Cutting Punches Defined

Written By Mark Batson Baril

A possible lead-in question may look like this.
As the purchasing agent at medium sized die-cutting house, I am responsible for the purchasing of punches for our dies. It seems that every year our company is purchasing and using more and more punches. It is very important that the punches I purchase are right for our application and are “quality punches”. As there are multiple vendors out there selling punches and there are so many punches available, I would like to know ... what exactly are the most common punches and what makes each a “quality punch”?

You have come to the right place! There ARE many types and qualities of punches available and your specific applications will constitute what types of punches you want to purchase. First, you need to educate yourself as to the most common punches and then what makes each a “quality punch”...

The most common punch is the tubular punch. Tube punches are the most economical of all of the punches and are used for the widest range of applications. Slugs cut by a tube punch do not feed thru the punch, but are left in the product being cut with the help of die ejection. A standard tubular punch by definition is a piece of 16 gauge tubing that has a bevel machined on one end to a specific cut size. Tube punch cut sizes span the decimal chart in both millimeter and inch measurements and can be machined into virtually any custom size. A quality tubular punch should have a chamfer on the bottom on both the inside and outside to aid in ease of insertion into the die board. The base size should have a .000" to +.003" tolerance, the cut edge bevel should be virtually free of tool marks and the cut edge should be razor sharp. Springs are available in tube punches to alleviate the need for die ejection. These springs should protrude approximately 1/16” from the cutting edge. A quality tube punch will also be clean of scale, free of burrs, have a case hardening depth of .003" to .005" and a surface hardness of 58-60 Rockwell.

Similar to the tubular punch is the straight wall punch. Straight wall punches are used for applications with minimum punch space allotment where the base size of a standard tubular punch would be too bog to fit. A straight wall punch has a base size that is only several thousandths of an inch larger than the cut edge. This small difference allows for a slight support bevel for strength. Straight wall punches cause less distortion of cut size in thicker materials. The slugs cut by this punch are left in the product through the use of die ejection or springs and share the tubular punches tolerances and quality guidelines.

Another common punch is the feed thru punch. Most people will confuse a “feed thru” punch with a “side outlet” punch. In a feed thru punch, the slug exits the punch through the bottom rather than the side as in side outlets. Feed thru punches are used when your application calls for the scrap to be removed from your product rather than being hand stripped at a later time in your manufacturing process. Feed thrus must be run on a bolster plate which supports the die while at the same time allowing the slugs to feed thru where they are vacuumed, blown away, or otherwise disposed of. Feed thrus are constructed from thin wall tubing which is spun or sized then re-machined to your specific cut size. This method assures the proper relief for slug ejection. A quality feed thru’s specs and sizes offered are much the same as a tube and straight wall except that the feed thru’s inside chamfer is minimal, the cut edge should have a slight support bevel on the inside for strength and they do not come with springs.

A side outlet punch is a punch who’s waste slug feeds through an exhaust chute machined into the side of the punch. Side outlet punches are used when your application calls for scrap to be ejected - as in a feed thru - but this punch does not require the use of a bolster plate. Other than the location of the exhaust hole for the slugs, differences between the feed thru punch and the side outlet are that the side outlet is machined out of a solid piece of steel and it’s use of a shoulder. A side outlet shoulder is defined as the machined area of the punch from the top of the cut edge to just above the exhaust chute.

The most common type of side outlets are standard and heavy duty. The heavy duty side outlet is used for thicker, heavier, abrasive materials, has an elongated shoulder and often includes a “knurl”. A knurl is a raised portion located at the bottom of a punch - similar in texture to a ratchet handle. It is approximately .005" to .010" larger than the base size of the punch and is .250" wide. The knurl is used to prevent the punch from spinning or becoming misalligned in the routed die board. The standard side outlet is used for easier to cut, medium to thin materials. It has a shorter shoulder than does a heavy duty and does not include a knurl unless specified. Again, a good quality side outlet should be razor sharp, free of tool marks, scale and burrs. It should include a slight support bevel on the inside for strength as well as an undercut which prevents the slug from jamming in the punch before it enters the exhaust chute.

All punches can be made in a variety of heights - the most common being .937" (23.8mm) and each can be altered to meet your specific application. The life of these punches is effected by the material being cut, the application for which the punch was designed and operator skill level. Typically, a punch should last as long - if not greater than - the rule used in the die.

Tubular punches, straight wall punches, feed thrus and side outlets may be the most common punches, but they are far from the only ones offered. Custom punches can be manufactured to virtually any shape or size and can be used to produce everything from high tolerance flex circuits to components used in military aircraft to the gasket in your car. Custom punches ... now THAT is another question altogether!!! I hope that you now have a better understanding of some of the more common punches and what makes each a quality punch.