How to Select Taps

Image Credit: Grainger | RS Components

Taps are used to form or cut female threads on the inside of holes. A tap is a screw-like tool which has threads and three or four flutes cut across the thread.  The edges of the thread, formed by the flutes, are the cutting edges. The lower part of the tap is somewhat tapered so that it can dig into the walls of the drill hole. The upper part of a tap consists of a shank ending in a square for folding the tap by a tap wrench. A tap wrench is a two-handled wrench that is either fixed or adjustable. Adjustable wrenches may be used with various size taps.

Cutting inside threads or internal thread forming is called tapping. After a hole has been drilled with a tap drill, it is ready for tapping. A tap and die set is used to cut threads on a round bar of metal, such as the threads on a bolt. An acme thread form can be produced only by taking precautions while tapping inside the threads. Taps are extremely hard and slender, so they can break easily if unnecessary force is applied. Taps are designed and manufactured to meet most industry specifications.

Types of Taps

There are three types of hand taps:

• Rougher- At the end of a rougher tap, the thread starts gradually when the tap is turned into the hole.

• Plug tap (intermediate)- An intermediate tap is tapered back from the edges. It has about three or four threads and is used after the taper tap has been used to cut the thread as far as possible.

• Bottoming tap (finisher)- The finisher tap has threads for its entire length and is used to finish the work prepared by the other two taps.

Tap Specifications

Taps consist of various specifications.

Material

Taps are made from carbon steel or high speed steel and are hardened and tempered.

Diameter and Depth

Tap threads have various diameters and depths. A tap drill size may be derived from the following formula: D = T – 2d, where D is the diameter of the tap drill size, T is diameter of the tap or bolt to be used, and d is the depth of thread. 

Application

Taps are necessary for repairing threads in stripped holes or bolts. All modern equipment has nuts and bolts, and cannot be made without taps.