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loft twisting

IsoThermIsoTherm Member Posts: 39 PRO
Greetings,
This is something of a general question about lofts. I've been using the loft tool quite a bit - enormously powerful, thanks - and I find myself fighting twists a lot. I try hard to keep the number of vertices the same, often use interim profiles and usually provide guides in the form of lines or curves. 
I've been through the videos several times and looked through other posts about this but I haven't seen much in the way of general guidance on what causes twisting and how to deal with it.
Thanks,
Isotherm
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Comments

  • Jake_RosenfeldJake_Rosenfeld Moderator, Onshape Employees, Developers Posts: 1,338
    @IsoTherm

    The "Match vertices" option is your friend here.  For example, this simple loft:


    Can be easily made to twist in any way you want by telling the system which vertices should be matched to each other on each profile:


    https://cad.onshape.com/documents/8d66251daa4ac8e0908fed76/w/67f39bfa99419fc69de20e49/e/82e121fdbb75f817648cb621
    https://cad.onshape.com/help/Content/loft.htm
    Jake Rosenfeld - Modeling Team
  • owen_sparksowen_sparks Member, Developers Posts: 2,333 PRO
    I believe the OP is seakng advice on avoiding unwanted twists.
    Owen S.
    Production Engineer
    HWM-Water Ltd
  • Jake_RosenfeldJake_Rosenfeld Moderator, Onshape Employees, Developers Posts: 1,338
    @IsoTherm

    Maybe it would be good to see an example of a loft that is causing you problems?  Instructions here on how to post your document in a forum thread:
    https://forum.onshape.com/discussion/9107/forum-post-checklist-read-this-first

    @owen_sparks My suggestion comes from the idea that "Match vertices" can create a wanted twist just as easily as it can eliminate an unwanted twist.  It seems like IsoTherm is frustrated that even though they have the same number of vertices on each profile, there is a twist from one profile to the next, so that the right vertices aren't matching up, and a "Match vertices" selection could solve that.
    Jake Rosenfeld - Modeling Team
  • owen_sparksowen_sparks Member, Developers Posts: 2,333 PRO
    @Jake_Rosenfeld ; Ah, hey, my bad :+1:
    Owen S
    Production Engineer
    HWM-Water Ltd
  • IsoThermIsoTherm Member Posts: 39 PRO
    Mostly true. Jake is right that the match vertices tool can (seemingly) create twists. Many of my profiles are round or elliptical, so I've had to create vertices in order to match them. Why would two circular profiles (one larger than the other) want to twist when I loft along a 2d spline as a path and both profiles are perpendicular to the ends of the spline?

    Here's an example - although I think I figured it out, it's a good illustration of the sort of guidance that would be helpful to find somewhere rather than trial and error, and error, and error.

    I lofted the vertical profiles from left to right, with splines for guides. My first attempt twisted because the small vertical line coming from the center of the circle in the left hand profile was not initially a construction line. While it didn't interrupt the closure of the profile I guess it added a vertices that threw off the count between them? When I changed it to a construction line, no problem. I never could get it to take the circle as part of the left hand profile, so I had to loft it as a solid, then cut the circle afterwords.

    Like I said, kind of a general request. I love this tool and I'm using it more often. Unfortunately I find it twisting my mind for an hour (or more) with each new loft while I try different things to unwind it. What I'm wondering is what things makes the tool assume a twist so that I can learn to avoid creating the issue in the first place? A bulletin list on this would be appreciated.

    Many thanks,
    Isotherm



  • IsoThermIsoTherm Member Posts: 39 PRO
    here's my most recent fight.
  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 2,345
    @IsoTherm if you could post a public link, we could take a look at it for you. If you don't want to make it public, you can make a copy of just that Part Studio (right click element tab > copy to clipboard, create a new document, then Paste Tab from the + menu), delete all the features after Sketch 9 and make it public. Cheers.
    Neil Cooke, Director of Technical Marketing, Onshape Inc.
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