Thursday, June 1, 2017

Acrylic Figure Sculpture

I recently completed a new figure sculpture. This one was produced from CNC router cut 0.22" thick acrylic sheets.

Modeling

The form was sculpted in ZBrush from a live model as part of my work with the West Huron Sculptors.

Given the simplification about to happen it was kept loose. It was decimated down to 200K polygons. It is then sized and exported to Rhino where the ReduceMesh command further brings it down.

Next, a Grasshopper definition was run which contours the form and lays out the form for fabrication. Grasshopper is a visual programming editor which runs inside the Rhino modeler. Here's the full program, called a definition. Obviously you can't see anything - but you can get an idea of complexity:

It's broken up into small chunks of functionality. 1) Contour the form 2) Get rid of small contours 3) Rebuild the curves, 4) Group together with neighboring piece, 5) Layout flat for cutting.

Here the contours have been extruded to visualize the result:


When Grasshopper contours a mesh it puts control vertices at every triangle intersection. That's a lot of verts which makes extruding slow and faceted.

In order to speed things up and smooth them out a little the Grasshopper definition rebuilds the curves. The number of control vertices used in based on the curve length. These are smooth but nicely represent the form.

Layout for Fabrication

The contours are then laid out and distributed on the ground plane. Each contour is drawn on top of the previous contour in the stack. This provides guidance of alignment from layer to layer. One obvious solution is to drill holes between the pieces and align them with a dowel. I didn't want to use acrylic dowels in the alignment because I didn't want them showing up piercing the piece - ouch! So the alignment diagrams were plotted on cheap bond paper and cut out. In the end this turned out to be less useful than I thought.

In order to CNC route these the nesting of parts needs to be much more efficient! Here are the contours laid out for cutting using RhinoNest:

Three sheets - each 48" x 46" (which I got from the off cut section of Alro Plastics). They look reasonably well packed, yeah? Actually it's about 48-50% efficient per sheet. Not great - but - a really good nesting is usually about 60% efficient. And that's not great. If you want it better - larger sheets, more small parts!

Cutting

The parts are cut with a plastic cutting endmill, one flute, sharp point, scalloped back edge - specially made for acrylic and other hard plastics - available here: Spiral O Flute from Tools Today. The single flute is useful so the plastic doesn't melt during cutting. Some advice: Wear a dust mask and sealed eye protection. Even with good dust collection I find this stuff pretty nasty to cut.

After cutting the parts were number by hand. Better would be to make a simple tool holder for a Sharpie and let the machine label them.

Starting to sort them.

Trimming off the tabs at the router table with a piloted straight cutting bit. That's super easy on the big parts. The smallest parts I had to do at the sander.

An oscillating spindle sander tended to break the tabs off too easily. So instead I clamped a random orbit sander into my bench, hooked up the dust collection and held the parts over 80 grit paper. That makes pretty quick work of it. It's messy though. Again, dust mask and eye protection essential!

After too much of that I switched to using a Dremel tool. This is much more localized and leaves the surface looking closer to the router cut edge. Much better.

Assembly

With all the parts cut and sanded they just need to be layered up and glued just like the computer model. That should be easy, right?!

Here we go... feet upward...

I kept the computer handy. One side view and one perspective on the monitor are helpful to figure out who is going where.

Drawings showing a small stack of layers are also extremely useful:

I could usually stack 3 or 4 pieces, glue them up, then glue on the previous stack:

Once the form starts to tuck under itself I need to mirror the drawings and build the other way. That allows them to be glued together with generally smaller parts on top of larger ones.

Last to go in was the arm going beneath her hair and against her neck. The last piece was too tight for 0.22" acrylic. I cut pieces at 0.125 and 0.1875" and went with the latter which was a nice fit.

Assembled

Whew! All put together, no clean-up of ANY kind! Some Dremel tool assisted polishing is in order.
 





LED Lights 

Here's an updated post about adding LED lights to this piece.

Lessons Learned

Well that was easy to cut but hard to assemble. Here are a few things I'd do differently next time:
  • I should have used a 1/8" diameter bit instead of 1/4" to cut the contours. In some spots, in particular the face, the added definition to the form would have been worth it. My thought at the time was keeping the resolution the same in every direction (1/4" thick acrylic) would look best. Having seen it I don't think so. 
  • The tabs need to be trimmed off with a cutter similar to the one used to cut the edges. Otherwise the surface looks different where the tabs were. This is apparent on the finished form. This would require making a custom router bit - basically adding a bearing to the existing tool. Of course if you have a large laser cutter you don't need tabs and the edges will all be polished!
  • Initially I used a feed of 150 with a spindle speed of 18000. Later I used 80 and 7500 and I prefer the later. The chips are much bigger and less sliver-y. 

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