The larger and smoother the relief hole is, the less likely it  will be that a crack will form in the corner. Generally, the  radius of the relief hole is specified on the drawing. A box  and pan brake, also called a finger brake, is used to bend the  box. Two opposite sides of the box are bent first. Then, the  fingers of the brake are adjusted so the folded-up sides ride  up in the cracks between the fingers when the leaf is raised  to bend the other two sides.  intersection of the inside bend tangent lines. This keeps any  material from interfering with the bend allowance area of the  other bend. If these bend allowance areas intersected with  each other, there would be substantial compressive stresses  that would accumulate in that corner while bending. This  could cause the part to crack while bending. The size of relief holes varies with thickness of the material.  They should be no less than  1 ⁄ 8-inch in diameter for aluminum  alloy sheet stock up to and including 0.064-inch thick, or  3 ⁄ 160-inch in diameter for stock ranging from 0.072-inch  to 0.128-inch thickness. The most common method of  determining the diameter of a relief hole is to use the radius  of bend for this dimension, provided it is not less than the  minimum allowance ( 1 ⁄ 8-inch). Relief holes must touch the intersection of the inside bend  tangent lines. To allow for possible error in bending, make  the relief holes extend  1 ⁄ 32-inch to  1 ⁄ 16-inch behind the inside  bend tangent lines. It is good practice to use the intersection  of these lines as the center for the holes. The line on the inside  of the curve is cut at an angle toward the relief holes to allow  for the stretching of the inside flange. The positioning of the relief hole is important.  [Figure 4-145] It should be located so its outer perimeter touches the  Open and closed bends present unique problems that require  more calculations than 90° bends. In the following 45° and  a 135° bend examples, the material is 0.050-inch thick and  the bend radius is  3 ⁄ 16-inch. Lay out the basic part using traditional layout procedures.  This determines the width of the flats and the bend allowance.  It is the intersection of the inside bend tangent lines that index  the bend relief hole position. Bisect these intersected lines  and move outward the distance of the radius of the hole on  this line. This is the center of the hole. Drill at this point and  finish by trimming off the remainder of the corner material.  The trim out is often tangent to the radius and perpendicular  to the edge.  [Figure 4-146]  This leaves an open corner. If the  corner must be closed, or a slightly longer flange is necessary,  then trim out accordingly. If the corner is to be welded, it  is necessary to have touching flanges at the corners. The  length of the flange should be one material thickness shorter  than the finished length of the part so only the insides of the  flanges touch.  4-73 Figure 4-146.  Relief hole layout.  Figure 4-145.  Relief hole location.  Layout Method Relief Hole Location Open and Closed Bends 2" Flat Flat Flat Area of bend (BA) Area of bend (BA) 1" 2" Bend relief radius Intersection of inside bend  tangent lines 11 ∕ 16 1  11 ∕ 16 11 ∕ 16 7 ∕ 16 7 ∕ 16 2  13 ∕ 16 2 1 2  13 ∕ 16 Notice overlapping mold lines (by 1 MG) R  = 0.250 ( 1 ∕ 4) T  = 0.063 ( 1 16) SB = 0.313 ( 5 ∕ 16) BA = 0.437 ( 7 16) MG = 0.191 ( 3 ∕ 16) Normal trim  tangent to radius If necessary for flanges to touch If  5 ∕ 16 R is required, punch  5 ∕ 8 hole