This week for the assignment 'make something large' with for Large Format Milling, I once again ran into a familiar situation of my ambition out stripping my design ability. I guess that is partially the point of this class, to push your abilities as far as you can.

Since our stock this week was a piece 8' x 4' Oriented Strand Board (a.k.a. chip-board), a material that does not have much employ in laboratories, I again opted to make something for real life rather than lab-life

My original idea for this week was to make a modernist rocking-chair. I liked this idea (and still do), but I realized that I don't actually need a rocking-chair, and if I made a rocking-chair, I wouldn't have anywhere to put it. What I do need at my house, is a bench for our entry-way. My wife and I have a 'shoes-off' house, but we don't have anywhere to sit to take shoes on and off. I am particularly aware of this when we have guests over. Watching friends who have not mastered the 'in-step to in-step' flick off of the shoes, or have their Sambas tied 'just so' that they're effectively slip-ons, and instead seeing them lean awkwardly against the wall, or worse, sit on the floor to put on their shoes, makes me feel like a pretty crap host. We do have guest slippers, so we aren't total animals, but still we need a bench.

At first, I had hoped to make an L-shaped bench in order to make the most of the limited space in the entry-way. Also, since this area of the house is in a state of perpetual cluster-f%#@ I wanted to make a combination bench/shoe-rack. The photographic evidence clearly shows that we have over-run the standard-issue shoe-rack that we have in this area.

Unfortunately, this is where my ambition began to outstrip my design ability. After making a few sketches of my ideas, I realized that incorporating the bend in the 'L' was going to substantially complicate the design, and if successful would only add about an extra foot of bench length. Also, I was envisioning that building the angle may require some mitre cuts, which I was not entirely sure I could do with the milling instrument. Additionally, I decided to jettison the incorporated shoe-rack for this version, and instead design it to sit over the existing shoe-rack. For starters, the version made this week would only be an initial prototype, which I would remake with 1/2" sanded plywood. I could not in good conscience have guest see a item made of chip-board be the first thing they witness as they walk into the house. So there would be an opportunity to incorporate the shoe-rack later. Second, the design of the shoe-rack would require generating a greater number of complex shapes in Autodesk Fusion360, which I am still getting the hang of, so getting the design of the bench itself would pose sufficient challenge.

I designed the bench using AutoDesk Fusion 360. Even though I planned to design the bench for press-fit assembly, and thus would comprised of primarily 2-D shapes I resisted the urge for expediency and simply design the bench as vectors in Illustrator because I know that designing and rendering in 3D, even if the fabrication is performed is a skills I need to developed. I also want to become more familiar with the parametric design capabilities in Fusion360

While I did scale back on some of the design complexity , I still wanted to continue the theme from week-01 to explore the technique of generating contoured surfaces from flat stock through the use of flexures, kerfing, and selective cutting. Since I am planning to make the final version of the bench using sanded plywood, which supposedly can be used to form beautifully contoured surfaces using kerfing, I hoped to incorporate an arching benchtop into the design.

Before going too far with the design, I wanted to get some idea of the degree of curvature I could obtain in 1/2" plywood using kerfing. Of course not knowing how densely I should place kerfs along the board, I did what any good scientist would do when trying to determine the unknown value of a continuous parameter, a titration!!!

Using a piece of scrap 1/2" plywood, I tested a series of kerfs along the board: from 2 passes/inch, 3 passes/inch, 4 passes/inch, 5 passes/inch; with 5 inch segments for each density. The Shopbot was fitted with a 1/8" inch endmill, and the cut depth was set to 0.35" or ~75% of the material thickness. The cut path was set using Vcarve software, with the tool set to move 'On' the vector path (as opposed to 'Inside' or 'Outside' the path).

3 passes/inch was clearly the optimal density for kerfing for 1/2" plywood, achieving remarkable bend of almost 90-degrees over the fairly short distance of 5". Interestingly both the less dense condition of 2 passes/inch and higher densities of 4 and 5 passes/inch left the board rigid.

This exercise taught me a few lessons: