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When we air bend sheet metal, the size of the die V-opening determines the inside radius of the bend. The metal forms to a natural radius based on the size of the die V-opening. The punch tip radius usually has little influence.

Does this mean that one size punch tip radius will work well with every application? Not quite.

Minimum punch radius ≥ 63% of the material thickness

If the punch tip radius is smaller than 63% of the material thickness, it tends to penetrate the surface of the material which can cause rapid wear at the tip of the punch as the material folds around the punch tip like a nut cracker.

It’s easy to spot this type of damage as upon close inspection you will notice that the punch tip radius appears to have been pinched.

The punch tip radius should be no less than 63% of the material thickness. The smallest inside radius that can be achieved by air bending sheet metal is .63 x material thickness.

Maximum punch radius ≤ the natural radius of the bend

When is the punch tip radius too large?

When planning to air bend a part, the best practice in developing the blank size is to use the naturally formed inside radius of the bend to calculate the bend deduction.

The punch tip radius used to form the bend should not be greater than the natural inside radius of the bend.If the punch tip radius is larger than the natural inside radius of the bend, the bend will conform to the punch tip radius resulting in larger than expected finished dimensions.

When the punch tip radius is larger than the natural inside radius of the bend, the inside radius of the bend will be controlled by the punch tip radius instead of the die V-opening.

Optimum punch radius = the natural radius of the bend

With a punch tip radius that is equal to the naturally formed radius of the sheet metal, you will get consistent angles and the punch tip will experience the least amount of wear.

How to predict the natural inside radius of a bend:

The die V-opening (VO) multiplied by a material factor will give you the inside radius (IR).

Material factors: Aluminum (AL) = .14, Cold rolled steel (CRS) = .16, and stainless style (SS) = .21

Example 1:        .059″ CRS formed in a .375″ VO                      (CSR material factor) .16 x .375 = .060″ IR

Example 2:        .125″ SS formed in a .875″ VO                         (SS material factor) .21 x .875 = .184″ IR

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