The relationship of the larger die hole size to the punch size is die clearance and is stated as a percentage of the thickness of the material being punched. The range of clearances varies from 10 percent for thin materials to 20 percent for thicker materials. For 3/4″ material the total die clearance is .150″. Clearance should always be specified when there is any reason for doubt.
Die Clearance has the following effects:
TOO MUCH CLEARANCE
1. Extra roll-in at top of the hole.
2. Too much burr at the bottom of the hole.
TOO LITTLE CLEARANCE
1. More punching pressure is needed. Can reduce tool life.
2. High stripping force causes part distortion and extra punch wear.
1. Straighter hole through material.
2. Minimum distortion at top of hole.
3. Minimum burr at bottom of hole.
Effects of die clearance are more noticeable in thicker materials (such as 1/4″) than in thinner material (such as 16 gauge). Roper Whitney stocks #28 style dies with .006″ clearance. For punching 24 gauge through 14 gauge mild steel or most grades of aluminum, we recommend that you order the #28 style dies for general satisfactory holes and fast delivery. For other gauges and material thicknesses and minimum burr, specify the type and thickness of material being punched and the exact clearance (see Chart 5).