Radius (R) Leg Setback (90° bend) R + 1 Base measurement Mold point Bend allowance (BA) Mold line (ML) F L A T F L A T Thickness (T) Bend tangent line (BL) FLANGE Bend tangent line dimension (BTLD) MLD BTLD MLD R T SB SB Mold point A B C 4-59 Figure 4-120.  Bend allowance terminology. Figure 4-119.  Crimping metal. Layout and Forming Terminology  Grain of the metal—natural grain of the material is formed  as the sheet is rolled from molten ingot. Bend lines should be  made to lie at a 90º angle to the grain of the metal if possible. Bend allowance (BA)—refers to the curved section of  metal within the bend (the portion of metal that is curved in  bending). The bend allowance may be considered as being  the length of the curved portion of the neutral line.  Bend radius—the arc is formed when sheet metal is bent. This  arc is called the bend radius. The bend radius is measured  from a radius center to the inside surface of the metal. The  minimum bend radius depends on the temper, thickness, and  type of material. Always use a Minimum Bend Radius Table  to determine the minimum bend radius for the alloy that is  going to be used. Minimum bend radius charts can be found  in manufacturer’s maintenance manuals.  Bend tangent line (BL)—the location at which the metal starts  to bend and the line at which the metal stops curving. All the  space between the band tangent lines is the bend allowance. Neutral axis—an imaginary line that has the same length  after bending as it had before bending.  [Figure 4-121]  After  bending, the bend area is 10 to 15 percent thinner than before  bending. This thinning of the bend area moves the neutral  line of the metal in towards the radius center. For calculation  purposes, it is often assumed that the neutral axis is located  at the center of the material, although the neutral axis is not  exactly in the center of the material. However, the amount  of error incurred is so slight that, for most work, assuming  it is at the center is satisfactory. Mold line (ML)—an extension of the flat side of a part  beyond the radius. Flange—the shorter part of a formed angle—the opposite of  leg. If each side of the angle is the same length, then each  is known as a leg. Leg—the longer part of a formed angle. Base measurement—the outside dimensions of a formed part.  Base measurement is given on the drawing or blueprint or  may be obtained from the original part. The following terms are commonly used in sheet metal  forming and flat pattern layout. Familiarity with these  terms aids in understanding how bend calculations are used  in a bending operation.  Figure 4-120  illustrates most of  these terms.