![]() It allows the input of many different factors in the calculation. I did construct my own tap drill calculator using Microsoft Excel. These are tap drills that are published in a great number of "accepted" 75% tables. ![]() I have seen some commonly suggested "75%" tap drills that actually provide less than 70% or as much as 78% of the full thread form. For some threads, too many in my humble opinion, that difference is all too large and the the actual percentage is quite different from the 75%. For most threads some compromise must be made. The evaluation of an actual tap drill is necessary because the calculated size and an actual, standard, available drill bit are only rarely the same. So I decided to construct my own tap drill calculator so I could both do the calculations myself AND evaluate the true percentage that a given tap drill would actually provide. That would be the only way to actually determine what those numbers were based on. It quickly became apparent that I needed to analyze the various numbers in the published and accepted tap drill tables. What I could not find anywhere was any definitive statement of precisely what numbers were used to calculate the size of the thread that constituted either a 75% or a 100% thread size. That drawing completely neglects tolerances and the different allowances that apply to the different classes of fit. Second, if you look into the standards, it is actually worse than the Wikipedia drawing that is shown in the original post. ![]() First, there are many tap drill tables out there and they do not always agree. I did some research and the deeper that I dug, the more confused I became. I saw different numbers being thrown about in posts like this so I became curious as to just what constituted a 75% thread and by extension, what constituted a 100% thread. This is a subject that has attracted my interest. Looking at the picture, it should be 5/8 H, and using smaller drills would not give you better threads but would make for harder threading and would break more taps.īy the way, we both found websites that supported the estimates we gave. And attributed this to historical reasons. The other fellow suggested that 100% threads are calculated using 3/4 of H, not 5/8. And hence I get 5.16 giving me 77% thread. That is, for 100% threads you want 6-1.082, or a 4.88mm drill. That theoretical thread height H, is sqrt(3)/2 * pitch. If you look at a picture of metric thread form, the height of the thread for an internal thread is about 5/8 of the theoretical thread height. The metric thread form specification is shown on Wikipedia (below). Obviously, 77% thread can't be both 5mm and 5.16mm. They estimated that this gives you 77% thread. Others suggested that the standard drill size for M6 is 5mm. I got a 13/64 (5.159mm) giving you about 77% thread engagment. I suggested number 6, 7, or 8, or a 13/64. On another site, someone wanted the proper drill size for a M6 (6mm x 1mm pitch) tap.
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