The bend allowance is the amount of extra material that is consume within the arc of the bend in the material. The bend allowance is the reason that the length of the flat portion of the finished part will not always be the same as the length of the leg of the part. When you measure the part after it is finished bending, you’ll find its flat length will be different than the length of the individual legs.
In order to ensure that the length of the flat portion of the metal that is cut from the stock material match the dimensions of the finished part, the bend allowance must be calculated. Several variable affect the bend allowance, including the bend angle, the inside radius of the bend, the thickness of the material being bend, and the K-factor of the material. The bend angle is the angle of the bend, and the inside radius is the radius of the bend.
What Is Bend Allowance and How to Check It
The thickness of the material impact the bend allowance because thicker materials stretch more on the outer radius of the bend, yet compress more on the inner radius of the bend. The K-factor is the location of the neutral axis within the material, which is the layer within the material that does not stretch or compress during the bending process. Each of these factor will impact the bend allowance; altering any of those variables will impact the bend allowance of the part.
For many metal fabrication operations, a K-factor between 0.33 and 0.50 are used to calculate the bend allowance. The exact K-factor that a worker uses in any operation, however, will depend upon the sharpness of the bend. Sharp bend will have the neutral axis closer to the inner radius of the bend, while gentler bends will have the neutral axis closer to the middle of the thickness of the metal part.
A chart of common K-factor for different types of bends can be used to select the correct K-factor for a given operation. Additionally, the method that the part is bent will impact the bend allowance that is use for that part. Methods like air bending, bottoming, and coining require different allowances than one another due to the different loading methods of the metal into the press brake.
Air bending allow the metal to remain suspended above the die, which creates more springback. Bottoming allows the metal to be force into contact with the die, which reduces springback. Coining allows for the metal to be forced against the die to such a hard squeeze that the metal does not move after the punch is lifted; however, the tonnage of the press brake must be increased for coining operation.
The type of metal that is used will impact the bend allowance for the part. Metals like mild steel tend to be the easiest to work with; however, metals like stainless steel require more overbend because stainless steel feature more springback. Additionally, metals like aluminum 5052 require a different bend allowance than metals like mild steel due to the difference in the relationship of the thickness and radius of the metal between the two metal type.
Additionally, it is recommended to test the bend allowance of any given part. Even if the calculations for the bend allowance are accurate, the dimensions may not be accurate with the first batch of parts cut. By cutting a test strip of the material to the same dimensions as the part to be made, the bend allowance can be tested.
If the dimensions of the test strip are not to scale, the K-factor for the part can be adjusted or the radius of the bend can be adjusted before cutting the remaining portions of the material needed for the batch of part. Testing the bend allowance reduces the waste of material if the calculations for the bend allowance are incorrect. Another variable to consider is the width of the die.
The wider the die opening, the less tonnage is required to bend the material. However, a wider die opening create a larger inside radius of the bend. One rule of thumb for die width is that the width of the die opening should be eight times the thickness of the metal part being cut.
However, another consideration for the width of the die is the variation in the inside radius of the finished part. Accurate bend allowance will create accurate dimensions for the flat part that will be bent. Accurate dimensions for the flat part will allow the part to load well into the press brake.
If the flat part load well into the press brake, the press brake will create accurate angles for the cut metal parts. Accurate angles will allow for the parts to snap into place with the other parts of the project without the need for force or filler metal. Each step in the metal fabrication process rely upon the accuracy of the previous step; thus, each of the variable in the process should of been measured rather than guessed.
