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Sweep along helix: bug? [Not a bug.]

laird_broadfieldlaird_broadfield ✭✭Member Posts: 41 ✭✭
edited June 2015 in General

In this part, I have used the method in https://www.onshape.com/cad-blog/tech-tip-creating-a-thread to create a thread in my cylinder.

https://cad.onshape.com/documents/c2c1f6cdabc149bf987fad3f/w/62ef4e04724449ad831323ed

Why does it end abruptly before the helix actually ends?  
Why does the (unexpected) end of the sweep move (further unexpectedly) if I chamfer the top of the cylinder?





Best Answers

Answers

  • laird_broadfieldlaird_broadfield ✭✭ Member Posts: 41 ✭✭
    @laird_broadfield ;

    The problem is that your triangle is not correctly related to the helix. The video is not clear on this point: it tells you to create a pierce relation between the top of the triangle and the helix

    Here's the more detailed version: There needs to be a pierce relation between a top corner of the triangle and the helix. This requires picking two entities only: the corner, and the helix entity away from the helix endpoint (you are creating a relation between the triangle endpoint and the ENTIRE helix, not an endpoint of the helix. The entire helix should highlight when selected). Then pick "Pierce"

    Other hints: Press the spacebar immediately before picking the two entities to clear any selected entities, and hide the Part temporarily in order to better be able to pick the helix.

    You should re-create the triangle first, to be sure you capture only the following additional constraints:
    1) The top line of the triangle should be horizontal
    2) The three sides should all be equal
    That's the key, along with (based on jakeramsley's hint) reordering the cylinder chamfer to be after the helix is specified.

    Wait... what's the difference between constraining using the top line of the triangle vs the top corner of the triangle?  It doesn't appear to make a difference in practice.

    (Yeah, I would need those triangle constraints if I were doing it "right", but I was experimenting with the triangle's flexibility anyway.  P.S.: there's a fillet for sketch corners, but not a chamfer?)
  • andrew_troupandrew_troup ✭✭✭✭ Member, Mentor Posts: 1,584 ✭✭✭✭
    A pierce constraint is between an infinitely thin two-dimensional entity (a line, a spline, a helix, or an edge) and a one-dimensional entity (a point)
    It cannot be solved if the latter entity is also two-dimensional (in other words, has length).

    Another way of saying it is this: you need a specific location on the triangle to line up with the point at which the helix pierces the sketch plane. Otherwise you end up with the sort of situation you started with, because when you create a constraint to a line, it's effectively to an infinite line.

    Constraints to a line are effectively saying: Make this point line up with the direction of this line, like lining up the bullseye of a target with the barrel of a gun.
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