a vise. The wooden forming blocks should have one edge  rounded as needed for the desired radius of bend. It should  also be curved slightly beyond 90° to allow for spring-back.  round, soft-faced mallet, strike the flange directly over the  V portion with light, even blows while gradually forcing it  downward into the V.  Bend the metal that protrudes beyond the bending block to  the desired angle by tapping lightly with a rubber, plastic,  or rawhide mallet. Start tapping at one end and work back  and forth along the edge to make a gradual and even bend.  Continue this process until the protruding metal is bent to the  desired angle against the forming block. Allow for spring- back by driving the material slightly farther than the actual  bend. If a large amount of metal extends beyond the forming  blocks, maintain hand pressure against the protruding sheet  to prevent it from bouncing. Remove any irregularities by  holding a straight block of hardwood edgewise against the  bend and striking it with heavy blows of a mallet or hammer.  If the amount of metal protruding beyond the bending blocks  is small, make the entire bend by using the hardwood block  and hammer. Begin at one end of the flange and form the curve gradually  and evenly by moving the strip slowly back and forth,  distributing the hammer blows at equal spaces on the  flange. Hold the strip firmly to keep it from bouncing when  hammered. An overly heavy blow buckles the metal, so keep  moving the flange across the V-block, but always lightly strike  the spot directly above the V.  Lay out a full-sized, accurate pattern on a sheet of paper or  plywood and periodically check the accuracy of the curve.  Comparing the angle with the pattern determines exactly  how the curve is progressing and just where it needs to be  increased or decreased. It is better to get the curve to conform  roughly to the desired shape before attempting to finish any  one portion, because the finishing or smoothing of the angle  may cause some other portion of the angle to change shape.  If any part of the angle strip is curved too much, reduce the  curve by reversing the angle strip on the V-block, placing the  bottom flange up, and striking it with light blows of the mallet. Both formed and extruded types of angles can be curved (not  bent sharply) by stretching or shrinking either of the flanges.  Curving by stretching one flange is usually preferred since  the process requires only a V-block and a mallet and is easily  accomplished. In the stretching method, place the flange to be stretched in  the groove of the V-block.  [Figure 4-149]  (If the flange is  to be shrunk, place the flange across the V-block.) Using a  Try to form the curve with a minimum amount of hammering,  for excessive hammering work hardens the metal. Work- hardening can be recognized by a lack of bending response or  by springiness in the metal. It can be recognized very readily  by an experienced worker. In some cases, the part may have  to be annealed during the curving operation. If so, be sure  to heat treat the part again before installing it on the aircraft. Curving an extruded or formed angle strip by shrinking may  be accomplished by either the previously discussed V-block  method or the shrinking block method. While the V-block  is more satisfactory because it is faster, easier, and affects  the metal less, good results can be obtained by the shrinking  block method. In the V-block method, place one flange of the angle strip  flat on the V-block with the other flange extending upward.  Using the process outlined in the stretching paragraphs, begin  at one end of the angle strip and work back and forth making  light blows. Strike the edge of the flange at a slight angle to  keep the vertical flange from bending outward.  Occasionally, check the curve for accuracy with the pattern.  If a sharp curve is made, the angle (cross-section of the  formed angle) closes slightly. To avoid such closing of the  angle, clamp the angle strip to a hardwood board with the  hammered flange facing upward using small C-clamps. The  Shrinking With V-Block and Shrinking Block Methods Formed or Extruded Angles Stretching with V-Block Method 4-75 Figure 4-149.  V-block forming. 