What's new
What's new

NPT threads generating shavings and flakes during use

JesseMENG

Plastic
Joined
Feb 15, 2016
Greetings,

I'm hoping to get another set of eyes on an issue. We're trying to solve an irritating problem that has plagued a lot of our NPT threaded solenoid vacuum valves. The aluminum body of the valve has been tapped to 1/4" NPT. Threads have been checked with a 1/4" NPT plug gauge and are within spec. When we screw in a male threaded quick-release valve (brass, chrome plated) into the solenoid valve, we end up with small metal shavings that make their way down into the valve, which contains multiple O-rings. We're finding this debris is causing small leaks. We've also checked the male 1/4"-NPT threads with a ring gauge, and it too measures within spec.

Here are a couple photos of the shavings we're finding:

1708370022963.png 1708370064733.png

Some additional info on how we're installing our male QR valves:
- We have tried various wraps of teflon tape, from 1 to 4 wraps, all yield shavings
- We've applied torque from hand-tight, to 10-20in-lbs, up to 80in-lbs (our standard torque for this application) - all generate some amount of shavings
- We've tried various liquid thread sealants. Some help encapsulate the shavings, but we still get shavings.

Is there some standard way of installing NPT threaded components into aluminum that we don't know about? Is it normal to generate such shavings? I imagine most applications may not be as sensitive as ours, so perhaps this issue isn't even noticed most of the time. I'm open to any ideas to try to alleviate this issue.

Thanks guys!

Jesse
 
Are the shavings aluminum or chrome? If chrome, you may have a plating problem, if aluminum it might be a thread-form issue on the plated part. Do the male threads seat to a 'normal' depth? If not, perhaps the male thread was not formed with subsequent plating in mind.
It is not normal to see such shavings in aluminum fittings.
 
Are the shavings aluminum or chrome?
Good question. We wondered that too. It's tough to tell even with our microscope. So we did a similar test and took a brass fitting with female threads and inserted our QR into it using the same methods. We found brass colored shavings. So we believe the metal is coming from the female aluminum threads.
 
You have surely done this, but inspect all threads and look for standing burrs at the internal and external thread crests. Make sure you are using high quality taps.

I hate tapered pipe reamers, but using one might help to produce a crest on the female thread, instead of leaving it all to the tap. Reamer depth control is critical. It's easy to overcut and create an oversize crest.

I always try to threadmill female threads to reduce burrs, and the little flap that forms from tap reversal in ductile materials.

Even with standing burrs, it's hard to understand why they would be pushed ahead of the fitting and not stay in the assembled part of the thread. It almost seems like a burr on the leading part of the male thread is cutting a chip that is moving ahead of the fitting.
 
Can you tell if the parts had a taper cut prior to thread
That I can not tell.

You have surely done this, but inspect all threads and look for standing burrs at the internal and external thread crests. Make sure you are using high quality taps.

I will certainly double check this, though nothing has stood out as being a significant concern. We don't machine these ourselves, but are working with the supplier to "fix" these. But first need to understand what is causing it. Is it that way we're installing the QR's? Are the threads incorrectly milled? Maybe they had a worn out tap? Or, small metal shavings are inherent to using NPT on soft metal?

Fun times! ;-)
 
. . . Or, small metal shavings are inherent to using NPT on soft metal? . . .
I've never encountered that over many years. The most common change in soft-metal threads is simply a 'form-to-fit', marked by a polished appearance of the threads.

As others have said, if you were using rough steel against aluminum, ok I see that, but normally, no.
 
Are either of the threads rolled or formed threads?
The female thread is machined/tapped based off the four vertical lines we can see (assume from the tap four flutes), one of which coincides with the end of the thread: 1708376953254.png

I am unsure of the manufacturing technique for the off-the-shelf QR valve, which is just like this one off McMaster. Here is a photo of ours:

1708377086361.png
 

Attachments

  • 1708377031729.png
    1708377031729.png
    1.2 MB · Views: 13
The female thread is tapped. It looks ok.

At 6 oclock on the male thread you can see the culprit I think. Just to the right of the crest a burr is pointing to the right. There may be nicks on the edge of the starting chamfer. Also tiny burrs on the crests.

The supplier should ensure no burrs on the starting thread.
 
The female thread is tapped. It looks ok.

At 6 oclock on the male thread you can see the culprit I think. Just to the right of the crest a burr is pointing to the right. There may be nicks on the edge of the starting chamfer. Also tiny burrs on the crests.

The supplier should ensure no burrs on the starting thread.
I do see what you see there. I just went out to reinspect that one (still had it sitting under microscope). The crest on the leading thread sure has a lot of dings on it (all the threads for that matter). I can feel the slight ridges with my fingernail. While I don't see any residual burrs from manufacturing, it is very apparent these are getting banged-up. They're shipped in bags that allow these to bang against each other. We've noticed the tiny dings on the threads, but being so small, didn't believe we were causing the shavings we were seeing. But, perhaps that is indeed the culprit.
1708379344247.png
 
Looks like they recut the chamfer to eliminate the burr from threading, but all they did was push it into the first thread.
 
You sure you are not having problems with the cast aluminum in the valve bodies? Did your engineers or suppliers changed something in the alloy cast?
 
You sure you are not having problems with the cast aluminum in the valve bodies? Did your engineers or suppliers changed something in the alloy cast?
We still aren't sure about anything at this moment. We're in the process of doing an RMA with the manufacturer for their inspection of the aluminum valve bodies. They claim the most recent batch was produced in the same way as the previous ones. However, being able to duplicate the problem on independently produced female NPT fittings with our same QR fittings has me suspicious that it may not be the valve's fault.

Last NPT fittings I got came with plastic thread protectors installed and were perfect when new.

That is a much better way of packaging. I like it.
 
100% the male thread is the problem!

Threaded parts should never be thrown together in a bag. Quite a few packing methods exist to prevent damage. Some of the clinging chips are due to plain don't give a damn or don't know any better.

You need another supplier! Let me quote the male part for you.
 








 
Back
Top