OffsetSrf, Compound Miters on a polysurface with only planar faces

I’m trying to speed up design and fabrication of complex forms using plywood and MDF sheet.

I’ve found that both the Rhino command Grasshopper components for OffsetSrf frequently fail to produce good results. Specifically, with polysurfaces with only planar faces, when the polysurface has a combination of triangular and ngon faces and/or a combination of convex and concave edges, the results are buggy and unusable.

I have been able to get good results from a tedious process of offsetting individual planar surfaces, scaling them up in 2d relative to their plane, finding intersections, and manually trimming. A further tedious manual process can split up the results into parts that can be fabricated with compound miter cuts. I talk about it in this thread: Trying To shell this shape

My questions:

  1. Are there are any grasshopper definitions out there that can do this automatically? I am interested in attempting to create such a definition, but if there’s anything out there that can already do it, I don’t want to duplicate efforts.

  2. Is there a component or simple definition that can reliably derive all the crease angles of the exterior of a polysurface with all planar faces?

  3. Are there grasshopper plug-ins that I should look at as possible paths to achieving either of the above goals? NGon looks like it might be promising. Who is a frequent poster on this forum who is an expert on NGon.


PS: I know ideally I would post a little code here to begin the conversation, but I don’t even know where to start, so I decided to start by asking questions.

R is a Surface modeler - he can do well what such a modeler can.

Thus I would strongly suggest to find a friend who operates an AEC/MCAD solid modeler and ask him for a rather convincing demo on that issue of yours.

But Grasshopper can:

A. Calculate crease angles and put them in a tree.

B. Individually offset the faces of a polysurface with all planar faces.

C. Calculate the intersections of infinite planes derived from the offset planar faces from B. above.

D. Create planes parallel to the crease angles in A. above

E. Create solids from boundary surfaces.

These, combined, tell me that Grasshopper can do what I want it to do.

There may be some conditions that require an additional step that I haven’t thought of, but I can’t see how the overall concept would not be possible.

My manual approach is just me performing an algorithm for manipulating simple planar geometry, and algorithms for manipulating geometry is what Grasshopper does.

Well … Is not impossible … but more or less is kinda re-inventing the wheel. And … assume that after the Holly Grail you want to fillet all the edges (as we do in real-life with most of solids - obvioulsy not ones made via MDF … but you get the gist of my meaning I do hope). What exactly are you going to do?

I’d expect to chamfer, not fillet, in some cases. Or, in other cases, not chamfer or fillet, but instead use a 5-axis CNC router with an acute angled V bit to cut both sides of variable angle miterfold V-channels. Either way, a roughing pass followed by a V bit on a head with an A and B axis allows for quick and clean cutting of variable angles in sheet stock on a vacuum table.

What I’m trying to do is to create irregular polyhedra out of plywood using a 5 axis cnc router with a tool changer, and two-sided machining. Cut fillets on one side, flip, cut miters. In some cases, it’d be a mixture of fillets and miters on each side. With a tool changer, there are also options for cutting joinery.

It would also be possible to do outside fillets on a variety of angles, if they can be modeled, though it would be slower, as there’s really no way to do them other than with multiple passes with a ballnose bit.

Simpler forms could also be cut on a sliding table saw, smaller simple forms on a chop saw.