the thickness of the metal by using the two adjusting screws  located on the bottom of each frame. The two most common  of these forming machines are the slip roll former and the  rotary former. Available in various sizes and capabilities,  these machines come in manual or powered versions.  The slip roll former in  Figure 4-60  is manually operated and  consists of three rolls, two housings, a base, and a handle.  The handle turns the two front rolls through a system of gears  enclosed in the housing. The front rolls serve as feeding, or  gripping, rolls. The rear roll gives the proper curvature to  the work. When the metal is started into the machine, the  rolls grip the metal and carry it to the rear roll, which curves  it. The desired radius of a bend is obtained by the rear roll.  The bend radius of the part can be checked as the forming  operation progresses by using a circle board or radius gauge.  The gauges can be made by cutting a piece of material to  the required finished radius and comparing it to the radius  being formed by the rolling operation. On some material,  the forming operation must be performed by passing the  material through the rolls several times with progressive  settings on the forming roll. On most machines, the top roll  can be released on one end, permitting the formed sheet to  be removed from the machine without distortion. The front and rear rolls are grooved to permit forming of  objects that have wired edges. The upper roll is equipped with  a release that permits easy removal of the metal after it has  been formed. When using the slip roll former, the lower front  roll must be raised or lowered before inserting the sheet of  metal. If the object has a folded edge, there must be enough  clearance between the rolls to prevent flattening the fold. If  a metal requiring special care (such as aluminum) is being  formed, the rolls must be clean and free of imperfections. The rear roll must be adjusted to give the proper curvature  to the part being formed. There are no gauges that indicate  settings for a specific diameter; therefore, trial and error  settings must be used to obtain the desired curvature. The  metal should be inserted between the rolls from the front of  the machine. Start the metal between the rolls by rotating the  operating handle in a clockwise direction. A starting edge is  formed by holding the operating handle firmly with the right  hand and raising the metal with the left hand. The bend of  the starting edge is determined by the diameter of the part  being formed. If the edge of the part is to be flat or nearly  flat, a starting edge should not be formed. Ensure that fingers and loose clothing are clear of the rolls  before the actual forming operation is started. Rotate the  operating handle until the metal is partially through the rolls  and change the left hand from the front edge of the sheet to the  upper edge of the sheet. Then, roll the remainder of the sheet  The drop hammer forming process produces shapes by the  progressive deformation of sheet metal in matched dies under  the repetitive blows of a gravity-drop hammer or a power- drop hammer. The configurations most commonly formed  by the process include shallow, smoothly contoured double- curvature parts, shallow-beaded parts, and parts with irregular  and comparatively deep recesses. Small quantities of cup- shaped and box-shaped parts, curved sections, and contoured  flanged parts are also formed. Drop hammer forming is not a  precision forming method and cannot provide tolerances as  close as 0.03-inch to 0.06-inch. Nevertheless, the process is  often used for sheet metal parts, such as aircraft components,  that undergo frequent design changes, or for which there is  a short run expectancy. Stretch forming is normally restricted to relatively large  parts with large radii of curvature and shallow depth, such as  contoured skin. Uniform contoured parts produced at a faster  speed give stretch forming an advantage over hand formed  parts. Also, the condition of the material is more uniform  than that obtained by hand forming.  In the process of stretch forming, a sheet of metal is shaped  by stretching it over a formed block to just beyond the elastic  limit where permanent set takes place with a minimum  amount of spring-back. To stretch the metal, the sheet is  rigidly clamped at two opposite edges in fixed vises. Then, the  metal is stretched by moving a ram that carries the form block  against the sheet with the pressure from the ram causing the  material to stretch and wrap to the contour of the form block.  The rotary machine is used on cylindrical and flat sheet  metal to shape the edge or to form a bead along the edge.  [Figure 4-62]  Various shaped rolls can be installed on the  rotary machine to perform these operations. The rotary  machine works best with thinner annealed materials. through the machine. If the desired curvature is not obtained,  return the metal to its starting position by rotating the handle  counterclockwise. Raise or lower the rear roll and roll the  metal through the rolls again. Repeat this procedure until  the desired curvature is obtained, then release the upper roll  and remove the metal. If the part to be formed has a tapered  shape, the rear roll should be set so that the rolls are closer  together on one end than on the opposite end. The amount  of adjustment must be determined by experimentation. If the  job being formed has a wired edge, the distance between the  upper and lower rolls and the distance between the lower front  roll and the rear roll should be slightly greater at the wired  end than at the opposite end.  [Figure 4-61] Rotary Machine Stretch Forming Drop Hammer 4-24