A tap wrench is a hand tool used to make screw threads. These tools typically have a removable bit called a tap, although some heavy-duty models have a fixed end. These taps have threads that make them look much like a bolt. The user uses a tap wrench by drilling a hole into a surface with a secondary tool, then screwing the tap into the hole. There are two main styles of tap wrench; the double-ended wrench looks like a screw driver with a tap on either end, and the T-handle has a cross bar at the top to allow greater torque when it is used.
The tap wrench was a staple of manufacturing before the 20th century, as it was one of the only reliable methods of threading holes. Since automated industrial manufacturing took prominence over manual manufacturing processes, these tools have become much less common. Even still, there are many situations where a tap wrench is the preferred method for making screw holes.
Screws and screw holes are made in a joined process called tap and die. Tapping is the process of drilling and threading a hole, and the die is a mold used to fashion a screw. In some cases, the die-cast screw is used to make the threads instead of the tap; in this case, the process is called threading.
There are two main methods of tapping, called manual and machine. Manual tapping uses a human-powered tap wrench. This process is common on softer items, like those made of wood or plastic. Most tap wrenches have a steel bit, making them impractical for any heavier applications. Manual taps are usually solid metal bits.
In general, woodworking is one of the most common uses for modern tap wrenches. Not only is the material soft enough for the wrench, but many wooden items are still hand-made. Outside of woodworking, tap wrenches are used for small repair work on a number of different items and for various light crafting projects.
Machine tapping uses industrial machines to create the threads. These machines can thread nearly any substance, including metal and stone. In general, these machines use bits that are harder than normal steel, such as a carbide or steel alloy. The taps used by industrial machines are often different than those used by manual processes. Since the machine threads so quickly, it needs to have a method of ejecting the extra material so it doesn’t get caught up in the thread groves.
Taps are built generally just a little larger than the screw used in the hole. This allows a small amount of freedom for the screw. The tolerances are still quite specific though, since a hole that is too small is easy to damage, but a hole that is too large will result in a loose connection.