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    Default Need help with thread technical dimentions

    hey guys,
    I'm an engine machinist so I know numbers but I don't have to deal with technical drawings and those sorts of specifics. I'm teaching myself CAD and getting better at it all the time and I have a part that I want to have made which has a custom male and female thread. What I need is the correct dimensions to give to the machinist so that the parts will fit together properly. What I'm looking for is the male thread diameter is 72mm with a 3mm thread pitch and a class 3 fit ( I want it a bit loose) What information do I need to give to the machinist for both parts, male and female, given the above requirements? Thanks for any help.
    -Nathan

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    Metric thread tolerances are specified as a "grade" and a "position", not a class - btw, not relevant but FYI - class 3 for UN threads is tight, not loose.

    You can read all about it here.

    However, a good general purpose choice is 6g/6H, and that's what you would expect to get if you don't specify.

    Your male thread should be called out on your drawing as "M72 x 3 6g", female as "M72 x 3 6H"

    You'll need to read the link to determine if you really need something else.

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    Quote Originally Posted by gregormarwick View Post
    Metric thread tolerances are specified as a "grade" and a "position", not a class - btw, not relevant but FYI - class 3 for UN threads is tight, not loose.

    You can read all about it here.

    However, a good general purpose choice is 6g/6H, and that's what you would expect to get if you don't specify.

    Your male thread should be called out on your drawing as "M72 x 3 6g", female as "M72 x 3 6H"

    You'll need to read the link to determine if you really need something else.
    I will, Thanks so much!!

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    Machinery's Handbook is also a good source for this. They do go into depth on it so fill your largest coffee cup first.

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    Here find a thread calculator you can put in your thread.
    Metric screw thread: M Profile calculator

    *Nope looks like this does not work.. sorry
    Where is Gordon when we need him?

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    Quote Originally Posted by nathanhouse View Post
    hey guys,
    I'm an engine machinist so I know numbers but I don't have to deal with technical drawings and those sorts of specifics. I'm teaching myself CAD and getting better at it all the time and I have a part that I want to have made which has a custom male and female thread. What I need is the correct dimensions to give to the machinist so that the parts will fit together properly. What I'm looking for is the male thread diameter is 72mm with a 3mm thread pitch and a class 3 fit ( I want it a bit loose) What information do I need to give to the machinist for both parts, male and female, given the above requirements? Thanks for any help.
    -Nathan
    why 3 mm pitch?

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    Quote Originally Posted by janvanruth View Post
    why 3 mm pitch?
    I have a part that I want to have made which has a custom male and female thread.

    just because I want it.

    Still I recommend a student and anyone who wants to be a lathe hand to design a nonexistent thread. But not/never order a part with that thread.

    Here find a M72x3
    Internal Metric Thread and Fastener Sizes M50 - M72 | Engineers Edge

    Metricthread

    https://www.natool.com/wp-content/up...atp101-102.pdf
    Last edited by michiganbuck; 03-28-2020 at 05:56 AM.

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    I'm using a 3mm pitch bc it's a standard pitch and honestly it's not too fine so that hopefully it won't be easily cross threaded but for the diameter of the part the thread length is fairly short.
    The part is an access/inspection hole for a vintage motorcycle oil pan that is welded right below the oil pickup screen. The male part holds a 2" flat magnet with an o-ring. The whole thing has to be relatively low profile as to not protrude too far toward the ground or get in the way of the exhaust.
    I'm using an on-line machining company to make a quantity of 10 pairs so I have to be able to give them the thread specifications. I've 3d printed the parts and all works well but that's not the same as machining the parts in aluminum.

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    Thanks for the links to the thread calculators. I was looking for some of these the other day but didn't find one this good 👍

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    Most here think an odd/special thread is a bugger from hell or a engineering folly when likely a standard can be found to do most needs..but to make a antique somewhat original I guess that is OK.

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    Quote Originally Posted by nathanhouse View Post
    hey guys,
    I'm an engine machinist so I know numbers but I don't have to deal with technical drawings and those sorts of specifics. I'm teaching myself CAD and getting better at it all the time and I have a part that I want to have made which has a custom male and female thread. What I need is the correct dimensions to give to the machinist so that the parts will fit together properly. What I'm looking for is the male thread diameter is 72mm with a 3mm thread pitch and a class 3 fit ( I want it a bit loose) What information do I need to give to the machinist for both parts, male and female, given the above requirements? Thanks for any help.
    -Nathan
    All you need to give is M72x3 and, if the machinist knows anything about metric threads, he'll know that the external thread tolerance is 6g and the internal 6H. There's an allowance between the two tolerances so avoid specifying "loose" or similar. That'll make it non standard and that usually costs more.

    m72x3.jpg

    How you know about "numbers" but not technical drawings is a mystery to me.

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    Quote Originally Posted by Gordon B. Clarke View Post
    All you need to give is M72x3 and, if the machinist knows anything about metric threads, he'll know that the external thread tolerance is 6g and the internal 6H. There's an allowance between the two tolerances so avoid specifying "loose" or similar. That'll make it non standard and that usually costs more.

    m72x3.jpg

    How you know about "numbers" but not technical drawings is a mystery to me.
    I just haven't ever done anything with thread tolerances in my career so I'm pretty ignorant on that front. Much like most excellent machinist here wouldn't have a clue as to what RvK and Rpk numbers you would want to see on the cylinder walls in a top alcohol drag race engine or a nascar cup engine. They may know about the values but maybe not the specifics because they have never had the need.
    My comment was to say I'm not a novice machinist but the machining I do is fairly different from what most here do and that's why I asked since I know enough to know I don't know enough.

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    IMO, the above advice to go with a standard metric call-out is good. Once you get into customizing the dimensions of a thread it gets complicated and the odds of it not being done right get pretty high.

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