(Sorry if I’ve placed this in wrong category! - I’m new )
I’m trying to make a program which constructs a ruled surface by using the sweep2 command. I.e. I give it two boundary curves (the rails) and as cross section curves I use lines between the curves. The output seems to always be a ruled surface - but I’m not sure if this holds for any choice of curves and lines? Can someone verify this?
A surface created using Sweep2 and straight line(s) for the sections will always create a ruled surface. Any ruled surface can be created by sweeping a straight line along two boundary curves.
Are you trying to create a developable surface which can be unrolled without distortion? Developable surfaces are ruled surfaces but not all ruled surfaces are developable surfaces. Usually the best tool for creating developable surfaces in Rhino is DevSrf which is Lab plug-in which can be downloaded for free.
Great 
! No, I’m not trying to create developable surfaces, but thanks anyway for the tip.
However, I can’t see how a cone, which is a ruled surface, can be generated by two boundary curves. In this case one of the boundary curves must be a point.
A cone is a special case, available as one of the primitives on the surface menu.
For a general cone use RailRevolve with the start of the revolve axis at the apex of the cone. Or use Loft with the Point option for one of the rails and style of Straight Sections.
Hi @davidcockey! Could you show such an example:
Erroneously I thought all ruled surfaces are developable
Is it still the best way? And why the DevLoft is better than the surface created using Sweep2 and a straight line(s).
_devLoft
is quite nice tool to create developable surfaces
you may also want to look at other topics searching for developable
for example here:
Rhino 7 has DevLoft which I usually use but only after determining “end” ruling lines and trimming/splitting the edge curves as needed. Search for a method I posted to find exact ruling lines between pairs of curves.
Two examples of ruled by non-developable surfaces.
RuledNonDevelopExamples.3dm (1.5 MB)
and sometimes stuff is not developable (left, screw / thread that works mechanically) but small adaptions allow developable design (right - not a nice screw-surface in terms of mechanic…)
develop_or_not.3dm (3.3 MB)
kind regards -tom
Thank you @Tom_P! I have got the idea 
