What's new
What's new

Urgent - please help if you can! Female Metric Thread Question

NVIDIA

Plastic
Joined
Jan 12, 2018
Hello,

I hope someone can be so kind as to help me with a query.

Thread Problem.jpg

I have recently checked a batch of products from a new Chinese manufacturer. The first crate of goods passed all of my tests so I approved it and moved it out of quarantine. We also had it tensile tested for strength - it passed.

So it was sent to the customer.

But today I found significant defects but none that can kill people! I was coming to the end of the final random sample and I can see that they have machined out the initial female thread recess with a much, much bigger diameter drill than they should.

Resulting in the label B being cut like it is.

Now I don't know if I have found 1/5000 like this but this can be very serious for us if one was to be like this. I need to do a full product recall and 100% inspection.

The threads in question are Female M16, M20,M24,M30

I am extremely worried about this and I do not know how to find out the minimum/maximum dimension this should be for British standards. I hope people understand what I'm asking?
 
Quite common when you send out work without specifications. Maybe the drill even wobbled. What I would do is test it with a higher class fastener if possible.
 
I hope this isn't rude, but it doens't sound like you have an engineering background. The threads you listed are fairly large, but we don't know the materials, loads, or application of the parts, and so can't actually determine if there's any risk to human life from the defect.

I think it would help if you could either provide more information, or contract with an engineering service to review the parts and application to determine whether there's a risk or not.
 
I am extremely worried about this and I do not know how to find out the minimum/maximum dimension this should be for British standards. I hope people understand what I'm asking?

Threads aren't typically inspected by diameters directly - you use a go/no-go thread gage specific to the thread you're inspecting. The typical classification for fit on a metric thread is 6H, although there are others. The 'go' side of the gage has to thread in smoothly, and the 'no-go' side cannot thread in more than a certain number of turns. 3 complete turns comes to mind.

Someone smarter than me can probably tell you the allowable number of turns a no-go gage can go into a threaded hole and still be considered acceptable, but I don't know off the top of my head. It may vary by classification and/or size.

https://www.grainger.com/product/6DHV0?cm_mmc=PPC:+Google+PLA&s_kwcid=AL!2966!3!56542783677!!!g!102360821157!&ef_id=WBtdSQAABdqpQB6t:20180112192718:s&kwid=productads-adid^56542783677-device^c-plaid^102360821157-sku^6DHV0-adType^PLA
 
Ha Ha Ha....Oh this is a good un.

So your listing as a "Buyer", you farmed stuff to China,
and now they are junk.

You got rid of too many people.

You need a proper draftsman/design/engineer, to properly document your tolerances/quality.

You need an proper "Incoming Inspector" to verify the parts are made "To Spec".

BTW a small hint for you.
When buying form overseas, don't try to use "Sample Lot" techniques to inspect.
You have no control on where they were made, and many times, more than one
small vendor made the parts, all dropping them into one box.

Your going to have to check 100%
 
Metric isn’t a British standard, it’s an ISO standard. You can find all the info you need for standard metric threads in the bible and if it’s a non standard you can work it out from info given.


Sent from my iPhone using Tapatalk
 
Hello,

The threads in question are Female M16, M20,M24,M30

I am extremely worried about this and I do not know how to find out the minimum/maximum dimension this should be for British standards. I hope people understand what I'm asking?

You didn't describe the threads you are working with completely so a definitive answer is gonna be hard to come by. M16x???

Plenty of sources for this info though. It's by no means some obscure trade secret. If you don't own a Machinery's Handbook, just Google minor diameter for the thread in question and you'll soon know all you wanted to know and much more.

Not meaning to offend, but if you don't even know how to find such basic information, are you sure you should be doing the inspecting yourself?
 
I hope this isn't rude, but it doens't sound like you have an engineering background. The threads you listed are fairly large, but we don't know the materials, loads, or application of the parts, and so can't actually determine if there's any risk to human life from the defect.

I think it would help if you could either provide more information, or contract with an engineering service to review the parts and application to determine whether there's a risk or not.

Hello,

No this is not rude at all. The material is 304 Stainless steel. The loads on the M24 are 2500kg - with a 3:1 safety factor. As I previously said it passed at testing at 7.5 tonne. It is a fixing plate for hanging a beam.

