Sheet Metal Blanking Punch & Die Clearance Calculator

Sheet Metal Blanking & Piercing Die Clearance Fundamentals

In sheet metal stamping, the clearance between the punch and die cavity is the single most critical geometric factor determining tool life, edge quality, press tonnage, and burr formation.

1. Blanking vs Piercing Dimensioning Rule

Because sheet metal springs inward upon piercing and outward upon blanking:

• Blanking: The slug is the finished product. The die opening governs part size:

  $$D_{\text{die}} = D_{\text{nominal}}, \quad D_{\text{punch}} = D_{\text{nominal}} - 2c$$

• Piercing / Punching: The remaining strip is the product, and the hole is the feature. The punch governs hole size:

  $$D_{\text{punch}} = D_{\text{hole}}, \quad D_{\text{die}} = D_{\text{hole}} + 2c$$

2. Optimal Clearance per Side

Optimal radial clearance (c) allows the crack propagating downward from the punch edge to meet the crack propagating upward from the die edge in a clean fracture line:

• Mild Steel (1008-1018): 7% to 8% of sheet thickness (t) per side.
• Austenitic Stainless (304/316): 9% to 12% of (t) per side (prevents severe work hardening and punch galling).
• Aluminum Alloys (5052, 6061): 4% to 6% of (t) per side.
• High-Strength Steels (DP600+): 10% to 14% of (t) per side.

3. Shear Force & Press Sizing Formula

Cutting force without shear angle is calculated from perimeter (L), thickness (t), and material shear strength (\tau_s):

$$F_c = L \cdot t \cdot \tau_s$$

When roof shear or bevel angle (\theta) is ground onto the punch, cutting force drops by a reduction factor, reducing shock loads on mechanical presses.