the type of metal being used determine what type of the  holding device is needed. Clamps and vises hold materials in place when it is not  possible to handle a tool and the workpiece at the same  time. A clamp is a fastening device with movable jaws that  has opposing, often adjustable, sides or parts. An essential  fastening device, it holds objects tightly together to prevent  movement or separation. Clamps can be either temporary  or permanent. Temporary clamps, such as the carriage  clamp (commonly called the C-clamp), are used to position  components while fixing them together. The C-clamp is shaped like a large C and has three main  parts: threaded screw, jaw, and swivel head.  [Figure 4-70] The swivel plate or flat end of the screw prevents the end from  turning directly against the material being clamped. C-clamp  size is measured by the dimension of the largest object the  frame can accommodate with the screw fully extended.  The distance from the center line of the screw to the inside  edge of the frame or the depth of throat is also an important  consideration when using this clamp. C-clamps vary in size  from two inches upward. Since C-clamps can leave marks  on aluminum, protect the aircraft covering with masking tape  at the places where the C-clamp is used. Two of the most commonly used vises are the machinist’s  vise and the utility vise.  [Figure 4-71]  The machinist’s  vise has flat jaws and usually a swivel base, whereas the  utility bench vise has scored, removable jaws and an anvil- faced back jaw. This vise holds heavier material than the  machinist’s vise and also grips pipe or rod firmly. The back  jaw can be used as an anvil if the work being done is light.  To avoid marring metal in the vise jaws, add some type of  padding, such as a ready-made rubber jaw pad. Reusable sheet metal fasteners temporarily hold drilled sheet  metal parts accurately in position for riveting or drilling. If  sheet metal parts are not held tightly together, they separate  while being riveted or drilled. The Cleco (also spelled Cleko)  fastener is the most commonly used sheet metal holder.  [Figure 4-72]  Vises are another clamping device that hold the workpiece  in place and allow work to be done on it with tools such as  saws and drills. The vise consists of two fixed or adjustable  jaws that are opened or closed by a screw or a lever. The size  of a vise is measured by both the jaw width and the capacity  of the vise when the jaws are fully open. Vises also depend  on a screw to apply pressure, but their textured jaws enhance  gripping ability beyond that of a clamp.  Figure 4-71.  A utility vise with swivel base and anvil. Figure 4-72.  Cleco fastener. Figure 4-70.  C-clamps.  Vises Reusable Sheet Metal Fasteners C-Clamps Clamps and Vises  4-29