Tag: Improvements

Making the world a better place, one piece at a time

Thing-O-Matic Stepper Extruder: First Steps
Over the past few weeks I’ve printed the gears and plate from TheRuttmeister’s Coloso-Gear MK5 extruder Thing and flatted the shaft on a moderately husky (but not hyperthyroid) NEMA 17 stepper motor. While tearing the Thing-O-Matic down to add thermal switches to the Extruder Head, I converted the MK5 Filament Drive into a stepper extruder. Much to my astonishment, when I plugged the cable in and fired up ReplicatorG … It Just Worked!

Even more amazing: the first pinout arrangement turned the motor in the correct direction!

Coloso-Gear Stepper Extruder

Some nasty pincushion distortion makes the larger gear look misaligned, but it’s parallel to the mounting plate and correctly engaged with the drive gear.

The motors arrived with short stubs of thin yellow wire on the IDC motor connectors, which I soldered directly to a much longer cable. The Parts Heap disgorged a chubby 8-conductor signal cable; I used pairs of wires for each motor connection, although one conductor would have entirely enough copper. The two cable ties around the motor prevent flexing those delicate wires as the Z stage moves.

Two tweaks to the MK6 Stepstruder profile in thingomatic.xml produced the right answers:
- Set motor_steps = 1456
- Set stepspermm = 48.2
Running the motor at 2.0 rpm for 30 sec should produce exactly 1 revolution of the big gear. I marked and counted the teeth on the larger gear as it rotated, and came up with 56 teeth. It’s a 51 tooth gear, so reducing the default 1600 steps/rev by 51/56 produces 1457. A defunct MBI stepper driver board that now only does full steps provides power; I resoldered all the chip pins and the fault isn’t due to external causes like no-lead solder.

Then run it for 60 seconds at 2.0 rpm and it’s under by maybe 1/10 of the tooth-to-tooth spacing. Adjust 1457 x 101.9/102 = 1456. Run it for another minute and it’s spot on.

I measured 60.45 mm for two revolutions of the big gear, so it’s 30.23 for one rev, which requires the aforementioned 1456 steps. Averaging more revolutions would yield more digits, but given the rubbery nature of molten filament, three significant figures seems entirely sufficient. I suspect this depends greatly on how deeply the extruder drive embosses the filament, so it’ll require some fine tuning.

Back of the envelope for the DC extruder at 255 PWM: feed = 45 mm/s, 0.35 mm thickness, w/t = 1.7 = 0.56 mm width gives 6.9 mm³/s. The filament is about 2.9 mm dia = 6.6 mm³, so it passed through the extruder at a bit over 1 mm/sec. There’s some windage involved in all those numbers and the extruding rate obviously depends on the temperature.

The stepper (from the usual eBay seller) is a Minebea 17PM-K150, which doesn’t appear in their catalog listing, so it’s likely one of their many custom motors. The stack length resembles the 17PM-K3xx series, which means roughly 1 A rated current. Setting the driver current to 500 mA (V_REF = 1 V) produces enough torque that I cannot pull the filament back hard enough to stop it.

The step rate at 2 rpm is:
```
48.6 step/s = (2 rev/min) x (51/7) x (1 min/60 s) x (200 step/rev)
```
At that lethargic pace, the K3xx motors have something like 0.250-0.300 N·m of torque at rated current. At half current, call it 0.100 N·m and multiply by 51/7 to get 0.700 N·m = 100 oz·in.

The effective drive diameter is 30.23/π = 9.6 mm, so the available force on the filament is 0.7 N·m / 0.01 m = 70 N ≈ 7 kgf = 15 lb. Yeah, but that little 7-tooth gear will snap right off …

The reversal plugin cranks the big gear backwards at 35 rpm, which works out to 850.5 step/s. That ought to work, particularly seeing as how it’s not actually pushing anything.
2011-03-21
Thing-O-Matic: Flatting Motor Shafts

The NEMA 17 steppers I picked up from eBay as part of the stepper extruder upgrade project have round shafts; that’s not surprising, as they came with pressed-on timing gear pulleys. In their new application they’ll sport plastic herringbone gears and those have setscrews.

