the load on the non-cutting edge of the tool.
There is some spiraling of the tool as it cuts, so the form at the bottom of the work piece may be rotated with respect to the form at the top of the hole or profile. 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. If reversing the machine is not practical, then interrupting the cut is another possible solution.
In general, a rotary broach will not cut as accurately as a push or pull broach. However, the ability to use this type of cutting tool on high-production machinery such as a screw machine, and eliminate secondary operations, makes this a desirable manufacturing method.