Tuesday, July 1, 2014

Pixelated Vessel

When working with CNC cut Baltic birch plywood it is a very common method of construction to use a stack lamination technique. Normally the edges of the layers are cut smooth by the router, and the discontinuities between the stacked layers are sanded smooth. Thus the resulting piece matches the designed smooth form.

I wanted to try something different:

I wanted to accentuate the fact the form was made from stacking. And to visually demonstrate that the vertical edged stacking is an approximation of the curved form. To make this more apparent I chose to stair-step or "pixelate" the forms horizontal curves as well. Below is an example:

The original form - a simple twisting pedestal:

The typical method of fabricating this form. Section it into layers, cut them, assemble (left image), and sand the edges from one piece to the next smooth (right image):

The stack lamination, "pixelated" version of it. Here the layers are 3/4" thick (corresponding to the plywood thickness) and the horizontal stepping happens in 3/4" increments as well.

For the vessel I wanted to create here's the source geometry I started with. It's a hollow closed polysurface in Rhino:

I wrote a Rhino Python script to section the form horizontally in layers which correspond to the thickness of the plywood. These intersection curves are then "pixelated" on each edge. That is, instead of using smooth curves like the original form, I have it step across in a user specified amount. This makes it look as if it was constructed out of curved edge Legos. 

The filleting of the curves is done so the CNC can cut into the corners. I actually find this less compelling than the more pure, true to concept, square edge pixels. So that's a compromise. However it is easier to sand, and a lot nice to touch once finished!

It would be possible on a 5-axis router to use an endmill parallel to the face of the plywood to clean out the corners. That's an entirely doable but bigger task for another day... This vessel is made by a 3-axis router.

In plan the curves look like this. You can see the implied grid and how they all align, layer to layer:

Here's a single curve shown in blue. You can see how it twists through the grid but will always align with its adjacent layer above and below:

In terms of the form approximation, here are the two forms overlay-ed on one another:

A property of the script controls how the volume of the original form relates to the pixels. You specify the number of corner points of each pixel (a number from 1 to 4) that are required to be within the form for inclusion in the result. In the example above 4 points were required. In the example below only 1 point was required. Thus, in the example below the pixels sit mostly outside the form rather than within it:

In order to align the parts during assembly some points are generated. These become holes which are drilled in each piece to allow dowels to precisely align them up. The points are generated by intersecting curves with each horizontal section. You can see the curves in the 3D form (in red) and the points on the laid out curves (in white) below:

And of course the layers are numbered in the 3D model to indicate which piece goes where.

Fabrication / Assembly

The parts were cut at Taubman College on one of the Onsrud 3-axis routers.

Shown below is the drilling of the piece to piece alignment holes. In each location there's a hole to align with the piece below and a hole to align with the piece above:

A 3/8" compression bit was used to cut the contours. This bit cuts all the way through the material in one pass. Tabs hold the piece in place so they don't move when fully cut. These were removed with a chisel and mallet. Here you can see two of the tabs at the bottom of the cut (circled):

I'm always trying to find out the smallest size I can get away with and still hold adequately. I'm currently at only 0.5" wide and 0.05" thick, two per piece. So it takes a single chop to get a perfectly clean edge. Much quieter, cleaner, and quicker than a trim router.

Back at my shop the parts were sorted during a dry fit. Then edge sanded to smooth them. If you look carefully you can see a few of the tabs still need to be chiseled off.

Assembly was simple. Quarter inch dowels were sliced up and used to align the forms. An easy glue up which could be done in sections.

Finished Vessel

Here's the fully assembled form prior to applying a finish. 

Here's the finished vessel: