designed specifically for the shape of the area being  formed. These blocks are made in pairs similar to those  used for straight angle bends and are identified in the same  manner. They differ in that they are made specifically for  the particular part to be formed, they fit each other exactly,  and they conform to the actual dimensions and contour of  the finished article. The forming blocks may be equipped with small aligning  pins to help line up the blocks and to hold the metal in place  or they may be held together by C-clamps or a vise. They also may be held together with bolts by drilling through form  blocks and the metal, provided the holes do not affect the  strength of the finished part. The edges of the forming block  are rounded to give the correct radius of bend to the part, and  are undercut approximately 5° to allow for spring-back of the  metal. This undercut is especially important if the material  is hard or if the bend must be accurate. In  Figure 4-158 , the nose rib is formed by crimping, beading,  putting in relief holes, and using a formed angle riveted on  Holes Crimps Figure 4-158.  Nose rib using a combination of forms. Figure 4-157.  Nose rib with crimps. Figure 4-156.  Nose rib with relief holes. The nose rib offers a good example of forming a curved  flange because it incorporates both stretching and shrinking  (by crimping). They usually have a concave flange, the  inside edge, and a convex flange, the outside edge. Note  the various types of forming represented in the following  figures. In the plain nose rib, only one large convex flange  is used.  [Figure 4-155]  Because of the great distance around  the part and the likelihood of buckles in forming, it is rather  difficult to form. The flange and the beaded (raised ridge  on sheet metal used to stiffen the piece) portion of this rib  provide sufficient strength to make this a good type to use.  In  Figure 4-156 , the concave flange is difficult to form, but  the outside flange is broken up into smaller sections by relief  holes. In  Figure 4-157 , note that crimps are placed at equally  spaced intervals to absorb material and cause curving, while  also giving strength to the part. each end. The beads and the formed angles supply strength to  the part. The basic steps in forming a curved flange follow:  [Figures 4-159  and  160] Figure 4-155.  Plain nose rib. Flange 4-78