A tap and die are tools used for cutting threads on blank stock and in holes to accommodate screws and bolts. Tap and die sets typically consist of matching taps and dies in all common screw sizes, along with wrenches to assist the user in holding and turning the tap and die. The tap is a metal rod resembling a drill bit that has a cutting edge machined into the rod for forming the female threads below a surface. A die is a round piece of metal with a hole in the center; the sides of the hole are machined with a cutting edge for forming male threads on a blank screw or rod. Depending on the type of set, threads can be cut into metals, plastics, and wood.
To form the female threads to accept a screw, a shallow recessed pilot-hole is drilled into the surface. When hard materials are tapped, a hole slightly smaller than the diameter of the tap is drilled to the desired depth of the screw. The tap is placed into the holding wrench supplied with the set, and the tip is placed against the hole.
The tap is then turned and the cutting edge bores into the hole to form the threads. For larger holes, or when especially hard material is tapped, smaller taps are sometimes used for the initial hole. Subsequent taps using progressively wider tap diameters are then used until the hole reaches its target size.
To form the male threads on a rod or blank screw, a die with threads matching the tap is used and placed into the holding wrench designed for the die. The rod or blank screw is inserted into the die and the die is turned until the desired number of threads is cut into the exterior of the rod or screw. The threaded piece is now ready to be screwed into the threaded hole made by the tap.
For commercial and industrial use, tap and die sets are used with power tools and work in the same manner as hand-operated sets. Taps are inserted into a drill press or power drill and dies are inserted into a lathe or similar spinning tool to accommodate the rod or screw. Complex multiple tap and die set-ups and specialized machinery is often used for volume work in factories.
A machinist's tapping guide is sometimes used to assist the operator in hand-tapping straight holes into a surface. When working with metal, an oil-based lubricant typically is used on the tap and die. This generally helps to lessen friction and stress on the tool.