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Bizarre loft - what am I doing wrong?

ian_harris952ian_harris952 Member Posts: 12

I am no expert, I only use Onshape as a hobbyist to knock up designs to 3D print or laser cut. I would like to use lofting as I think it would highlight errors in my designs. However I am having a problem getting it to do what I am expecting it to do.

I have had some success with lofting in some of my other drawings, but in my test drawing, in which I am trying to get a handle on what it does and how it works, I have had the least success.

In the top loft, even if I just loft between Former 1 and Former 2, one side works as expected, but the other side has a strange crease down it. I really want to loft from the first point (vertex) to the last point. When I do this one side works, the other side looks like the Elephant Man. The lower loft works slightly better, but I cannot get it to loft from the first point to the last point at all, instead red lines are drawn, and I cannot understand what they mean or how to correct the problem.

I have made sure all the faces in the loft are identical in every way except size. They are very simple shapes consisting of two mirrored splines.

There are no error messages displayed or any indication other than the red lines of what the problem is, and I don't understand what the red lines mean.

I've tried adding guides and paths and matching connections, but none of those efforts have rectified the problem.

Is there something basic I am missing, or have misunderstood?

All advice gratefully received,



  • ian_harris952ian_harris952 Member Posts: 12
    Well, it looks like I might have answered my own question, sort of.
    I tried creating a whole new drawing to test some other ideas about lofting, which worked (lofting shapes defined with straight lines), then tried splines again, which worked, (also lofting splines and rectilinear shapes together) and ended up back at my original shape, which now works.
    The original drawing (linked to above) started out with some other artifacts in it, which I had added, modified, deleted, recreated, etc. in my experiments, so maybe something there has corrupted the drawing and created the problems.
    Seems the old "turn it off and turn it on again" strategy works for lofting too. Just delete the whole drawing and start again.
    I have noticed in the past that Onshape seems to be better at creating new things than modifying existing things. I have found a few times that, having tried to modify a part the way I want it, and having Onshape do odd things (like moving other things around that I didn't want moved) the optimal solution is to delete the whole mess and start again.
    However I have to admit that most of my problems may be created between keyboard and seat.
  • shawn_crockershawn_crocker Member, OS Professional Posts: 90 PRO
    I know what you mean by it somtimes working better by just restarting. Overtime, you start developing mental patterns a predictions for what you are about to do. It starts to become easier to go back and modify because your workflow is nailed down and you don't have to keep trying to remember why you put this thing is here or why this line there. Feeling the need to delete and restart I have found stems from having tried too many things in too short a time span none of which worked. Because everything you look at starts to have no meaning due to a lack of an experience driven work flow, deleting ends up being more efficient. I think it's normal and aids in the learning process and speeds up the building of intuition and foresight into what where and when.
  • matthew_stacymatthew_stacy Member Posts: 270 PRO
    @ian_harris952, fully constraining your sketches is an important CAD practice to establish.  Apply geometric constraints (coincident, perpendicular, midpoint, Etc.) and dimensions to fully define the geometry and express design intent (makes editing the model much more controllable).  If the sketch is blue, you're not done yet (example below).

    Under-constrained sketches invite a plethora of problems in subsequent features (e.g. lofts).  It's too easy to miss gaps between entities.   Think of the sketch as the foundation for all subsequent features in your model.  Engineers don't like Jenga.
  • ian_harris952ian_harris952 Member Posts: 12
    @matthew_stacy thanks for the response. I have to admit I'm a bit lazy in this respect, when I have tried to constrain everything sometimes it seems to take longer than actually creating the sketch in the first place. I've used the "fix" function to avoid problems (like redimensioning a line, which then causes the whole drawing to be moved/distorted rather than just making the line longer), but otherwise, because my drawings tend to be pretty simple I'm not that rigorous about constraining everything. I'll only do it if I'm forced to! I'll do it for my example and see if that improves things.
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