Urgent - please help if you can! Female Metric Thread Question - Page 2
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  1. #21
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    ^ 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!

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    Just order some chinese bolts to fit the chinese holes...

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    Question - so long as there is sufficient material (the threads are strong enough on test), does it really matter what the standard is? A lot of the time minor diameters are overbored intentionally (slightly) in order to make the threading op easier. If I remember/understand right, this can have a surprisingly small effect on the functional strength of the thread.

    I'm guessing that you need to make it belt and braces though, since it's a safety critical part...therefore standards have to be adhered to.

    Question is, like others have asked - how did you get to be inspecting safety-critical parts, without knowing what the allowable tolerances were? Again, I don't mean to be rude about it, but something needs some serious review there!

    I hope you get it sorted properly, and well done for catching it before any parts were put into service.


    T

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    Quote Originally Posted by adama View Post
    ^ 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!
    I don't know where you got your info, but here in the USA the various
    standards are a wee bit more than that.

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    Quote Originally Posted by Tam. View Post
    Question - so long as there is sufficient material (the threads are strong enough on test), does it really matter what the standard is?
    His test rig probably has a very nice fitting male thread.

    What would happen when a male thread, right at the low limit on pitch and major diameters, is used in the tensile test. Might give a different result. Ultimately isn't that the reason for standards?

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    Quote Originally Posted by Tam. View Post
    Question - so long as there is sufficient material (the threads are strong enough on test), does it really matter what the standard is? A lot of the time minor diameters are overbored intentionally (slightly) in order to make the threading op easier. If I remember/understand right, this can have a surprisingly small effect on the functional strength of the thread.

    I'm guessing that you need to make it belt and braces though, since it's a safety critical part...therefore standards have to be adhered to.

    Question is, like others have asked - how did you get to be inspecting safety-critical parts, without knowing what the allowable tolerances were? Again, I don't mean to be rude about it, but something needs some serious review there!

    I hope you get it sorted properly, and well done for catching it before any parts were put into service.


    T
    They have done the minor diameter on the M24 at 21.44mm (edit - spoke to manufacture and they used 21.5mm drill)- I'm not prepared to take the risk and will be scrapping the lot. We have some consultants who have been guiding our inspection procedure. I checked everything I was told to do - so a full review needs to take place there. For the M30 they used 27.5mm drill.

    There were engineers drawings and draftsmen but I now know that the drawings are no way near detailed enough in relation to threading tolerances.

    Steep learning curve!

  8. #27
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    Quote Originally Posted by NVIDIA View Post
    They have done the minor diameter on the M24...

    the drawings are no way near detailed enough in relation to threading tolerances...
    Really?

    An M24 is an M24.
    Period.
    The End.
    There need be no tolerance indications on the print unless that M24 is a special in either Pitch, Minor, Major or Class!
    ( though I still say that the pitch should be clearly stated on metric threads, regardless of ISO or not .... )

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    Quote Originally Posted by SeymourDumore View Post
    Really?

    An M24 is an M24.
    Period.
    The End.
    There need be no tolerance indications on the print unless that M24 is a special in either Pitch, Minor, Major or Class!
    ( though I still say that the pitch should be clearly stated on metric threads, regardless of ISO or not .... )
    Yes M24 is M24 period. But clearly if we do not control manufactures in China they will do what they want. An M24 thread cut into a M24 whole isn't going to have the same outcome. They used a 21.5mm drill because the stainless steel material was hard and by making it larger than they should have it stopped the threading tap breaking and it was easier to tap.

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    A quick fix, provided you have material around the hole will be to redrill and tap the holes for an insert like the helicoil, it will end your current grief and will give you extremely strong threads.

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  12. #30
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    Quote Originally Posted by NVIDIA View Post
    They have done the minor diameter on the M24 at 21.44mm (edit - spoke to manufacture and they used 21.5mm drill)- I'm not prepared to take the risk and will be scrapping the lot. We have some consultants who have been guiding our inspection procedure. I checked everything I was told to do - so a full review needs to take place there. For the M30 they used 27.5mm drill.