Herringbone gears with nut inserts

Both nuts have epoxy potting to prevent moving / rotating under duress. Remember to load the screw threads with beeswax and run it all the way through before you pot the nuts, lest the screw become one with the nut. Yes, the left gear fits a NEMA 23 stepper.

(Those are 14-tooth gears. I’ll actually use a 7-tooth gear, but I printed a bunch of gears to get the hang of it.)

Any time you tighten a setscrew on a motor shaft, it’ll raise a burr on the shaft. You can pull a plastic / printed gear off a ruined shaft because the burr will simply carve a gash through the plastic. A metal-hub gear or pulley will jam solid on the burr; you definitely don’t want that to happen.

The solution, which comes standard on many motor shafts, is a flatted section where the screw can raise a burr without causing a problem. In addition, the flat prevents the screw from sliding around the shaft and producing a circular scar that makes the gear impossible to remove.

Adding a flat requires a few minutes of Quality Shop Time, but will save you considerable hassle later on. Just Do It!

Mummify the motor in masking tape to keep grinding grit and metallic dust out of the shaft bearings, then grab the shaft in a smooth- or soft-jaw vise. I grabbed a machinist’s vise in the bench vise, but use what you have.

Masked motor in vise

Apply a Dremel grinding stone / cutoff wheel along the shaft to produce a flat about the same width as the tip of the screw. The object of the game is to make the flat wide enough to keep the burr on the flat, but not grind half the shaft away.

Don’t grind the shaft without clamping it, because the vibration will destroy the bearings. Clamp the shaft to stabilize it and isolate the motor, then do the grinding.

Flatted shaft with screw

Here’s the shaft after installing & removing the gear. Notice the burr:

Flatted shaft with screw scar

And a detail of the burr:

Flatted shaft scar – detail

It’s not like I’m over-tightening the screw, either: that’s what a hardened screw does to a soft motor shaft.

2011-03-20
Building an LED Floodlight Into a Task Lamp

LED Gooseneck Floodlight

Eks forced me to take a pile of ~~crap~~ useful make-froms, including a gooseneck task lamp that was probably bolted onto a machine tool in its former life. It sported a 20 W halogen bulb, but looked to be just about exactly the right size for those LED floodlights, which is why I didn’t put up much of a fuss about taking it off his hands.

The LED lamps are much bigger than the halogen bulb, but they fit neatly into the housing diameter. All they needed was a bit more front-to-back room, which looked a lot like a chunk of PVC pipe. The housing screws together with a 1.5 mm thread that I can’t produce on my inch lathe; I’m still not set up for thread milling. This being a low-stress application with a lamp that ought to outlast me, I figured I’d just make the belly band slip-fit the two threads, glue it in place, and move on.

I sawed off a length of PVC pipe, faced off the ends in the lathe, then CNC milled a recess to clear the male threads on the gooseneck part (I hate precision boring in the lathe). Given the rather tenuous grasp of that 3-jaw chuck, I made two passes around the perimeter: pipe ID 52.1, thread OD 54.5, remove 1.2 mm all around, about 9 mm down.

Milling top recess

On the other end, the female thread ID = 52.2 and the pipe ID = 52.1, so I glued another ring of PVC pipe inside to provide enough meat to turn it down. Once again, saw off a ring, face the ends, then cut out a segment so that the OD circumference of the inner ring is just slightly smaller than the ID circumference of the outer pipe. The result looked like this:

PVC insert sizing

Apply a heat gun to the inner ring until it’s soft enough to stuff into the pipe, clamp it until it hardens, apply PVC cement, and clamp overnight. Contrary to appearances, the ends of the two pipes are flush at the surface. Once again, you cannot have too many clamps:

Clamped PVC insert

Turning down the outside to fit the threads shows just how little meat was left on that pipe:

Skinning down to the insert

While it was chucked up (and despite my dislike of boring) I bored a bevel to accept the LED lamp and adjusted the OD so the lamp fit snugly between the end of the belly band and the lens holder on the front of the housing:

Floodlight in holder

The switch comes from the Parts Heap. A D drill puts a slightly undersized hole that’s just right for the threaded switch; I simply turned it in by hand. A length of zip cord carries the power up the gooseneck, where various ends get soldered to the switch and lamp.

