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why are helix startpoints so unstable?

peter_morrispeter_morris Member Posts: 43 ✭✭
When trying to model a thread and to get the precise extent of it correct I have had to change the length of the surface extrude on which the helix is drawn.  This then misaligns the helix with the thread profile which is meant to be in a perpendicular sketch and hence the sweep fails. 

This is so frustrating - I've now redrawn the sketch multiple times and in my latest iteration the terminal point of the helix doesnt align with a plane!  I must be missing something so any pointers to some advice will be grately appreciated.

Many thanks.

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    thomas_kozakthomas_kozak Member Posts: 38 ✭✭
    I've had this happen also.  For instance when modeling a worm gear a plane is created normal to a helix and the tooth profile is sketched on that plane.  Some modifications to the base cylinder seem to cause the created plane to rotate 90 degrees, which completely reorients the tooth profile.
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    peter_morrispeter_morris Member Posts: 43 ✭✭
    Hi Tim,

    No I wasn't aware of the thread generator, I'll now have a look.

    The model I'm working on is here: https://cad.onshape.com/documents/e6d527b6f8f4ca57eb42141f/w/647c2cdf99d90ae6405339ed/e/281ccfa7a874cb35e667d2a9 but its getting pretty complicated and the threads are between the nuts on the two taper bearings (no taper bearing housing yet drawn!).

    Thanks for the advice.

    For info to Thomas, I'd done the same as you creating a sketch on a profile perpendicular to the helix, but whenever I resized the surface the helix was drawn on the start point moved, but I did discover that it only moved in increments of 45 deg.  The real confusion arose because i'd 'Used' the start point of the helix as the drawing reference and when the helix moved so the point transferred into the sketch from the helix (by 'Using') moved to match the new helix start point.  Now I know this I move the angle of the helix to realign the helix with the plane that the profile is drawn on and this seems to be a practical, but somewhat annoying solution.  HTH

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