Bending Allowance Hemming Sheet Metal

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.

Bending allowance hemming sheet metal.

When working with a brake press hems are always created in a two step process. 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. When sheet metal is bent it stretches in length. Take the following example.

We are going to review three bending scenarios with three different bending angles. A scrap tool or production run can be very costly much more so that a test piece. The material thickness will be measured in decimal form not by the gauge number. This page also includes a link to a on line sheet metal bend allowance calculator.

We want to know the final length of the detail. You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k. You seem to be working with a closed hem which has a zero inside bend radius. The following illustration shows the equation calculation for determining the bend allowance when forming sheet metal.

The total amount of elongation for such a hem is 43 percent of the material thickness under perfect conditions. In sheet metal hemming means to fold the metal back on itself. The bend allowance formula takes into account the geometries of bending and the properties of your metal to determine the bend allowance. The bending angle is 90 sheet thickness 5 mm and the inside radius is 6 mm.

In materials with a thickness less than 080 the hem allowance factor is 43 of the material thickness. The bend radius refers to the inside radius. 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 blank size is computed through the use of the hem allowance ha.

First we must start with the k factor. These leave residues that sometimes can be very difficult to remove. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside edges of the bend. Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in figure 1.

The formed bend radius is dependent upon the dies used the material properties and the material thickness. The key word here is perfect. 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. Most sheet metal materials will conform to the calculations.

Sheet metal calculator bend allowance equations and calculator.