We have an entire post relating to hems if you are interested in their design and uses. Because of this there are different considerations given to hem allowances. This will throw off any Bend Allowance or Bend Deduction formulas you are trying to use to calculate your flat pattern. The Outside Setback is only considered in angles up to about 170 ° because as the angle approaches a 180 ° bend the values of the Outside Setback and Inside Setback approach infinity. As you can see these values are heavily dependent on the Bend Angle. The below image shows the difference between the Outside Setback (OSSB) and the Inside Setback (ISSB). The Outside Setback is closely related to, but should not be confused with, the Inside Setback. Because this is a geometrical function it does not change with a metal’s K-Factor the way Bend Allowance and Bend Deduction do. When the sheet metal is bent the inside radius pulls the edge of the material away from the apex of the bend. The Outside Setback is the distance from the apex of the outside mold lines to the tangent point of the outside radius. Understanding the Outside Setback of the workpiece is an important part of designing sheet metal parts.
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