Metric threads seem crazy. There are too many pitches.

[J Tiers]

Take 8mm...... there are taps available easily (Victor Machinery) for 8 x 0.5, 8x 0.75, 8x 1.0, 8x 1.25, and 8 x 1.5. FIVE easily available pitches?

Same with 5mm...... 5 x 0.5, 5 x 0.75, 5 x 0.8, 5 x 0.9, 5 x 1.0. Again, FIVE apparently fairly common pitches. And I have a part that seems to be 5 x 0.65. It's not 0.5, and it's not 0.75, nor 0.7, not 0.6, as near as I can tell with only a few threads to judge the fit of the gauge by. That would make 6 pitches.

Maybe someone familiar with the metric system can explain why the flaming red devil has devised so many apparently common pitches for one thread diameter. Common enough that I can buy taps for them without trying hard, in the US.

With US threads, you have coarse, fine, and sometimes extra fine. But there are normally just two of those in reasonably common use, the extra fine being rare. Sure, companies like Starrett have legacy odd US threads, but they are not commonly available as taps or dies.

This does not align with the "metric system" being so simple and easy. Is there some reason, or is it the usual "because we can"?

[CalM]

The reality is, for most of the metric world, only the "coarse thread" is common.

except!...

The auto makers seem to choose otherwise for an endless variety of "special applications" .

That really confuses us Imperial users.

So just consider the metric standard as the coarse thread option for any and all "fastening" requirements. BUT,, when dealing with the cap screw that hold the seats to the floor boards of your pickup truck, DOUBLE CHECK THE THREAD PITCH!

;-)

[MattiJ]

Take 8mm...... there are taps available easily (Victor Machinery) for 8 x 0.5, 8x 0.75, 8x 1.0, 8x 1.25, and 8 x 1.5. FIVE easily available pitches?

Same with 5mm...... 5 x 0.5, 5 x 0.75, 5 x 0.8, 5 x 0.9, 5 x 1.0. Again, FIVE apparently fairly common pitches. And I have a part that seems to be 5 x 0.65. It's not 0.5, and it's not 0.75, nor 0.7, not 0.6, as near as I can tell with only a few threads to judge the fit of the gauge by. That would make 6 pitches.

Maybe someone familiar with the metric system can explain why the flaming red devil has devised so many apparently common pitches for one thread diameter. Common enough that I can buy taps for them without trying hard, in the US.

With US threads, you have coarse, fine, and sometimes extra fine. But there are normally just two of those in reasonably common use, the extra fine being rare. Sure, companies like Starrett have legacy odd US threads, but they are not commonly available as taps or dies.

This does not align with the "metric system" being so simple and easy. Is there some reason, or is it the usual "because we can"?

Don't blame us if Victor has unusually large selection of weird taps for some unknown reason
Nobody around here stocks such a selection of taps.

As CalM mentions "metric coarse" is really used for 99% of things.
Auto makers maybe being the biggest exeption and even they use std pitch for most of the fasteners except engine internals.
And con rod bolts and head bolts are sort of speciality that you shouldn't replace with garden variety hardware store fasteners anyways..

[J Tiers]

That sort of is not the point.

I don't give a tinker's damn what the usual pitch is. I am typically looking at, say, a 5mm threaded hole, or a 5 mm threaded stud or similar part. It is what it is, and that's what I need to find out. The possible pitches, from Victor, or according to pitch charts, are so close as to be very hard to tell apart. But obviously different enough that they won't interchange.

I happen to have a number of such things in the 4mm to 8mm range to determine at the moment. It appears that one of them may not even be a choice I can get from Victor, but the choices I found are bad enough. The differences are small enough that without a goodly length of undamaged OD thread, it is not possible to be sure which is on the part.

I need to match the thread, in order to make a missing part. I don't have room to recut it larger. I want to use the original thread on the replacement part. And whoever made it chose a reasonably fine thread, but I don't know which one.

So..... with only a relatively few threads to do a gauge check, can YOU easily determine if it is a 0.75mm, 0.8mm, or 0.9mm thread pitch? How about a 0.6mm or a 0.65mm pitch? I surely cannot be certain of it on an inside thread, where the fit in the threads is not easy to see, it's pretty much "feel". On an outside thread it is a bit easier, but even there if the existing thread you have on a broken part is damaged, fairly short, or both, it's a problem.

Even if I had all the possible taps, my confidence in using a tap to find the hole thread pitch is low. Taps have a nasty habit of altering the thread to fit themselves (it's what they do), and with small fine pitch threads, that is worse.

To put it in perspective, a 0.75mm pitch is 33.8tpi, while a 0.8mm pitch is 31.75 tpi. Pretty close together, and not easy to tell apart, particularly if you don't have many threads on the part, or they are torn up on the part with the outside threads.

For US threads, your choices are not generally that closely spaced, and using really odd threads is reserved for guns, or specialized instruments. Anything else will be coarse, fine, or pipe threads.

If the part fits a hole in an item that needs to fit well and seal, there is an added issue. Just guessing, or "using the closest" is pretty much a kludge, with a fair chance of damaging the thread and being stuck with no options.

