This article refers to the clamp used to hold together two workpieces. For clamps used to secure stage lighting instruments to battens, see C-Clamp (stagecraft). For the Chicago Indie Rock band, see C-Clamp

A C-clamp is a type of clamp device typically used to hold a wood or metal workpiece, and are often used in, but are not limited to, carpentry and welding. These clamps are called "C" clamps because of their C shaped frame, but are otherwise often called G-clamps or G-cramps because including the screw part they are shaped like an uppercase letter G.


C-clamps are typically made of steel or cast iron, though smaller clamps may be made of pot metal. At the top of the "C" is usually a small flat edge. At the bottom is a threaded hole through which a large threaded screw protrudes. One end of this screw contains a flat edge of similar size to the one at the top of the frame, and the other end usually a small metal bar, perpendicular to the screw itself, which is used to gain leverage when tightening the clamp. When the clamp is completely closed, the flat end of the screw is in contact with the flat end on the frame. When the clamp is actually used, it is very rare that this occurs. Generally some other object or objects will be contained between the top and bottom flat edges.


A C-clamp is used by turning the screw through the bottom of the frame until the desired state is reached. In the case that the clamp is being tightened, this is when the objects being secured are satisfactorily secured between the flat end of the screw and the flat end of the frame. If the clamp is being loosened, this is when a sufficient amount of force has relieved to allow the secured objects to be moved.


A C-Clamp can be a useful tool for woodworking but special care should be taken when working with delicate woods. The flat edges of the frame, generally no larger than a quarter (depending on the size of the clamp) can cause indentations and marring of the surfaces being clamped. This can be avoided by using 2 pieces of scrap wood. Each piece of scrap wood is directly in contact with the flat edges of the frame and with the items being clamped. This will allow the scrap wood to receive the damage from the clamping, while dispersing the clamping force across the piece of scrap wood into the clamped objects. Deep throated clamps are also available and provide far greater reach for smaller jobs.


C-clamps are also commonly used in welding. The heavy duty metal construction and the curved shape make it ideal for holding together metal pieces that need to be welded together. A screw, a type of simple machine, allows a tremendous amount of pressure to be applied with a fairly little amount of work.


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