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As I was browsing ISO 965-1 it defines tolerances on a plenty of variables regarding metric threads, but it gives the pitch of the thread as a simple number from an allowed set ( 3 – 2 – 1,5 – 1– 0,75 – 0,5 – 0,35 – 0,25 – 0,2 ), and no tolerances on these are ever given.

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Why is it so? Are they defined in some other standard or is there a specific reason why they aren't defined to a given tolerance? Specifically, when I'm using a threaded rod as a lead-screw actuator, I'd like to know what inaccuracy can I expect coming from imprecision of pitch - how accurately do 1000 turns of a M10x1 threaded rod convert to 1000mm travel distance of the nut (minus backslash etc)?

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  • $\begingroup$ Does "normalized" in your title have some special meaning in this context or are you using it as a synonym for "standardized?" If the latter, I would avoid "normalized" as it has many special meanings. $\endgroup$ – Air Feb 26 '15 at 20:33
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    $\begingroup$ @Air: I meant this as "standarized", a habitual language calque from my native language. $\endgroup$ – SF. Feb 26 '15 at 21:36
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I think the pitch is implied by the major and pitch diameter.

The tolerance of these $D_{maj}$ and $D_{pitch}$ are given in ISO 965-2

I'll attempt juggling with the math in the ISO metric screw wiki, (please correct if wrong): http://en.wikipedia.org/wiki/ISO_metric_screw_thread

$(3/8)H=(D_{maj}-D_{pitch})\cdot(1/2)$

$H=P\cos(30^\circ)=P\sqrt{3}/2$

$P = \underbrace{(2/\sqrt{3})\cdot(8/3)\cdot(1/2)}_{1.5396}\cdot (D_{maj}-D_{pitch})$

For an M10x1.25 fine thread (could not find the M10x1.00) this would result to:

M10x1.25: $D_{maj}=[9.972...9.76]$ $D_{pitch}=[9.16...9.042]$ --> $P=[0.92..1.43]$

So for a $1000mm$ rod, this would be a nominal $800$ turns with a minimum of $698$ and a max of $1083$.

Quite a range I think! I also found this picture of a mismatch failure (src): broken

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  • $\begingroup$ I'm pretty sure you've got a couple errors in here (at least one is that the difference in major diameter and pitch diameter is 3H/8, not 5H/8) but once I started into it I started getting crazy numbers. $\endgroup$ – Trevor Archibald Feb 27 '15 at 0:45
  • $\begingroup$ True, I corrected this and also the diameter to radius. Tnx $\endgroup$ – Walter Feb 27 '15 at 8:37

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