Tapping Drill sizes

[VicParticipant @vic]

[VicParticipant @vic]

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?

[EmgeeParticipant @emgee]

Vic

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.

**LINK**

Emgee

[Adrian R2Participant @adrianr2]

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.

[Anonymous]

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

Andrew

[DMBParticipant @dmb]

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.

[DMBParticipant @dmb]

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.

John.

[DMBParticipant @dmb]

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 yetParticipant @notdoneityet]

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

[Anonymous]

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.

[Anonymous]

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.

Andrew

[VicParticipant @vic]

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

[John PurdyParticipant @johnpurdy78347]

Vic

I've sent you a PM.

John

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