Wires pt. 3: Progress on all fronts

(And a few minor setbacks.)



Tests of our first prototype revealed some problems. Mainly, we haven't been able to heat the plastic effectively by convection / radiation alone. We tried making contact with the PLA surface, but this didn't work either: If we let the heater cool and then try to lift it, it stuck to the PLA. If we lifted it before it cooled, the wire pulled out from the still-hot plastic. The heat didn't diffuse very far into the PLA.

The ineffectiveness of the first extruder at heating the plastic lead us to machine some new ones to replace it:



Also, our 0.3mm pencil has proven itself very unreliable at feeding the wire; clicking the pencil doesn't always result in advancing the wire. It could be a problem with the wire, or the pencil itself. Fortunately 0.5mm pencils have proven much more effective, but they are more difficult to click - our solenoid will certainly not have the power. This should be solvable with some re-engineering.


Heater:

So, heat transfer first. I machined two more extruder tip designs. We've abandoned the flange for now, and are using a narrow tip instead. The tricky part is to try to get more heat going down to the PLA than moving up along the steel tube. Thinking about the heat transfer situation we're facing led to this second design, which uses a flared conical tip. Thermal conductance is proportional to cross-sectional area, so the conductance gradually increases along the length of the cone toward the bottom. To do this properly I'd like to do a pen-and-paper calculation, combined with finite element modelling in SolidWorks. But for now I'm just going on intuition. To put it in the language of circuits, the heater tip is like a current divider. I could make a much more complicated and accurate model, but this one illustrates the theory very well:


The PLA surface is assumed to be a heat sink, as is the stainless tube length above the heater. Heat will tend to flow both up and down from the nichrome wire, but we can skew things to make it prefer to flow downward by having good thermal contact to the PLA and having a high thermal resistance going up. I hope that the cone's bottleneck will act as a thermal resistor to keep heat moving downward. Ideally I'd make the bottleneck much longer, but we'll start with this and see if it works before we move to more fragile designs.

So our other new tip uses the same cone design, but the cone is made of alumium and screws onto the stainless shaft. This is done for the same reason; aluminum's thermal conductivity is about 15 times better than stainless, so now the stainless itself should have a high thermal resistance compared with the aluminum path to PLA. Aluminum has the advantage of being easy and quick to machine compared with stainless, but I'm apprehensive about making thin structures from it because it's pretty weak. Also, its high thermal conductivity downward comes at the cost of high thermal conductivity upward. I'm not sure the cone will make much difference here; without a doubt a lot of heat will flow up the aluminum. The thermal contact between the aluminum and stainless is quite poor, but the aluminum piece might be large enough to act as its own heat sink anyway:

So I tried to make it as thin as I could. We'll see. Sometimes it's quicker to just do the experiment than to over-analyze these things.


Mo used a screw jack to manually simulate the RepRap's Z-bed, and mounted the heater nozzle on a clamp. Bing did it up with nichrome and fibreglass just like the real thing, so this test would be more authentic (no more bic lighters). We started with the all-stainless nozzle. Again convection didn't seem to be enough to heat the surface, but when we made contact, the heat penetrated very deep into the PLA. And so did the wire. Penetrating a few millimetres means we can remove the tip while it's still warm, because the wire won't pull out.

(Sincere apologies to SparkFun Electronics for using their logo as a test piece).

Here's a wire bonded this way, by our tip, to a coat hook.



The wire here is 24-gauge (0.5mm) tinned copper, from McMaster-Carr. It fits beautifully in a cheap 0.5mm pencil, but the best thing is that it's not insulated like magnet wire, so it might actually be useful. The downside is that it's quite stiff, which might make it hard to print with. Our impression is that if we bond it at regular intervals and when going around bends, we shouldn't have too many problems.


The aluminum tip will be tested next.




Cutter:

These tips don't yet have cross-drilled holes for the rotating cutter. But with Bing's observation that the cutter seems to act as a powerful heat sink, it might be worth considering a back-up plan. I'm thinking of using the solenoid directly; solenoid bars come with holes drilled in them already.


We'd lose the mechanical advantage, but when it comes to solenoids I'm not sure that's such a bad thing. A solenoid ideally has a force proportional to the inverse square of the pull distance (although for very small pull distances, magnetic saturation makes it more linear). With a mechanical advantage of 2, we'd amplify our force by two but need to pull over twice the distance, so at the far extent the force the solenoid can provide will drop by a factor of four. It's hard to tell at this point whether it's beneficial or not, because the strongest force is really needed right at the end of the pull, when the wire gets cut, where the mechanical advantage and solenoid non-linearity work together to provide a strong force.

So it could go either way. At least it's worth keeping this alternative in mind. It is, after all, a fair bit easier to build.

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

Heating:

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.



Cutting:

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).