•  Enter chart at the bottom on the appropriate scale with  the sum of the radius and material thickness.  •  Read up to the bend angle.  •  Find the setback from corresponding scale on the left. •  Material thickness is 0.063-inch. •  Bend angle is 135°. •  R + T = 0.183-inch.  •  Read up to a bend angle of 135°. •  Locate the setback at the left hand side of the graph  in the middle scale (0.435-inch).  [Figure 4-125] Find 0.183 at the bottom of the graph. It is found in the  middle scale. Example:  of bend affects the overall length of the metal, whereas the  thickness influences the radius of bend. Bending a piece of metal compresses the material on the  inside of the curve and stretches the material on the outside  of the curve. However, at some distance between these  two extremes lies a space which is not affected by either  force. This is known as the neutral line or neutral axis and  occurs at a distance approximately 0.445 times the metal  thickness (0.445  × T) from the inside of the radius of the  bend.  [Figure 4-126] The length of this neutral axis must be determined so that  sufficient material can be provided for the bend. This is called  the bend allowance. This amount must be added to the overall  length of the layout pattern to ensure adequate material for  the bend. To save time in calculation of the bend allowance,  formulas and charts for various angles, radii of bends,  material thicknesses, and other factors have been developed.  Formula 1: Bend Allowance for a 90° Bend To the radius of bend (R) add  1 ⁄ 2 the thickness of the metal  ( 1 ⁄ 2T). This gives R +  1 ⁄ 2T, or the radius of the circle of the  neutral axis.  [Figure 4-127]  Compute the circumference  of this circle by multiplying the radius of the neutral line  (R +  1 ⁄ 2 T) by 2π (NOTE: π = 3.1416): 2π (R +  1 ⁄ 2T). Since a  90° bend is a quarter of the circle, divide the circumference  by 4. This gives: The flats, or flat portions of the U-channel, are equal to the  mold line dimension minus the setback for each of the sides,  and the mold line length minus two setbacks for the center  flat. Two setbacks need to be subtracted from the center flat  because this flat has a bend on either side.  The flat dimension for the sample U-channel is calculated in  the following manner: This is the bend allowance for a 90° bend. To use the formula  for a 90° bend having a radius of  1 ⁄ 4 inch for material 0.051- inch thick, substitute in the formula as follows. When making a bend or fold in a piece of metal, the bend  allowance or length of material required for the bend must be  calculated. Bend allowance depends on four factors: degree  of bend, radius of the bend, thickness of the metal, and type  of metal used. The bend allowance, or the length of material required for  the bend, is 0.4327 or  7 ⁄ 16-inch. This formula uses two constant values that have evolved over  a period of years as being the relationship of the degrees in the  bend to the thickness of the metal when determining the bend  Formula 2: Bend Allowance for a 90° Bend The radius of the bend is generally proportional to the  thickness of the material. Furthermore, the sharper the radius  of bend, the less the material that is needed for the bend. The  type of material is also important. If the material is soft, it  can be bent very sharply; but if it is hard, the radius of bend  is greater, and the bend allowance is greater. The degree  4-64 Flat = MLD – SB MLD = mold line dimension SB = setback Flat dimension = MLD – SB Flat 1 = 1.00-inch – 0.2-inch = 0.8-inch Flat 2 = 2.00-inch – (2  ×  0.2-inch) = 1.6-inch Flat 3 = 1.00-inch – 0.2-inch = 0.8-inch Step 4: Find the Bend Allowance Step 3: Find the Length of the Flat Line Dimension The flat line dimension can be found using the formula:  Bend allowance = (2  ×  3.1416)(0.250 +  1 ⁄ 2(0.051)) 4 = 6.2832(0.250 + 0.0255) 4 = 6.2832(0.2755) 4 = 0.4327 2π (R +  1 ⁄ 2T) 4