Advantages and Limitations of a Broaching Assembly.

The concept of broaching can be traced back to the early 1850s, with the first applications used for cutting keyways in?pulleys?and?gears. After World War 1, broaching was used to rifle gun barrels. In the 1920s and 30s the tolerances were tightened and the cost reduced thanks to advances in?form grinding?and broaching machines.

Broaching?is a?machining?process that uses a toothed tool, called a?broach, to remove material. There are two main types of broaching:?linear?and?rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to affect the cut. Linear broaches are used in a?broaching machine, which is also sometimes shortened to?broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axisymmetric shape. A rotary broach is used in a?lathe?or?screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient. Broaching is used when precision machining is required, especially for odd shapes. Commonly machined surfaces include circular and non-circular holes,?splines,?keyways, and flat surfaces.

Broaches are shaped similar to a saw, except the height of the teeth increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one for finishing. Broaching is an unusual machining process because it has the feed?built into the tool. The profile of the machined surface is always the inverse of the profile of the broach. The rise per tooth (RPT), also known as the?step?or feed per tooth, determines the amount of material removed and the size of the chip. The process depends on the type of broaching being performed. Surface broaching is very simple as either the workpiece is moved against a stationary surface broach, or the workpiece is held stationary while the broach is moved against it.

Internal broaching is more involved. The process begins by clamping the workpiece into a special holding?fixture, called a?work holder, which mounts in the broaching machine. The broaching machine?elevator, which is the part of the machine that moves the broach above the work holder, then lowers the broach through the workpiece. Once through, the broaching machine's?puller, essentially a hook, grabs the?pilot?of the broach. 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.?The broach usually only moves linearly, but sometimes it is also rotated to create a spiral spline or gun-barrel?rifling.

Broaching was originally developed for machining internal keyways. However, it was soon discovered that broaching is very useful for machining other surfaces and shapes for high volume workpieces. Because each broach is specialized to cut just one shape, either the broach must be specially designed for the geometry of the workpiece or the workpiece must be designed around a standard broach geometry.

Because all of the features that are built into the broach, no complex motion or skilled labour is required to use it.?A broach is effectively a collection of?single point cutting tools?arrayed in sequence, cutting one after the other; its cut is analogous to multiple passes of a?shaper.

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,?and that the workpiece is strong enough to withstand the forces involved.

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