Lofts and Sweeps that fold back on themselves

steven_armstrong

Hello All,

I am modelling an air cooled cylinder head. It is time to do the fins. I am using the outer edge of the fin as the guide. I find that if the curvature of the guide is too great the loft or sweep folds back on itself and fails. Should I be doing this a different way?


https://cad.onshape.com/documents/812e1b48d1f642ed1f9ad8df/w/f18e3a8396163861513668e6/e/9b6aed276b422a12477c493b

tim_hess427

I'm guessing here, but would it be possible to create a path at the base of the fin (along the cylinder head) and select both paths? 

An alternative approach could maybe be to: 1) sketch the whole fin on the horizontal plane (same plane as your "Fin Guides Outer" sketch is using), 2) extrude up/down to create a constant thickness fin intersecting with the head, 3) use draft to create a taper, 4) fillet, chamfer, or sweep the along the outside edges as needed. 

matthew_stacy

@steven_armstrong try filling the bottom (or top) of this region and then thicken the newly created surface.  I think you will find that the length of the "Face of Sweep 2" is too long relative to the radius of curvature of the "Edge of Fin Guides Outer".  See how the "Face of Sweep 2" intersects itself as Onshape attempts to construct "Sweep 4"? 

I believe that the resulting overlapping geometry is the issue.

Regards,

-Matt

EvanReese

A loft with guides instead of a sweep seems to give the result you're looking for. Here's my copy of it

steven_armstrong

Thanks Guys,
I now understand the proper use of guide curves. I have realised that to exercise full control of the loft there needs to be a guide curve for each vertex that needs to be controlled. In addition the guide curve form can't  be in contradiction to profile start and end conditions. To do this fin in one go I need a start and end profile and a guide curve for  5 end vertices and two root vertices. That's 7 planes!