    There were engineers drawings and draftsmen but I now know that the drawings are no way near detailed enough in relation to threading tolerances.

    Steep learning curve!
    If it hasn't been pointed out, the size drill they used does not necessarily equal the size hole they ended up with before threading.

    mtd.jpg

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    Quote Originally Posted by NVIDIA View Post
    Yes M24 is M24 period. But clearly if we do not control manufactures in China they will do what they want. ...

    And there is where your problem lies.

    An M24 is an M24, no further clarification is needed..

    Here, from this side of the pond ( as the dumbfuck Americans we are ) you may have gotten a quick question to specify the pitch, otherwise nothing else.
    An M24 x nn is an M24 x nn ( where "nn" stands for the pitch or elevation ), no need for tolerances unless they are special for your needs.

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    It goes to show that you get what you pay for with chinese manufacturers, I've seen too often parts disappear to chinese vendors only to come back to us 6 months later due to poor quality or excessive lead time. Best one I've seen is 1000's of rusty SS316 brackets fully assembled on a prestigious building, all had to be taken down and replaced costing many thousands of pounds all in the name of saving a few quid getting them made in china (they even had a fake material cert with them). I got to make the replacements and charged even more than my original quote on them making for a very costly mistake by the buyer.

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    NVIDIA, one other thing I'd like to bring up is how the fasteners that mate with these tapped holes are going to be used. If they'll be in tension (load hanging from the axis of the fastener) then it's more critical that the thread be to spec, as design strength is based on a certain amount of engagement of male and female thread. If they're in shear (load perpendicular to bolt axis) then there's a "little" more leeway, again, depending on design, as you're not simply dependent on thread pullout or similar failure.

    Another aspect is fatigue, if these fasteners will see regular flexing loads the joint design is more critical. And if you're using stainless steel fasteners in stainless blocks, there's the risk of seizing (galling) between mating threads. This can be a real problem, including preventing proper torquing, and can make the bolt impossible to remove after it's been fully secured. Depending on the joint you may want to use anti-seize or thread locking compounds for additional safety.

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  18. #34
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    Default Urgent - please help if you can! Female Metric Thread Question

    Metric threads are very simple to determine what the hole size should be. Standard M24x3 metric course should be a 21mm hole give or take a bollock hair. M16x2 is a 14mm hole etc. So even if it is a non standard or standard fine pitch it is simple to determine the hole size just take the pitch from the nominal thread diameter and there you have it. If a thread is stated on a drawing as M24 it would always be assumed to be the standard course pitch unless otherwise stated.


    Sent from my iPhone using Tapatalk

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    Quote Originally Posted by SeymourDumore View Post
    Really?
    An M24 is an M24.
    Period.
    The End.
    There need be no tolerance indications on the print unless that M24 is a special in either Pitch, Minor, Major or Class!
    ( though I still say that the pitch should be clearly stated on metric threads, regardless of ISO or not .... )
    Hold on, isn't there a number attached to the thread spec to specify fit? eg 'M24 x 3.0 H6' Or is that what you mean by class? If it's a safety critical part threads should surely be comprehensively called out, especially when the drawing is going to the other side of the world. Sure, convention would default to metric coarse pitch etc, but I wouldn't count on that for a safety part... Due dilligence would require comprehensive specs, I'd imagine.



    Quote Originally Posted by Vancbiker
    His test rig probably has a very nice fitting male thread.

    What would happen when a male thread, right at the low limit on pitch and major diameters, is used in the tensile test. Might give a different result. Ultimately isn't that the reason for standards?
    Fair point. I was envisaging a worst case scenario (smallest, within tolerance) bolt in the rig, but standards are there to offer a degree of tolerance to other parts being out of spec too. I fail this time!

    T

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    Quote Originally Posted by adama View Post
    ^ 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!
    nope.
    on so many levels...

  21. #37
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    Chinese machined threads, the reason why teflon tape was invented

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