Bending Allowance Hemming Sheet Metal

The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside edges of the bend.

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. You will need to know your material thickness mt the bend angle b the inside radius ir and the k factor k. These leave residues that sometimes can be very difficult to remove. When sheet metal is bent it stretches in length.

When working with a brake press hems are always created in a two step process. 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. You seem to be working with a closed hem which has a zero inside bend radius. The blank size is computed through the use of the hem allowance ha.

Most sheet metal materials will conform to the calculations. First we must start with the k factor. 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.

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 key word here is perfect. Sheet metal calculator bend allowance equations and calculator. 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 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. 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.

In sheet metal hemming means to fold the metal back on itself. The formed bend radius is dependent upon the dies used the material properties and the material thickness. 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 materials with a thickness less than 080 the hem allowance factor is 43 of the material thickness. Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in figure 1. We want to know the final length of the detail. The bending angle is 90 sheet thickness 5 mm and the inside radius is 6 mm.

Take the following example.