The size and condition of a hole before tapping has a direct bearing on the success of the tapping operation and the quality of thread produced. For best results, holes should be smooth and free of burrs. Drills should be kept sharp so that they cut freely. This is especially important in materials that work harden, such as some alloys and stainless steel. Reaming before tapping is desirable in all cases, and is a necessity for quality work, especially in the case of tapered threads.
Often, the size of the hole before tapping is not clearly specified, or in many instances is specified without proper consideration of the factors involved. Misinformation on this matter can result in very high threading costs.
American National and Unified thread specifications provide for a maximum of 83 1/3% thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread. In actual practice, however, the minimum percentage of thread required is generally determined by the design requirements of the thread involved. If the length of thread in the tapped hole is very short, it may be necessary to produce a minimum of 75% or 80% thread to develop the required strength. However, if the length of thread produced is several times the diameter, a 50% or 60% thread will usually provide a greater strength than that of the mating screw. Tapping costs increase very rapidly as the percentage of thread increases and it is poor economy to specify a greater percentage of thread than is actually necessary.To provide for practical tapping, the percentage of thread must be kept to a minimum in the cases of long thread lengths, fine pitches and small diameters.
For example, it is entirely practical to produce a 1/4″-20 UNC thread of 80% in 1/8″ thick sheet metal; however, it would not be practical to produce this percentage of thread in a hole tapped to a 1/2-inch depth. Very fine pitch threads are difficult to tap with a high percentage even in the case of shallow holes.
For the most economical tapping, the percentage of threads produced should always be as low as permissible, which means that the tap drill used should be the largest diameter that the specifications will permit. Minor diameters for various percentages of American National or Unified form of thread can be obtained by using the constants shown below – It must be remembered, however, that the values obtained are minor diameters and not drill sizes.
It is a well known fact that drills generally produce a larger diameter hole than the drill diameter, depending upon the condition of the drill, accuracy of the sharpening, etc., and also the material which is to be drilled.
When tapping some materials, especially with fine pitch threads, a spin-up occurs at the crest of the thread so that the actual minor diameter of the thread after tapping is smaller than the size of the drilled hole. For this reason, the minor diameter of the tapped hole should be checked to make certain that it is within the limits specified. In other words, it may be necessary to experiment with various drill sizes to find the exact size that is satisfactory.