Broaching 101

Wikipedia – Broaching is a machining process that uses a toothed tool, called a broach, to remove material

For steel rule die making the process of reducing the thickness of the steel rule, typically within a few thousandths of an inch of the cutting bevel, we call this broaching. This will allow the die maker to bend the cutting rule a little further or into a tighter radius. This is typically done to prevent cracking or edge distortion of the steel rule. 

There are two typical methods for broaching, using a grinding wheel and the other is using a series of teeth to progressively scrape away the side of the rule to a specific depth. Broaching can be done with special offline equipment also with newer technology it can be accomplished inline on a rule bender, saving time and improving accuracy.

When bending a small radius without a broach, the rule is compressed on the inside and stretched on the outside. This distorted steel can create issues for the die maker and the die cutter. The stress on the rule can create a crack in the rule to the point of breakage. The other area that can be affected is in the height of the rule, making the radius area slightly taller causing that area of rule to hit the cutting plate harder. 

Broaching the tight radius areas can also allow the rule to be inserted into the die board a little easier by having less material to fit into a small area. 

For die makers, understanding the customer’s goals is very important to making the right tool for the job. If a faster make-ready and longer die life is a goal, broaching might be one way to help them save some time in make-ready as well as increase die life. Looking at the folding carton industry, we see press manufacturers focused on helping their customers run machines to their full potential. We also see those customers working toward faster make-ready times and longer die life. Taking the time to broach areas with tight bends may help prevent a high spots in the die allowing for faster make-ready by reducing patch up time but also extending the life of those knives by preventing them from cutting into the cutting plate.

While there are reasons and benefits to broaching, the process is not always an exact science. It may take some time and finesse to get it just right since the hardness of different rules vary. One suggestion was to test the rule you are using for its ability to broach and bend a tight radius before you begin.