It seems that one needs an extensive sample box, with all sizes and all known threads in the range you work with, so that the best fit can be selected. That's just crazy.

OK, it is what it is. But who thought it was a good idea to have so dang many thread pitches all close together?

[Peter.]

Take 8mm...... there are taps available easily (Victor Machinery) for 8 x 0.5, 8x 0.75, 8x 1.0, 8x 1.25, and 8 x 1.5. FIVE easily available pitches?

Same with 5mm...... 5 x 0.5, 5 x 0.75, 5 x 0.8, 5 x 0.9, 5 x 1.0. Again, FIVE apparently fairly common pitches. And I have a part that seems to be 5 x 0.65. It's not 0.5, and it's not 0.75, nor 0.7, not 0.6, as near as I can tell with only a few threads to judge the fit of the gauge by. That would make 6 pitches.

Maybe someone familiar with the metric system can explain why the flaming red devil has devised so many apparently common pitches for one thread diameter. Common enough that I can buy taps for them without trying hard, in the US.

With US threads, you have coarse, fine, and sometimes extra fine. But there are normally just two of those in reasonably common use, the extra fine being rare. Sure, companies like Starrett have legacy odd US threads, but they are not commonly available as taps or dies.

This does not align with the "metric system" being so simple and easy. Is there some reason, or is it the usual "because we can"?

I've been using metric my whole life and I have never yet seen a 5mm fastener thread that wasn't 0.8mm pitch. I've never yet seen ANY metric thread that was more coarse than the standard 'coarse' thread (except lead/feed screws that is). The reality is that 'apparently fairly common' is actually fairly rare although M8, 10, 12, 14 come in fine threads fairly regularly in automotive applications.

I see this as no different to inch sizes TBH. Ask for a 1/4" bolt you're going to expect a 20TPI UNC but you could get 28TPI UNF, 32TPI UNEF of any of seven UNS pitches. The only difference is that UNF is actually quite common where a fine pitch M6 or M8 quite rare.

[Paul Alciatore]

Short answer: metric thread standards were written by many different countries before the present day more or less standardized series of pitches were adopted.

Long answer is a lot more complicated and probably involves choices made by various lathe manufacturers over time which is tied in with those different standard pitches in the many countries.

Part of this is in the way metric threads are defined by the linear pitch. A metric thread is defined by the distance from ONE thread crest to the crest of the next, adjacent thread. It is a linear measure and, of course, it is in mm. When metric standards were being invented someone probably thought this was a more logical method than the threads PER inch (TPI) used for English measure threads. In my humble opinion, that person or persons was WRONG. Dead WRONG.

If you ever look carefully at a chart of English thread pitches, you will notice that the common threads are all multiples of a single, eight thread sequence: 8, 9, 10, 11, 12, 13, 14, and 15 TPI. This is then multiplied by factors like 2, 4, 8, etc. to get additional ranges like 16 - 30 TPI, 32 - 60 TPI, etc. And it is also divided by 2 to get 4 through 7.5 TPI on many QC gear boxes. Why was this sequence chosen? MATH!

When change gears were being made for a lathe to cut English measure threads, the math dictates that a simple sequence of gears can produce the base sequence of eight threads and then a second set of change gears can easily multiply or divide that basic sequence by the multiples of 2 factors that I mentioned above. The math is simple and the set of gears can be just a basic assortment. So manual change gears are selected to obtain the needed prime numbers of teeth and this can be done with a gear set where the gear tooth counts are all multiples of 4 or 5. So 4 x 4 = 16, 4 x 5 = 20, 4 x 6 = 24, etc. is an example of a sequence of gears "by fours".

Likewise, when quick change gear boxes were devised, it was easy to make one section which produced the basic sequence of eight thread pitches and another section did the multiplying. Over time additional numbers were added in either the basic sequence (11.5 TPI) or in the multipliers, but that was the basic idea.

Metric threads, on the other hand are not measured in threads PER mm or threads PER cm. Instead they are measured directly in mm and I am sure that this, at first glance, appears more elegant to some. But it makes the choice of gears a lot more complicated. I am sure that at first, this led to the different "standard" metric threads as different lathe manufacturers in different countries and regions, came to different conclusions about just how the manual change gears and the quick change boxes were going to be made. And those different conclusions meant different gear sets and different "standard" sets of thread pitches. Another thing that was difficult in metric threads, measured in linear mms was that the idea of creating a basic sequence and then multiplying or dividing it by factors was just not so easy. Play with the numbers yourself and try to come up with a sequence of perhaps five or six metric threads and then multiply or divide that sequence to get additional ones that carry on from the low and high ends of the base sequence and see what you can get. The math is not as easy.

Another thing that is a lot more complicated on metric lathes is the thread dial. A thread dial that has to synchronize English threads, measured in threads PER inch is based on a four inch travel of the carriage in cutting the thread. This means that you simply multiply the TPI of the lead screw by four and that is the gear you need to mesh with that lead screw for a thread dial. But many, dare I say all, metric thread dials need multiple gears in order to cover even a limited range of metric threads. Again, the math favors threads that are measured in threads PER some unit of length instead of just in that unit of length directly. Math again.

