I still have a bit of work to do on that last section.Īfter I had partially completed this very complete tap drill calculator, I tried the sizes of the tap drills suggested by the many published tables and these trials convinced me that the published tables are calculated for threads with 1/8H flats on the crests and 1/8H fill at the roots. It shows that bit's diameter and the actual percentage of a full thread that it will produce for your thread. Finally, it has a section where you can enter the actual drill bit that you wish to use. Then it displays the actual diameter of that drill bit in case you can't remember what the diameter of a L size bit is. It shows that suggested drill bit size as a letter, number, fractional, of mm size. It then shows a suggested tap drill bit using the one of the above, two choices that is closest to the calculated size. It shows these nearest drill bit sizes in the same units that were selected as the first choice above (inches or mm). It then uses table lookup functions to find actual, real world drill bits both the nearest size under the calculated amount and the nearest size over the calculated size. It then calculates the Height of a full, sharp Vee thread, the Height of an actual, full thread, the Height of a thread of the desired percentage, a calculated Tap Drill Diameter in inches, a calculated Tap Drill Diameter in mm. It takes into account the:Įach of these factors can be entered separately. It does allow the calculation of tap drills for Vee threads of any cross sectional angle (60, 55, etc.) It does not use just a simple formula like the common one of Major Diameter minus the Pitch. 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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