I applied some hot-melt glue to the threads and pushed everything together:

Finished LED Floodlight

The glass lens on the front fits in a molded holder with an annular air gap. The LED lamp housing has all those fancy cooling fins against the inner pipe, so there’s a bit of cooling air flow around the lamp and out through the rear black section. A thermocouple reports the lamp temperature gets up around 75 °C in a 14 °C shop; a 50 °C rise might be a tad warm in the summer, but we’ll see what happens.

The power supply came from the Parts Heap: a 12 V 1 A wall switching power supply in the shape of a wall wart. For now, the zip cord from the lamp terminates in a coaxial power jack that (amazingly enough) fits the wart’s connector, but I’ll eventually put a box in there somewhere.

Clamped the butt end of the gooseneck to the backsplash on the countertop under the mill and It Just Works!

2011-03-19

Tour Easy: Zzipper Fairing Upper Mount Plates

The stock Zzipper fairing handlebar mount consists of an aluminum bar with a plate welded to each end at more-or-less the correct angle to match the fairing curve. The plate has a 1/4 inch hole in one end, wherein a 1/4-20 nylon machine screw clamps the fairing to the plate, with a nylon washer distributing the stress. That doesn’t cope well with the vibrations caused by riding around here, let alone our summer vacation trips on crushed-stone rail trails, and the fairings tend to stress-crack at the holes.

These 3D printed plates are just the latest in a long series of attempts to distribute the stress over a larger area. The outside view:

Fairing mount - outside — Fairing mount – outside

The open hole gets another screw to hold the plates in position. The bump on the far side is an Oozebane turd, about which more later.

The view from inside the fairing:

Fairing mount - inside — Fairing mount – inside

You can’t see the layer of black foam rubber salvaged from a mouse pad between each plate and the fairing. That should prevent any local stress concentration at the screw and ease the transition to the tapered plate edges.

The solid model looks about like you’d expect:

Fairing Mount Plates - Upper — Fairing Mount Plates – Upper

The hole position depends on the fairing position, as the fairings have three holes. The pictures show the fairing on my bike; it’s in the lowest position, with the screw in the topmost hole. The OpenSCAD file has an option to put the holes where you need them.

The plates are only 8 layers thick, printed with 4 solid layers top and bottom to eliminate any fill. You could do the same by setting the fill to 100%, I suppose. Using 4 outer shells (3 additional) makes the flanged edge nice and flat and uniform.

The layer height is 0.33 mm, with w/t=1.7 for a width of 0.56 mm. Feed rate = 43 mm/s and flow rate = 255. DC Extruder, alas.

Running the first layer at feed = 0.5 and flow = 0.75 produces some fluffing in the fill, but there’s no way to get a lower flow from the DC extruder motor. Flow = 0.75 corresponds to PWM=191; anything lower sometimes fails to start the motor. If it starts, it’ll run, but … that’s not dependable.

I printed them on an aluminum plate for a nice flat bottom surface.

The OpenSCAD source code:

// Clamp plates for Zzipper fairing on Tour Easy recumbents
// Ed Nisley - KE4ZNU - Mar 2011

// Build with...
//	extrusion parameters matching the values below
//	4 outer shells
//	4 solid surfaces at top + bottom
//  slow feeds to ensure hole perimeters stick to fill

include </home/ed/Thing-O-Matic/lib/MCAD/boxes.scad>
include </home/ed/Thing-O-Matic/lib/MCAD/units.scad>

// Select hole layout
// The if statement seems to work only for CSG object trees
// Fortunately, I need only two different layouts...

