Keith's Electronics Blog

At the end of March, I broke my MakerBot CupCake idler wheel while trying out my new filament drive worm-pulley. In April, my friends Scott Smith and Ben Wynne in San Diego machined me some aluminum replacement wheels, which are totally awesome.

Old and busted; new hotness. Yeah, baby!

This wheel press-fits perfectly onto the bearing, starting by hand and then leaning heavily on it on a countertop. It’s thicker than the original wheel, making precise alignment with the drive pulley less important. It even has a knurled edge — showoffs!

I’d been experiencing “plaid” printing, in which the filament feed rate dropped at very regular intervals, and attributed it to the (tangible) irregularities of the acrylic idler wheel. But I’m still getting plaid prints, and I don’t think my aluminum wheel (hot off the lathe) is irregular, so the problem must be elsewhere.

Hm, look at the deflection of my filament feed motor shaft, all the way to the left of the hole in the enclosure.

Hm, look at the deflection of the shaft now, when the flat hits the edge. That would explain why the weak feed is so regular — recovery doesn’t rely on the drive pulley maybe grabbing the filament and maybe being able to spin the idler wheel to a different spot before it gets going again.

Looks like I need a bearing on that motor shaft. I printed a shoulder washer/bushing for it and it helped for a while, but not enough. I think a real, metal bearing is in my future. And perhaps a different drive geometry. A lot of good filament feed designs are being uploaded to Thingiverse.

As I discovered a couple of weeks ago, Glastherm HT is not a good enough insulator to protect the acrylic mounting plate under my MakerBot CupCake heated build platform. Last night I tried it with glass standoffs between the hotplate and the Glastherm.

I had some leftover glass from having a window cut recently. I bought a glass cutter and cut a half-inch strip off the end of one of the pieces, then cut that into smaller tiles. I smoothed the sharp edges on a diamond file, then pried apart the hotplate sandwich and stuck the glass tiles onto the leftover kapton tape.

I repeated my earlier test, standing the sandwich on edge on another piece of insulating material and running the heated build platform up to (and past) operating temperature. (Sorry no laser dot in the picture — I’m not quite coordinated enough to hold the thermometer’s capture button, point it the right direction, hold my hand still, and take a picture all at the same time.)

The air gap did a considerably better job of insulating than the Glastherm alone. Also when I remembered to measure the temperature separately under the glass feet, the spots were 15-20°F hotter than the rest of the Glastherm, suggesting that smaller glass dots could provide a significant further reduction in heat transfer.

135°F on the back side is pretty manageable; 200°F is a bit warm to be in contact with plastic. However, as Leon pointed out last time, it’s not just about how hot the back side gets, but about how rapidly the heat is transferred into the backing material.

I set the heated platform onto a spare piece of plexiglass and also dropped some scrap filament onto the top of the platform. Already above operating temperatures, the filament softened nicely and the plexiglass stayed cool to the touch and didn’t warp. Leaving the setup unattended for a while and letting the platform drift up to 340°F, the plexi got a little soft.

I think this is usable for the CupCake, but I’ll need to exercise a little care managing the plate temperature (and I’m overdue hooking up the thermistor for closed-loop temperature control). I think this may not yet be enough insulation to tape my SMT hotplate directly on top of a plastic enclosure and run 290°C (~550°F) soldering temperatures.

I got a care package this week from my friend Scott in San Diego. Half of it was two pieces of Glastherm HT that he cut to fit my soldering hotplate and my MakerBot CupCake heated build platform. Scott and his buddy Ben Wynne are building hotplates and were also planning to use Glastherm to insulate them.

Last night I pried the warped acrylic off the underside of my platform, cleaned the surfaces with alcohol to ensure a good stick, and affixed the Glastherm to the heater PCB with kapton tape. I then set the assembly on edge on another piece of Glastherm and ran it up to operating temperature.

The Glastherm was not nearly as good an insulator as I expected. The “cold” side of the sandwich lagged only 25°F behind the hot side as I was heating it from room temperature and once it stabilized at 250°F, (at which point the “cold” side reach equilibrium around 225°F).

I’m surprised that the Glastherm provided so little insulation. Perhaps it’s not intended to be used in direct thermally-conductive contact with a hot surface — although the diagram on their web site certainly suggests that it is.

At any rate, the “cold” side of my sandwich is still far too hot to be in contact with my acrylic mounting plate. And since Glastherm is supposed to be pretty good stuff, I think I’m resigned to finding a way to assemble my sandwich that leaves an air gap between the heater and the mounting plate, which I hadn’t wanted to do.