Although many small form applications are well suited for rotary broaching, there are times you may decide to use the broach as a punch. Some precision machine operations will be improved with this method, because the possibility of a twist or spiral in the form created when rotary broaching will be eliminated.
The greatest disadvantage to using the rotary broach as a punch broach is that instead of moving pressure from tooth to tooth, punch broaching or shaping pushes the entire form into the part at once. This shaping technique creates much more pressure than rotary broaching, and some machines may not be able to handle it. The increased pressure may also cause failure of the broach, and possible tearing of the material.
However, due to the relatively low cost of rotary broaches compared to other broaching methods, use of the broach as a shaping tool can be practical. This is most often in the case in smaller, shallow, blind holes. If the volume of precision parts to be produced is low, rotary broaches made from high speed steel can stay within tolerance for many cycles.
If there is too much pressure when creating a double square or other symmetrical form, as shown in the illustration, a form with less teeth can be used. For example, shaping or punch broaching with a square can be done, and then rotate the work piece or the cutting tool 45 degrees and broach the square again. The result is a double square. A rectangle shaped broach can be manufactured for cutting hexagons, and so on.
Many modern CNC and screw machines also have the ability to machine in a peck cycle or shaping mode. Interrupted motion can also be accomplished manually or through programming. The lighter cuts may create a longer overall cycle time, but may also reduce pressure and improve tool life.