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Problem comprehending metric fasteners...

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  • Problem comprehending metric fasteners...

    The four bolts holding down the column of my milling machine show inconsistencies in measurement.

    I assume they are described as 16mm but when I measure them all four are different measuring 15.75, 15.77, 15.78 and 15.80mm across the threads.
    Head diameters and heights also show discrepancies.
    The only consistent measurement is length.

    These are also not OE fasteners as I changed them out from a dependable vendor and I've noticed discrepancies with other sizes as well .

    Were I to turn or mill something to a metric dimension I'd try to get it to the specified dimension.
    Is metric really that cavalier?
    Len

  • #2
    bolts are rolled.

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    • #3
      Originally posted by dian View Post
      bolts are rolled.
      And pie are squared...so does that mean anything between 15 and 17mm is 16mm?
      Len

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      • #4
        I appreciate your pain. Your experience with metric threads is similar to what has kept me confused about them for decades. Unfortunately, as a young machinist I never had the time, need, or inclination to study the subject, so to this day I remain ignorant on the subject.
        “I know lots of people who are educated far beyond their intelligence”

        Lewis Grizzard

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        • #5
          Here you can find your screw tolerance.
          http://www.amesweb.info/Screws/IsoMe...rewThread.aspx

          How much deviation on thread diameter are you used to see on your imperial screws?

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          • #6
            All bolts, inch and metric, are a few thousandths under the nominal diameter. When measuring the threads on a 1/4" bolt, for example, you will find it to be around .246 - .247 major diameter. The crest of the thread is usually not sharp, but has a small radius or flat at the top of the triangle. If it was full diameter and sharp, it would bind up in the mating thread and not screw together easily. It is simply a matter of clearance.

            What matters is the diameter on the flanks or sides of the thread, somewhere between the major and minor diameter. This is measured with a thread mic or a regular mic and thread wires, etc.
            Kansas City area

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            • #7
              The major diameter of a male thread is always less than the nominal diameter since the crests are specified with a flat.

              The allowable minimum and maximum OD for an M16x2 is 15.682mm and 15.962 a difference of 0.28mm or 11 thou.

              Your bolts show a variation of 0.05mm, which is about 2 thou.

              The allowable minimum and maximum OD for a 5/8" by 11tpi thread is 0.6113" to 0.6234" a differnce of about 12 thou.
              Paul Compton
              www.morini-mania.co.uk
              http://www.youtube.com/user/EVguru

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              • #8
                Paul has it right,on a bolt that diameter either SAE or Metric the OD tolerances can be fairly wide.
                I just need one more tool,just one!

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                • #9
                  You might have bought these bolts from a reputable dealer but that doesn't mean that dealers don't buy lower grades as well as better ones. From the variation you are noticing on the heads it would appear that these fasteners are certainly of a grade point. The cheaper grades in inch sizes would show much the same sort of tolerance issues. And to put it into perspective with the maximum difference being .05mm we are talking about a .002" worst case variation on a bolt which is up around 5/8" diameter. That's not what I'd call a big deal on lower cost fasteners.

                  Up this way metric is about a 50-50 sort of deal. So I see a fair amount of metric hardware and have also bought my share of it. I've had no problems with finding decent quality fasteners for either metric or SAE when I pay a similar price.
                  Chilliwack BC, Canada

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                  • #10
                    Originally posted by QSIMDO View Post
                    And pie are squared...so does that mean anything between 15 and 17mm is 16mm?
                    I find it easiest to just round up to the next whole MM. So a bolt that measures 16.724 would be a 17mm - not 16mm. What sucks is having to buy extra taps and dies when your set only comes with one 11mm and it's the wrong pitch for what you need.

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                    • #11
                      STOP IT
                      Your bolts are WELL WITHIN TOLERANCE.End OF STORY. Edwin Dirnbeck

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                      • #12
                        The underlying cause it simply that nuts and bolts are not machined. It might be more accurate to say they're forged.

                        To make a bolt, a piece is snipped off a roll of wire, clamped in a vise, and successively bashed until a head is formed. The other end is then squeezed between two ribbed plates in an action a lot like rolling out a snake of Play-Doh, which presses threads into the shank.

                        The only part that could really be called "machining" is when the head is forced through a punch to form the flats.

                        And, this is all done at surprisingly high speeds, on machines that are more likely than not decades old, which have made many millions of bolts each.

                        None of it is precise, nor does it need to be.

                        Doc.
                        Doc's Machine. (Probably not what you expect.)

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                        • #13
                          Originally posted by Doc Nickel View Post
                          The underlying cause it simply that nuts and bolts are not machined. It might be more accurate to say they're forged.
                          .
                          .
                          .
                          None of it is precise, nor does it need to be.

                          Doc.
                          I would add "and if it does, the cost goes up markedly". Fasteners are available, off the shelf or custom, if needed, to much tighter tolerance than commodity graded parts. Even the cheapest box store graded parts are generally in the middle 20% or so of the tolerance range for grade and class, though. Even the counterfeit graded parts. It is quite a mature manufacturing process.

                          Of course, the cheapest of all might not meet any grade at all, and the counterfeits might fail on mechanical properties.

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                          • #14
                            While on the discussion of how bolts are made I thought some here would enjoy a look at a US facility that is well known for manufacturing high quality fasteners. These aren't cheap hardware store grade fasteners, although much of the processes of manufacturing them are similar.

                            Below is part one of a three part series on how ARP fasteners are made

                            Home, down in the valley behind the Red Angus
                            Bad Decisions Make Good Stories​

                            Location: British Columbia

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                            • #15
                              Originally posted by dian View Post
                              bolts are rolled.
                              Yeah, the difference between 15.8mm and 15.75mm is .05mm which equates to just a little less than .002". That's nothing. One of the things that beginners have trouble with is understanding that not everything has to be made to super tight tolerances. Lots of times "working right" is all that matters.

                              I've seen people agonize over the size of a drilled hole when they're putting a thread into it. Ninety-nine per cent of the time if the chart says to use "X" drill for a certain thread that's all you need. Drill your hole--with decent drill bit--run in the tap and you're done..
                              Keith
                              __________________________
                              Just one project too many--that's what finally got him...

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