The result of this is that there are far fewer standard metric threads than English threads. Some will say this is to make things simpler. Well, they are right. It keeps the price of a lathe down as fewer gears are needed. But this situation also produced a wide variety of metric threads in the beginning and it produces problems when you want to cut a metric thread that is not part of the gear set of your lathe. Almost every non standard metric thread is going to wind up needing at least one additional gear to the set that any present day lathe manufacturer supplies with the machine.

And the wide variety of taps and dies is a leftover from the earlier days when every country and manufacturer did their own thing in metric threading.

[J Tiers]

heh..... the thing in question is from Germany..... the country that standardized everything. So much for standards.

[The Artful Bodger]

heh..... the thing in question is from Germany..... the country that standardized everything. So much for standards.

Maybe Löwenherz thread?

[PStechPaul]

As for lathe change gears, assuming a leadscrew with 1mm pitch, a 1:1 gear ratio (20-20 tooth) produces a 1mm thread. And then with a 20 tooth secondary gear you can use primary gears of 16, 17, 18, 19, 20, 25, 28, and 30 tooth for 8 thread pitches of 0.80, 0.85, 0.90, 0.95, 1.00, 1.10, 1.25, and 1.50. Of course, to cut English TPI threads with a metric leadscrew, it involves a lot of gear changes and approximations.

Conversely, with a 16 TPI leadscrew, a 30/60 gear set with standard 9x20 lathe change gears of 16, 18, 19, 20, 22, 23, 24, 26, and 28 will produce threads of 32, 36, 38, 40, 44, 46, 48, 52, and 56 TPI, and with a 60/30 gear set they will be 8.0, 9.0, 9.5, 10.0, 11.0, 11.5, 12.0, 13.0, and 14.0.

Here is an extended chart I made (borrowed from somewhere):



And the metric equivalents for this lathe which has a 16 TPI leadscrew


The official threading chart for this lathe:


I have found that the usual metric coarse threads are much finer than the Imperial coarse threads, and often closer to Imperial fine, as a ratio of diameter to pitch:

[Bruno Mueller]

In the metric system there are different tables.
standard screws have the so-called coarse thread, although this can not be compared with the inch screws.
Here is a comprehensive table of thread standards.

https://www.gewinde-normen.de/en/iso-fine-thread.html

[Black Forest]

heh..... the thing in question is from Germany..... the country that standardized everything. So much for standards.

Germany is getting a bad rap for being so precise! I know lots of Germans that are not precise.

[RB211]

Call me crazy, but this American would prefer to use the Metric system, thinks it is superior. Look at drills for example, they go by .1mm. They are labeled by size, none of this stupid numbering or letter sizes. 3mm tap is paired with a 2.5mm drill. Seems simple enough.

[Bruno Mueller]

The data in the table for the drill diameter should not be taken too seriously. In Germany, there is hardly a drill with a diameter of 2.46mm for an M3 thread, so you use the next largest diameter available.
So, the Germans do not take it so exactly then again.

[RB211]

I mean, if you want to dial in your threads to however you want them, your only real option is thread milling, since it allows you to do exactly what you want.

[J Tiers]

I hate the "metric system". Mostly because there is no "system" to it.

I really like SI. SI is nominally like "metric", but it is an organized and rational system that happens to use the same units as the "metric system".

They are different.

The problem with "metric" (which simply means using units of metres, kg, etc, nothing more), is that there is German metric, Swiss metric, chinese metric, Japanese metric, russian metric, etc. Pick your "metric standard" when it comes to fasteners, there is sure to be one you like.

Again, it is of NO CONSEQUENCE to me what is "usual" with metric. That is not the problem at hand at present.

The issue is that there are so many existing thread pitches, that it is very difficult to determine what the thread that must be matched is. I have a number of "metric" threads to be matched, and the available choices are so close that it is quite difficult to identify the one that matches what I have.

I have the "usual" metric size taps and dies, same as many of you do. None of them are even close to the correct pitch. So far I have found only ONE "standard metric" pitch thread on the thing. And I did not have to replace that.

The point about the available taps is simple, and most missed it....... Many of you delight in saying that the US is non-metric (that is totally false, but let that go). So, it follows the choices of pitch for metric threads available in the US are presumably limited to the most standard. But there are at least 5 pitches available easily, for several "metric standard" size fasteners etc, even here in the backward US.

Obviously, the few metric taps available in the "non-metric" US are only a sub set of what is out there. They must be the "most standard" of the "non-standard" pitches. So it follows that there are even more "common odd pitches" than I see in the range of taps.

And that seems to be true. One of the threads is, as far as I can determine, either 5 x 0,6, or 5 x 0,65. Neither is included in even the "excessive" range of available taps/dies. And the item having the thread is nothing special (I am not going to confuse the issue by describing it).

Apparently, there may be a "standard", but it is widely ignored in favor of whatever the designer thought was a good idea. Apparently any increment of 0,05mm is fair game as a thread pitch.

The "metric" thread pitch "system" makes the US threads seem refreshingly simple.