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Help Determining Thread Fit (ISO 965-1) 1

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DaPinci

DaPinci

Mechanical
Mar 26, 2022
3
I am designing a cutting device to cut a threaded wire (M1.4x0.3) via counter-torque (torque applied to a nut, causes the wire to fail in tension).

ISO 965-1 states that normal engagement length is 0.7 < l < 2mm and long engagement would be anything over 2mm. The current design for the cutter internal thread is 5mm long for various reasons.

ISO 965-1 states that for long engagement threads it is better to use grades 6 or above.

ISO 965-1 also states that for M1.4 and smaller the combinations 5H/6h, 4H/6h or finer shall be chosen.

So in this instance, M1.4 with long engagement, what is the ideal thread fit?

Additionally, my manufacturer is telling me they can only get a hold of a 6H/6g machine tap. Will 6H/6g work or should I try to use 4H/6h per ISO?

Am I fretting unnecessarily? If you can recommend any textbooks or other relevant sources of information, I would appreciate it.


 
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It is typical where the modulus of elasticity is roughly the same between the threaded parts that only 3-4 turns carry the majority of the load; longer engagement doesn't make much difference.

I'm not familiar with ISO fits, but I would expect a tap would not have noticeable pitch variation for a given tap even though the clearance changes, so the sharing of the load among the turns won't be changed on fit alone.

Why tension and not the traditional shear? The end of the thread will likely need follow-on operation to make it suitable for
further use.
 
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Thank you @3DDave!

By traditional shear, do you mean some way to twist it to failure or use a wire cutter? In this application, space is limited so I don't think a chuck to provide clamping force is viable. Is that what you were thinking or something else?

I've double checked all my work and am certain now that I'm using the correct figures and formulae but at this diameter, the formulae for shear area of the external (bolt) threads (from B1.1 Appendix B) yield negative results.

Additionally, the bolt tensile strength figures all exceed the stripping forces at these small diameters.

Regarding compatibility, ISO 965-1:1998 2009 pg 15 has guidance on compatibility. I think any of the above fits are compatible. The manufacturer suggested 6H/6g so we'll go for that for now.

Thank you for your help @3DDave. Cheers!
 
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@3DDave Right, that's what I thought. I had been thinking that is too large for this application but if I could design something like that, that is aligned with the axis of the threaded rod, it may work.

Is anyone on this forum familiar with the equations for shear area/stripping strength of threads, provided in B1.1 9.3? Even with the figures for limiting dimensions provided in tables in Machinery's Handbook, they give negative values for areas for small diameters such as 1.6 with a fit of 6H/6g. So are they not applicable? Is there any other standard that provides different equations for small diameters?
 
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