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6mm tap, US/standard drill bit size?

Old 05-22-20, 09:36 PM
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loubikes
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6mm tap, US/standard drill bit size?

I have tap for a 6mm hole but don't have the correct, metric drill bit (5.2mm).

13/64in = 5.159mm

​​​​​ Can I use a 13/64in bit? The answer seems like a yes to me but I would love an answer from someone with experience. First time using a tap.

Thanks!

*I am tapping a brazeon on my seat stay that stripped. Previously 5mm.
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Old 05-23-20, 05:25 AM
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Close enough especially given the slop of using a hand drill.
Use cutting oil, tap straight, back in and out often
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Old 05-23-20, 05:56 AM
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According to this chart, assuming M6x1 you would be better off using a #8 or #4 drill. For M6x0.75, #4 or 7/32. I always err on oversized, tapping into an undersized hole greatly increases the risk of breaking the tap. https://littlemachineshop.com/images...DrillSizes.pdf
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Old 05-23-20, 07:16 AM
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The 13/64 should work fine. I found a set of metric combination drill/ tap bits at harbor freight that work great for through hole tapping. I put them in my makita impact driver.
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Old 05-23-20, 07:17 AM
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Try a practice piece first.
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Old 05-23-20, 07:37 AM
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The M6 tap is a 6mm x 1mm per thread. Not .75mm per thread. MY understanding is that the "M" references a single "standard" thread pitch, in this case 1mm per thread. Andy

BTW this type of info is readily available by a quick Google search for tap drill size. (Just like a lot of other info) Andy
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Old 05-23-20, 07:38 AM
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.041mm is .00163".
I doubt you could drill 2 consecutive holes with the same drill bit and get that close on both.
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Old 05-23-20, 08:17 AM
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Originally Posted by Andrew R Stewart View Post
The M6 tap is a 6mm x 1mm per thread. Not .75mm per thread. MY understanding is that the "M" references a single "standard" thread pitch, in this case 1mm per thread. Andy

BTW this type of info is readily available by a quick Google search for tap drill size. (Just like a lot of other info) Andy
M6x1 is coarse thread, M6x0.75 is fine; both are available, but coarse thread is more common. M5 and some of the larger sizes have 3 pitches to choose from, as do some of the larger SAE sizes. So best not to assume.
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Old 05-27-20, 08:37 PM
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Thanks to all for help. This worked great, rear rack is back on.

For anyone else needing to do this here's what I did.
  • M6 x 1 (coarse) tap
  • 13/64 bit - made a couple passes after going through.
  • used 3-in-one oil to lubricate tap but WD-40 would probably be fine and i saw that suggested in my research.
  • I went really slowly and backed out every 1/4 turn or so and cleaned tap and re-oiled. Overkill I'm sure but not hard to do.
My only concern was that I hadn't realized that some taps are 'tapered' and some are 'bottom'. It seems my tap was slightly tapered but reached the correct width (6mm) within about 1/4" of tip. Had the taper been any longer (not sure if this is standard), it would have been a problem because it would have maxed-out against the inside of the seat stay prior to reaching full width.

Thanks again!
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Old 05-28-20, 08:13 AM
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IIRC, taps came in starter, full & plug.
Plug for finish threading blind holes.
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Old 05-28-20, 12:18 PM
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You worked faster than I typed! Glad it came out ok. Glad the 13/64 dill worked. Generally, 3in1 or WD40 DON'T work as well as tap cutting oil, but this is water over the dam now. And for a through (and not a blind) hole, a tapered tap is fine. Running it into the hole to the point where its not tapered is what you want to do.

Typed before I read what you'd done:

Just a point of correction: the nominal correct size for an M6 cutting tap is often given as 5mm. That's for 100% thread depth. As a nod to practicality, most holes drilled for tapping specify about 65-70% of full thread depth. That's where you 5.2mm comes in (one formula is drill size = nominal thread OD - Percent thread engagement/76.98). A bit more accurate estimate using the formulas in the webpage below is between 5.1556 and 5.089mm. 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. I would target a little less than the 5.09-5.16 target. For this reason, I'd use a number 8 drill (note that this is a NUMBER drill, and the number 8 is not mm). Or a number 7 drill if I had it. But you'd probably be fine with a 13/64 as well. With a number 6 drill of good quality and steady hand you'd be close, but still be too big for my taste. It might work. Use tapping fluid (normal oil doesn't do as good a job).