HoleSelect = 1;						// 0 = his, 1 = hers

HolesTop 	= (0 == HoleSelect) ? [0,1,1] : [1,0,1];
HolesBottom = (0 == HoleSelect) ? [0,1,1] : [1,0,1];

// Set these to match the extrusion parameters for successful building

ThreadZ = 0.33;						// extrusion thickness
ThreadWidth = 0.57;					// extrusion width = ThreadZ x w/t

HoleWindage = ThreadWidth;			// enlarge hole dia by extrusion width

// Plate dimensions

HoleDia = 0.25 * inch;				// these are 1/4-20 bolt holes
HoleSpace = (1) * inch;				// center-to-center spacing
									//  usually 1 inch, but 15/16 on one bike

CornerR = 5.0;						// corner rounding

Layer1X = 90;						// against fairing surface
Layer1Y = 32;
Layer1Z = 2*ThreadZ;

Layer2Margin = 1.5;					// uncovered edge
Layer2X = Layer1X - 2*Layer2Margin;
Layer2Y = Layer1Y - 2*Layer2Margin;
Layer2Z = 3*ThreadZ;

MountX = 46.3 + HoleWindage;		// handlebar mounting bracket end plate
MountHoleSpace = 13.0;				//  end to hole center
MountY = 16.3 + HoleWindage;
MountZ = 4*ThreadZ;					// recess depth
MountCap = 3.0;						// endcap arc height
MountR = (pow(MountCap,2) + 0.25*pow(MountY,2)) / (2*MountCap);	// ... radius

Layer3Margin = 1.5;
Layer3X = Layer2X - 2*Layer3Margin;
Layer3Y = max((Layer2Y - 2*Layer3Margin),(MountY + 8*ThreadWidth));
Layer3Z = 3*ThreadZ;

PlateZ = Layer1Z + Layer2Z + Layer3Z;

// Convenience settings

BuildOffset = 3.0 + Layer1Y/2;		// build Y spacing between top & bottom plates

Protrusion = 0.1;					// extend holes beyond surfaces for visibility

//---------------
// Create plate with selectable holes

module Plate(hs) {

  difference() {

	union() {
		translate([0,0,Layer1Z/2])
		  roundedBox([Layer1X,Layer1Y,Layer1Z],CornerR,true);
		translate([0,0,Layer1Z + Layer2Z/2])
			roundedBox([Layer2X,Layer2Y,Layer2Z],CornerR,true);
		translate([0,0,Layer1Z + Layer2Z + Layer3Z/2])
			roundedBox([Layer3X,Layer3Y,Layer3Z],CornerR,true);
	}

	if (0 != hs[0]) {
	  translate([-HoleSpace,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

	if (0 != hs[1]) {
	  translate([0,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

	if (0 != hs[2]) {
	  translate([HoleSpace,0,PlateZ/2])
		  cylinder(r=(HoleDia + HoleWindage)/2,
					h=(PlateZ + 2*Protrusion),
					center=true,$fn=10);
	}

  }

}

//---------------
//-- Build the things...

translate([0,BuildOffset,0]) Plate(HolesTop);

translate([0,-BuildOffset,0])
  difference() {
	Plate(HolesBottom);

	translate([-(HoleSpace + MountHoleSpace - MountX/2),0,PlateZ - MountZ/2 + Protrusion/2])
	  intersection() {
		cube([MountX,MountY,(MountZ + Protrusion)],center=true);
		union() {
		  cube([(MountX - 2*MountCap),MountY,(MountZ + Protrusion)],center=true);
		  translate([ (MountX/2 - MountR),0,0])
			cylinder(r=MountR,h=(MountZ + Protrusion),center=true);
		  translate([-(MountX/2 - MountR),0,0])
			cylinder(r=MountR,h=(MountZ + Protrusion),center=true);
		}
	  }
  }

I loves me my Thing-O-Matic, despite its annoyances…

[Update: Stepper extruder parameters and a tweak to make the mount plate track the hole position correctly.]

2011-03-15

Thing-O-Matic: Thermal Lockout Circuit
Thermal lockout control box

This is a proof-of-concept lashup of a circuit to shut off the Thing-O-Matic’s power should the Thermal Core overheat. It vaguely resembles those doodles, but with the thermal switch cases grounded and an indicator for the main thermal switch.

