Avoiding the 7 Deadly Sins of Stamping: Part 7—Understanding the Physics Behind Sheet Metal Cutting
August 21, 2025Comments
In part six of this series (MetalForming, May 2025), we discussed the history of the 10% cutting-clearance rule, as well as the advantages of using a clearance value with respect to the sheet thickness and the material’s mechanical properties. Here we take a look at what happens when stampers use different clearance values when piercing holes, and discuss the advantages of using the correct cutting clearance.
Whenever I conduct training for metal formers, I always tell people to start visualizing the material being formed not as sheet metal but as a substance that behaves similarly to Silly Putty—a compound with a rubbery, elastic consistency. When stretched rapidly, it would snap and break, but when pulled slowly, it would stretch. Materials used in stamping behave in a similar fashion.
Metal-Cutting Theory: Compressive vs. Tensile Failure
In most sheet metal-stamping operations, the material deforms to the point of failure between two bypassing die sections or components. To cut the sheet metal, the die requires a cutting punch and a mating die section into which the punch enters. The distance between these two working sections—the cutting clearance—will vary with respect to the sheet metal grade and thickness, its hardness, and the desired edge quality.
Cutting clearance often is expressed as a percentage of sheet thickness. Although clearances can range from 0 to as much 25% of the sheet thickness, the most common cutting clearance used is about 10% of the sheet thickness. For example, when designing a die to cut 0.050-in.-thick sheet, the distance between the upper and lower cutting sections would be 0.005 in. per side, for a total diameter difference of 0.010 in. Excessive or insufficient clearance between cutting sections could produce an excessive burr on the part.
Cutting in Steps
Step 1: The punch travels downward and hits the sheet metal, transferring a great deal of shock up through the punch. As the punch begins to enter the sheet, the sheet begins to deform slightly—the amount of deformation depends on the material’s ductility, sheet thickness and the clearance between the mating die sections. Using excess clearance value can result in tensile failure (and pull the slug out), while insufficient clearance value can lead to compressive failure (and push the slug out)—Fig. 1 and 2.
Step 2: As the punch continues to travel downward, it begins to push out, or cold-extrude, the sheet metal. This creates the sheared portion of the cut, or the “cut band,” Fig. 3.
Step 3: Once the shear strength of the sheet metal has been met, it begins to fracture or break away. Shear strength, a value similar to tensile strength, represents the maximum shear stress that a material can withstand when subjected to two bypassing planes. This process produces a cut edge with a shiny portion—the cut band or shear zone, and a portion called the breakout or fracture zone. With most conventional cutting operations, the thickness of the cut bend typically is 20 to 40% of the sheet thickness, Fig. 3.
Step 5: At this stage, the punch direction has been reversed; the punch retracts from the hole—called punch stripping.
