Broach (metalwork)
From Wikipedia, the free encyclopedia
A broach is a series of progressively taller chisel points mounted on a single piece of steel, typically used to enlarge a circular hole into a larger noncircular shape such as a square or other desired shape.
Another typical use of a broach is to cut 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.
[edit] Wobble or Rotary Broach
A somewhat different design of tool that can achieve the irregular hole shape of a broach is called a wobble broach. This type of tool is often used on rotating machines such as lathes. The wobble broaching process is also called rotary broaching.
The tool has a contour similar to the desired final shape but the leading edge of the tool is wider than the body. The tool is free to rotate 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. As the work rotates, the broach is pressed against it and rotates synchronously with it. However, since the axis of rotation is different, the leading (cutting) edge of the broach "wobbles" with respect to the work.
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. 1/2*sin(1). 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 bottom of the hole may be rotated with respect to the top of the hole. Spiraling may be undesirable because it binds the body of the tool and prevents it from wobbling freely. 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 wobble broach will not cut as accurately as a push or pull broach. However, the ability to use a wobble broach on high production machinery such as a screw machine makes this a desirable manufacturing method.
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