Tag: Improvements

The Automatic Build Platform rollers have a small gap in the middle where pegs on the platform support the shaft. The belt must be sufficiently taut that it’s flat across the entire length & width of the platform, which means it’s so tight that it collapses into the gap and forms wrinkles in the most critical area.

Prior to installing an aluminum plate build surface, I wondered if adding a support in the gap would reduce the wrinkling, so I cooked up a small OpenSCAD script to print these things out:

They’re at about the finest resolution the printer can produce; getting the fill between the walls seems iffy at best. The top set has obvious gaps that come from having walls too close together.

I finally printed them at 0.4 mm thickness and a width of 0.5 mm (w/t = 1.2) and that produced the lower set with adequate fill:

Unsurprisingly, the holes are too small, but that’s easily fixed with a drill just slightly over 1/8 inch. The length of the stem also required a bit of fine-tuning; you can always make it shorter with a file:

Some preliminary testing says that the motionless supports might produce too much friction on the belt, but that was with a paper backing. Running slick tape around the middle of the belt’s inside surface might help, plus it would add a bit of stiffening. Adding some ridges to reduce the surface area in contact with the belt would probably just score the belt.

I’ve been experimenting with Kapton-on-paper belts, which remain much flatter than the endless belt, but are much more sensitive to the gaps. More fiddling is in order, after I get some one-off parts built on the aluminum plates.

The OpenSCAD script:

// ABP Central Shaft Spacers
// Ed Nisley - KE4ZNU - Feb 2011

//--- Extrusion dimensions
//		Must be tall and skinny to get fill around the hole

ExtThickness = 0.50;		// extrusion thickness
WTRatio = 1.20;				// width-over-thickness

//--- Spacer dimensions

ShaftDia = 3.175;			// metal rod = hard 1/8 inch = 0.125 inch
ShaftRad = ShaftDia/2;

RollerDia = 8.0;			// approximate OD of in-situ rubber rollers
RollerRad = RollerDia/2;

OverAllLen = 7.6;			// length along shaft = hard 0.300 inch

TaperLen = 2 * ExtThickness;	// make it a few layers thick

Faces = 10;					// polygonal shape for outside cylinders

//--- APB Interface

PlatformZ = 5.0;			// thickness of ABP support under heater
PlatformGap = 2.0;			// distance from roller to APB

//--- nophead's polygonal hole correction
//		http://hydraraptor.blogspot.com/2011/02/polyholes.html
//		Adapted to center the resulting cylinder

module polyhole(h, d, c) {
    n = max(round(2 * d),3);
    cylinder(h = h, r = (d / 2) / cos (180 / n), $fn = n, center=c);
}

//--- Odds & ends

FinagleLen = 1.0;			// enough to make holes obvious

//-----------------------
// The Spacer!

difference() {
	union() {
		translate([0, 0, OverAllLen/2-TaperLen/2])
			cylinder(r1=RollerRad, r2=RollerRad-TaperLen, h=TaperLen,
					  center=true, $fn=Faces);
		cylinder(r=RollerRad, h=OverAllLen-2*TaperLen,
				  center=true, $fn=Faces);
		translate([0, 0, -(OverAllLen/2-TaperLen/2)])
			cylinder(r1=RollerRad-TaperLen, r2=RollerRad, h=TaperLen,
					  center=true, $fn=Faces);
		translate([(RollerRad+PlatformGap)/2, 0, 0])
			cube(size=[RollerRad+PlatformGap, PlatformZ, OverAllLen],
				  center=true);
	}
	polyhole(OverAllLen+2*FinagleLen, ShaftDia, true);
}

While I had the Thermal Core out and everything disconnected, I drilled a mounting hole in the tombstone of epoxy around the thermocouple bead, hand-twisting a small drill gripped in a pin vise.

That makes mounting the thermocouple much easier when the MK5 head gets tucked in place inside the Thing-O-Matic case. The washer is smaller than I’d used before, too. There’s no thermal compound under the brick, but I’ll probably add some the next time it comes out.

I pushed the insulating blanket back around the thermocouple and wire, then added a fuzzy button (punched out for the nozzle) atop the mess and taped it all in place. The thermocouple certainly runs a bit cooler than the Thermal Core, but I have no way of measuring the difference.

In any event, I think consistency is more important than absolute accuracy, because you’re tuning the whole affair for best printing at a given temperature, rather than picking an absolute temperature and adjusting everything else to suit.

