What is broaching

Broaching is a machining process that uses a serrated tool called a broach to remove material. There are two main types of broaching: linear and rotary. In linear broaching, the most common method, broaching is performed linearly on the surface of the workpiece to create 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 pushed into the workpiece to cut an asymmetric shape. A rotating chuck is used in a lathe or screw machine. With both methods, the cut is made in one pass of the broach and is therefore very efficient. Broaching is used when precision machining is required, especially for irregular shapes. Commonly machined surfaces include round and non-round holes, grooves, splines, and flat surfaces. Typical parts include small to medium sized castings, forgings, screw machine parts and stampings. Although broaching can be expensive, broaching is generally preferred over other methods when used for high-quantity mass production.
Broaches are saw-shaped, except that the height of the teeth increases with the length of the tool. In addition, the broach contains three separate sections: one for roughing, one for semi-finishing, and one for finishing. Broaching is an unusual machining process because the feed is built into the tool. The profile of the machined surface is always the opposite of the broach. The rise per tooth (RPT), also called pitch or feed per tooth, determines the amount of material removed and the size of the chips. The broach can be moved relative to the workpiece or vice versa. Since all functions are built into the broach, no complicated movements or skilled workers are required to operate it. T A broach is effectively a set of single-pointed cutting tools that are lined up and cut in sequence; its cut is analogous to the multiple passes of a milling machine.

The process depends on the type of broaching being performed. Surface broaching is very easy as either the part is moved over a fixed surface broach or the part is held stationary while the broach is moved over it.
Internal broaching is more complicated. The process begins by clamping the workpiece in a special fixture called a workpiece holder that is mounted on the broaching machine. The broaching machine, the part of the machine that moves the mandrel over the workpiece holder and then lowers the broach through the workpiece. Once through, the broaching machine's puller, essentially a hook, grabs the pilot of the broach. The lifter then releases the top of the follower and the puller pulls the broach all the way through the workpiece. The part is then removed from the machine and the broach is raised to reconnect to the elevator. The broach usually moves only linearly but is sometimes rotated to create as spiral spline or barrel rifling.
Cutting lubricants are used for three reasons:
1. for cooling and broaching workpieces
2. for lubricating cut surfaces
3. Rinse shavings from teeth.
 
Oil-based coolants are the most popular. However, heavy-duty water-soluble cutting fluids are being used because of their superior cooling, cleanliness, and non-flammability.

Broaching was originally developed for machining internal grooves. However, broaching quickly proved very useful for machining other surfaces and shapes of mass-produced parts. Because each broach is designed to cut a single shape, the broach must be designed specifically for the geometry of the part, or the part must be designed based on standard broach geometry.
  Broaching speeds range from 20 to 120 surface feet per minute (SFPM). This results in a total cycle time of 5 to 30 seconds. Most of the time is spent retracting, broach handling, and loading and unloading the workpiece.

The only limitation of broaching is that there are no obstacles along the entire length of the machined surface, the machined geometry does not have multiplane planes, and the part is strong enough to withstand the forces. In the case of interior broaching in particular, there must first be a hole in the part into which the broach can penetrate. There are also limitations on the size of the inner cups.
Broaching works best on softer materials such as brass, bronze, copper alloys, aluminum, graphite, hard rubber, wood, composites and plastics. However, it still has a good machinability index for mild and free-cutting steels. When broaching, the machinability rating is closely related to the hardness of the material. For steel, the ideal hardness range is between 16 and 24 Rockwell C (HRC); Hardness above HRC 35 will quickly dull the broach. Broaching is more difficult with the harder
materials of stainless steel and titanium, but still possible.