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  1. #1
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    Default Circle calculation

    Trying to draw a plastic thread in autocad but i just can't get my head around the geometry to work this out.
    It's to 3D print a new fuel cap for my ride on mower.
    In the following exaggerated picture, the thread is like a sine wave. I know the pitch is 4mm and the thread depth is 1.2mm.
    As is drawn with circles touching each other to form the wave, what diameter are the circles ?

    Circles.jpg

  2. #2
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    Default

    How about 4.1761226mm?

  3. #3
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    Quote Originally Posted by Neil317 View Post
    How about 4.1761226mm?
    Nah too large.

  4. #4
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    Default

    Slightly larger than 2. Doesn't AutoCAD tell you the circle diameter?
    Chris

  5. #5
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    Default

    Quote Originally Posted by jack620 View Post
    Slightly larger than 2. Doesn't AutoCAD tell you the circle diameter?
    It would if I've already drawn it, but I need to know the diameter so I can draw it.

  6. #6
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    Default

    Did some fiddling with trial and error and a radius of 1.134 on a 4mm pitch gives you a thread depth of 1.1985mm

    Radius 1.1335 gives a depth of 1.1996

  7. #7
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    Default

    I did the same with Sketchup and came up with a radius of roughly 1.15, so I reckon elan is right.
    Chris

  8. #8
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    Wrong first time, doubled the pitch.
    I need a another go!
    Whoopsie... still wrong

    deleted image

  9. #9
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    Default

    Got it !
    The usual light bulb moment is all it took.

    The circles centers make an isosceles triangle that can be divided in half making a right angle triangle.
    The right angle triangle has a base of 2 (B) and the height of 1.2 (H), so the radii of the circles is 1/2 the hypotenuse (HY).
    R = Root (B squared + H squared) /2 ...answer : 1.16619.............(hard to do this stuff on a standard keyboard)

    Circles.jpg
    Even after entering these figures into autocad, the circles don't quite line up properly. very strange.

  10. #10
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    How are you doing that Neil?
    When I plug your numbers into an AutoCAD drawing, I only get a thread depth of 1.135

  11. #11
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    Quote Originally Posted by phaser View Post
    and the height of 1.2 (H)
    Unfortunately not. The centres of the right column of circles are not on the line that touches the peaks of the thread pitch, so H is not actually your depth

  12. #12
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    Quote Originally Posted by phaser View Post
    Even after entering these figures into autocad, the circles don't quite line up properly. very strange.
    I reckon that's because your triangle's base is on the centre of the circle, but the pitch (1.2) isn't on the centre.
    Chris

  13. #13
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    Yes Elan, I am still wrong.

  14. #14
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    This gets you within 0.4 micron of your required depth

    Clipboard01.jpg

  15. #15
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    Default

    what diameter are the circles ?

    2.2332381Ø

    Cheers John


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