It’s worth noting that the J-B Industro Weld epoxy in that block was in fine shape, despite roasting at nearly its maximum rated temperature for a few tens of hours. That’s not a lifetime test, but it’s encouraging.

The MBI assembly instructions blithely direct you to:

Using superglue, or ideally acrylic cement, you’ll want to attach the spacer feet to the bottom of the supports.

As it turns out, though, the tabs on the Support sides stand just a bit proud of the Bottom plates, so that any attempt to glue the Feet in place will simply attach them to the side tabs and nothing else. Not what you want…

So I rubbed the Bottom plates on a sheet of coarse sandpaper until everything was nice and flat:

Then the spacer feet glued neatly in place:

I tried to keep the acrylic cement off the tabs, so it’s theoretically possible to dismantle the whole thing, but I suspect that’ll never happen.

Hot ABS plastic gives off a characteristic stink odor smell aroma that’s hard on the nose and probably not particularly good for the lungs. Even in the basement, it seems like a Bad Idea to stink up the place, so I added an exhaust fan and charcoal filter to blot up the odor.

The key step is to add the fan provided with the TOM (which they recommend you don’t use!): outside the box, oriented backwards, and running on +5 V instead of +12 V. The general concept: free up some precious space inside the box, shove the exhaust through a filter, and do it with a gentle breeze rather than a mighty blast.

Although it’s not a part of this sub-project, the heatsink holds a 2 Ω 25 W resistor that serves as a 12.5 W dummy / minimum load on the +5 V supply to keep it within tolerance. Right now, the heatsink is just jammed between the screws, because I’m probably going to add a similar dummy load to the +12 V supply when I move to a stepper extruder.

In case you’re hypersensitive to overheated resistors: the heatsink runs at 65 °C, the resistor at 75 °C, and the specs give a permissible dissipation of 20 W. You could work it out…

The first step is to route the 4-pin ATX power connector (which popped off the big connector block plugged into the Motherboard) out the left-rear hole in the acrylic floor under the XY stage. I don’t have a mating connector, so I conjured up something from the same square pins as I used in the Extruder power supply modification and some wire harvested from a dead ATX supply. The black heatshrink tubing holds the four wires and their pins in the proper configuration. Obviously, you want matching colored wires, because the “connector” isn’t polarized!

On the other end, a four-pin screw terminal block provides a convenient way to attach a variety of gadgetry. At last count, it serves the exhaust fan, +5 V dummy load, LED platform light, and a cooling fan. More details on those later…

The fan frame required a small gouge to route the wire inward through the vent hole in the side of the TOM case:

Four nuts secure the fan to the frame. Fortunately, the fan’s motor housing sits on the exhaust end, so the filter material rests against the hub support and spider. Here’s what the whole arrangement looks like, with the filters pried away from the fan.

A trip to the local Big Box home warehouse produced a $10 20×25-inch activated charcoal air filter intended for a whole-house air conditioner. I now have a large plastic grid, a sheet of open-cell foam air filter, plus a generous supply of charcoal filter material. I cut a 4-3/4-inch strip from one side, chopped it into 4-3/4-inch squares (that’s 120 mm everywhere else in the world), trimmed off the corners, tucked two layers behind the TOM filter holder frame, and added four more nuts-and-washers.

The spare filter material goes in a sealed plastic bag, because activated carbon has a limited lifetime when exposed to free air. That’s what it does for a living: adsorb smelly molecules from passing air!

The final step is to close off all the TOM’s openings, thus restricting air flow through the case. This has the happy side effect of warming the build area and reducing drafts, both quite important in a wintery 50-ish °F basement. Taking pictures of clear acrylic sheet is essentially impossible, but you can see the front piece there and the paper seals around the filament spool there. I make no apology for the masking tape; after everything’s working, I’ll formalize the arrangements.

Incidentally, don’t get too secure with the front window, because the ABP pokes through the opening to disgorge finished parts. In fact, the front of the ABP whacks the window when the nozzle reaches the back of the ABP, so you don’t want a mechanical latch holding the window closed.

I’m thinking a magnetic latch is in order.

There’s enough leakage around the windows to keep the fan happy, although it sucks the last one closed. Those four square cable holes in the acrylic sheet between the upper and lower chambers provide the only air channels, so the exhaust fan probably doesn’t compete with the ATX supply’s cooling fan.

While the filter doesn’t kill off all the stink, the TOM is a much better companion now…