The penalty for too big a hole is a weak threaded connection. For too small a hole is difficult tapping and possibly a broken tap.

The advice above is for tapping into steel. If you have a notion to try your hand and at tapping titanium, my suggestion would be ... don't. Even drilling Ti can be problematic - it work hardens something fierce and the drill tends to wander as a result. Get someone who's done this. 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. Emuge, Prototyp... etc. And use moly-dee or some other good lubricant.

No. 8, 5.05mm, 0.1990 inch
No. 7, 5.11mm, 0.2010 inch
Fractional 13/64, 5.16mm, 0 .2031 inch
No. 6, 5.18mm, 0.2040 inch

For tap drill sizes:
https://www.natool.com/wp-content/up...atp113-116.pdf
For available drill sizes:
https://www.imperialsupplies.com/pdf...DrillChart.pdf

Last edited by WizardOfBoz; 05-28-20 at 12:34 PM.
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Old 05-28-20, 12:18 PM
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Originally Posted by loubikes View Post
My only concern was that I hadn't realized that some taps are 'tapered' and some are 'bottom'. It seems my tap was slightly tapered but reached the correct width (6mm) within about 1/4" of tip. Had the taper been any longer (not sure if this is standard), it would have been a problem because it would have maxed-out against the inside of the seat stay prior to reaching full width.

You can turn a tapered tap into a bottoming tap by grinding the tip as needed.
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Old 05-28-20, 11:49 PM
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Originally Posted by WizardOfBoz View Post
Just a point of correction: the nominal correct size for an M6 cutting tap is often given as 5mm. That's for 100% thread depth. As a nod to practicality, most holes drilled for tapping specify about 65-70% of full thread depth. That's where you 5.2mm comes in ...
A 5 mm drill is the standard tap drill for M6 x 1 threads:
4.7 mm drill => 100% threads
5.0 mm drill => 77% threads
5.2 mm drill => 61% threads.
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Old 05-29-20, 07:31 AM
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Originally Posted by tomato coupe View Post
A 5 mm drill is the standard tap drill for M6 x 1 threads:
4.7 mm drill => 100% threads
5.0 mm drill => 77% threads
5.2 mm drill => 61% threads.
We're getting information from different sources apparently. I'd like to know 1) the correct sizes (and yours may be), and 2) the source. I put links in my post - do you have source info, or a citation for the above?
The metric thread form specification is shown on Wikipedia (below). I suspect that we agree that the theoretical thread height H, is sqrt(3)/2 * pitch. Pitch being 1mm, then, H equals 0.866mm. The thread form does not contain the top H/8 of the thread crest, nor the bottom H/4 of the thread root. This makes it stronger than the early American National Standard which used sharp root and crest. The current Unified (flat root) or Metric (allows for rounded root) standards are stronger. Anyway, the internal thread radius is 5/8H less than the nominal, or 5*0.866/8 mm, or 0.541mm. That's 1.082mm less in diameter. So 4.88mm should be 100%. The 13/64 drill bit is 5.159mm. I get this as being 77.6%. 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.
But where is this discrepancy coming from? 4.7 vs 4.88 for 100%, and 5.16 being either 77% or about 63%?
Can you share your source? I'm going to dig out may Machninists Handbook to see if I can find the numbers in there.


Last edited by WizardOfBoz; 05-29-20 at 08:03 AM.
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Old 05-29-20, 10:35 AM
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Don't ask me why, but 3/4H is usually considered 100% thread percentage rather than 5/8H. (I think it's actually due to an historical issue in the U.S.). The differences you note arise from whether you choose 3/4H or 5/8H as 100%.

Using the 3/4H convention, the simple rule of thumb for 77% threads is: (Drill size = Thread diameter - Thread pitch). That's the basis for the vast majority of thread charts, with sizes rounded to the nearest common drill size.

