Thread Tightness Specification? 5

[Helepolis]

Hi all,

I had to improve an existing polymer part, on this part was a threaded hole for a ball point plunger and the plunger had some resistance wile screwing it in (something like a nylon-insert locknut).

The thread on the part that i made is an exact match to the original plunger (confirmed the thread with a gauge before modeling the part CAD and after manufacturing) but it feels very loose when screwing it in and out.

So i was wondering, is there a standard for defining the "tightness" of the thread or i need to manually override the pilot hole diameter to a smaller size, for example the thread it metric M5 and the pilot hole Dia. will be 4.20 mm (by ISO standard) so to make it a tighter fit i need to override it to 4 mm (just guessing).

Thanks!

 

[Compositepro]

I think Nescius makes an excellent suggestion for this application. Use an unthreaded hole that is only a few hundredths of an inch smaller than the outside diameter of the screw.

 

[MikeHalloran]

OR, wrap the plunger with teflon pipe thread tape.

You can get fancy and specify a minimum installation torque, with instructions to add wraps of tape until it's exceeded.



Mike Halloran
Pembroke Pines, FL, USA

 

[Helepolis]

mold a suitably sized plain hole in the plastic part, no threads, and just screw the fastener in.

Well, it might be a good idea but the quantity of the manufactured part will be a couple thousand. So maybe the specific application of the plunger is low precision, I'm still worried of inconsistent assembly.
I might go with the idea KENAT suggested, to leave a small portion of the hole unthreaded.
And thanks for the good luck!

Any chance the original part left at least part of the thread engagement length unthreaded to act a bit like a Nyloc or something along those lines?

I have absolutely no way to check this out, but this can be a great solution. Thanks for the idea!

You can't rely on Solid Works for everything- this is the stuff that gets engineers into trouble- thinking that the computer knows everything that there is about every possible product feature.

You got me all wrong, i definitely do not rely on SW and it's automated features. As i said before I'm new to real world engineering and so far the default settings of the ISO standard in SW were enough to design what i needed, and I'm still learning.

wrap the plunger with teflon pipe thread tape.

It sounds like a good and simple solution, i'll look into it. Thanks.

 

[Tmoose]

"If you can't measure the thread in the mating part you will be chasing this for a long time."

I'd say that comment by Screwman is among the most important so far, if the goal is to duplicate the original design's function and feel.

======================

Would a certain amount (quantified) of "prevailing torque" suffice?

 

[CWB1]

I agree with Tmoose and Screwman above. You are trying to establish a mating fit between parts so without knowing dimensions of the threaded hole you are flying blind and wasting time. Either contact the manufacturer to request the thread class, measure a few parts, or guesstimate based upon the fit of your part.

Not to belittle, but I would highly recommend some time studying manufacturing and machine operations texts, and bouncing these basic manufacturing questions off local senior engineers. Process is everything, depending on how the threads on your part are manufactured (cut vs formed vs molded etc) answers to your questions may vary. Also, pilot holes don't necessarily affect the thread, hence why prints don't get a drill diameter on a tap callout.

 

[pylfrm]

Helepolis,

If the external threads are M5 × 0.8 - 6g, then .1900-32 UNF-2B internal threads should provide an interference fit. I'm not saying this a good idea for your application, but perhaps it's worth considering.


pylfrm

 

[Helepolis]

I've managed to recreate the same feel for the plunger locking, the only thing that bothers me is the looseness of the plunger.
It doesn't impair the function of the lock, I'm just worried that from the frequent assembly and disassembly of the parts the plunger might move and i don't want to use any thread locking compounds to keep the possibility of adjustment in case of wear.

Anyway, you guys gave me a lot to think about and excellent ideas.

Thank you!

 

[Tmoose]

"I've managed to recreate the same feel for the plunger locking...."

In the beginning that sounded to me like prevailing torque. It still does.

What internal thread provided the correct "feel?"

The description of "looseness of the plunger" has me confused.

 

[Helepolis]

"I've managed to recreate the same feel for the plunger locking...."

In the beginning that sounded to me like prevailing torque. It still does.

What internal thread provided the correct "feel?"

The description of "looseness of the plunger" has me confused.

I'll try to clarify, the original product consisted out of 2 main parts, a female and male part, that are designed to be assembled and disassembled frequently.
To prevent the two parts from disengaging after assembly, a ballpoint plunger is used to act as a latch.
For the sake of the discussion, the thread for the plunger is in the female part and when the plunger is screwed in the thread, it feels like prevailing torque (like screwing on a Nyloc nut), there is a resistance.

Now for what Iv'e managed to recreate.
The whole assembly and disassembly of the female and male parts, feels and behaves exactly like the original product.
My only concern is that when i screw in the plunger i don't feel the resistance like on the original part and it can be easily screwed and unscrewed, like a regular nut (with the flick of the finger it screws easily on the bolt).

 

[Tmoose]

Pictures and drawings would have been mighty useful in the very first post.
It is not too late,

 

[3DDave]

I don't get why the spring plunger doesn't have a nylock element.

 

[KENAT]

Plungers with plastic patches for thread locking tend to say not for use in plastic or 'soft' metals from what I've seen.

Doesn't mean it isn't work a try though.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[3DDave]

Pre-run them into a hard thread? It's no worse than believing in a secret thread form that allows a polymer to slightly grab a screw.

 

[Helepolis]

I'll add some pictures tomorrow from work, maybe it'll clarify some things.