Wednesday, March 10, 2010
Wires pt. 2: Tip Prototype #1 Machined
Less than three weeks remaining before our project is due! Time to get a move on.
After a few hours in the UBC Student machine shop, we've built the extruder tip. Machining this piece from a stainless steel screw required some special cobalt drills (available at the hardware store), because normal drills broke repeatedly. A lathe is highly recommended for doing the drilling, although conceivably it could also be done with a drill press.
As you can see, we've modified the end of the screw to keep nichrome wire as close as possible to the printed part. This required a significant amount of machining, but the benefit is not yet certain. If time permits, we will test out simpler variations (more easily produced in a basement) that reduce the machining requirements.
This tip serves two purposes - if it weren't for these requirements, we'd just hook up the mechanical pencil and be done with it all.
- Mount the nichrome wire heater close enough to the plastic surface to heat it effectively, and far enough from the spoolhead to protect it from the heat
- Mount the wire-cutting mechanism
The initial design was to simply have nichrome wire wrapped around a smooth tip very close to the end. However we found during our plastic extruder experiments that it can be tough to keep the nichrome fixed in place, so we added a lip/flange to the design to help wind the nichrome. Now since the material is stainless steel, the flange acts as an insulator between the nichrome and plastic, which is bad because we want as much heat to flow downward as possible. So the flange was made to be as thin as possible (<0.5mm), since the thermal resistance of a part is proportional to its thickness in the direction of heat transfer. Then, since we had the tools available and it would only take a few minutes extra, we thought we'd further reduce the flange's insulation by drilling holes in it. This led to the idea of weaving the nichrome through the holes themselves, in order to get it as close as possible to the plastic surface. But concerns about the wire biting into the insulation led to coating the nichrome with kapton. However, this combination led to rather sparsely-wound nichrome wires, so when we tried to heat the plastic build surface with this setup, it was not effective. It was also difficult to insulate the nichrome, because there wasn't much room. We'll try out some other configurations soon -- I think the original plan might still work the best.
For those who haven't read about SpoolHead on the wiki, the idea is to use a rotating cutter inside the main tube. Basically you drill a hole through the tube and put a narrow rod inside, which also has a hole in it (preferably wider at one end, so that it only cuts the wire in one place). The holes normally are aligned, but when the rod undergoes a rotation, the wire is sheared in half.
We could easily use more standard available parts, but being in a hurry, we could not wait for an order of 3mm diameter rods to arrive. So I quickly cut one from brass, filed it flat on either end and drilled the hole (halfway with a 2.3 mm drill, and the rest with a 1.2mm drill).
The cutter design would probably work best with a steel cutter. However, stainless-on-stainless gives very high friction, and I didn't have scrap tool steel on hand. I wouldn't want to use this brass piece to cut piano wire, but it seems to work just fine for copper. One potential modification would be to tap a 2mm screw thread in the brass cutter, and use a drilled-out set screw as the "blade". That would probably work quite well, combining the hard cutting edge of steel (replaceable, too) with the machinability and low friction of the brass rod.
We tried it out by hand. It worked! The mechanical advantage here is about 13 (1.5mm from the fulcrum to the cutting edge, 20mm to the pulling point) and the thin wire isn't that strong to begin with, so it sheared the wire cleanly and effortlessly. (It remains to be seen, of course, if our wimpy solenoid will be able to do it though).