Neutral line 4-60 The radius of bend of a sheet of material is the radius of the  bend as measured on the inside of the curved material. The  minimum radius of bend of a sheet of material is the sharpest  curve, or bend, to which the sheet can be bent without  critically weakening the metal at the bend. If the radius of  bend is too small, stresses and strains weaken the metal and  may result in cracking. Flat—that portion of a part that is not included in the bend. It  is equal to the base measurement (MLD) minus the setback. Sight line—also called the bend or brake line, it is the layout  line on the metal being formed that is set even with the nose  of the brake and serves as a guide in bending the work. When forming straight bends, the thickness of the material,  its alloy composition, and its temper condition must be  considered. Generally speaking, the thinner the material is,  the more sharply it can be bent (the smaller the radius of  bend), and the softer the material is, the sharper the bend  is. Other factors that must be considered when making  straight line bends are bend allowance, setback, and brake or  sight line. Setback (SB)—the distance the jaws of a brake must be  setback from the mold line to form a bend. In a 90° bend,  SB = R + T (radius of the bend plus thickness of the metal).  The setback dimension must be determined prior to making  the bend because setback is used in determining the location  of the beginning bend tangent line. When a part has more  than one bend, setback must be subtracted for each bend. The  majority of bends in sheet metal are 90° bends. The K-factor  must be used for all bends that are smaller or larger than 90°. SB = K(R+T) When forming straight angle bends, correct allowances  must be made for setback and bend allowance. If shrinking  or stretching processes are to be used, allowances must be  made so that the part can be turned out with a minimum  amount of forming.  K-Factor—the percentage of the material thickness where  there is no stretching or compressing of the material, such as  the neutral axis. This percentage has been calculated and is  one of 179 numbers on the K chart corresponding to one of  the angles between 0° and 180° to which metal can be bent.  [Figure 4-122]  Whenever metal is to be bent to any angle  other than 90° (K-factor of 90° equal to 1), the corresponding  K-factor number is selected from the chart and is multiplied  by the sum of the radius (R) and the thickness (T) of the metal.  The product is the amount of setback (see next paragraph)  for the bend. If no K chart is available, the K-factor can be  calculated with a calculator by using the following formula:  the K value is the tangent of one-half the bend angle. To prevent any waste of material and to get a greater degree  of accuracy in the finished part, it is wise to make a layout  or flat pattern of a part before forming it. Construction of  interchangeable structural and nonstructural parts is achieved  by forming flat sheet stock to make channel, angle, zee, or  hat section members. Before a sheet metal part is formed,  make a flat pattern to show how much material is required  in the bend areas, at what point the sheet must be inserted  into the forming tool, or where bend lines are located. Bend  lines must be determined to develop a flat pattern for sheet  metal forming. Mold point—the point of intersection of the mold lines. The  mold point would be the outside corner of the part if there  were no radius. Mold line dimension (MLD)—the dimension of a part made  by the intersection of mold lines. It is the dimension the part  would have if its corners had no radius.  Total developed width (TDW)—the width of material  measured around the bends from edge to edge. Finding the  TDW is necessary to determine the size of material to be  cut. The TDW is less than the sum of mold line dimensions  since the metal is bent on a radius and not to a square corner  as mold line dimensions indicate.  Open angle—an angle that is more than 90° when measured  between legs, or less than 90° when the amount of bend is  measured. Closed angle—an angle that is less than 90° when measured  between legs, or more than 90° when the amount of bend is  measured. Flat = MLD – SB Figure 4-121.  Neutral line. Layout or Flat Pattern Development Making Straight Line Bends