Unrolling a surface with a bevel cut,


#1

i’m unrolling all the parts for a dory project. the sides and bottom will have a bevel cut on the outside of the panels. i’m unrolling the inside surface and i’m pulling the outside curve to the inside surface. the unrolled bottom surface has scribe lines on the inside and after it gets scribe lines and cut out, it gets turned over for the bevel cut. just wanted to see what anyone thinks about this. i’m thinking it has to be right.

3 new photos added to shared album


(David Cockey) #2

Your approach should work depending on the level of accuracy/precision desired.

If you want to be even more accurate create the mid-surface using TweenSurfaces. ExtendSrf the mid-surface and Pull the inner and outer edges onto the mid-surface which is also known as the neutral surface. (In theory the neutral surface does not stretch or compress when the sheet is rolled/unrolled.) Trim using the pulled inner edges. UnrollSrf the mid-surface and pulled outer edge curve. In theory that will be more accurate for laying out the sheet material than unrolling the inner or outer surface. However for a shape with the radius of curvature very large compared to the thickness of the material the difference compared to unrolling the inner surface with the outer edge curves pulled onto it will be very small. Experiment and see how big the differences are for your project.

More considerations:

  • How close to exactly developable are your surfaces? Unrolling a not quite exactly developable surface will introduce discrepancies which may or may not be significant depending on the shape, how far the surface is from developable, and material properties.

  • Unrolling an exactly developable surface does not exactly model the actual physics of unrolling a physical sheet with thickness. One cause of the differences is the Poisson effect. Another for plywood and similar is the material properties depend on direction, ie an non-isotropic material. These differences may negligible compared to other cause of discrepancies.

  • How accurate can you cut the sheets? How accurate will your building form/setup be? This may be largest source of discrepancies for a physical build.


#3

David,

Thanks for your help with this. I looked at the tween surface method, did everything like you said and the bevel on the unrolled surface is very close to what I’m getting by using just the inside surface. The sides of this dory are developable surfaces and they are very simply compared to some surfaces. I tried to talk this girl that I’m doing this project out of the bevel business and just do it with square cut edges but she wants to do it like this. as you can see in the pictures, I designed four females for the set up and it has an assembly for rolling it over. So with the females and all the thwarts I think this boat should be very close to the model.

W