February 8, 2005
Having a pierce or perforating punch chip, or worse yet break, can cause severe die damage. It also can result in nonconforming parts because of burrs or undersize holes. Broken punch pieces can fall into the die and crush pads, as well as forming and cutting sections. This article discusses a few common reasons for pierce punch failure.
Obvious reasons for punch breakage are misloading, misgauging, or misfeeding the die. Having double metal in areas engineered for a single metal thickness can cause severe punch deflection and breakage.
Piercing half holes or cutouts also can cause pierce punch deflection, chipping, and breaking (see Figure 1). Make sure that when you engineer a die, especially a progressive die, that you avoid half forms and cuts. Doing so may require moving the first hit line to a different location.
Forming half of a part can cause the upper and lower die to shift sideways, which results in poor punch alignment. If half hits cannot be avoided, ensure that the die is fully heeled with respect to the direction of force (see Figure 2). Also make sure that all piercing and cutting sections are heeled, including pitch or french stops (see Figure 3).
When bending one side of a part, consider adding a balancing bend or tab on the opposing side. This helps balance forces and eliminates the need for heel blocks. The additional bend can be cut off later in the die (see Figure 4).
Be careful starting material into progressive dies. Watch carefully for scrap left in the tool, and be careful that you are not half loading pads with material. Half loading can tip the pad, deflect, and break the punches. If half loading is unavoidable, use pad balancers to prevent the pads from tipping (see Figure 5).
Whenever possible, avoid using solid, plate, or bridge strippers. These strippers are inexpensive to manufacture but do not hold the material securely to the lower cutting section during the piercing and cutting operation. They also allow the metal to bow up during stripping. Instead of a solid stripper, use a spring-loaded pressure pad/plate (see Figure 6).
Cutting typically should not be incorporated into a forming operation. However, it occurs more often than you would expect and presents a serious problem. Metal movement during drawing or stretching, when the pierce punch enters into the material, can and usually does cause severe punch deflection. If the punch is rigid enough to resist deflection, two things can happen: The hole in the part is elongated and/or the metal is not allowed to flow.
Simple bending die piercing does not present problems as serious as those created by drawing or stretching die piercing, primarily because there is very little metal movement during a simple wipe-bending operation. Avoid cutting in forming dies, but if it is absolutely necessary to do so, do it only in simple wiping and bending dies.
This is a big one. Using an old gap- or C-frame press to pierce and cut? Good luck, especially if you are maximizing the available tonnage. While gap-frame presses (also known as open-back or C-frame presses) work well for low-tonnage cutting operations, simple bending and other light duty operations, severe press deflection can result when the tonnage requirement increases.
When the tonnage on the ram increases, the amount that the ram deflects with respect to the bolster goes up, which causes poor pierce punch and cutting section alignment. Most gap-frame presses are rated for deflection on a per-inch-of-throat-depth basis. For example if you have a brand X gap-frame press with a 24-in.-deep bed rated at 100 tons, and you calculate ram to bolster deflection when 100 tons is applied to the press, the ram could be out of parallel to the bolster as much as 0.048 in. This calculation uses a 0.002-in.-per-inch deflection rate. Most older gap-frame presses are rated at 0.0015 in. to 0.002 in. deflection per inch of throat depth (see Figure 7).
Whenever possible, use a straight-side or box-frame press. These presses deflect considerably less, on average, 12 times less than a gap-frame press (see Figure 8).
Press vibration also is a problem with gap presses, especially when using solid carbide punches. Although carbide is extremely wear-resistant, its ability to absorb shock and vibration is very poor.
Keep in mind that pierce and cutting punches must have the ability to adsorb severe shock without breaking or chipping. They also must be abrasive- and adhesive-wear resistant.
You must consider several factors when selecting the best punch for your operation: the thickness and hardness of metal being cut, the cutting clearance selected, and the ratio of punch diameter to material thickness. These factors control the tool steel selection.
For example, piercing a 0.5-in. hole in low-carbon steel that is 0.062 in. thick using 10 percent per side cutting clearance requires a punch made from tool steel that has medium toughness and medium wear resistance. A-2 is a common tool steel selected for this application. If you change the material you are cutting to high-strength spring steel, you will need a punch made with ultrapremium tool steel with both impact strength as well as wear resistance. CPM and VANADIS® varieties are common.
Of course small punches are more volatile than large-diameter punches. When piercing very small holes, use a quill punch. These special punches are designed with a large shank diameter and a small, short punch point.
Remember, many factors contribute to punch breakage. The ones discussed in this article are only a few of the most common.