Make a ring/doughnut shape bigger inside and out


I’ve been getting on pretty well with Rhino for Mac over the past few months, but I’ve run into a problem tonight for something that would be really useful that I can’t work out how to do.

I have a ring shape (pretty much a doughnut with straight edges) that is to fit freely into a larger shape. So that it doesn’t stick when it’s 3D-printed I want to leave a 1mm gap both inside and outside and to do this my idea is to copy the ring, and transform it so the radius of the outside edge is about 1mm more and the radius of the inside circle is 1mm less then “difference” it with the larger shape.

But can I do this? If I transform then both the outside and inside edges get larger or smaller together, and if I put the “centre” between the outer and inner circle of the ring then it stretches ok in, say, the x direction, but the y-direction stretches way out.

If not then I can always create the “thicker” ring by hand, as it were, but of course it would take longer.

Here’s a simple “before” and “after” of what I mean.

What do you think?

Offset for curves and/or OffsetSrf for surfaces.

Brilliant. Thanks David. OffsetSrf works fine for what I need just now, and I see there’s a lot of scope for more complex offset commands for future use.