A cap screw is a kind of fastener used for making mechanical connections between mating objects, to be sure that they are held together securely. These screws are often used to fasten machine parts & lots of other types of objects, such as those inside home appliances or consumer electronic devices. It is important to pick the right size & type of cap screw for each individual application.
A cap screw is directly tightened in to a threaded or tapped hole, & is normally used without a nut. It's a giant head on finish & a cylindrical shaft with an outside thread - a spiral structure that allows the screw to be advanced when rotated. The tapped hole has an 
internal thread that matches the outside thread of the screw. When this screw is inserted & rotated in to a tapped hole, it advances. The screw is tightened & loosened by applying torque to the head of the screw using a tool.
A variety of Hex socket head cap screws are obtainable to permit different tools to be used for tightening or loosening the screw. The head has a bigger diameter than the threaded portion to provide a positive mechanical cease when tightening the screw, & to permit the head to be formed to accept a specific type of tool. Examples of often used cap screw heads are the hex head, which has a hexagonal shape for use with a spanner wrench; the socket head, which has recessed hexagonal hole for use with an Allen wrench; & the button head, which has a lower profile, dome-shaped head & recessed hexagonal hole for use in counterbored holes.
A hex cap screw can generate a high amount of clamping force when tightened. The mating surfaces between the screw & nut resist the force being applied by the screw, & help to prevent the mechanical connection from loosening over time. The type & size of cap screw chosen for a specific application depends primarily on the forces necessary to adequately secure the mechanical connection