[Update: You should read the rant at the bottom of that post to understand why this isn’t a firmware mod and doesn’t contain a microcontroller.]

Operation is straightforward:
- The black NO (Normally Open) momentary switch energizes the DPDT relay, one NO pole of which then holds the relay power on.
- The red NC (Normally Closed) momentary switch interrupts that circuit and releases the relay.
- An NC thermal switch detects an overheated Thermal Core, opens that circuit, and releases the relay.
The other NO relay pole connects / disconnects the ATX power supply’s -Power On line from the Thing-O-Matic Motherboard. That connection requires a circuit-board cut to splice the relay into the Motherboard.

The LEDs:
- Lower Green = ATX AC power on (from +5VSB power)
- Upper Green = +Power On signal active
- Red = Test / Fault (on = relay inactive)
- Yellow = low over-temperature alarm
- Orange atop box = high over-temp switch active
I included a second NO thermal switch that activates at a lower temperature, mostly because I had one, but that’s certainly not required. The multitude of LEDs makes for a happy-looking box; labels would be a nice touch, I agree.

When you turn on the ATX power supply, the Lower Green and Red LEDs turn on: the “Test” part of the “Test / Fault” indicator. Push the black button, the Red LED goes off, the Upper Green LED goes on, and the Thing-O-Matic is up & running. Push the red button, the TOM shuts down, and you’re back to the starting condition.

The Yellow LED goes on when the lower temperature switch goes on.

Shortly thereafter, presumably, the higher temperature switch opens, the Orange LED goes on, the TOM shuts down, and you’re left with the Lower Green, Yellow, and Orange LEDs: zowie! When the high temp switch cools off a bit, the Orange LED goes off and the Red LED goes on. After a while, the Yellow LED will go off, and you’re back to Square One again.

What’s not yet done: mounting the thermal switches to the Thermal Core in a way that’s mechanically solid, electrically isolated, and thermally dependable. I just got a bag of 100 °C NC switches, which make more sense than the 65 °C NC switches I’d been fooling with.

The wiring uses 4P4C and 6P6C modular phone connectors and cables, which are cheap & readily available, if not exactly proof against high temperatures. In normal use, failures tend to be open-circuit that will shut off the heater power. Take care not to position the cables so they melt first; they’re not intended as thermal switches.

Achtung: modular cable color codes are not standardized, particularly on the jack side, so pay more attention to the pin numbers than the colors. If I ever meet the guys who rearranged the jack colors, There. Will. Be. Gibbage.

A back view of the box shows a nice rectangular hole that’s obviously a manual CNC job on the Sherline, with no corner filing whatsoever. Hot melt glue holds the connectors in place, so I’m not showing off the inside:

Thermal lockout box – rear

The -Power On connection to the Motherboard requires the single cut shown in yellow:

Motherboard PCB Modification

It looks like this in real life, with the wire soldered to the Arduino header pin. Another dab of my Shop Assistant’s orange nail polish seals the PCB wound:

Motherboard -Power On modification

The remaining wires attach to the ATX power connector pins on the bottom of the board. The yellow wire passes through an unused mounting hole on its way to the top side, as above. Use a cable tie to tie the cable to the board, through a pair of otherwise unused RS-485 connector mounting holes.

Motherboard Connections – Bottom

While you’re chopping away at the Motherboard, add that isolating diode to keep +5 V USB power from turning the ATX fan with the power off.

The overall schematic (clicky for more dots):

Thermal Lockout Schematic

There is no corresponding PCB layout, because the circuitry forms a point-to-point hairball inside the box. If you were doing this for real, you’d want a PCB with a bazillion connections, but …

For example, here’s the FET driver for the Orange (it just looks Red) high temperature LED before a liberal application of heat ~~stink~~ shrink tubing:

Overtemperature LED driver hairball

You can test the thermal switches using a butane lighter.
2011-03-14

Sherline Collet Pusher Pin Holder

Although that collet pusher works fine, the locking pin holder often teleported itself inside the vacuum cleaner. It recently reappeared on the far end of the main workbench, a good 15 feet away from the Sherline as the swarf flies. This, to misquote Churchill, is an impertinence up with which I shall not put.

