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Thread: Iso metric Screw M8 Major Diameter Tolerance

  1. #1
    Associate Engineer
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    Iso metric Screw M8 Major Diameter Tolerance

    Hallo to you all,

    I am trying to a find a specification sheet where I can obtain the tolerance of the major diameter of a M8 Screw, my problem is that I don't find this tolerance field in any of the sheets I looked up on the internet.

    I need this because I am defining the size of a through hole, were the Screw goes through. It seems to me that the tolerance of this type of screw is very small that it can be neglected, but this doesn't go well with my project manager, who is insisting to find this tolerance field!

    Can someone help, or give me the tolerance field if he/she knows?

    Thank you

  2. #2
    Lead Engineer Cake of Doom's Avatar
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    Hi Anas and welcome to the forum.

    The Steel designers manual recommends making 2 mm larger than the named diameter of the bolt, so in your case, the hole would be 10 mm. This changes to 3 mm if bolting through more than 2 plies.

    The tolerance table start at M12 unfortunately but you get the general jist. This is listed on page 1236 under 'Bolt Data'. Thats the closest I can get to getting you something in writing I'm afraid but I hope it helps.

  3. #3
    Technical Fellow jboggs's Avatar
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    Anas - There are standards for the size of clearance holes for screws. Look in any Machinery's Handbook or many other places. The size of the clearance hole size is not based on the tolerance on the size of the screw. The screw is not located by the clearance hole. It is located by the threads of the tapped hole. Clearance hole sizes are based on commonly accepted best practices for the ability to perfectly match the locations of the threaded hole and the clearance hole.

    I can also tell you this. If you measure the actual OD of the threads on an M8 screw it will be less than 8mm. Look up screw thread standards and you'll see what I mean. So theoretically an M8 screw should fit through an 8mm hole, and probably will if it is the only screw involved. But getting a pattern of tapped holes to match that closely with a pattern of clearance holes requires a level of precision that is usually not productive, and easily leads to binding.

  4. #4
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    Thank you for your valuable and generous replies.

  5. #5
    Technical Fellow jboggs's Avatar
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    Anas - more thoughts on this. It sounds like you might be trying to precisely locate the parts using screws. Be aware that while screws do locate a part, the level of precision is limited, and often not enough. In the field of precision machine design, screws are just considered to be devices to hold parts together, not to precisely locate them. Use dowel pins, keys, or other methods for that purpose. Again, Machinery's Handbook can give you more guidance on how to do that.

  6. #6
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    see machinery's handbook, if you need some let me know...

  7. #7
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    Regarding ASME B1.13M-2005 Metric Screw Threads M Profile, On page 26 you can find this... anyway see attachment...M8.pdf.

    let me know if you need something else, hope with this information works...

  8. #8
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    I prefer say that is better work with norms instead of people say.... on this case you can find information on ASME B1.13M-2005 Metric Screw Threads M Profile....

  9. #9
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    I prefer say that is better work with norms instead of people say.... on this case you can find information on ASME B1.13M-2005 Metric Screw Threads M Profile....

  10. #10
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    my thoughts

    M8.pdfI prefer say that is better work with norms instead of people say.... on this case you can find information on ASME B1.13M-2005 Metric Screw Threads M Profile....

    Anas see attachment with the answer for your question

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