Thing-O-Matic: Aluminum Build Plate

This is a variation of Thing 6384: an aluminum plate sitting atop the Automated Build Platform’s bare heat spreader, minus the belt. HIs truly ingenious idea was to cover the plate with a thin layer of ABS to ensure adhesion: an ABS filament bonds very well to ABS!

Aluminum build plate in action
Aluminum build plate in action

I started with a big sheet of 3/32 inch aluminum, a bit thinner than the 1/8 inch sheet he used, which is what I had in the Parts Heap. Bandsawed three chunks to rough shape, squared up the edges on the Sherline with manual CNC:

Squaring the sheets
Squaring the sheets

That was complicated by the Sherline’s cramped work envelope. The 5/8 inch lathe bit on the right sits at exactly right angles to the X axis and serves as the reference plane. To make it happen:

  • Stack the three plates, clamp to table aligned against lathe bit
  • Whack off the far edge
  • Put clean edge against lathe bit
  • Whack off another edge
  • Measure / scribe 120 mm from each new edge (thus the blue stripes)
  • Align & cut

That actually worked quite well, although you’d think the angular error would build up as I rotated the plates. I checked and tweaked the angle after the first cut and it was all good.

Tight hole clearance
Tight hole clearance

Then drill six clearance holes for the socket head cap screws holding the heater plate to the ABP; a #1 drill gave a few mils clearance, which is all it needs. The holes are 4 mm in from the edges of the 120 mm square, with the two middle ones at, yes, 60 mm.

However, there’s not much meat between the edge of the plate and the holes: call it 1.1 mm. If you do this, using 122 mm plates would produce less scary-close results. That’s why I like manual CNC for this stuff: no need to lay it out, tap in the numbers and it just Works.

My APB heater has a static drain connected to the heat spreader, so I milled a 2 mm recess around the right-hand screws to clear the lugs, wires, and Wire Glue blob. The silicone wiper gets its own cutout, which I made a snug fit so that the rubber would push the plate against the screw heads and hold it in place.

Milled recesses
Milled recesses

I machined recesses on only one plate, so I could incorporate any changes in the other two. The initial setup was atop a scrap plastic sheet which, as it turned out, wasn’t particularly flat. The edges of that not-quite-complete hole on the left were nasty-sharp.

Thin-shaved plate edge
Thin-shaved plate edge

Then clean off the ink with xylene, scrub the plate with a 220-grit sanding sponge, and it looks really nice. Impossible to photograph a uniform gray surface, though: the autofocus goes nuts.

While all that was going on, I’d dumped some MEK into a polyethylene jar along with a handful of calibration cubes and similar debris. I used MEK, rather than acetone, because it’s somewhat less aggressively flammable while still being a good solvent for ABS. Right now, the gunk has the consistency of thin honey, which may be too thick to spread easily; I’m still figuring this out. I apply the gunk with a folded coffee filter: scrape the puddle around to cover the whole plate, then let it dry. This is best done outdoors, except that right now it’s well below freezing out there.

Here’s what the film looks like under the start of a quartet of dodecahedrons I ran off to see if they stuck properly:

ABS coating on aluminum build plate
ABS coating on aluminum build plate

The bottom surface looks like it was machined: dead flat,nice edges, good thread definition. The parts stick like they were glued to the surface, with no tendency to pull up at the corners.

The Outline thread shows some adhesion trouble for the first 10 mm or so. After that, it’s nailed right to the ABS film. That’s why I use Outline, at least until I figure out a better way to start the thread.

After I finish the next two plates, I’ll have a somewhat quick-change build platform: pull the hot plate off (holding it with pliers!) and slap a new one on. Not as convenient as the ABP, but much better for building precision parts like gears and extruder motor mounts.