Keith's Electronics Blog

I’ve been dissatisfied with having the CupCake’s extruder controller attached to the front of the extruder. The PC board (being opaque) blocked the view of the feed mechanism. As much trouble as I have with filament feed, it’s pretty important for me to be able to keep an eye on it.

Actually, as many times as I’ve had to disassemble the feeder (replace broken idler pulley, reglue idler pulley, remove broken feed pulley flange, replace broken plastic?, etc.), I also got tired of having to remove the PCB every time.

And <whine>the patch cord didn’t route nicely from the side of the machine facing forward to the top of the machine facing up.</whine>

I’d previously mounted the extruder PCB up on the highest two screws of the extruder housing, placing it above the feeder face instead of covering it, but that was pretty fragile and I was constantly worrying about bumping it and snapping it off. This weekend I moved the extruder controller to its new permanent home on the right side of the CupCake in the empty space above the motherboard. It fits nicely and shares a mounting screw with the upper Z-axis endstop.

It does block access to the SD card slot, but I don’t use SD cards anyway. Even if I do someday, I think I’m most likely to put a card in it once and leave it there for doing prints without a host computer connected. Now that I’ve eliminated printing stalls and blobs, I don’t have a lot of reason to try uploading designs to a card every time before printing, nor save designs to a card from the host to then install into the machine.

When I have time, I’ll make a shorter (4″?) patch cord to jumper from the motherboard to the extruder controller without all the excess I currently have coiled up.

Cable Lacing

I also got to try my hand at cable lacing, something I’ve long admired in old TVs and vacuum tube organs. I don’t know that it’s the right look for a modern rapid prototyping machine, but the waxed string cost only a pittance and it doesn’t have to be the permanent solution.

The cable clips I had on hand were too big to anchor the mobile end to the extruder properly, but they do help guide the cable until I can buy smaller clips. Initially I had thought I’d need to enclose a piano wire in the cable bundle to help it maintain its arch, but it’s flexing very nicely without additional support.

Extruder Wire Gauge and Current

I’ve been concerned about the ability to deliver enough power to the extruder controller over the ethernet patch cord, especially when people are running heated build platforms off it too. I realized after the fact that I should have checked the wire gauge I was using before extending the heater wires out the top of the machine, but let’s check it now.

Powerstream.com provides a wire gauge and current table and calculator. For the 24-gauge wire I used, they list the maximum amps for chassis wiring as 3.5A, which is a comfortable margin over my actual current of 12V / 6Ω = 2A, so I’m safe.

Toward the bottom of the page they have a load calculator. Entering 24-gauge copper, 12VDC, 1.5′ one-way length, and 2A, they show a round trip .158V (1.32%) drop in the wire and 11.842V delivered to the nichrome heater. Not too bad.

While we’re here anyway, let’s do the math for running both the nozzle and platform heaters from the extrusion controller in its original configuration. The interface uses three wires of the (almost certainly) 24-gauge patch cord for each of 12V and GND, so that’s equivalent to just over a single 20-gauge wire.

The nozzle heater was 2A and I see people talking about 4-6Ω platform resistance for another approximately 3A. The chart says 20 gauge is good for 11A for chassis wiring, which is a comfortable margin. The calculator shows a .314V drop over the 3′ patch cord, still delivering 11.7V to the extruder controller. Quite acceptable and nowhere near as bad as I feared.

In the last couple of months, I’ve worked through and fixed several major problems with my CupCake hardware and Skeinforge slicer settings that were preventing me from printing any useful models: warped build bed, infill density, printing stalls causing blobs, XY slowdown on curves causing blobs, and bizarrely bad printing.

Making a Less Warpable Build Bed

I switched a while back from the included foamcore build surfaces to a scrap of plexiglas, to which the extruded ABS sticks more securely and from which it releases after the build without damage. Initially I taped the plexi to the build platform at the corners; but as the the model being printed cooled and began to shrink, the plexi warped upward with it and the tape was insufficient to keep the plexi (and model) flat. The warped upper surface of the build then snagged the nozzle, the steppers lost steps, the next layer(s) got out of alignment, and builds were ruined.

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This post is for anyone thinking about a CupCake and wondering what the extrusion is really like. The rest of y’all, go away for a bit.

First extrusion, no CAM happening, just heating up the extruder and running filament.

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I received my CupCake kit around the end of April; it’s now August and I haven’t completed it yet. What’s up with that??? Well, I’ve been finishing the case.

That’s a pun. Here, let me spell it out: I’ve been completing the case by applying wood finish to it.

White “Stain”

I decided early on that I wanted the case to be as light-colored as possible so the interior would illuminate well to watch and photograph builds; but that I wanted to honor the fact that it’s made of plywood and let the wood grain show through.

That said, the back sides of some of the plywood pieces are a bit rough. (The fronts are all very nice, and both sides of the 1/8″ pieces are nice.) Although the rough sides all face the interior, the interior is where I’m going to be looking and photographing a lot. So a lightening agent with a bit of opacity –but not too much — seemed like a good idea.

Vicki the Paint Lady at Graber’s Ace Hardware led me to a Zar “country white” oil-based wood stain that she had used before and which very closely resembles an oil-based paint. It applies like a stain, but the color particles are white and more opaque than I’m used to from regular wood stain. I had to be a bit careful in the application not to leave it on so thick as to completely hide the wood grain; but I was pleased with the result.

The smaller, upper piece is untreated; the lower background piece has one coat.

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