Computer-Controlled Machining

This week's assignment: make (design, mill, and assemble) something big (~meter-scale).

[1] Tolerance & Dog Bones

Before I started milling anything large on the CNC, I first had to determine a reasonable tolerance for joint sizes as well as an acceptable dog bone size. I designed a small test piece to test out both of these properties, and I actually managed to get the values correct on the first try. These are images of the CAD model for the test piece (dog bone area in red), and what two of the pieces look like when fit together:

Here is a short video of the CNC milling out these two test pieces, using a 1/8" end mill:

[2] Design

For this week's actual project, I wanted to make a small stool that would be sturdy enough for me to sit on. I was inspired by some images I saw after searching for "flat pack stool" on Google. After some OnShape-ing, I came up with this design for three-piece stool that would fit together with two pieces joining to be the legs of the stool and one circular piece for the top of the stool:

I made sure to use the tolerance and dog bone size I had determined from my initial test in the CAD model for the stool.

[3] CNC Milling

After exporting the 3 pieces into separate DXF files and processing them using Aspire, I was ready to mill them out on the ShopBot CNC in the Harvard shop. The first piece I cut out actually had an error, since I had forgotten to switch between the "Outside/Right" and "Inside/Left" milling options, which caused the CNC to mill on the wrong side of the specified contours. This is what the failed piece looks like after I stopped the CNC when I realized my mistake:

After changing to the "Outside/Right" option in Aspire and generating a new ShopBot file, I restarted the milling process. This is what the first piece looks like being correctly milled by the CNC:

After milling out the three pieces, they came together effortlessly and I was quite pleased with the final product. Some beauty shots are below (yes, I did laser cut a stencil and spray-paint my name on the bottom):

[4] File Downloads

Tolerance/Dogbone Test (.DXF)
3-Piece Stool (.ZIP)