Unrolling a rectangle to round transition

I have a rectangle to round transition i’m trying to unroll but it keeps just giving me one triangle. Rectangle to round transition.3dm (74.5 KB)
I have the transition drawn the way i want and next to it is my attempt at lofting. It will be cut out of steel in 2 halves. It will need the seam (for each half) to be in the center of the narrow side of the rectangle. Then will be bent at each of the 12 lines going in between the circle and the rectangle and welded together. I think part of the problem is the arcs on the circle. They can be straight if necessary. Here is a hand drawn flat pattern for the part that i’m hoping to get

One half of the surface created and unrolled in the attached file. Rectangle to round transitionDC01.3dm (232.5 KB)


  1. Temporary curves and lines created to use as boundaries for surface creation. The curve circle is copied is split at the seams as needed. The rectangle polyline is copied and exploded. Seam lines created. Rectangle sides split.
  2. Loft command used to create the curved surfaces. Curves are portions of circle and apex point.
  3. EdgeSrf command used to create the flat surfaces. Boundaries are the seam lines, edges of lofted surfaces, and portions of circle.
  4. Surfaces joined into a polysurface…
  5. UnrollSrf command used to unroll the polysurface. Relative tolerance set to .001.

Creases are only needed between the flat and curved surfaces. Are you trying to solve this in the same way you would in 2D drafting?

Hi Fabrisol- see if the attached file helps at all - the closed elliptical curve is split at the lines; each surface is an EdgeSrf of three curves- a section of ellipse and two lines, (Not the ones that end on a point at the ellipse, of course, they are made the other way round) Choose the side where you want the center isocurve first (i.e. the ellipse segment) when picking the curves for EdgeSrf. The surfaces are joined, and the whole thing is split (red and blue), and then one of them unrolled. Am I close?

Rectangle to round transition_Maybe.3dm (111.2 KB)


I really appreciate your help but i’m still not sure what you did different. I’m not familiar with the edgesrf command. I think i might be having a problem with the arrows used with lofting.
Anyway, to answer your question, i’m not a draftsman, but i am a metal worker. The reason why its just a surface is on thin material like this, going off the i.d. works really well since when you form metal it stretches so my whole intent is making a flat pattern, not a realistic or ideal part. The creases are where my bend lines will be. Steel only wants to bend in one direction like paper.

Thanks, that is pretty much what i need. I’m not familiar with edgesrf. I was using the loft command. I think part of my problem was the direction of the arrows on the loft command.


Ben, the surfaces I created are “developable” which means that they only curved in one direction. The unrolled surface should form into the transition shape without any stretching.

EdgeSrf is a command which can be used to create a surface from a set of curves which define the edges.

So i’m drawing another square to round transition, this one is 24”x24” going into a 3” pipe at about 45 degrees. With edgesrf i can only make one section at a time. I can only unroll one surface at a time. When i use the unroll command i can only select one surface. I’m guessing i’m doing something wrong with edgesrf

Make the individual surfaces, then use Join to join the surfaces together. Then unroll the joined surfaces with UnrollSrf. Make sure Explode is set to No in UnrollSrf so that the joined surfaces will unroll together.

For a surface which wraps around and is joined all the way around UnrollSrf will decide where to make the split when unrolling. The simpliest way to make sure it splits where you want it to is to use the Split command and make a split in the surface before unrolling.