Filip is Making Anything...in MAS 863.11

Two weeks into the semester, I learned that I was one of the winners of a design competition, to build a Sukkah on the Washington University campus in St. Louis. A Sukkah is a temporary hut built to celebrate the harvest, or 'Sukkot', typically occuring in the Fall.

My design proposal was based on using 8"x96" heavy-duty paper tubes, which were to be of varying lengths and cut with a 60 degree bias. Each tube was aligned to a dia-grid and rotated in increments of 30 degrees. I was interested in seeing what kind of variety I could produce with a very simple set of rules.

Cutting Veneer with the Shop-bot

While the original design used cardboard, the cost of 100+ tubes as demanded by my initial proposal was beyond the budget I had to work with. I also wanted to take advantage of the fab equipment we were using in the class. A colleague had suggested I roll my own tubes to cut costs and I did some test layouts and found that I would get more components using a 4'x8'sheet material in comparison to a paper tube. I played with 1/32"polypropylene sheets first because they seem to be enjoying a kind of popularity and can be cut using the shop-bot. Unfortunately, the sqf cost was slightly more then paper tubes and would not save any money. The competition proposal made a point of capitilizing on the atmospheric lighting effects of a very simple material and I decided to re-visit wood veneer, which I had some previous experience with. I found that when rolled against the grain, even something as thin as 10 mil paper-backed veneer, had an impressive amount of structural stiffness.

Issues

Because the wood and paper-backer expand and contract at different rates, veneer is dimensionally unstable - it never lays flat!

This is not an issue of taping or fixing the veneer down. The NC Router in N51 and at Washington University both have vacuum beds which hold the material flat. This actually causes more problems, because it creases the material.

This is not an issue of taping or fixing the veneer down. The NC Router in N51 and at Washington University both have vacuum beds which hold the material flat. This can cause more problems, because it creases the material.

Because the material can float anywhere between .25 to 1.25 inches off the cutting bed, the retraction disctance and clearance plane of the end mill must be set much higher. This adds considerable amounts of time - typically the router stays as close to the material surface when moving in order to minimize the amount of time spent on each sheet. For example, if you are cutting 100 holes on 10 sheets of .0625" thick material and the retraction distance is 1.5" higher than normal, it will take 4-5 seconds longer to drill each hole. 5 secs * 100 holes * 10 sheets = 83 minutes of additional cut time. So, minimizing retraction distance is critical; Too high and the cut times are unnecesarily long; Too low and the material is inadvertently cut while the router is moving between cuts/holes.

Why not use a laser cutter?

The first issue with using a laser cutter, is that we don't have one with a bed as large as the Sukkah components. Second issue is that the material heights will cause similar issues. I tried cutting a smaller mock-up component with the Epilog 120W and found that in some places the material was catching fire while only scoring in other spots.

I also tried using the holes in the components as points through which I could fix the pieces to the cutting board with machine screws. This actually worked pretty well. I would cut the holes first, fix the veneer down with screws and then cut out the components. If super accuracy us needed, this is the best way to go, but it dramatically increases the amount of time it takes to process a sheet.

In the end, the solution was a mixture of things, I used a vacuum bed, but did not tape the material edges, minimizing the heights of the folds, while preventing creases in the wood. I set the retraction distance to .75" and used an .125 down mill.

In Praise of the ShopBot

I witnessed a lot of frustration generated by the ShopBot and Partworks software, but, after being forced to use three different machines within one week, I can say that the ShopBot is by far the easiest to program and service. The cultural impact of fabrication technology has less to do with its sophistication and capability than it does with public access to it - how easily can someone master the logic and interface of the machine. It takes just a few minutes to set-up a file in partworks, change the bit, set the z-height and origin and start the ShopBot cutting. Some of the machines I used this week had elaborate set-up processes and opaque interfaces. These were machines that require a huge investment in training infrastructure - it tooks me almost 30 min. to set the z-height and or origin on one of them...and I knew what I was doing at that point!

To zero the end mill on an AXYZ NC router, you use a piece of paper and lower the bit until it tears the sheet.

Prototyping

Fabrication at Washington University went smoothly, despite the learning curve associated with their equipment.

Fabrication at Washington University went smoothly, despite the learning curve associated with their equipment.

Special Thanks: The junior faculty at Washington University were the backbone of my effort. Ken Tracy, Forrest Fulton and Christine Yogiamon, spent countless hours explaining the work-flow to me - and many more late-night hours helping me cut and assemble components.

Tips for Cutting Veneer using a CNC router

1. Cut with the veneer face-down.

2. Layer multiple sheets on top of each other.

3. If possible, use a CNC router with a vacuum table .

4. If a vacuum table is not available, attach the components to the bed with screws, following the first machine path.

5. Use a straight or down-cut end mill.

6. Fly-cut or change the sacrificial layer often - ridges and grooves sometimes cause the veneer to 'drop'into the gaps, preventing it from being cut smoothly.