Step 5: Find the Total Developed Width of the Material The total developed width (TDW) can be calculated when  the dimensions of the flats and the bend allowance are found.  The following formula is used to calculate TDW: Note that the amount of metal needed to make the channel  is less than the dimensions of the outside of the channel  (total of mold line dimensions is 4 inches). This is because  the metal follows the radius of the bend rather than going  from mold line to mold line. It is good practice to check  that the calculated TDW is smaller than the total mold line  dimensions. If the calculated TDW is larger than the mold  line dimensions, the math was incorrect.  For the U-channel example, this gives: After a flat pattern layout of all relevant information is made,  the material can be cut to the correct size, and the bend tangent  lines can be drawn on the material.  [Figure 4-130] The pattern laid out in  Figure 4-130  is complete, except  for a sight line that needs to be drawn to help position  the bend tangent line directly at the point where the bend  should start. Draw a line inside the bend allowance area that  is one bend radius away from the bend tangent line that is  placed under the brake nose bar. Put the metal in the brake  under the clamp and adjust the position of the metal until  the sight line is directly below the edge of the radius bar.  [Figure 4-131]  Now, clamp the brake on the metal and raise  The J-chart, often found in the SRM, can be used to determine  bend deduction or setback and the TDW of a flat pattern  layout when the inside bend radius, bend angle, and material  thickness are known.  [Figure 4-132]  While not as accurate as  the traditional layout method, the J-chart provides sufficient  information for most applications. The J-chart does not  require difficult calculations or memorized formulas because  the required information can be found in the repair drawing  or can be measured with simple measuring tools.  NOTE: A common mistake is to draw the sight line in the  middle of the bend allowance area, instead of one radius  away from the bend tangent line that is placed under the  brake nose bar. the leaf to make the bend. The bend begins exactly on the  bend tangent line.  Figure 4-130.  Flat pattern layout.  Figure 4-131.  Sight line. Using a J-Chart To Calculate Total Developed Width TDW = Flat 1 + Flat 2 + Flat 3 + (2  ×  BA) TDW = 0.8 + 1.6 + 0.8 + (2  ×  0.27) TDW = 3.74-inches TDW = Flats + (bend allowance  ×  number of bends) Step 6: Flat Pattern Lay Out Step 7: Draw the Sight Lines on the Flat Pattern Bend tangent lines Brake Sight line The sight line is located one  radius inside the bend tangent  line that is placed in the brake. Bend tangent lines Sight line looking straight down the nose radius bar Brake nose 4-67 0.80 0.80 1.60 0.27 0.27 Bend allowance Flat 1 Flat 2 Flat 3 Bend allowance