Diecutting Punches – Terminology

Exhaust – Self Cleaning
The exhaust chute on a self-cleaning punch can be out the side or the end of the punch. An exhaust is a machined opening in the punch, which allows the material to exit through and out of the punch. The exhaust should be as high as possible from the bottom of the self-cleaning punch so that the scrap can exit as efficiently as possible. Each die cut slug pushes the next until they exit the punch.
Exhaust – Chisel
The chisel style exhaust is used when the bridge of a punch exceeds approximately 1-1/8″. Above this length, the punch may flex over the bridge area. The chisel depth is usually half of the material thickness so that the first time the punch is cut the whole slug is removed. During the second cut, the fore mentioned slug is cut in half and exists the punch in two pieces.
Exhaust – Feed-Thru
The feed thru exhaust is a standard + .030 larger than the cut size of the punch and usually has the same shape as the cut. All feed thru punches must be used in conjunction with a bolster plate.
A chamfer is a 45-degree angle machined onto the bottom of a punch. This allows the punch to be inserted into the dieboard without shaving the sides of the hole.
Knurls are mostly commonly found on self-cleaning punches. The knurl is machined onto the bottom of the punch and is approximately 1/4 long. A knurl resembles the rough surface of a ratchet handle. The straight knurl is cut into base of the punch and raises the surface approximately +.010 from the base diameter. The purpose of the knurl is to fold. First, it stops the punch from turning therefore keeping the correct alignment of the exhaust hole to the routed dieboard. Secondly, it keeps the punch tight in the dieboard, which can loosen after a re-rule with an un-knurled punch.
Cut Edge
The cut edge is the working end of the punch and usually varies from 20 -30 degrees per side. The lesser the cut edge bevel, the less pressure it takes to cut – therefore preventing a distorted cut. All cut edges are machined, unlike rule, which may be ground or shaved.
The base is the body of the punch, which supports the cut edge. Bases come in many shapes. The base should always have a plus tolerance from “on size” to +.003. A base should never be under size. This will make it a loose fit.
Usually there are three types of bevels… Center, inside, and outside. Center bevel is the most common of the three. On stock that is .014 or less, it does not matter which bevel you choose – as it will not influence the cut.
Support Bevels
A support bevel is very critical to the performance and life of the punch. This bevel will add strength as well as stop chipping and peeing of the punch’s cut edge. For example, a 1/2″ base punch can not measure an exact 1/2″ cut because of the 1/64″ outside support the bevel that is machined into the punch to achieve concentricity. The cut size becomes 31/64″ diameter. This also applies to a 1/2″ I.D. tube on a 9/16″ base. The 1/64″ inside support bevel make the punch size a 33/64″ cut diameter. If the support bevel on a self-cleaning punch is too big, the punch requires too much pressure to pick up the slug and will act as a cavity punch leaving the slug in the work rather than removing it.
A cavity is a machined pocket on the inside of the cut edge. This pocket can vary in depth depending upon the application or machining limitations. A standard cavity depth should be 5/16″ to accommodate standard die ejection.
A very common method of ejecting the scrap from the punch is the use of a spring. The spring usually protrudes 1/16″ above the cut edge of the punch and has a working range of about .100″ of the travel. Springs are most common in tubing punches, but can be used in virtually in a style of punch.
Pins are another method to keep a punch from turning in the dieboard. Most commonly, they are used to keep the punch from being pulled through the dieboard by the material. Pins can be used on a heavy solid base punch, which may hang upside down in the press, to keep the punch from falling out of the dieboard.
Height is the overall length from the top of the cut edge to the bottom of the punch. The height of the punch is usually precision ground to assure the proper tolerance.
The purpose of a nick, or a deliberate break in the cut edge, is so that the slug stays in the die cut part. As the slug remains partially connected to the material, it is easier to handle without the slug falling out. If more than one nick is required, they should be symmetrical in the spacing of the punch. A nick is usually about .010-.030 wide depending upon your requirements. Punches can be ordered with nick installed – but are usually put in by the die maker.
Just about any punch can be serrated. The purpose of the serration is to relieve the cutting pressure so that the punch does not crush or tear the material when cutting. The most common materials using serration are corrugated and foam rubber. The serration will minimize scaving (hour glassing) of the material. When cutting material containing string or threads the serrated punch often works best. Serrated punches always cut into soft pad. Any tooth per inch (t.p.i.) can be ordered. The most common teeth per inch are 8-12 for corrugated 12-14 “shallow profile” for cutting foam.
Heat Treat
Most common tube punches are made of low carbon steel and must have carbon added to achieve the desired hardness. This is known as “case hardening”. The case is usually .003 -.005 surface hardened and about 58/60 Rockwell “C” scale. High carbon / tool steel punches can be neutral hardened because the steel already contains carbon. These punches are through hardened and work best at about 48/52 Rockwell.
All self-cleaning and feed thru punches must have undercuts. This will reduce the pressure it takes for the punch to remove slugs. If the undercut is inadequate, the punch may break or work as a cavity punch leaving the slug in the work rather than removing it.
Wall Thickness
The most common wall thickness is .060 which is comparable to 4pt … Rule. This wall thickness is needed for maximum strength on the standard applications, but can vary depending on your specific application.
A shoulder is measured from the top of the cut edge of a punch down to the solid base. The ideal shoulder should be flush with the dieboard. Shoulders may vary in self-cleaning punches because of strength needed to support the cut edge. These punches are preferably machined with the shortest shoulder possible because the pressure required to push the slugs through a long shoulder could lead to breakage of the punch.
The bridge is the space between the shoulder ant the top of a self-cleaning exhaust. The longer the cut size – the thicker the bridge needs to be. If the cut length of a punch exceeds approximately 1-1/8″, a special exhaust must be designed.