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Thread: Circle calculation
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25th Sep 2018, 07:36 PM #1Senior Member
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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
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25th Sep 2018, 08:02 PM #2Senior Member
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How about 4.1761226mm?
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25th Sep 2018, 08:20 PM #3Senior Member
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25th Sep 2018, 08:36 PM #4Most Valued Member
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Slightly larger than 2. Doesn't AutoCAD tell you the circle diameter?
Chris
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25th Sep 2018, 08:43 PM #5Senior Member
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25th Sep 2018, 08:52 PM #6Gear expert in training
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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
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25th Sep 2018, 08:59 PM #7Most Valued Member
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I did the same with Sketchup and came up with a radius of roughly 1.15, so I reckon elan is right.
Chris
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25th Sep 2018, 10:35 PM #8Senior Member
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Wrong first time, doubled the pitch.
I need a another go!
Whoopsie... still wrong
deleted image
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25th Sep 2018, 10:39 PM #9Senior Member
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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.
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25th Sep 2018, 10:44 PM #10Gear expert in training
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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
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25th Sep 2018, 10:51 PM #11Gear expert in training
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25th Sep 2018, 10:51 PM #12Most Valued Member
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25th Sep 2018, 10:58 PM #13Senior Member
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Yes Elan, I am still wrong.
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25th Sep 2018, 11:03 PM #14Gear expert in training
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This gets you within 0.4 micron of your required depth
Clipboard01.jpg
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25th Sep 2018, 11:09 PM #15Senior Member
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what diameter are the circles ?
2.2332381Ø
Cheers John
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