Loft Developable Bad Result

(David Cockey) #1

Loft command with Developable option frequently produces surfaces which have significant differences from a true developable surface. These differences can be large enough to be unacceptable for work such as boat design. This particularly seems to occur when the middle of the developable surface is relatively flat. An example is attached.
Loft with Developable option surface:

Exact surface:

It was created so that the correct developable surface is known exactly and is two adjoining conic sections with continuous curvature between them.
Loft Fail 11.3dm (463.8 KB)
Both the exact developable surface and the result from Loft Developable are included along with the result from DevSrf which creates a correct developable surface. While the Loft Developable surface unrolls “within tolerance” (whatever the tolerance is) it differs considerable from the exact surface. Section and “waterline” curves are included which can be compared. This screen capture shows the difference. The gold curves are on the Loft Developable surface, and white curves are on the exact curve.

What's New - Loft
(Pascal Golay) #2

Hi David- Developable is going away as an option for Loft - as you point out, it is not reliable enough to be useful in very many cases.


(David Cockey) #3

Will DevSrf become a standard command rather than a Lab plug-in? My experience is that DevSrf generally gives good results when used with the Least Twist option.

(Pascal Golay) #4

Hi David- I do not know the answer to that- it has been discussed a good deal here but I don’t know the upshot- let’s ask @lowell


(Lowell Walmsley) #5

I don’t know that either.
It would be nice to have a good way to make those surfaces. DevSrf has never quite made it as a general use tool. Its pretty good at some things, but you do have to have good input for it to work right.
Often the first thing that happens when somebody tries to use it for the first time is that they give it rail curves that can’t possibly make a developable surface and get a mess out the other end and decide it doesn’t work.