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At times, threaded holes will “close in” or shrink due to heat shrinkage or perhaps “build-up.” In either case, the threaded hole becomes tighter in the process. The "fix" for this condition is to use a slightly larger tap to compensate for shrinkage. This practice is also common for parts to be plated.
If there is a buildup, the solution is fairly simple. We simply take the amount of the build up, multiply that by four, and increase the tap's H limit size accordingly. For example, if .00025" buildup, then .00025 x 4 = .0010. .001" is equivalent to two H limits, therefore, if you were using a H6 you should use a H8.
If there is shrinkage to heat, the solution is to use a larger tap, but the problem is trying to decide how much larger you should go. There is no formula that will give you an exact number. Generally, it is done by trial and error. Our recommendation is to use your GO gage as a guide. If the GO fits snugly but still enters all the way, one or two H limits should be plenty. If the gage locks up tight from the start, then three, four, or more is probably necessary. (Each H limit is .0005" larger than the previous one.) If it is heat shrinkage, there could be another problem. When parts shrink, they shrink in ALL directions. So a threaded hole gets smaller in diameter, but it also shrinks along the axis of the thread, producing less than the desired pitch thread. This would be like compressing a coil spring, the distance between each coil is now smaller. The effect would be similar to attempting to screw a 1.5mm pitch thread gage into a 1.25mm threaded hole. This kind of error isn't very significant, but it does happen. We have a few customers that buy taps with a long lead, anticipating it will shrink to normal after heating. Generally, the configuration of the part determines the amount of shrinkage. We recommend starting with a tap that is a couple of H limits larger and see how it gages.
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