A tap wrench is a hand tool used to operate any small tool that has a square driving head as part of its body. These are usually cutting tools, of which the most common are taps. Other small hand tools, such as hand reamers, might employ a tap wrench.
The image shows three tap wrenches. All have two handles with the cutting tool connection in the center. This helps balance the torquing forces and reduces lateral forces which could snap the hardened cutting tool.
The two left tap wrenches in the image are Bar style which are distinguished by the cutting tool securing method. One of the handles is threaded so tightening the handle exerts pressure on the head of the cutting tool. In the case of the lower bar tap wrench, this moves a jaw to apply clamping pressure.
The third wrench pictured is a '"T"' tap wrench. This model uses a collet design which uses a threaded nosecap to close several collet fingers, which exert clamping pressure on the cutting tool's drive head. This style is useful where restricted clearances or extra reach is required. T tap wrenches vary in length (reach) of the main body and the diameter of the body to accommodate various sizes of cutting tool. The hole which forms the collet fingers may be stepped (have multiple diameters, usually two) to increase the range of cutting tools which may be held by one wrench size.
Numerous sizes of bar tap wrenches are required to cover the available size range of tool heads. Generally speaking, the smallest size which accommodates the tool head is recommended because it reduces the risk of breakage from excessive force.
Taps and reamers are hardened by heat treatment during manufacture. Although this hardness promotes long tool life, it also induces brittleness, so the tool must be used with more care than other hand tools.
Unlike drills, hand taps don't automatically remove the chips they create. A hand tap cannot cut its threads in a single rotation because it creates long chips which quickly jam the tap (an effect known as "crowding"), possibly breaking it. Therefore, in manual thread cutting, normal wrench usage is to cut the threads 1/2 to 2/3 of a turn (180 to 240 degree rotation), then reverse the tap for about 60 degrees until the chips are broken by the back edges of the cutters. It may be necessary to periodically remove the tap from the hole to clear the chips, especially when a "blind" (closed bottom) hole is threaded.
For continuous tapping operations (i.e., power tapping) specialized spiral point or "gun" taps are used to eject the chips and prevent crowding.
Reamers, on the other hand, should not be reversed in use as this will tend to dull the cutting edges. To clear any chips, the whole tool is withdrawn and the chips carefully removed from the hole and between the reamer's flutes. Withdrawals must be often enough to prevent packing of the chips. The likelihood of breakage is reduced considerably if there is space for the chips to move out of the way. Reamers may be manufactured with either straight or spiral flutes to assist the cutting action as well as the movement of chips. A better finish and more accurate hole diameter is achieved in hard materials with spiral flute reamers.
Although a tap wrench and its tools are operated at low speed, the use of safety glasses is highly recommended. Tool breakage can cause metal shards to be released at high speed. Also, when a tap is used to thread a blind hole, compressed air may be required to clear chips from the workpiece, and this absolutely requires the use of protective equipment.
The use of a suitable lubricant is essential with most tapping and reaming operations. Recommended lubricants for some common materials are as follows: Carbon Steel : Petroleum-based or synthetic cutting oil. Alloy Steel : Petroleum-based cutting oil mixed with a small amount (approximately 10 percent) of kerosene or mineral spirits. This mixture is also suitable for use with stainless steel. Cast Iron : No lubricant. An air blast should be used to clear chips. Aluminum : Kerosene or mineral spirits mixed with a small amount (15-25 percent) of petroleum-based cutting oil. In an emergency, WD-40 may be substituted. Brass : Kerosene or mineral spirits. Bronze : Kerosene or mineral spirits mixed with a small amount (10-15 percent) of petroleum-based cutting oil.
In power tapping and reaming operations, the tool and workpiece should be continuously flooded with lubricant.