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Tapping Drill sizes

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Vic03/08/2021 15:36:58
2895 forum posts
8 photos

I’ve got various charts out in the shed but normally just Google it if I’m indoors. The problem is which source to trust? I have a project coming up in which I’m going to be using fasteners I don’t normally use, M4.5 coarse. I did a quick Google and the size quoted was 3.75mm. Looking in the shed I found I had a couple of 3.7mm drill bits. I did another Google and found both 3.7mm and 3.8mm quoted as being the correct size. Is there a definitive source of information for tapping drills etc? I seem to remember there was a chart that had several sizes for each thread and quoted a percentage thread engagement? Anyone have a link?

Emgee03/08/2021 16:01:53
2147 forum posts
265 photos


Using the sum of deducting the pitch from diameter (4.5-.75) gives the 3.75mm but the chart I use for thread data shows 3.8mm diameter tapping drill.



Adrian R203/08/2021 16:12:46
114 forum posts
5 photos

I would try what you have in a similar piece of scrap, see what happens. 3.7mm seems on paper too small but it might drill oversize and be just right. Obviously don't over stress the tap if it doesn't want to go, but assuming a non-safety related attachment then there is a fair range of tolerance.

Andrew Johnston03/08/2021 16:17:35
6235 forum posts
676 photos

Depending on material I'd use 3.9 or 4mm, with 75% or 60% engagement respectively.


DMB03/08/2021 16:50:45
1154 forum posts
1 photos

To; Andrew, without checking your figures but accepting, I agree and for most purposes 60% or so is adequate.

OP, vic, always go for % engagement. Your query relates to a small diameter tap which therefore lacks torsional strength. Dont push your luck with say 90/95% or you could snap it, especially a carbon one and small diameter as well.

A good example of what I think are often dodgy charts is where brass thread taps are shown as needing a 64th inch diameter drill less than the nominal tap size.When you use larger diameter taps, it becomes very obvious how much torque is needed to turn the wrench. Electric motor driven threaders are used on large-ish diameter pipes, e.g.

DMB03/08/2021 17:11:23
1154 forum posts
1 photos

vic, try to only use charts that give % engagement of threads. Beware of internet ones, try to use known to be reputable sources. Some model engineering suppliers use charts giving tight engagements. Cynically, if you bust it as a result, you will need to buy another - good for trade! Even my trusty ref Book "Screw threads and twist drills", Ian Bradley, 4th ed., gives 3.7mm. Appendix says metric drills are recommended in charts at 75%.Hmm.....Ithink that I would do the maths to check it. Andrew says above, depending on material with which I agree but also what it is, e.g., a casting or perhaps even a block of say brass upon which a lot of work has already been done, you would want to take more care than if it was a length of say steel for an unimportant job.

Basically, 60 -70 or even 75% try on scrap identical material if possible.


DMB03/08/2021 17:14:40
1154 forum posts
1 photos

If it don't want to go, don't force it. I have tried a drill and found the HSS tap starting to twist so I took tap out and consulted my drill sizes charts to use the very next larger and it went OK.

not done it yet03/08/2021 17:41:09
6279 forum posts
20 photos

Just use google. Google images shows dozens, if not hundreds, of such charts. Select a proper engineering chart, not a quick ebay seller suggestion.

Peter Greene03/08/2021 18:35:41
272 forum posts
2 photos

My "bible" for all this is Tubal Cain' s "Drills Taps and Dies" (Workshop Practice Series No. 12). All the drill sizes for various engagements for every thread system you are likely to need. Suggest copying and laminating, for workshop use, the charts you need.

Worth a read from cover-to-cover.

Andrew Johnston03/08/2021 18:47:35
6235 forum posts
676 photos
Posted by Peter Greene on 03/08/2021 18:35:41:

My "bible" for all this is Tubal Cain's "Drills Taps and Dies" (Workshop Practice Series No. 12)

Mine too.


Vic03/08/2021 19:00:53
2895 forum posts
8 photos

Thanks for the feedback folks. I can do a test run on some scrap so that’s probably the way to go.

John Purdy04/08/2021 01:30:49
270 forum posts
169 photos


I've sent you a PM.


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