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Thread: Metric thread dimensions :-?

  1. #11
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    Quote Originally Posted by Cenedd View Post
    .....
    I hate the sheer variety in imperial units - 12 inches in a foot, three feet in a yard, 1760 yards in a mile (seriously?! Yeah, I had to look that up) - and yet I'm conditioned to miles, mph, mpg, height of people in feet and weight of people in stone. .....
    But nobody uses most of them.

    Feet and inches. Feet for any long distance (yards in some sports measurements), inches for anything smaller. Carpenters and ordinary people use fractions, technical people using inches use decimal for anything of precision. Fractions come up as "about a half inch", or the like.

    It's the British who imposed that stuff on us, mostly because we were them at one time. And we can add "drachms, minims, firkins, furlongs, hogsheads, tuns", and a myriad of other odd old traditional measurements to the list.

    Folks ridiculing english measurements units like to point at those old units as if they were in constant use and a source of confusion, but the average person probably has never heard of any of them, and has no need for 'em.

    And, of course the folks poking fun like to forget that their OWN nation used to have measurement units that were likely not consistent even from town to town. Almost everywhere in northern europe there was a local variety of "inch" and "pound", etc. Maybe it equated with the one in the next town, maybe not. And that was true even fairly recently, as time goes, it isn't just something from 1100AD.
    Last edited by J Tiers; 01-12-2018 at 11:27 AM.
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  2. #12
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    Quote Originally Posted by Cenedd View Post
    Thanks MattiJ. That makes sense and is one of the formulae I'd shoved into Excel to run up my own chart. The information published online seems to be quite contradictory and finding a definitive standard seems to be impossible. As an example of what I mean:

    Maryland Metrics' metric thread data showing that Major diameter = nominal diameter and that it's measured to the flatted crest: http://mdmetric.com/tech/thddat2.htm
    Maryland Metrics' extended metric thread data showing that Major diameter < nominal diameter and giving it a range: http://mdmetric.com/tech/M-thead%20600.htm
    If there is something with a definitive standard, it's the common 60 degree metric threads. Every aspect of the thread is easily determined given only the major diameter and the pitch. Just takes a little very elementary math. You can find tables for all the common threads so that you don't even have to do the math. That's what you have with the charts from Maryland Metrics.

    Tolerances are there to allow for ease of assembly as well as wear of the tools used in mass production.

    Dan
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  3. #13
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    @J Tiers: Don't get me wrong, I'm not having a go at the system - and I can throw in rods, poles, chains, perches and acres to the list of units off the top of my head - its just that I've got to settle on one system (for my own sanity...or lack thereof) and, despite "English" measurements being our fault, most stuff is now metric here....except the pint. But that's mainly because the 1/2 litre is slightly smaller and you just know it would cost the same!

    Danlb: It's that that I'm trying to understand. I can cope with the maths - as long as I'm not required to do it in my head - as long as I've got the definitions clear. All that I've found stated clearly appears to be wrong...or at least it is unless you add tolerances into the mix. I've not found anything that clearly states that Dmajor is Dnominal minus some non-zero* variable** for tolerance. What I'm after really is confirmation that this is correct - or wrong - as I'm piecing it together from what I can find.

    *If I make up an M6 bolt with a Dmajor of 6mm, it's not going to fit anything as it will be too large.
    **I get that there are different tolerance classes for different fits.

  4. #14
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    There is thread FORM and thread DIAMETER, and PITCH diameter.

    Same pitch diameter can have different actual major diameters, depending whether it was cut full V, or with the flat, etc. And the pitch diameter can vary depending on fit class. So, just figure that ALL threaded fasteners will be undersized from nominal as to actual OD, BUT that they ALL will go through a hole that has clearance over the nominal "size"....

    So a 6.1mm hole should pass ANY properly made M6 fastener.

    Quote Originally Posted by Cenedd View Post
    @J Tiers: Don't get me wrong, I'm not having a go at the system - and I can throw in rods, poles, chains, perches and acres to the list of units off the top of my head - its just that I've got to settle on one system (for my own sanity...or lack thereof) and, despite "English" measurements being our fault, most stuff is now metric here....except the pint. But that's mainly because the 1/2 litre is slightly smaller and you just know it would cost the same!

