![]() ![]() It is simply formed by the movement of material inward from the diameter of the drilled hole. The minor diameter apparently NEVER reaches the point where the tap is actually forming it. As the form tap gets larger, it displaces more and more material, but again, the only place the material can go is inward, so the minor gets smaller and smaller.Īs such, one fix for borderline too-small minor is to choose a lower D-value form tap. Consider the limit where the form tap OD is the same size as the drilled hole - no displaced material, so the minor (not really a minor since there is no thread. For a given drilled hole size, a larger form tap will make a smaller minor since it displaces more material, and the only place for the material to go is inward. **Ī form tap, OTOH, displaces material from the outside in. ** Exception - if you attempt to cut tap with a significantly undersized drilled hole, at some point the tap will attempt to open up the minor. You need a smaller starting hole.įirst of all, a cut tap does nothing to the minor - whatever the hole size before tapping that's the hole size after tapping (modulo maybe a little bit of burr). You can go too far- too much over on the tap, you can get a perfect minor but the fail the gage because the PD is over. On all form tapped holes, bringing in the minor to nominal-I make the big adjustments with the drill size, and fine tune with the D value (or H if it's an inch thread) of the tap. A too-far oversize tap won't make an oversize minor, it will just jam and snap off in the hole. The minor of the tap is under the low end of the tolerance for the thread, and it doesn't see metal unless you jam it into an undersize hole. At some point the gage no longer goes, because it's gaging the pitch diameter. As the tap wears, the major and pitch diameters decrease, and the minor diameter increases. The tap will wear on the OD and flanks, and that means you will displace less material, and the size of the hole will stay closer to the original drill size. Tapping it larger means you are displacing more volume, which shrinks the minor. The minor is already oversize before the tap goes in the hole. The loosest fit screw must fit in the tightest fit hole and vice-versa.Ĭlick to expand.I do a lot of form tapped holes, and I've spent a lot of time tweaking them in where I want them. In theory, all taps are larger than any screws of the same thread size. None of them will get any larger.Ī tap, cutting or forming, is made oversized so that there is some clearance for the nominal sized screw, that is one that meets any of the tolerance specs for any class of fit, to fit in the hole. In fact, with wear, all three diameters, OD, pitch, and minor will get smaller. Now if the minor diameter of the tap gets smaller, then the corresponding minor diameter of the threaded hole ALSO GETS SMALLER with wear. It is still smaller than the pitch diameter, just like on an external thread. ![]() Now the minor diameter of a threaded hole is still the smallest diameter present. However that does not necessarily mean that they will all decrease at the same rate. The tap will get SMALLER with wear and that applies to all the dimensions of the tap: OD, pitch diameter, minor diameter, whatever, they all get smaller. We are talking about a tap and the only thing that wear can do to a tap is make it smaller. But fortunately, I stopped and thought about it. ![]() I almost fell into that trap when I posted. ![]()
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