Metric taps and dies - and pitches

Hi,

I am a little bit confused with the metric taps and die choices available. I have loads of "imperial/English" taps and dies and am more familiar with feet and inches to be honest from my apprenticeship days long ago. But more and more drawings are referring to metric "stuff".

Now I don't mind metric stuff - I have a metric Emco mill, and an "English" Myford!

So to my question.

If a drawing says tap or thread for M5, does it actually mean M5 x 1mm pitch or M5 x 0.8 pitch etc.....? For example Tracey Tools show that there are 5 different pitches with an M5 thread.......and I thought metric was supposed to be easier!

I am sure there is a table showing the preferred/standard metric tap sizes and pitch for Model Engineers somewhere....otherwise do I just use the Metric "Coarse" range? (it seems so much easier with the "old" stuff e.g. tap 2BA or thread 3/8" BSF for example.......)

Hopefully one our our resident metric thread persons on here will be able to advise which of the 5 available choices of M5 I should buy - in fact what metric taps and dies should I buy and what pitch?

Confused

Derek

Generally, use Metric coarse unless specifed otherwise.

regards Martin

Edited By Martin Kyte on 16/04/2020 12:04:07

Metric thread normal pitch range is usually defined as metric course. That is in your M5 example 0.8mm pitch.

The alternative pitches are usually finer, but coarser pitches are used in some industries. You will see that there are numerous pitches particularly in larger sizes. I only use metric threads, coarse range normally, but finer pitches are used on threaded adaptors and I use M8x1 and M10x1. the finer threads are suited to thiner wall items such as pipe fittings.

As far as I can work out, when metric threads are nominated simply by (say) M10 with no additional information, it refers to coarse thread pitch which in this case is 1.5mm. If a fine pitch thread is called for then it is nominated by M10 - 1.25 meaning a thread pitch of 1.25mm. In general, most of the commercially available metric screw threads less than M8 are coarse. The reason for having differing pitches is hard to identify, but the Nord-Lock website has a useful summary here: **LINK** which might help. I suspect that in the smaller sizes, the tolerances required for fine thread cannot be easily maintained in manufacture so the coarse thread prevails. I'm happy to be enlightened by anyone with more accurate information.

There are a few frequently encountered exceptions to the rule, such as Emco's M12 x 1 and M14 x 1 on Unimats and Ronson's M4.5 x 0.75 for lighter filler valves, but apart from these as Martin says Metric coarse should cover most things. Personally, apart from these specials (and M8 x 1LH for leadscrews), I have only ever needed the preferred sizes M3, M4, M6, M8, M10 and M12.

Brian G

Sort of related. When we make equipment it is common in imperial to use a 40tpi thread to give 25 divisions on the dial and the ME range of threads covered that, or a 20 tpi with more divisions on the dial.
In metric the coarser selection would automatically be 1mm pitch but what is the 'favourite' for a fine pitch? 0.5mm is getting a bit fine?

0.75mm, I've used that to match standard gas fittings and not far off 32tpi ME. But then again I have also use M30 x 1 which is relatively fine when used at that diameter.

If the OP is just going to buy some taps for general use then metric coarse will cover 95% of cases.

What you have worked out is correct M? with no further info is coarse used to be standard practice to only put pitch if non coarse although sum machine shops ask for pitch it in all cases now days.

Steve

Tubal Cain, "The Model Engineer's Handbook" Third edition. Page 47.

If you search online long enough, you will doubtless find a pdf copy.

Fantastic response guys thanks.

I have Tubal Cains book...why didn't I look in there first?......doh

Much clearer now. I guess its metric coarse by default if the instruction/drawing says to "tap M5" for example, otherwise it would be specifically stated if the metric thread pitch was to be any different ...say "metric fine (MF?)

Thanks once again everyone

Stay safe

Regards

Derek

As said, coarse unless otherwise specified. But worth knowing the reason why other pitches are available.

For most purposes coarse-pitched threads are preferred because they take less effort to make, are less likely to be damaged by rough handling and much faster to assemble because they don't have to be aligned carefully. They also suit soft materials.

However, fine pitched threads are usually stronger and less likely to come loose due to heat cycling or vibration. Extra-fine perform better again, but using them slows assembly and maintenance down and they're more likely to get cross-threaded. Fine pitched threads are more trouble than they're worth unless there's a specific need.

For 'it matters' applications like aerospace there are more tightly specified metric variants - tighter size tolerances, additional pitches, and strength requirements etc.

An ordinary DIY store M12 bolt will have a coarse pitched thread (x1.75) crudely rolled from common steel. But for grown-up fastening where the wheels might come off, four different pitches are available for M12 - 1.75, 1.5, 1.25 and 1.0 in various improved tolerances. A fully specified aerospace M12 bolt might be X-ray inspected and five times stronger than the best I've ever seen, with prices to match.

A basic metric tap and die set covers all my needs, and it contains a few I've never used. Much to be said for buying taps and dies as the need arises. Most of my jobs are done with M2, M4, M10 and M12 in coarse pitches. M4 predominates, but it depends on the type of work being done. I'm sure others have their favourites!

Dave

Machinery's Handbook has tables of coarse, fine and constant pitches for metric threads, as well as 1st, 2nd and 3rd choices for nominal thread diameter.

Andrew