fingers not removed for an operation must be securely seated  and firmly tightened before the brake is used. The radius of  the nose on the clamping fingers is usually rather small and  frequently requires nose radius shims to be custom made for  the total length of the bend.  Since most cornice brakes and box and pan brakes are limited  to a maximum forming capacity of approximately 0.090- inch annealed aluminum, 0.063-inch 7075T6, or 0.063-inch  stainless steel, operations that require the forming of thicker  and more complex parts use a press brake.  [Figure 4-59] The press brake is the most common machine tool used to  bend sheet metal and applies force via mechanical and/or  hydraulic components to shape the sheet metal between the  punch and die. Narrow U-channels (especially with long legs)  and hat channel stringers can be formed on the press brake  by using special gooseneck or offset dies. Special urethane  lower dies are useful for forming channels and stringers.  Power press brakes can be set up with back stops (some are  computer controlled) for high volume production. Press brake  operations are usually done manually and require skill and  knowledge of safe use. With the exception of the brake, the slip roll is probably  used more than any other machine in the shop.  [Figure 4-60] This machine is used to form sheets into cylinders or other  straight curved surfaces. It consists of right and left end  frames with three solid rolls mounted in between. Gears,  which are operated by either a hand crank or a power drive,  connect the two gripping rolls. These rolls can be adjusted to  Figure 4-59.  Press brake. Slip Roll Former Press Brake Figure 4-60.  Slip roll former. Operating handle Housing Grooves Upper front roll Lower front roll Base Grooves 4-23