Neutral axis Shrinking Stretching 0445T Distance from  inner radius of bend 90° +  1 /2 T Radius T B C Figure 4-126. Neutral axis and stresses resulting from bending.  Figure 4-127. Bend allowance for a 90 °  bend. allowance for a particular application. By experimentation  with actual bends in metals, aircraft engineers have found  that accurate bending results could be obtained by using the  following formula for any degree of bend from 1° to 180°. Bend allowance = (0.01743R + 0.0078T)N where: To use this formula for a 90° bend having a radius of .16- inch for material 0.040-inch thick, substitute in the formula  as follows: The L-bracket shown in  Figure 4-129  is made from 2024-T3  aluminum alloy and the bend is 60° from flat. Note that the  bend angle in the figure indicates 120°, but that is the number  of degrees between the two flanges and not the bend angle  from flat. To find the correct bend angle, use the following  formula:  The actual bend is 60°. To find the correct bend radius for a  60° bend of material 0.040-inches thick, use the following  procedure. If the bend is to be other than 90°, use the lower number in  the block (the bend allowance for 1°) and compute the bend  allowance.  Several bend allowance calculation programs are available  online. Just enter the material thickness, radius, and degree  of bend and the computer program calculates the bend  allowance. Reading across the top of the bend allowance chart, find  the column for a radius of bend of .156-inch. Now, find the  block in this column that is opposite the material thickness  (gauge) of 0.040 in the column at the left. The upper number  in the cell is (0.273), the correct bend allowance in inches  for a 90° bends. Example: The material thickness of the U-channel is 0.040- inch and the bend radius is 0.16-inch. In  Figure 4-128,  the radius of bend is shown on the top line,  and the metal thickness is shown on the left hand column.  The upper number in each cell is the bend allowance for a  90° bend. The lower number in the cell is the bend allowance  per 1° of bend. To determine the bend allowance for a 90°  bend, simply use the top number in the chart. Use of Bend Allowance Chart for a 90° Bend Use of Chart for Other Than a 90° Bend Example: Bend Angle = 180° – Angle between flanges R = the desired bend radius T = the thickness of the metal N = number of degrees of bend  Bend allowance = (0.01743  ×  0.16) + (0.0078  ×  0.040) × 90 = 0.27 inches 1.  Go to the left side of the table and find 0.040-inch. 2.  Go to the right and locate the bend radius of 0.16-inch  (0.156-inch).  3.  Note the bottom number in the block (0.003034).  4.  Multiply this number by the bend angle: 0.003034 × 60 = 0.18204 4-65