My first Shopbot experience
Computer-aided milling: working with Shopbot
I was looking for a tool that allows cutting very precise slots in wood. Working with a mold is a reasonable solution, but not precise enough. The slots are needed for connecting two pieces of wood without using glue or screws. The thickness of the wood is 6 mm, so the slots should be exactly 6 mm wide, and completely straight. Shopbot(SB) at the Fablab is designed to do this trick.
Before I started cutting, I visited the Fablab three times to gather information about SB. The fourth time I came with a Adobe Illustrator document to the Waag and finally SB did exactly what I wanted it to do.
In a first experience, nothing comes naturally. In this article I will shortly describe the process and the problems I encountered. First you have to create a file the SB-computer can use. In my case it was an Adobe Illustrator document. With the line-drawing tool you draw the paths the bit has to follow.(I chose lines with 1 pixel thickness). Then there are three options you should consider: does the bit has to move on, inside or outside the drawn lines? For me it was: outside the lines.
Next issue; the bit is round, which means on inside edges it will not cut right angles. For my slots this meant that they would not be long enough. How it exactly works is too complicated for me to explain, but there are documents on the Fablab-site that do explain this problem very good. To overcome the round-edges-problem I had to add half the thickness of the bit to the length of the slots. (In illustrator I designed everything perfectly, but machines have their own rules!)
I am cutting a sheet of 125/250cm /6 mm birch. The illustrator document should have the same size (no thickness of course), which meant scrolling a lot. Note that it is better to do the layout in millimeters. I used centimeters, but when the document was translated to the SB-PC it was suddenly one tenth it's size! This was easy to fix, but starting in millimeters is probably better.
The next issue is the thickness of the wood. You buy as much as 6 mm, but is it 6 mm? Measure before your mill! My sheet of wood turned out to be 6.2 mm thick. With a slot width of 6 mm this is not gonna work.
A test-drive, in which SB was told to cut a 6.2 mm slot, resulted in a slot that was still too narrow (5.7 mm)! This was not supposed to happen, but it did, so SB was not doing what it was was told to do. We still do not understand why this happened (suggestions?).
We worked around the problem. The Smart Guys from FabLab fooled the SB-computer: there was a 1/8 inch (3.17 mm) bit in the machine and the computer was told that it was a 2.5 mm bit. It worked and ShopBot went to work at full speed and suddenly everything went without saying. In just over an hour SB milled nearly 40 meters slot in 6 mm birch. Great!
Birch is well suited for Shopbot, but for a perfect finish the new surfaces still have to be sanded.
