Bending Allowance Hemming Sheet Metal

In materials with a thickness less than 080 the hem allowance factor is 43 of the material thickness.

Bending allowance hemming sheet metal.

First we must start with the k factor. 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. We want to know the final length of the detail.

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. A scrap tool or production run can be very costly much more so that a test piece. The blank size is computed through the use of the hem allowance ha.

The material thickness will be measured in decimal form not by the gauge number. Take the following example. The key word here is perfect. You seem to be working with a closed hem which has a zero inside bend radius.

Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in figure 1. You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k. The total amount of elongation for such a hem is 43 percent of the material thickness under perfect conditions. The bend allowance formula takes into account the geometries of bending and the properties of your metal to determine the bend allowance.

When working with a brake press hems are always created in a two step process. We are going to review three bending scenarios with three different bending angles. Formula for the bend deductions outside setbacks and bend allowances cease to be of value when the bend angle exceeds 174. These leave residues that sometimes can be very difficult to remove.

Most sheet metal materials will conform to the calculations. 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. Sheet metal calculator bend allowance equations and calculator. The bending angle is 90 sheet thickness 5 mm and the inside radius is 6 mm.

The following illustration shows the equation calculation for determining the bend allowance when forming sheet metal. The formed bend radius is dependent upon the dies used the material properties and the material thickness. This page also includes a link to a on line sheet metal bend allowance calculator. 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.