WEEK06: computer-controlled machiningThis week we learned about computer-controlled machining. One can consider this section a natural extension of the learnings from cutting week and electronics production week. In those cases, we were mostly concerned with 2D (or 2D-ish) subtractive machining on relatively small substrates using an auto-generated toolpath. Our machine of choice this week is the 60x120" ShopBot CNC router, which (a) is much larger than the SRM-20 or Epilog; (b) is far more dangerous than previous machines I've used in the class; and (c) requires quite a bit more planning to set up cuts. Following a familiar pattern, our assignments were twofold: (a) characterize the ShopBot by testing runout, feeds/speeds, and toolpaths; and (b) make something big. Thanks Tom for teaching us how to use the ShopBot!
a note on safetyI tend to wear safety glasses whenever I'm in the shop, because I quite frequently find myself jumping up and running over to do something potentially hazardous like soldering or drilling. Sometimes choosing to wear PPE is a matter of personal preference; for instance, most folks don't wear specs while soldering. Assembling an apparatus is another example; simple press-fits are safe enough, but it's worth donning the gear if you're going to gently bash things together. Other times PPE isn't an option. The ShopBot is one of those times. Unlike the SRM-20 and the Epilog, the working bits of the ShopBot are exposed to the user, and the spindle is quite a bit more powerful. Broken end mills quickly turn into projectiles. Wear your safety glasses!
machine characterizationWe had a fairly large group for machine characterization this week. This was good, because we all came with ideas and helped troubleshoot when stuff went wrong, but also meant we had to be extra careful to ensure we worked safetly around the machine. Alfonso is building a wind tunnel for his final project and brought a design to cut out of 1/2" OSB that we could measure. We also designed few known shapes to check, including a 5" square with a 3" internal cut and a number of longer thinner pieces. We used VCarvePro to set up the pocket cuts as follows:
- 1/4" up-cutting two flute end mill
- two passes to 0.55" depth (vs ~0.47" OSB thickness)
- two 1/2" wide, 1/4" tall tabs per piece
- 10,000 RPM spindle speed
- 120 inches per minute cut XY, 60 inches per minute Z
|outer box width||5.000"||4.986"|
|inner box width||3.000"||3.027"|
make something bigI initially intended to build a chair. The design would be elegant, sturdy, delicate, robust, curvy, delightful... such ambition. But at some point the week started to slip away, and I realized that my (somewhat) familiar design workflow wasn't well suited for the organic curves I wanted to make. Someday, probably soon, I'll dip my toe in Rhino/Grasshopper and spend some time learning Blender. But for now, I decided to build a plywood box using 6mm birch plywood. Since I still wanted a bit of curviness in the design, I started by cutting out a few test coupons to get a feel for the material's flexibility. I drew a parametric sketch of one flexure segment in Fusion 360: Then I arrayed the flexure feature across the part, coming up with two sets of parameters to test: The two designs shown above have the following parameters:
|test piece||slot radius||web width||slot spacing||flexure copies|
|top||4 mm||10 mm||12 mm||5|
|bottom||1.75 mm||8 mm||7 mm||8|