Ordinary cut broaches for machining previously drilled holes consist of the following elements:
- Pull End – This is designed to permit engagement of the broach with the broaching machine through the use of puller head.
- Front Pilot – This centres the broach in the hole before the teeth begin to cut.
- Roughing and Semi Finishing Teeth – These remove most of the stock in the hole.
- Finishing Teeth – These are for sizing the hole and must have the shape required for the finished hole.
- Rear Pilot and Follower Rest – These support the broach after the last tooth leaves the hole.
- Land – The top portion of a tooth is called land and in most cases it is ground to provide a slight clearance.
- Back Off or Clearance Angle – This corresponds to the relief angle of a single point tool. This is 1.5 to 20 for cast iron and steel. Finishing teeth have a smaller angle ranging from 0 to 1.5O.
- Rake or Hook Angle or Face Angle – This corresponds to the rake angle of a lathe tool. The rake angle varies according to the material to be cut, and in general, increases as the ductility increases. Value of this angle for most steels ranges from 12 to 15.
- Pitch – The linear distance from the cutting edge at one tooth to the corresponding edge on the next tooth is called pitch ‘t’ and differs for cutting (roughing and semi finishing) and finishing teeth. For the cutting, the pitch is selected in accordance with length ‘L’ of the hole being broached (t = 1.25 √ L to 1.5√ L). On an average, the pitch at finishing teeth is usually equal to half of the cutting teeth pitch. The pitch should vary by 0.2 to 0.3 mm after several teeth.
The height of the roughing and semi finishing teeth gradually increases from the shank to the finishing teeth. This increment is called the cut per teeth & it depends upon the material being machined and the hole size. The cut per teeth is usually taken from 0.01 to 0.2 mm.
Most broaches are made from 18% tungsten, 4% chromium and 1% vanadium alloy steel ground after hardening. Carbide broaches are used extensively in the broaching of cast iron in the automotive field. They are also used for surface broaches, high production and finishing broaches.