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!

 

[monkeydog]

Just out of curiosity, what specification did you use when you designed the threads?

 

[Tmoose]

By "pilot" diameter, do you mean minor diameter, as might be created by a tap drill on the internal thread ?

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.jpg

 

[Screwman1]

Thread fit is controlled by the class of fit which controls both the tolerances and allowances for the male and female threads. I hope that you used an existing thread form that has been worked out correctly for this- bad things generally happen when engineers try to create their own unique thread profiles without putting a serious amount of time into understanding the tolerances and allowances that are needed for a functional fit.

 

[Helepolis]

@monkeydog
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!

 

[JNieman]

Since the question about thread function has been addressed, I'll add another bit of a suggestions:
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.

 

[KENAT]

"polymer part"

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.

Posting guidelines faq731-376

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What is Engineering anyway: faq1088-1484​

 

[Screwman1]

Interesting; you don't often see molded parts use standard 60 degree thread profiles when field removal is required because of the tightness of fit and the bending from the limited cross sectional area at the pitch diameter. I have seen many more acme and stub acme threads used when disassembly is required in the end product because of the increase in cross section and the reduction in radial stress when tightened which can cause binding in polymers with 60 degree threads.

 

[Compositepro]

How did you create the threads in the plastic part? Using a tap to thread plastic is highly variable. The type of plastic and its hardness will result in differing dimensions when using a tap designed for metals. The temperature of the plastic and sharpness of the tap will also be important factors. If the thread is molded, the molding conditions will change the dimensions.
Deforming the threads on the insert will tighten the fit.

 

[monkeydog]

Without knowing more that "a feature in Solidworks" how do you know what you are designing

 

[SwinnyGG]

Without knowing more that "a feature in Solidworks" how do you know what you are designing

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.

 

[Tmoose]

"Thank you for the first link, just what i was looking for!"

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.

 

[JNieman]

Is this a molded part? I did not see anywhere the OP stated that. Given the broad term of "polymer" and the mention of taps and drills, I assume we are talking about machinable plastics, like UHMW, PEEK, or something like that.

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.

 

[Helepolis]

Typically when using ball plungers, one would apply a thread locking compound rather than creating a special or unusual thread form.

Yes I'm aware about the thread locking compounds, but i think the tight thread was there for a reason, as the plunger is there to create a low force click-in lock for another part, the two parts will be assembled and dissembled quite often.
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.

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.

I have absolutely no info about the original part, as it is military product and not intended for the general public at all.

Interesting; you don't often see molded parts use standard 60 degree thread profiles when field removal is required because of the tightness of fit and the bending from the limited cross sectional area at the pitch diameter. I have seen many more acme and stub acme threads used when disassembly is required in the end product because of the increase in cross section and the reduction in radial stress when tightened which can cause binding in polymers with 60 degree threads.

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.

How did you create the threads in the plastic part?

All of the post-processing operations on the part are done using machinery, including the thread.

Without knowing more that "a feature in Solidworks" how do you know what you are designing

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.

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?

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.

That chart refers to PITCH diameter, which is NOT the same thing as MINOR diameter.

Maybe i understood incorrectly, but the pitch diameter defines the tightness of the female thread, right?
If so, this chart is what i need.

 

[Screwman1]

There is a lot more than just pitch diameter that determines "tightness" of the threads. You need to have the flank angles, major, minor, lead angle, drunkeness root radius etc, all taken into account to control the 'fit' of threads as well as the tolerance and allowance of those parameters on the mating threads. If you can't measure the thread in the mating part you will be chasing this for a long time. Are you using a variable element gage or pitch wires to measure the pitch diameter?
Check out ISO 898-1 for the full dimensional standards for 60 degree metric threads.

 

[Helepolis]

There is a lot more than just pitch diameter that determines "tightness" of the threads. You need to have the flank angles, major, minor, lead angle, drunkeness root radius etc, all taken into account to control the 'fit' of threads as well as the tolerance and allowance of those parameters on the mating threads.

Thank you for the reference, i'll absolutely check it out, but maybe there is a simpler solution to make the thread tighter.
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.

 

[Nescius]

In a low force, low precision application where I needed removable threadlocking behavior between a molded plastic hole and a metal fastener, I'd investigate the possibility of eliminating the secondary threading operation...mold a suitably sized plain hole in the plastic part, no threads, and just screw the fastener in. Like I said, if the forces are low, you might get enough holding power without a heavy interference fit that distorts or stresses the plastic part.

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.

 

[3DDave]

There aren't any commonly used interference-fit straight threads. There are a few self-wedging thread forms that depend on fastener tension to make work, but that won't work for an item that isn't put into tension.

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.

 

[KENAT]

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?

Posting guidelines faq731-376

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

 

[Screwman1]

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. What you are trying to do is to deal with the different classes of fit that are available for threads; if you are going to do this with precision fit you will be working with something like a 4h tolerance grade and position, rather the the standard 6g (4H, 6G for internal threads). You will for sure want to drop down to the "h" allowance to reduce clearance. If you don't understand this stuff you should get the ISO thread standard and study it until you do.