Press tools and die making
Press tools and die making covers blanking, piercing, bending, drawing, compound and progressive dies, and the forces and clearances that make a press shop safe and productive.
Core rules
- Blanking cuts the workpiece to shape; piercing cuts a hole, and the slug is scrap. In blanking the die opening determines the blank size; in piercing the punch determines the hole size.
- Clearance is the gap per side between punch and die. It depends on sheet thickness and material strength. Correct clearance gives a clean cut band and limited burr.
- A die set provides alignment through guide pillars and bushes. The punch, die block, stripper, back gauge and knockout must be robust and safely guarded.
- Press force for shearing is approximately F = perimeter times thickness times shear strength. Add allowance for stripping, bending or drawing and for machine capacity.
- Bending needs allowance for springback and bend radius; drawing needs blank-holder force, draw radius and lubrication.
Worked example
A rectangular blank has a cutting perimeter of 100 mm and is 2 mm thick mild steel with shear strength 300 N per mm squared. Shearing force F = 100 times 2 times 300 = 60,000 N = 60 kN. A press rated above this, with allowance for stripping and shock, is selected. The die clearance is set from the material thickness and grade, not from the press tonnage.
Traps
Reversing blanking and piercing clearance gives a wrong-size part; ignoring stripping force underestimates tonnage; a worn stripper lets the strip lift and jam; and missing guards around the press point of operation is a serious safety failure.