Trapezoidal tap drill size ?

[jim1956Participant @jim1956]

I’ve bought a T8 x 2 Trapezoidal tap and was wondering about the Tapping drill size.

Online search is not helpful, one source told me 6.9mm which doesn’t seem right since an M8 x 1.25 has a 6.8 mm tap drill and a bigger pitch should mean a smaller minor diameter.

Another source which looked to be AI generated assured me that the correct tap drill size for a T8 trapezoidal thread is 3.3mm (screen capture attached in case you don’t believe me)

I found a formula online for imperial ACME tap drills.

Tap drill size for acme taps
Based on a 100% of thread as per ANSI to calculate tap drill size for acme taps.

step 1

1 Divided by TPI ( threads per inch)

step 2

subtract this amount from the OD. for pitches 10 and greater always add .005″ for the drill size for pitches less than 10 TPI take the amount from step 1 and multiply x.05 ( 5 %) and add this to the drill size. This is for 95% to 100% threads.

For my 8mm x 2 Trapezoidal in imperial would be .315 x 12.7tpi

1/12.7=.079

.315 – .079 = 0.236 +.005 = 0.241” = 6.12mm tap drill size

Does this seem sensible?

[mark smith 20Participant @marksmith20]

using the guhring tapping calculator.

6.051mm for a 75% thread

5.922mm for 80%

5.792 mm for 85%

5.662mm for 90 %

etc…. Not sure if any special considerations for trapezoidal threads.

https://guhring.com/Tech/tapdrill

[Clive FosterParticipant @clivefoster55965]

jim1956

If you have a CAD program best way to sort this sort of thing out is to do an accurate drawing with the sizes on it. Hopefully the CAD program will have basic trapezoidal or ACME model already in it that you can modify.

Harder to do on paper but if you have a drawing board with protractor it’s doable.

Once you’ve got the whole thing in front of you on paper it’s usually easy enough to sort out things. Hard part can be getting your head round the clearances involved. Not as simple as H6 or whatever taps for ordinary threads.

As ever total pain in  the rump first time through. By teh fourth time it’s “Why did I ever find it hard.” But who does 4 in their lifetime.

Hate the modern on-line way of just giving numbers and no context.

Clive

[peak4Participant @peak4]

If you have a look here, the tapping diameter would appear to be D1
https://roymech.org/Useful_Tables/Screws/Trapezoidal.html
Your thread isn’t listed, as the nominal pitch for T8 is 1.5mm, however, for all 2mm pitch threads, the D1 size seems to be 2mm smaller than the designated male thread diameter

[Neil AParticipant @neila]

Peak4 beat me to it.

I would only add that I would think of D1 as being be the minimum tapping size drill to use.

It will depend on the material you are tapping, I think a little experiment might be in order to see how hard it is to tap.

Neil

[Neil A]

On Neil A Said:

Peak4 beat me to it.

I would only add that I would think of D1 as being be the minimum tapping size drill to use.

It will depend on the material you are tapping, I think a little experiment might be in order to see how hard it is to tap.

Neil

Clearly I don’t have the kit in question, but what I did wonder, was being a non preferred size and a smallish diameter rod, whether they have gone for a shallower thread depth the same as the T8 1.5mm for more rigidity.
I suppose a caliper or blade micrometer might give a clue there.

Bill

[jim1956Participant @jim1956]

Thanks everyone for your input and some very useful links.  My first attempt will be in mild steel at 6mm.

Thanks again

Jim

[old martParticipant @oldmart]

I would try a test on aluminium first, less likely to break the tap.

[bernard towersParticipant @bernardtowers37738]

That’s not necessarily so, be careful what grade and use GOOD quality cut lube

[Author]

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