When I try to create a flat form of a archimedes screw using Solidworks it gives me a diameter different from what is calculated manually basically using a Pythagoras Theorem. Can anyone explain the reason as to what is used by any 3D Software in order to calculate the diameter and essentially what factors am I assuming when I calculate using the pythagoras theorem only.enter image description here

I tried to calculate a screw with pitch- 400mm, OD - 400mm and Id - 268mm
Using Pythagoras theorem gives me a OD - 430mm and ID - 297mm circle
whereas Solidworks gives a OD - 450mm and ID - 318mm circle.
  • I'm not familiar with these jargon, 'Id, OD, ...' could you please edited it up to a bit and please add at least a screenshot of your CAD showing us how you use smart dimension. – Sam Farjamirad Sep 12 at 13:48
  • OD is outer diameter and ID is internal diameter... – Solar Mike Sep 12 at 14:07
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    if you could show your working with an appropriate diagram we may be able to help – DrBwts Sep 12 at 14:16
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    i bet the software is just calculating a diffdrent thing than you think its calculating – joojaa Sep 12 at 14:46
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    When you are using solidworks' sheet metal tools, you should specify the material as I believe it uses this information to help it generate the flat pattern, based on the deformation behaviour of the material. – Jonathan R Swift Sep 13 at 17:14

A flat helix (helicoid) is a ruled surface, but it is not a developable surface. So you are going to have a flattening problem. Try using the middle fiber radius and compare the hand calc to the software and see if they aren't a lot closer.

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