Keith's Electronics Blog

On January 5, nophead wrote of his serendipitous discovery that ABS sticks well to hot kapton tape and releases well from warm kapton tape. Coupled with difficulties I’d been experiencing with my plexi build platform not being rigid and flat enough, I knew immediately that I wanted the top of my heated build bed to be aluminum plate covered in kapton.

When Tom McGuire built me a PCB heating element, he offered me some sheet aluminum for the top; but although not thin, it wasn’t thick enough to be as rigid as I wanted it to be. I contacted my friend Scott Smith who has taken up machining and he offered to mill me a piece from some 1/4″ plate he had on hand. I sent dimensions and he did the rest, including taking pictures and sending me the following commentary.

February 6:

Having rough-cut a 100mm square piece of polished flat 0.25″ aluminum on the band saw, it’s time to square off the edges.

After one edge is flat, I can flip the piece over and mill the opposite side. Once done, I have at least two sides that are guaranteed parallel.

When I rough-cut the piece, I left some extra so that I could square the piece off without losing too much material. Now both sides are parallel.

Next, I lay the piece flat in the vise with the non-parallel edges on the left and right sides. I can run the mill in the Y direction to square these last two edges off.

Using a macro lens in rather low-light, so I had to crank up the ISO and open up the aperture. Still, my shutter speed was too slow to catch individual chips flying around…

Once the piece is square, I use an edge finder to zero out the CNC at the center. The stop is placed in the vice so that when I flip the piece over, I don’t have to re-center the CNC again.

The crosshair is made using an 60 degree engraving bit at high speed.

To zero out the Z axis, I wiggle a piece of paper back and forth under the engraving bit as I slowly lower the spindle. When the paper stops moving freely, I set the Z position to the thickness of the paper.

After engraving there were burrs around the crosshairs. Before I use Scotch Brite to clean it up, I take a red sharpie and ink in the grooves.

The Scotch Brite takes any excess ink off of the surface, but leave the ink in the grooves. Hard to see in this photo, but the crosshair is red.

Over the holiday break, I had written about using a hairdryer to heat my CupCake build chamber.

On January 11, 2010, Tom McGuire wrote:

Hey Keith, I haven’t caught up with your blog for some time. Looks like you got a lot done in the last couple weeks. It’s good to see you’re getting results from your CupCake.

Can I help you with a hot plate? I could make you a thin aluminum plate about 4″ square that gets up to 200°C. I guess you would need it to be insulated on the bottom to keep from burning your stage. Do you have a low voltage with lots of current available, say 12V @ 10A or may be 5V @ 20A. What do you think?

What did I think??? I thought it sounded great! The heated chamber was helping my prints, but I was still getting some warp. I knew a heated platform was the way to go and it had been my goal all along.

We started discussing the way other folks were making their heated platforms and Tom went on to say:

Oh 100°C would be no problem for PC board material. I was thinking at first to place a bunch of low ohm surface mount resistors around on a 4″ square PC board but then I remember we did this other project that was supposed to be in inductive coil for an induction heating system. It turned out to be more of a heater coil itself. I could do something like this but in a square pattern and leave openings for your screws and magnets.

Because I wasn’t sure yet whether the heater PCB would mount fairly directly to the Y stage (and thus need holes for screws and magnets) or be raised above it (far enough that it wouldn’t need holes), I pointed Tom at the CupCake CAD files on Thingiverse.

He downloaded the DXF for the original build platform to get the hole size and spacing, then laid out this heater PCB with traces fitting around all the necessary openings.

Then he showed up for lunch with a milled heater PCB that ended up measuring 4Ω resistance (an extremely convenient value) and with an SMT thermistor already soldered in the middle.

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.