![]() ![]() ![]() If the threads are gauging too tight, the H limit number is increased. These are just recommendations so if the threads in the part are gauging too loose, use a smaller H-Limit. The difference in size from one H limit to the next is 0.0005″ increments for taps through 1″ diameter. They are, essentially, the same thing.Īs the following table shows, each H-Limit has a 0.0005” tolerance H-Limits are used for Imperial (or Inch) thread designations and D-Limits are used for Metric threads. H-Limits are used to make small adjustments to the pitch diameter. It should be noted that you need different H-Limits for cutting versus forming threads. Forming threads require a larger H-Limit because they form the threads exactly to the size of the tap, while cutting taps cut a thread that is larger than the tap. For example, on a ¼-20 2B fit, the recommended H-Limit is H5 and on a ¼-20 3B fit, the recommended H-Limit is H3. Each combination of tap size and pitch has a recommended H-Limit for each Class of Fit. For Inch threads, the most common classes of fit are 2B and 3B. This is the critical element to ensure the part gauges correctly and choosing the number ensures that the threads will gage within the required tolerance. Working outside of the “Traditional” H-Limits may be the answer to both of those questions.
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