9/5/2023 0 Comments Forming tap chart![]() ![]() ![]() If the tapped hole is going to be plated after tapping, then you would want to use a higher H limit or even a tap pitch diameter that does not fall into an H limit at all, such that after plating it will fall within the specified class of fit. Another example is, if the material being tapped is very ductal, and closes in after tapping, then you may want to use a higher H limit than that recommended. For example, if your machine is older, and the spindle runs out a little, you may want to consider a lower H limit than that recommended. When tapping conditions are not normal, the 40% may need to be adjusted. The recommended H limit for a 1/4–20 UNC-2B is H5 and for class 3B is H3 as shown in the figure above. You can find our recommended H limit table in the technical section of our Special Taps, Dies and Gages Catalog. In some cases it may be slightly more or less than the 40%. The recommended H limits found in manufacturers’ catalogs and other industry literature, are based on it being at approximately 40% into the tolerance zone for the class of fit. Conversely, if you want tool life, you do not want to use an H limit close to the minimum pitch diameter. Therefore, to avoid tapping oversize, you do not want to choose an H limit near the maximum pitch diameter for the class of fit being tapped. Under normal tapping conditions a tap will cut larger than its measured size. The figure below shows the 0.0005 incremental difference between H and L limits as well as their relationship to the basic pitch diameter. Either designation with or without the G, will work. We should mention that H and L limits are also expressed as GH or GL, with the G meaning that tap has a ground thread. In this second part, we will explain the relationship of tap sizes and class of threads. Welcome to another post on our Tapping Tips! This is the second part of a two-part series on the relationship of inch tap size and class of thread. ![]()
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