Tooling by Design
Depending on the hardness and thickness of the material being cut, the ideal clearance can vary anywhere from 5 to 20 percent per side. Burr size, die rollover, burnish band and fracture zones are commonly used to characterize the quality of the shear edge. The image in Fig. 1 exemplifies a quality sheared edge exhibiting these characteristics (SAE paper 2007-01-0340, Konieczny, et al).
When the cutting clearances are small, press and die alignment become very critical. If this alignment is not maintained properly the punch and die details may contact each other and the cutting edges may be damaged. In addition, an edge defect known as secondary shear—sometimes referred to as a double-break —often will occur. Secondary shear results when the stamping fracture-zone angles from the punch edge and die edge do not meet. As a result, an additional band of material is sheared, producing a second burnish zone as the two fracture zones try to join (Fig. 2B); thus the term secondary shear. Small cutting clearances also require greater punching forces and cause greater punch wear, due to abrasion, as the pierced material grips firmly around the punch. As a result, increased stripping forces are needed to extract the punch.
Larger cutting clearances make press and die alignment less critical and also require less cutting and stripping forces. But if the cutting clearance becomes too great, extreme rollover occurs and undesirable burrs develop. In extreme cases, the material may actually tear or break open in the rollover zone if the surface is stretched beyond its ultimate tensile strength (Fig. 3; Eary and Reed). Soft, thick metals will sometimes extrude a thick heavy burr into the large cutting clearance gap.
Cutting clearances are applied to the offal material. Offal is the portion of the material which is not part of the end product and is sometimes referred to as engineered scrap. When a hole is pierced in a stamping, the punch size equals the desired hole size in the stamping and the cutting clearance is applied to the die opening, which is larger than the punch. This in turn produces a larger-diameter slug that is two-times the cutting clearance larger than the punched hole.
In blanking operations, the slug is retained and the resulting hole is left in the offal material. In this case the cutting clearance is applied to the punch by making it two-times the cutting clearance smaller than the die opening.
A special note for high-strength-steel applications:
The image in Fig. 1 is from SAE paper 2007-01-0340, On Formability Limitations in Stamping Involving Sheared Edge Stretching, by A. Konieczny and T. Henderson (US Steel Corp.). Konieczny and Henderson recently experimented with various grades and strength levels of HSLA, dual-phase and TRIP steels, applying cutting clearances from 1.1 percent to 20.8 percent. The objective was to investigate the influences of cutting clearance on the quality of sheared-edge conditions and further establish the extent to which the resulting sheared-edge conditions limit edge-stretching operations, such as stretch flanging and hole expansions. The influence of cutting clearance, burr height, burr location and expansion-punch geometry on the different types and grades of materials were studied to determine critical product design considerations, particularly for the advanced high-strength steels. This is a valuable body of work for any company that is stamping advanced high-strength steels. MF