Herewith, a replacement offering several advantages:

Won’t fit up the vacuum’s snout
Easy to grip
Perfect pin alignment
3D printing FTW!

It’s a flat block resting on the flat top of the pulley, with a nice arc matching the pusher’s OD. A small hole for the pin at exactly the right altitude makes the whole thing rock-solid stable: it slides firmly into position.

The 3D model looks like you’d expect:

Pin holder - OpenSCAD model — Pin holder – OpenSCAD model

The finger grips were just for pretty, as you don’t need that much traction to extract the thing.

A similar view of the real object with the bottom surface up and some flash around the edges:

Locking pin holder - spindle end view — Locking pin holder – spindle end view

The as-printed block put the pin about 0.2 mm above the spindle hole, so I rubbed it on Mr Belt Sander (with the power off) until it fit. I printed the block on the aluminum plate platform; the Z height home setting evidently needs a tweak. However, the hole was exactly the correct distance from the top surface: flipping the block over fit perfectly.

The OpenSCAD source code:

// Collet extractor locking pin holder for Sherline spindle
// Ed Nisley - KE4ZNU - Feb 2011

include </home/ed/Thing-O-Matic/lib/MCAD/boxes.scad>
include </home/ed/Thing-O-Matic/lib/MCAD/units.scad>

PusherOD = 17.35;					// Shell of collet pusher

PulleyOD = 65.5;					// For 3k rpm head

PinHoleCtr = (3/16) * inch;			// pin hole center above pulley surface
PinDia = 2.50;						// pin is about #40 drill
PinHoleDepth = 10.0;				// hole depth from PusherOD

HoleWindage = 0.55;					// Approximate extrusion width
Padding = 0.1;						// A bit of spacing to make things obvious

HolderWidth = 2 * PusherOD;						// Overall holder width
HolderProtrusion = 15;							// sticks out beyond pulley
HolderLength = PulleyOD/2 + HolderProtrusion;	//	... length
HolderThickness = 2*PinHoleCtr;					//	... thickness
HolderRounding = HolderWidth/5;					// corner rounding

GripLength = 0.70 * HolderWidth;	// grip notch
GripWidth = 0.25 * GripLength;
GripIndent = HolderProtrusion/2;

difference() {

// main slab

  translate([-HolderLength/2,0,0])
	roundedBox([HolderLength,HolderWidth,HolderThickness],HolderRounding,true,$fn=4*8);

// pin hole

  translate([-(PusherOD/2 + PinHoleDepth/2 - Padding),0,0])
  rotate([0,90,0])
	cylinder(r=PinDia/2,h=(PinHoleDepth + Padding),center=true,$fn=8);

// upper grip

  translate([-(HolderLength - GripIndent),0,(HolderThickness/2)])
  rotate([90,0,0])
	cylinder(r=GripWidth/2,h=(GripLength - GripWidth),center=true);

  translate([-(HolderLength - GripIndent),((GripLength - GripWidth)/2),(HolderThickness/2)])
	sphere(r=GripWidth/2,$fn=10);

  translate([-(HolderLength - GripIndent),-((GripLength - GripWidth)/2),(HolderThickness/2)])
	sphere(r=GripWidth/2,$fn=10);

// lower grip

  translate([-(HolderLength - GripIndent),0,-(HolderThickness/2)])
  rotate([90,0,0])
	cylinder(r=GripWidth/2,h=(GripLength - GripWidth),center=true);

  translate([-(HolderLength - GripIndent),((GripLength - GripWidth)/2),-(HolderThickness/2)])
	sphere(r=GripWidth/2,$fn=8);

  translate([-(HolderLength - GripIndent),-((GripLength - GripWidth)/2),-(HolderThickness/2)])
	sphere(r=GripWidth/2,$fn=8);

// spindle shaft

  cylinder(r=(PusherOD/2)+HoleWindage,h=(HolderThickness + 2*Padding),center=true);

}

2011-03-12

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

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

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

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

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

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

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.
2011-03-04