Understanding the Bend Allowance and consequently the Bend Deduction of a part is a crucial first step to understanding how sheet metal parts are fabricated. When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched. As this happens you gain a small amount of total length in your part. Likewise when you are trying to develop a flat pattern you will have to make a deduction from your desired part size to get the correct flat size. The Bend Allowance is defined as the material you will add to the actual leg lengths of the part in order to develop a flat pattern. The leg lengths are the part of the flange which is outside of the bend radius. Unfortunately sheet metal bending is not always going to be the same in every shop. The largest variations come from the materials themselves. Protective coatings, variations in the alloy and thickness as well as many other small factors all add up to give you Bend Allowances unique to your operation. This chart will get you close enough for most applications and may not require fine tuning on your part.

 

Bend Allowances

Bend allowance is the amount of metal to be added to the total layout. The most important considerations when bending metal is the min. bend radius. When bends are made smaller than the required min. radius, metal will crack at the outside heel of the bend. The min. radius of a bend is determined by the hardness and thickness of the metal. There are four methods for finding bend allowances. In the form below uses the empirical formula. It is the most accurate because it takes into account the shifting of the neutral line, slightly inward, when the metal is bent. The following form must be filled in with decimal equivalants.

 

Radius of Bend:  
 Inches (decimel equivalent)
  
  
Thickness of Material:  
 Inches (decimel equivalent)
 
Angle of Bend:  
 (degrees)
 
Bend Allowance:  
 Inches