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Thread: Understanding machine screw sizes

  1. #1
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    Default Understanding machine screw sizes

    Well I couldn't find the answer searching the Internet so I thought I'd try here. I need to order some machine screws from McMaster for a microscope project I'm working on. It's a slotted round head machine screw and not metric. My thread gauge says it 40 tpi, and my caliper reads 0.107" to 0.108" for the diameter, with a length of 0.220" (probably 0.25"). I know how to measure and read bolts etc., but not when they get this small.

    Struggling,
    -JW:


  2. #2
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    First hit on an internet search comes up with this:

    https://www.engineersedge.com/screw_threads_chart.htm

    Looks like you have a very common #4-40 pan head screw.

  3. #3
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    That would be a #4-40 x 1/4" screw. For reference, the nominal major diameters for the most common number size screws are as follows:

    #4 .112"
    #6 .138"
    #8 .164"
    #10 .190"

    I believe #12 is .216" or thereabouts.

    See also the thread chart at https://www.engineersedge.com/screw_threads_chart.htm

  4. #4
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    Quote Originally Posted by Brian H. View Post
    That would be a #4-40 x 1/4" screw. For reference, the nominal major diameters for the most common number size screws are as follows:

    #4 .112"
    #6 .138"
    #8 .164"
    #10 .190"

    I believe #12 is .216" or thereabouts.

    See also the thread chart at https://www.engineersedge.com/screw_threads_chart.htm

    Thanks guys, that's what I couldn't figure out. I actually went to that site PStechPaul but didn't see TPI on the chart and got even more confused. I didn't want to end up with a box of screws that weren't correct.

    -JW:

  5. #5
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    4-40 means #4 with 40 TPI.

  6. #6
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    It can be difficult to identify screws as they are usually slightly smaller than the nominal size in the thread charts.
    An example are 10-32 UNF and 5mm metric coarse, The 10-32 is 0.19" diameter and the 5mm is 0.1968" diameter. The thread pitch is even closer, the 5mm is 31.8tpi (0.8mm). A thread gauge will look ok at first glance both ways round with only the slightest rock.

  7. #7
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    Here's the way the numbering scheme works: A #0 would be .060, then for each number higher add .013.
    Last edited by lynnl; 06-23-2019 at 06:49 AM.

  8. #8
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    Quote Originally Posted by old mart View Post
    It can be difficult to identify screws as they are usually slightly smaller than the nominal size in the thread charts.
    ...

    That is because screws are not measured by the nominal diameter.
    They are measured to the major diameter.

    -D
    DZER

  9. #9
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    Quote Originally Posted by Doozer View Post
    That is because screws are not measured by the nominal diameter.
    They are measured to the major diameter.

    -D
    That's true, but who carries around a set of thread wires or a thread micrometer wherever they go?

  10. #10
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    Quote Originally Posted by SLK001 View Post
    That's true, but who carries around a set of thread wires or a thread micrometer wherever they go?
    You dont need those for major diameter measurement. Only for pitch diameter.

    1/4 screw has major diameter tolerance range from about 0.240 to 0.249 so you might expect to find most of the screws to measure about 0.245

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