You will have to change by at least two H limits. By changing an H limit by only one increment will not solve an over or under size tapping problem. So, the tap pitch diameter size of a H3 could be the same as H4. This is due to the tolerance used in the manufacture of the tap. In addition, the minimum of each H limit is the same as the maximum of the next lower H limit. But you may find, because you are tapping so many holes, that purchasing a special tap with a higher H limit can increase tap life and will pay for itself, resulting in a lower cost per tapped hole.įrom the figure above, notice that the maximum of each H limit is the same as the minimum of the next higher H limit. Tap manufacturers cannot afford to stock every H limit for every size, so they only carry the recommended H limit for the standard class as well as a few others. 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. They work well in many types of materials due to the shearing action of the spiral grind, and the fact that chips exiting through the bottom of the hole virtually eliminates the issue of backing out over broken chips on reversal.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. Spiral point taps are also very popular because of their versatility. The hole being tapped should be a thru hole, or have plenty of clearance to allow for chip collection. This is especially effective for deep hole tapping. Spiral point taps, also known as "gun taps" because they "shoot" chips forward (clever, huh?), are very effective at clearing chips ahead of the cutting edge of the tap and pushing them out the other end of the hole. Spiral pointed taps, on the other hand, are not even closely related since they produce an effect quite the opposite from spiral flutes. Using this same reasoning, you should rarely - if ever - use spiral flutes of any kind in materials with fine or powdery chips, i.e.
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