Thread Tightness Specification?
Thread Tightness Specification?
(OP)
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!
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!





RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
pitch diameter controls the thread "looseness" not the minor diameter.
http://www.newmantools.com/taps/fig9.gif
What did your measuring consist of testing with a thread gage ?
like this -
https://i.ytimg.com/vi/Gdvtw0pTAOs/maxresdefault.j...
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
I use Solidworks 2016 so i didn't exactly designed the thread from scratch, the software comes with predefined standards which one of them is the ISO, which i use to make threads.
All you need is the center of the hole and position it on a plane and the software makes all the rest, of course you can modify manually the parameters of the the thread, but this time it wasn't necessary.
Works like a charm but this is the first time that i needed the thread to be a tighter fit around the thread of the plunger.
@Tmoose
Yes, by "pilot" i mean the minor diameter. Used that term as this hole will guide the tap drill.
And yes, used a thread measuring gauge similar to the one in the link made by INSIZE.
Thank you for the first link, just what i was looking for!
RE: Thread Tightness Specification?
Typically when using ball plungers, one would apply a thread locking compound rather than creating a special or unusual thread form. This will prevent it from backing out once set at the appropriate depth. This is, of course, assuming it is not installed into a blind hole in which is bottoms out in.
RE: Thread Tightness Specification?
Do you not have the drawing/information for the part you are re-creating? It may be that it had some kind of deformed or 'locking' thread form not standard M5.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
Deforming the threads on the insert will tighten the fit.
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
Unless you tell it differently, Solidworks will automatically define threads per ISO or ASME standards with correct tolerances and callouts.
Unless an engineer is very very familiar with the intricacies of fastener design (as someone already said) threadforms are not a place to re-invent the wheel.
There are a couple potential sources of error I can see.
OP, are these parts tapped by hand? Hand tapping in plastic is likely to yield results with high variation. Plastic is much less stiff than metals, so even the speed at which the tap is turned will have a noticeable effect on final thread dimensions.
Was the thread actually cut in the model, and then the mold for the new part taken from model geometry?
If so, this is a problem- thread tolerances are very fine. Conventional thread forms need not, and should never be, modeled for use as the master. Specify the thread type and class, and allow the molder to figure out how to hit the tolerance window.
RE: Thread Tightness Specification?
That chart refers to PITCH diameter, which is NOT the same thing as MINOR diameter.
http://www.newmantools.com/taps/fig9.gif
Unless the minor diameter (incorrectly) ends up bigger than the pitch diameter.
RE: Thread Tightness Specification?
As mentioned before, however, the method of tapping can have a large effect on the resulting thread form due to the mechanical properties of some of these materials. Gaging will tell if it's the thread form that is causing the issue of course.
RE: Thread Tightness Specification?
My guess is that the tight thread is to maintain the plunger in place but without any locking compounds so it can be adjusted in case of wear.
This part is indeed a molded polymer part.
I have absolutely no info about the original part, as it is military product and not intended for the general public at all.
If i understand you correctly, the purpose of the plunger is only to create a low force locking latch, so the axial forces are very low, only during assembly for an instance when the ball is retracted before popping back in place into a notch on the second assembled part.
All of the post-processing operations on the part are done using machinery, including the thread.
I'm using solidworks for well over 6 years (before i started my degree) and i believe that I'm quite familiar and proficient with this software including the "Hole Wizard" feature. Non the less iv'e graduated a couple of months ago and still learning the trade, on top of it all this part was a high priority, time wise, so i didn't study the topic of threads thoroughly enough.
The parts are taped using a semi-automated process, where the parts are assembled on to a machine to create the threads.
The thread was defined using the dimensions i took from the plunger, the threaded hole in the original part was unreachable for measurement tools.
Maybe i understood incorrectly, but the pitch diameter defines the tightness of the female thread, right?
If so, this chart is what i need.
RE: Thread Tightness Specification?
Check out ISO 898-1 for the full dimensional standards for 60 degree metric threads.
RE: Thread Tightness Specification?
This isn't a high precision feature, so i hope that for cases like this there is a more general way to make tight griping threads.
For example the (auto) hole callout in SW for the thread for technical drawings, shows only the minor diameter, thread size (i.e. M5, M10), pitch, depth, basic pitch tolerance and some other features not directly related to the thread itself (i.e. counter sink), so I'm guessing that if this info is enough to create a fully functional thread, there is a simpler way (maybe some other metric standard or a default tight thread type in the ISO standard), because it seams that by changing all (or some) of the parameters you mentioned will no longer be an ISO standard.
RE: Thread Tightness Specification?
I happen to be wearing my mass-production thinking cap right now, so this idea may not apply to your application at all. Good luck.
RE: Thread Tightness Specification?
The way to know this is true is to notice the limited search results from Google.
The most common way of retaining threaded items is with deformed threads, but that isn't easy with plastic parts. If it was me I'd look at pushing a press-fit pin alongside the threaded hole to deform the thread that way.
RE: Thread Tightness Specification?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
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
RE: Thread Tightness Specification?
I might go with the idea KENAT suggested, to leave a small portion of the hole unthreaded.
And thanks for the good luck!
I have absolutely no way to check this out, but this can be a great solution. Thanks for the idea!
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.
It sounds like a good and simple solution, i'll look into it. Thanks.
RE: Thread Tightness Specification?
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?
RE: Thread Tightness Specification?
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.
RE: Thread Tightness Specification?
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
RE: Thread Tightness Specification?
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!
RE: Thread Tightness Specification?
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.
RE: Thread Tightness Specification?
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).
RE: Thread Tightness Specification?
It is not too late,
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?
Doesn't mean it isn't work a try though.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Thread Tightness Specification?
RE: Thread Tightness Specification?