Bending Allowance Hemming Sheet Metal

The bending angle is 90 sheet thickness 5 mm and the inside radius is 6 mm.

Bending allowance hemming sheet metal.

When sheet metal is bent it stretches in length. Often bend allowances are calculated for a sheet metal part and used to make costly tooling or production parts that require a lot of labor to produce. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside edges of the bend. A scrap tool or production run can be very costly much more so that a test piece.

You seem to be working with a closed hem which has a zero inside bend radius. In materials with a thickness less than 080 the hem allowance factor is 43 of the material thickness. We are going to review three bending scenarios with three different bending angles. The formed bend radius is dependent upon the dies used the material properties and the material thickness.

Let s say you have a similar part to the one on the image above it has a straight leg of 20 mm and another of 70 mm. 60 90 and 120 and we will calculate k factor bend allowance and bend deduction for them. You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k. When working with a brake press hems are always created in a two step process.

The total amount of elongation for such a hem is 43 percent of the material thickness under perfect conditions. Most sheet metal materials will conform to the calculations. The key word here is perfect. Formula for the bend deductions outside setbacks and bend allowances cease to be of value when the bend angle exceeds 174.

In sheet metal hemming means to fold the metal back on itself. Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in figure 1. The following illustration shows the equation calculation for determining the bend allowance when forming sheet metal. Create a bend with acute angle tooling in the metal 30 is preferable but 45 will work for some circumstances.

The blank size is computed through the use of the hem allowance ha. The bend radius refers to the inside radius. These leave residues that sometimes can be very difficult to remove. Take the following example.

Mark the two lines on the sheet metal where the bend will begin and end the material between these two marks the bend allowance will deform to create the angle. The bend allowance formula takes into account the geometries of bending and the properties of your metal to determine the bend allowance. The material thickness will be measured in decimal form not by the gauge number. To create the bend manually you can use a shaped form perhaps made of wood which conforms to the bend radius you want to achieve.

We want to know the final length of the detail.