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I am looking to make a design update to the threading of the main body hand piece (piece in which you hold) to the different size standoffs (basically pieces that position the laser beam at different distances). The current thread is a 3/4"-18 triple start thread but is causing a lot of issues when customers go to install the different stand offs. I am looking to have a thread that is easy to install while also provides a secure fit. UNF while provides the strength could present issues with installment because the customer could cross thread AND it also takes longer to thread. What is a threading you may recommend for this scenario? Any links or references is always appreciated. Thanks for your time!

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    $\begingroup$ Why not bayonet fitting? Much faster. No alignment issues. 1/4 turn to lock. $\endgroup$
    – Transistor
    Jan 22 at 15:26
  • $\begingroup$ Are you using a 60 degree thread form? You might want to consider square or trapezoidal. I don't know the name of it, but there's a way to machine the start of the thread so it's much harder to cross-thread. $\endgroup$
    – TimWescott
    Jan 22 at 15:54
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    $\begingroup$ @TimWescott maybe you mean higbee cut $\endgroup$
    – joojaa
    Jan 22 at 16:13
  • $\begingroup$ @joojaa -- yes, that's it. My dad was a firefighter, which is where I learned of it. All I remembered was "dat thread treatment whut is on dat hose end". If the OP is implementing this as molded parts it won't cost extra -- ymmv on machined parts. $\endgroup$
    – TimWescott
    Jan 22 at 18:06

2 Answers 2

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Looking at a the bottle cap of a 1/2 gallon plastic (blow molded) milk jug, for inspiration. Is skinny, and happens to have a triple-start thread like yours. So maybe the triple-start thread is a good thing to have?

aa

The thread doesn't start all the way at the top, and there's a little bit of plain cylinder which mates with a round groove (or annular/cylindrical protrusion) in the cap -- noted above as [A].

Not 100% sure, but it seems to give the user some extra tactile feedback about alignment before the threads engage. Possibly [B] and [C] would bump into each other (in addition to the side walls) if it were out of alignment.

ab

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    $\begingroup$ this is similar to a higbee cut tread. Irs sort of the same thing in injection form $\endgroup$
    – joojaa
    Jan 22 at 16:30
  • $\begingroup$ @joojaa - cool! you should make that into an answer with a reference or pic of it $\endgroup$
    – Pete W
    Jan 22 at 16:53
  • $\begingroup$ i am trying to get access to ASME B1.7-2006 which apparently covers this. But my standards collection contract does not allow me to see ASME publications online. I might be able to go to the standardisation offices library later this week they have all publications available for reading for free on premises as long as you do all your notes with pen and paper. $\endgroup$
    – joojaa
    Jan 24 at 8:08
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People cross threading joints is usually due to alignment, consider a sleeve to control alignment then engage the thread.

Check out the types of joint used in pool cues - uniloc etc that cannot be cross-threaded.

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