![]() ![]() In nearly all other countries, metric drill bit sizes are most common, and all others are anachronisms or are reserved for dealing with designs from the US. In the U.S., fractional inch and gauge drill bit sizes are in common use. They can be made in any size to order, but standards organizations have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors. Use a number 7 drill! It's 5.Drill bits are the cutting tools of drilling machines. Will report back if there's any clarification. I asked the question: 5/8 or 3/4 on a site for professional machinists I visit. And 5.16 is either 77 or 64% All reasonably good numbers (tho 92%, if true, might cause some taps to break). Not being contrary - the issue could be the difference between really hard tapping and breaking taps (if 3/4 results in too small of a hole) or very weak threaded joints (if 5/8 results in too big a hole). The distance inside the major diameter to the root is 5/8 H. Still, when I look at the drawing, there is a theoretical distance outside the major diameter of 1/8 H for a putative sharp crest. A horrible design - an invitation to fatigue failure. The historical deal might be that older American National Standard threads had sharp roots and crests. The differences you note arise from whether you choose 3/4H or 5/8H as 100%.The 3/4 vs 5/8 difference would explain it. (I think it's actually due to an historical issue in the U.S.). I put links in my post - do you have source info, or a citation for the above?ĭon't ask me why, but 3/4H is usually considered 100% thread percentage rather than 5/8H. I'd like to know 1) the correct sizes (and yours may be), and 2) the source. We're getting information from different sources apparently. Given that most folks target 65 to 70%, and a hand-held drill is gonna make a hole that is a bit oversize I think the OP chose just about the perfect setup.īut where is this discrepancy coming from? 4.7 vs 4.88 for 100%, and 5.16 being either 77% or about 63%?Ĭan you share your source? I'm going to dig out may Machninists Handbook to see if I can find the numbers in there. Anyway, the internal thread radius is 5/8H less than the nominal, or 5*0.866/8 mm, or 0.541mm. The current Unified (flat root) or Metric (allows for rounded root) standards are stronger. This makes it stronger than the early American National Standard which used sharp root and crest. The thread form does not contain the top H/8 of the thread crest, nor the bottom H/4 of the thread root. I suspect that we agree that the theoretical thread height H, is sqrt(3)/2 * pitch. The metric thread form specification is shown on Wikipedia (below). I put links in my post - do you have source info, or a citation for the above? And use moly-dee or some other good lubricant.Ī 5 mm drill is the standard tap drill for M6 x 1 threads:ĥ.2 mm drill => 61% threads.We're getting information from different sources apparently. If you must, read up first on drilling Ti, then on tapping Ti, and do some research and get a really good drills and taps. Even drilling Ti can be problematic - it work hardens something fierce and the drill tends to wander as a result. If you have a notion to try your hand and at tapping titanium, my suggestion would be. The advice above is for tapping into steel. For too small a hole is difficult tapping and possibly a broken tap. The penalty for too big a hole is a weak threaded connection. Use tapping fluid (normal oil doesn't do as good a job). ![]() With a number 6 drill of good quality and steady hand you'd be close, but still be too big for my taste. But you'd probably be fine with a 13/64 as well. For this reason, I'd use a number 8 drill (note that this is a NUMBER drill, and the number 8 is not mm). ![]() I would target a little less than the 5.09-5.16 target. But here's the thing: if you use a hand drill, and/or a poor quality drill bit, your hole diameter is going to be larger than the drill. A bit more accurate estimate using the formulas in the webpage below is between 5.1556 and 5.089mm. That's where you 5.2mm comes in (one formula is drill size = nominal thread OD - Percent thread engagement/76.98). As a nod to practicality, most holes drilled for tapping specify about 65-70% of full thread depth. Just a point of correction: the nominal correct size for an M6 cutting tap is often given as 5mm. Running it into the hole to the point where its not tapered is what you want to do. And for a through (and not a blind) hole, a tapered tap is fine. Generally, 3in1 or WD40 DON'T work as well as tap cutting oil, but this is water over the dam now. You worked faster than I typed! Glad it came out ok. ![]()
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