    ...
    Why not? Have at it. It's a full firkin of fun!

    No worries.... I can poke fun at any. I prefer metric, or, I should say , SI, for most engineering purposes, but many things are still expressed in ways that make english units easier, especially here in US. For a place that officially made metric legal for trade about 150 years ago, we have not made the changeover very fast.
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    Quote Originally Posted by Cenedd View Post
    Danlb: It's that that I'm trying to understand. I can cope with the maths - as long as I'm not required to do it in my head - as long as I've got the definitions clear. All that I've found stated clearly appears to be wrong...or at least it is unless you add tolerances into the mix. I've not found anything that clearly states that Dmajor is Dnominal minus some non-zero* variable** for tolerance. What I'm after really is confirmation that this is correct - or wrong - as I'm piecing it together from what I can find.

    *If I make up an M6 bolt with a Dmajor of 6mm, it's not going to fit anything as it will be too large.
    **I get that there are different tolerance classes for different fits.

    I recommend that you study the wiki page that describes the thread. It has all the formulas. https://en.wikipedia.org/wiki/ISO_metric_screw_thread

    Note that several measurements are not based directly on the major diameter, but are instead based on the theoretical thread height. That theoretical thread extends outside the bolt's major diameter.

    Note also that the root of the male and female threads is defined as a radius that extends beyond the major or minor diameter. With that radius, you can cut the major to the spec and everything will work. Without that radius you are likely to end up with scrubbing between the root and crest.
    Measure twice. Cut once. Weld. Repeat.
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  6. #16
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    The OP seems to be focusing on certain dimensions to the exclusion of others.

    The REAL CONTROL as far as fitting is concerned is the PITCH DIAMETER. if that is wrong, then nothing works. So that is the basic thing to get right. While it is shown in the link as derived from the major diameter, that works the other way also, and when checking the screw, with wires, which is a standard way, you are checking the PITCH diameter.

    If you look at screw specs, the pitch diameter will be closely controlled. It is THE thing to get correct in terms of diameter. The rest is more or less details, although they are important ones.

    Focusing on the OD, or major diameter is not correct, and may be very misleading during the cutting process, since burrs etc will foul it up even when the pitch diameter is correct. Get the pitch diameter right, clean up the Dmin and Dmaj to spec, and the thing should fit.

    You can usually measure pitch diameter even if there are burrs etc.
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  7. #17
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    Quote Originally Posted by J Tiers View Post
    The OP seems to be focusing on certain dimensions to the exclusion of others.

    The REAL CONTROL as far as fitting is concerned is the PITCH DIAMETER. if that is wrong, then nothing works. So that is the basic thing to get right. While it is shown in the link as derived from the major diameter, that works the other way also, and when checking the screw, with wires, which is a standard way, you are checking the PITCH diameter.

    If you look at screw specs, the pitch diameter will be closely controlled. It is THE thing to get correct in terms of diameter. The rest is more or less details, although they are important ones.

    Focusing on the OD, or major diameter is not correct, and may be very misleading during the cutting process, since burrs etc will foul it up even when the pitch diameter is correct. Get the pitch diameter right, clean up the Dmin and Dmaj to spec, and the thing should fit.

    You can usually measure pitch diameter even if there are burrs etc.
    You have it bass ackwards Jerry. Everything is related to the Major/Minor diameter. It's easy to turn the outside of a rod to the major diameter or the bore to the minor diameter. So start there. Once that's done, you cut the threads to the proper depth (depending on the profile of your cutter) and double check by measuring the pitch diameter. It's been done that way for most of my life.

    This works because the thread is defined by the geometric relationship between the depth of a 60 degree cut and the major and minor diameters.
    The thread depth is always 0.614 pitch.

