Bending Allowance Hemming Sheet Metal

In sheet metal hemming means to fold the metal back on itself.

Bending allowance hemming sheet metal.

Create a bend with acute angle tooling in the metal 30 is preferable but 45 will work for some circumstances. Sheet metal calculator bend allowance equations and calculator. The key word here is perfect. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside edges of the bend.

The bend radius refers to the inside radius. 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. In materials with a thickness less than 080 the hem allowance factor is 43 of the material thickness. 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.

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. 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. The blank size is computed through the use of the hem allowance ha.

A scrap tool or production run can be very costly much more so that a test piece. The bending angle is 90 sheet thickness 5 mm and the inside radius is 6 mm. 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.

Formula for the bend deductions outside setbacks and bend allowances cease to be of value when the bend angle exceeds 174. You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k. This page also includes a link to a on line sheet metal bend allowance calculator. Most sheet metal materials will conform to the calculations.

The total amount of elongation for such a hem is 43 percent of the material thickness under perfect conditions. First we must start with the k factor. 60 90 and 120 and we will calculate k factor bend allowance and bend deduction for them. We are going to review three bending scenarios with three different bending angles.

Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in figure 1. When working with a brake press hems are always created in a two step process. We want to know the final length of the detail. These leave residues that sometimes can be very difficult to remove.

Take the following example.