# Unrolling curved, closed, solid Poly surface accurately

With a view to generating a lazer/water jet cutting file, for example using 2 and 4mm thick material, I have created a model that resembles this exactly. For the sake of explaining the model, imagine a hollow cylinder with a 3m radius modelled wall thickness of 4mm as a solid closed poly surface. Then imagine small holes radiating the wall of the cylinder puncturing both the inner and outer face (kind of like the spoke holes in a bicycle wheel rim) of the 4mm thick material. When using the unroll tool, the inner and outer surface are different sizes based on the larger circumference of the outer face and as such do not align. With a view to lazer cutting the flat faced material with holes in it, is there a way of generating a cut file whilst navigating this difference of the material thickness as if it were a solid and not a shell with 4 faces?

If I only consider one of the faces Iâ€™m thinking that the cut will either be to big or to small. This gets more complicated when trying to layer multiple bands of material whilst aligning perforations.

And help much appreciated.

bending a sheetmetal with 4mm thickness - there will be compression on one side and expansion on the otherâ€¦ and somewhere in the middle there is a zone that has the size you need to cut.
A rule of thumb is to use the mid-Surface - but depending on the used alloy and bending technique this might not be precise.
if you layer multiple bands, i would guess that the mid-surface will work, as the offset is equal to the material thickness - so the error on the first layer will repeat on the second - so at least the layers will fit to each other.
so you unroll a single surface, not a polysurface.
you will need 2d-Data anyhow for the laser / watercut.
hope that helps - kind regards -tom

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Tom provided a good description of what to do. Where the â€śneutralâ€ť surface should be located depends on both the fabrication method and the material properties.

Intermediate surfaces can be created using several methods. If it is mid-surface then TweenSurfaces is usually the simplest. If it is at at an arbitrary fraction of the thickness then using OffsetSrf with the appropriate offset distance can be used.

Thank you for you feedback, both responses have been most helpful.