For other thread percentages, there are many online calculators:
https://www.guhring.com/tech/TapDrill/
https://www.kennametal.com/us/en/res...rill-size.html




Originally Posted by WizardOfBoz View Post
We're getting information from different sources apparently. I'd like to know 1) the correct sizes (and yours may be), and 2) the source. I put links in my post - do you have source info, or a citation for the above?
The metric thread form specification is shown on Wikipedia (below). I suspect that we agree that the theoretical thread height H, is sqrt(3)/2 * pitch. Pitch being 1mm, then, H equals 0.866mm. The thread form does not contain the top H/8 of the thread crest, nor the bottom H/4 of the thread root. This makes it stronger than the early American National Standard which used sharp root and crest. The current Unified (flat root) or Metric (allows for rounded root) standards are stronger. Anyway, the internal thread radius is 5/8H less than the nominal, or 5*0.866/8 mm, or 0.541mm. That's 1.082mm less in diameter. So 4.88mm should be 100%. The 13/64 drill bit is 5.159mm. I get this as being 77.6%. 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.
But where is this discrepancy coming from? 4.7 vs 4.88 for 100%, and 5.16 being either 77% or about 63%?
Can you share your source? I'm going to dig out may Machninists Handbook to see if I can find the numbers in there.

Last edited by tomato coupe; 05-29-20 at 10:39 AM.
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Old 05-29-20, 12:09 PM
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Originally Posted by tomato coupe View Post
Don't ask me why, but 3/4H is usually considered 100% thread percentage rather than 5/8H. (I think it's actually due to an historical issue in the U.S.). 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. The historical deal might be that older American National Standard threads had sharp roots and crests. A horrible design - an invitation to fatigue failure. But that was the spec. 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. The distance inside the major diameter to the root is 5/8 H. 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).

I guess the key point is that 0% is 6mm. And 5mm is either 92 or 77%. And 5.16 is either 77 or 64% All reasonably good numbers (tho 92%, if true, might cause some taps to break).

I asked the question: 5/8 or 3/4 on a site for professional machinists I visit. Will report back if there's any clarification. In the mean-time... Use a number 7 drill! It's 5.105mm!
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Old 05-29-20, 12:39 PM
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Originally Posted by WizardOfBoz View Post
I asked the question: 5/8 or 3/4 on a site for professional machinists I visit. Will report back if there's any clarification. In the mean-time... Use a number 7 drill! It's 5.105mm!
The topic has been discussed numerous times on Practical Machinist. Search the website ...

The two charts you referenced in a previous post list 5.00 mm and #10 (4.91 mm) as the drill size for M6 x 1 threads.

Last edited by tomato coupe; 05-29-20 at 12:51 PM.
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Old 05-29-20, 12:53 PM
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Just got a note from an experienced machinist. He's in England, so presumably knows metric specs. He says that while 5mm is the standard, "However 5mm = 0.1969'' ……….on tough / hard / horrible material, where I can, I use 13/64 = 0.2031'' the extra 0.0062'' making life easier."
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Old 05-29-20, 12:56 PM
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Originally Posted by tomato coupe View Post
The topic has been discussed numerous times on Practical Machinist. Search the website ...

The two charts you referenced in a previous post list 5.00 mm and #10 (4.91 mm) as the drill size for M6 x 1 threads.
Hmmm. Don't know how I morked that up. But the 4.91mm corresponds to my 4.89mm for 100%. Having some talent for breaking taps, I think I'm going with the larger holes!
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Old 05-29-20, 01:24 PM
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Also noted that you have questions on PM regarding tapping M6x0.5, and tapping Invar... (Assuming that you use the same Tomato Coupe name). Above my experience.

But, as I said, 13/64 gives you a pretty good (77% or 65%) thread engagement either way. And the OP (who has long ago gone out for a bike ride) got his rack mounted.
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Old 05-30-20, 11:26 AM
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Old trick my Dad a machinist , (& WW2 Vet), showed me,, hold the tap up and the twist drill ,

right size you see the valleys of the teeth in the tap behind the drill you are holding in front of it..
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Old 05-30-20, 11:49 AM
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Originally Posted by fietsbob View Post
Old trick my Dad a machinist , (& WW2 Vet), showed me,, hold the tap up and the twist drill ,

right size you see the valleys of the teeth in the tap behind the drill you are holding in front of it.
Are you sure you heard him correctly?
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Old 05-30-20, 11:54 AM
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You an expert you opine away..
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