The cutting sections of a broach can be broken down into three basic parts. (1) The front section of teeth, or roughing teeth, remove most of the stock. (2) The next section is referred to as the semi-finishing teeth, which gradually reduce the amount of stock taken out. (3) The rear portion of the broach is comprised of the finishing teeth, which create the finish dimensions on the part.
A typical tooth form is shown in Figure 1 below. The pitch is the distance from one cutting tooth to the next. The land is the length of a particular cutting tooth. (Tool life is expressed as a function of the land length.) The depth determines the amount of area available for the part chip, and the face (hook) angle gives a smooth surface for the chip to curl. The face angle radius should be kept constant throughout the life of the broach tool to allow the gullet to perform as required. The back radius of the tooth must blend smoothly with the face angle radius since a mismatch could cause the entering part chip to catch and stick, thereby leading to damage of the broach tool.
The face angle is a variable that is designed to the hardness and type of material being broached. When the broach tool is sharpened, it is critical that this angle be held. The back-off angle provides clearance for the land of the tooth form and is a variable determined by material and part configuration. The back-off angle also needs to be held constant throughout the life of the broach tool.
A Typical Broach Tooth Form.