If you’re used to rotary broaching hexagon forms, the idea of broaching squares may seem fairly straightforward. However, due to the smaller number of corners and larger corner-to-corner dimension, broaching squares does have some challenges. Here’s a couple ideas that make broaching squares easier:
1. Larger pilot hole diameter
When broaching hexagons, we recommend using a 3% oversize pilot hole. This pilot hole diameter reduces chip size, separates the chips better, reduces pressure and ultimately improves tool life. However, the chips on square forms are much larger. Therefore, we recommend using a 10% oversize pilot hole when rotary broaching a square. This is not an absolute rule, but it’s a good idea to start here and then make the pilot hole smaller if it seems feasible.
2. Spun ground diameter
Typical rotary broaches remove up to .025″ of material per side. When changing from a 1/4″ hex to a 1/4″ square, the amount of material changes from about .020″ per side to .050″ per side. Although it’s still possible to broach a 1/4″ square in many materials, the extra material will prove to be bothersome and likely reduce tool life.
A common option when broaching a sharp corner in a square hole is to leave a small amount of material in the corner. This is because the mating piece also is missing material in the corner or includes a favorable tolerance. Although adding an individual radius to each corner of the broach is not practical, adding a spun ground diameter removes the material and is a standard option from Polygon Solutions. Adding a spun ground diameter reduces chip size, separates the chips better, reduces pressure and ultimately improves tool life.
Some customers desire to use a rotary broach to finish a square hole even though the size is too large for rotary broaching. The solution includes milling out the material and then using the rotary broach as a finishing tool. The challenges with this solution include aligning the rotary broach because broach holders to not include an alignment feature. As a result, a guide post or broach brake must be used to hold alignment.
These points are useful for setting up a rotary broach holder up to make square holes. Although each set-up is different, we hope you can use them to see that when rotary broaching, squares are not bad.