![]() ![]() That’s because the 1/4-20 UNC-2B No-Go limit is 0.2224" at the maximum the high side of a GH-11 pitch diameter for this tap size is 0.2230". Knowing the constant heat-treatment distortion, we are virtually guaranteed not to have an oversize condition. ![]() The larger tap size will increase our chances of having a gageable part after heat treating. This change in thread size will be the same, even if we use a +0.005" oversize tap. The threaded part is distorted by 0.002" to 0.003" after heat treating. A +0.005" (GH-11) tap is recommended in this case. Generally, a part requires more of a significant change than +0.001" before heat treating to fall within the class of fit after heat treating. In this situation, a typical response is to request a tap with a GH-7 limit, which is 0.001" larger than the GH-5 limit.īut that might not do the trick. And a tap is 0.002" over basic pitch diameter at the minimum GH-5 limit. Since the zero point is basic pitch diameter, a tap with a GH-5 limit’s maximum diameter is 0.0025" over basic. increment between successive H-limits, and for taps 1" in diameter or smaller the H-limit is held to a 0.0005"-dia. However, because of the heat treatment, the GH-5 tap will produce an undersize hole. As a result, H-limits for larger taps overlap. The proper tap size can be determined by consulting the Metal Cutting Tool Institute (MCTI) or any tap manufacturer’s catalog.įigure 1: Note that H-limits for taps measuring between 1" and 1 1/2" in diameter (right) have twice the tolerance of H-limits for taps smaller than 1" in diameter (left). The G indicates that the tap is precision ground, and the H-5 limit puts the tap size in the middle range of the Class 2B tolerance level. Normally, we would use a GH-5 tap limit for this class of fit. Situation 1: Tapping a part with a small-diameter internal thread requiring heat treatment.Ĭonsider a threaded hole that has to gage to a 1/4-20 UNC-2B class of fit after heat treating. Given all these variables, how do you select the proper tap size? The following examples will help solve some commonly encountered problems. Therefore, each lot should be evaluated for machinability and the effects of other processes to which the material will be subjected. Some material will react differently from lot to lot because of a difference in chemical composition. Adding to this unpredictability is the problem of variation between material lots received. When the job is first set up, the tap size is an important factor and should be established through testing and trial runs. Unfortunately, the effects of material movement and heat treating are difficult to predict. The part is distorted and/or scale or other residue accumulates in the threaded hole, again causing an undersize condition.
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