Loft with guide and path making a weird wave

WaldPinkler

Why is loft doing this crazy s#$%?

I could draw a nonplanar path for the top but I thought it would be easy enough as one solid piece.

(The model)

Here is the loft with the base curve to fit to.


If I add guides, the top doesn't follow.

A path gets the general idea right but I can't use 2 paths.

Combining both makes a weird wave.

S1mon

I would try adding 1 or 2 or maybe 3 intermediate cross-sections, or consider creating guides for all the edges.

If you really aren't sure what those missing edges should look like, you might try lofting just a face at a time. Keep in mind that lofting from one straight line to another with one guide curve will give a different result than lofting the curve with two straight guides. You may like one better than the other. Once you've got the missing edges, you can loft that face, and then create the flat ends (use offset of 0 with the sketch, or create a fill surface).

GWS50

This is using Evan Reece's Freeform Spline, a fantastic tool, one which I use a lot and am most grateful for.

https://cad.onshape.com/documents/76fb929696ca21c1ac9d86f4/w/08da4f8c9cbb8cfbfd8e68b1/e/1830a337975d29daaadb5dc1

Freeform Spline by Evan Reece
https://cad.onshape.com/documents/cd51f29c6937305f86a9df95/v/e7c4be973fdab957ca9c5497/e/44de484ba0acba52f99fcfc5

GWS50

Sorry ....that's Reese not Reece....apologies

EvanReese

GWS50 said:

Sorry ....that's Reese not Reece....apologies

ouch! my feelings!  jk thanks for the plug.

I messed with it and found that Simon's suggestion is best. I did separate lofts. If you need more control, then making a guide for each corner with freeform spline (or any other way you like) would be what you need. https://cad.onshape.com/documents/24f49100190c77289e7761df/w/7cd885f7dab14f951602271c/e/55e73bffa89321fe6279152b

S1mon

Here's a link to another version. 




It's not very well documented, but a couple key points:

1. Loft (and sweep) are not great at knowing how to make things that look good, especially when they have to make up a lot of stuff. I ended up building the top curves (sketch 6+7).
2. In many situations, it's important for the curves to be built in a similar way so that the parameterization isn't all warped. Sketch 6+7 use the same type of splines with parallel tangents, similar weights and the through points are placed to make a visually similar pseudo-offset of the "Top - curve path" splines.
3. Even with the similarity, some cross curves are helpful to give loft an idea of what to do (sketch 2 locates the cross sections, sketch 3+4 are the cross sections, using pierce relationships).
4. The top curves (sketch 6+7) could be completely 3D, but many curves that look good on cars and other very swoopy 3D objects, are actually 2D, but just at some weird angle. This is important so that as you rotate the object around in 3D, there's never a view where the curve wiggles in an ugly way. I constructed the planes (3+4) for sketches 6+7 using a helper sketch (#1). (When 3D curves are truly 3D they often keep the tightest parts of the curve 2D and twist along straighter sections of the curve or where there are inflections)
5. I tried lofting the whole shape in 1 go, but even with the added cross sections, loft twists things too much. Instead I lofted the 3D faces individually as surfaces. Then I enclosed those with some sketch faces to make a solid.

WaldPinkler

Thanks, sometimes I think things should be simpler than they are. I drew the top and side guides then a ruled surface and a projected curve to get the 3d curve. All good and more control.

WaldPinkler

This is just for fun modelling my own yacht. 4 guides works just fine although a bit more work.

This is the basic stern.