    The outermost ​1⁄8 and the innermost ​1⁄4 of the height H of the V-shape are cut off from the profile of the theoretic sharp thread. Thus the truncated crest and root will be equal to the major and minor diameters. If you have a 60 degree cutter with the proper profile and you start with the proper Dmaj and cut to the proper depth the pitch diameter will HAVE to be within the proper ranges. It can't be anything else.

    I know that Jerry will protest mightily, but I challenge him to describe the step by step process for single pointing a 13x1.25m thread using just the pitch diameter. All math is, of course required. No charts, since given those two parameters all others are derived. I want to hear how you can single point a male thread without knowing where the Dmaj is.

    Dan
    Last edited by danlb; 01-12-2018 at 09:57 PM.
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  8. #18
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    Quote Originally Posted by Cenedd View Post
    Danlb: It's that that I'm trying to understand. I can cope with the maths - as long as I'm not required to do it in my head - as long as I've got the definitions clear. All that I've found stated clearly appears to be wrong...or at least it is unless you add tolerances into the mix. I've not found anything that clearly states that Dmajor is Dnominal minus some non-zero* variable** for tolerance. What I'm after really is confirmation that this is correct - or wrong - as I'm piecing it together from what I can find.
    Cenedd, The problem that you (and many) have with the "nominal diameter" is that it is based on an older standard where the threads were sharp at the crest and at the root. This is represented in the variable H which stands for a theoretical thread height if it was a sharp V thread. Unfortunately, the new ISO standard calls for a truncated crest. The male thread (screw or bolt) is truncated by 1/8 of H and the female is truncated by 1/4 of H.

    Here's the math for an 8x1.25M bolt.

    Nominal is 8mm. Pitch is 1.25mm

    H= .866 * P which equals .866 * 1.25 which equals 1.0825 mm

    Major is truncated by 1/8 of H on both sides.
    Dmaj = 8 - (1.0825 / 8 + 10825 / 8 ) which equals 8 - (1.0825 / 4 ) which equals 7.7294 .


    Dmin is figured in a similar manner. It will always be Dmaj - 2 * 5/8 H

    In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.

    Make sense?
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  9. #19
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    There is how you turn the thread, the procedure used.

    There is also how you measure the thread to be sure the size is correct, the dimension that must be correct for the thread to be correct.

    Do not confuse the two.

    The first is just a practical method of making the thread. Sure... cut to the desired specified "Dmajor" and then thread until you reach the correct pitch diameter. That is the least work.

    There are many dimensions for the major diameter that will work and fit well. The range of outside diameters that are OK is much more than the range of correct pitch diameters for a given thread. Get the OD too small, it still screws in OK. Get the pitch diameter too small, and it is loose.

    Most QC on threads is for pitch diameter and thread form. OD is incidental.

    That is what I am saying....

    Of course you could start with a large diameter, and keep threading until the pitch diameter is correct. That would take a long time, many threading passes that would be cut away entirely before the correct PD was reached. You would be turning down the part with the threading tool. It is not sensible, so you start with a reasonable OD and then thread.

    BUT THE GOAL IS CORRECT PITCH DIAMETER. The OD can be perfect, but if the pitch diameter is wrong, it's no good. However, if the pitch diameter is right, and the thread form is within the maximum limits, it will fit and can be a good thread even though the OD is not perfect.

    Put a different way, the pitch diameter is a primary characteristic not dependent on the OD, the OD is dependent on the pitch diameter, thread form, and the actual pitch.

    Do not get obsessed about the OD, so long as it is not too large. Make the OD correct, by all means, but the thing to focus on getting right is the pitch diameter. Frankly, if you get the pitch diameter correct, and have the right thread form. it is impossible to have the wrong OD.
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  10. #20
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    Quote Originally Posted by J Tiers View Post
    Do not get obsessed about the OD, so long as it is not too large. Make the OD correct, by all means, but the thing to focus on getting right is the pitch diameter. Frankly, if you get the pitch diameter correct, and have the right thread form. it is impossible to have the wrong OD.
    That's a lot of nice side stepping. It also skips over the corollary. If you have the right Major diameter and the right thread form it's impossible to have the wrong pitch diameter. I defy anyone to prove that wrong.

    Dan
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

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