Three wire thread masurements for taps

Having just watched a YT video by Artisan Makes, he is making an M20 x 1 tap.

In the video, he states that there are online calculators for three wire thread measurements. So far, so good.

But if you are making a tap, does it not have to be larger than the bolt that screws into the female thread that the tap produces?

So how do you arrive at the correct three wire measurement for a TAP?

There is some information from Sandvik on the class of thread produced by a standard tap here:

https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/thread-standards-and-thread-tapping-tolerance-classes

and it quotes H6 as a typical female thread tolerance.

What would be the shaft tolerance on the tap to produce this hole?

No three wire calculator I can find alllows an oversize tolerance.

Thanks.

Screw threads are normally not made at their nominal diameter but bolts a bit smaller and nuts a bit larger. The depth of thread is always the same. Doing this results in play between nut and bolt and that is needed for normal operation.
The play between nut and bolt depends on the thread tolerances. Normal hardware store metric nuts and bolts have a diameter about Nominal diameter minus 10% of nominal thread depth. Nominal thread depth equals the thread pitch.

I make my taps (command and roll taps) at the nominal thread diameter. I use the same thread depth as when threading a normal bolt using that tool/insert. That works fine for me when tapping aluminium (normal taps) or wood (roll taps).

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An interesting question [and a very informative link, thanks]
I think the key to finding the answer will be a thorough appreciation of those tolerances … from which it should be possible to deduce the relevant three-wire measurement.

[quote]
Normal tap tolerance is ISO 2 (6H), which generates an average quality fit between screw and nut. Lower tolerance (ISO 1) generates a fine fit without a gap on the flanks between screw and nut. Higher tolerance (ISO 3) generates a rough fit with a large gap. This is used if the nut is coated, or if a loose fit is preferred. Between tolerances 6H (ISO2) and 6G (ISO3), and between 6G and 7G, there are also taps with tolerance 6HX and 6GX. “X” means the tolerance is outside the standard and is used for taps working with high strength or abrasive materials such as cast iron. These materials do not cause oversize problems so higher tolerance can be used in order to increase tool life. The tolerance width is equal between 6H and 6HX. Forming taps are usually produced with a 6HX or 6GX tolerance.

[/quote]

… My brain hurts already !

MichaelG.

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Edit: __ This might be a good rabbit-hole to start exploring: 

https://thegauge.co.uk/thread-tech-specs

Edited By Michael Gilligan on 13/08/2023 05:02:29

Thanks for the reply. There is some shorthand going on in Sandvik's descriptions.

ISO limits and fits (e.g. https://simplybearings.co.uk/shop/Info-Pages-ISO-Limits/c4746_4779/index.html?page=1) are fussy about nomenclature, hence my confusion.

Any tolerance with uppercase is for a hole rather than a shaft, so in a very strict sense a 6H tap is non-sensical. The short hand translates to 'a tap designated 6H will produce a female thread to 6H tolerance'.

Thanks to some words from the link you kindly provided, I found ISO2857:

https://cdn.standards.iteh.ai/samples/7858/fb4ec4f1d31d4e02b38f44d39c4bd111/ISO-2857-1973.pdf

So, as you say, use the dimensions from ISO2857 and perform the three wire calculations on the basis of these.

Probably best to do all the measurements before cutting three flutes into the tap.

Agreed

MichaelG.

I wondered what the UN and Whitworth thread series equivalents to ISO2857 (also called EN22857) might be.

Some info. (but not a formal standard) here:

http://www.tapmatic.com/tapping_questions_class_of_threads_h_limits.ydev

Note that the number after the H is the multiple of 0.0005" that the pitch diameter is oversize for that class of tap.

This puts it a little more clearly (but note that the link below is for an STI tap, so ignore the actual numerical values):

https://katofastening.com/article/taplimits.html

Best I can find for Whitworth form is ISO5969, which is for ISO pipe threads (which are Whitworth form).

https://cdn.standards.iteh.ai/samples/12163/1c8129d4e26a45a9a5452ae960f61f91/ISO-5969-1979.pdf

I noticed that stock parts wobble when I did a lot of my own threading

If I wanted a tight fit thread I always cut one part myself to fit the stock nut/bolt part