There will be a product recall, the products have not been used yet. But I will be doing dimensional checks tomorrow on 100% of batch to test if anymore have been done like this.

Full dimensional drawings were sent, however they have used the wrong drill bit for the initial hollowing out of the female threading - clearly far too big. All the threads have been cut correctly.
 
You didn't describe the threads you are working with completely so a definitive answer is gonna be hard to come by. M16x???

Plenty of sources for this info though. It's by no means some obscure trade secret. If you don't own a Machinery's Handbook, just Google minor diameter for the thread in question and you'll soon know all you wanted to know and much more.

Not meaning to offend, but if you don't even know how to find such basic information, are you sure you should be doing the inspecting yourself?

Thank you!

Metric Coarse (Standard)

Could you please help me with the allowable tolerances?
 
Interesting discussion. I'm not really familiar with go/no-go gauges. My understanding is that a go gauge would certainly thread in. Would a no-go gauge capture the excessively large minor diameter on the female thread? Or if the major diameter is correct, would the no-go indicate that the thread is OK?

And how far can you go? 75% is routine. I did find a Kennametal reference for a 50% thread that states:

A common internal thread, drilled so that it results in 50% of full thread will break the external thread, before the internal thread will strip. A common internal thread drilled out so that it contains 100% of full thread is only 5% stronger than a 75% height of thread, yet it requires 3 times the power to tap.
 
Per Machinery's Handbook......

M16x2 internal minor diameter 13.835mm-14.135mm
M20x2.5 internal minor diameter 17.294mm-17.649mm
M24x3 internal minor diameter 20.752mm-21.152mm
M30x3.5 internal minor diameter 26.211mm-26.661mm

For class 5H fit
 
Thanks so much for that guys! Obviously the recall is underway already and I will be consulting with engineers. I'l be sure to update people on the outcome.

Very interested to see if a go-no go guage would have picked this up?
 
Hello,

No this is not rude at all. The material is 304 Stainless steel. The loads on the M24 are 2500kg - with a 3:1 safety factor. As I previously said it passed at testing at 7.5 tonne. It is a fixing plate for hanging a beam.

There will be a product recall, the products have not been used yet. But I will be doing dimensional checks tomorrow on 100% of batch to test if anymore have been done like this.

Full dimensional drawings were sent, however they have used the wrong drill bit for the initial hollowing out of the female threading - clearly far too big. All the threads have been cut correctly.

Thanks for the additional information. From what you mention, I would be very concerned about the incorrect threads in these parts, and doing a full recall is appropriate. You should not only ensure all parts are received, you may have to initiate a formal scraping method to ensure that any bad parts do not somehow enter inventory again (i.e. a destructive action to prevent use).

Are the thread loads in tension or shear? Could the parts be tapped deeper and a longer bolt used? Even if there are possible fix methods it might be best to have new parts made, and perform not just dimensional checks when the new parts arrive, but materials testing too (strength, ductility, chemical composition) to ensure the vendor did not compromise anywhere.

And perhaps check again to see if a British shop could provide the parts at a reasonable cost. Somewhat greater up-front cost can be amortized through lower risk of failure or lawsuit down the road.
 
Thanks so much for that guys! Obviously the recall is underway already and I will be consulting with engineers. I'l be sure to update people on the outcome.

Very interested to see if a go-no go guage would have picked this up?


A GO-NOGO only checks the pitch diameter. It might only catch it if the holes were drilled larger than the pitch diameter. On critical threads, thread percentage should also be inspected, meaning checking the minor diameter against documented tolerances.
 
Thanks so much for that guys! Obviously the recall is underway already and I will be consulting with engineers. I'l be sure to update people on the outcome.

Very interested to see if a go-no go guage would have picked this up?

Hi,

You work for NVIDIA?

Cheers.
 
I do a lot of work with Asia-based vendors. While this sounds like a true mistake (someone used the wrong size drill), often vendors will cheat their tap drill size large in order to improve tap life. I've learned that when a thread counts, always use a nogo pin gauge for the minor diameter along with your go/nogo for the pitch diameter.

Regards,
Henry
 
^ if that makes you nervous, you better a void a whole lot of things in life. Hell most bridges don't even get 2:1. Pressure vessels only get 1.5:1 think you need to recalibrate your worryometer scale!
 








 
Back
Top