Category: Machine Shop

Having added a dummy load to pull a minimum current from both the +5 V and +12 V supplies, it seemed reasonable to connect the ATX +Power Good signal to the Arduino’s -Reset input. That ensures a power glitch will force a hard reset, rather than produce random crashes / instability / weirdness, and make the problem obvious.

Of course, that presumes the power supply notices the glitch and drops the +Power Good line. That’s probably an incorrect assumption, but the only way to test it requires hitting the power supply with a crowbar and I’m just not going to go there.

Anyhow.

This also ensures the Arduino gets a hard reset when I turn the power off by triggering the manual shutdown button on the thermal lockout control box. The Arduino draws power through the USB cable from the PC (at least on the Foxconn box I’m using) and sometimes starts up crazy; that will no longer happen.

The +Power Good signal arrives through the Gray wire on ATX pin 8 (it also drives an LED on the lockout box) and the -Reset signal is the outside pin on the Motherboard’s manual reset pushbutton. The new connection looks like this:

Being no fool somewhat cautious, however, I added a switch that can disconnect the two lines; if it turns out the +Power Good signal has any glitches, I can use the original mode while scratching my head.

A giant blob of hot melt glue holds the switch in position:

A wire burrows through one of the unused RS-485 connector mounting holes under the switch on its way to the ATX connector. It’s the Blue wire below the board in the previous picture, enters from the top right here, and terminates on the third pad over with the Black wire that joins the cable on the way to the Lockout box

Now I’ll see whether the Thing-O-Matic begins resetting at random moments. After doing all the various mods & improvements you’ve seen over the past few months the printer has been quite reliable, so I have some hope that this change will produce … no change.

As a quick test, I let the printer sit all day with the Thermal Core and HBP temperatures cycling around 50 °C and all four steppers enabled. Any reset would disable the steppers and make itself obvious, but after two days it’s all good.

One good sign: the LED ring around the extruder head just barely changes brightness as the heaters cycle.

Oh, and the steppers don’t overheat, either. This thing is starting to behave like a real 3D printer should!

If it does start resetting, however, I’ll add a latch inside the Thermal Lockout box that captures short +Power Good glitches and lights Yet Another LED.

The ATX power supplies commonly used in PCs generally require a minimum load of about an amp on the +5 V and +12 V lines to ensure good voltage regulation, but the Thing-O-Matic Motherboard has only a 30 Ω power resistor that draws 170 mA. They’re also designed for relatively constant-current loads, which the Thing-O-Matic is not, by any stretch of the imagination, so it’s no wonder the voltages jump all over the place.

I think, but cannot prove, that many of the random problems plaguing long-duration prints arise from power glitches. That load resistor on the +5 V line was, at best, a stop-gap measure, and this is what I had in mind from the start. There are additional constant loads here and there throughout the Thing-O-Matic, but it really does not apply a known minimum load to the power supply.

I got a stock of 6 Ω resistors for that heater project and used a trio here: one draws 800 mA from +5 V (about an amp, including the MB load) and the other two in series draw 1 A from +12 V. The little fan runs from +12 V, although I may connect it to +5 V to make it quieter; the 16 W total power dissipation is too high for convection cooling.

Note that these are 50 W resistors dissipating 6 W apiece while running at room temperature. Yes, they look like extruder heater resistors, but this is an application they’re designed for.

A bit of machining mated a junked CPU cooler to a half-inch slab of aluminum that serves as a heat spreader. The cooler had many tabs and protuberances on its bottom surface that I simply sliced off with an abrasive wheel. Two pieces of brass shim stock filled in a mysterious recess along the right edge in this picture. The cooler’s spring clamp engages a pair of wire tabs screwed to the spreader and the force smushes a layer of thermal compound into the air gaps.

The holes atop the cooler didn’t match up with any fans in my collection, but that’s why I have a Thing-O-Matic. I had to rotate the fan case to get the holes to fit, which was trivially easy in OpenSCAD.

And then it built about like you’d expect:

Yes, it really needs a finger guard, but that’s in the nature of fine tuning…

The OpenSCAD source code, with a rather fat ThreadWidth setting. IIRC, this was fallout from a random walk through the Skeinforge parameter space.

// Fan adapter plate for CPU cooler
//  Used for Thing-O-Matic minimum current loads
// Ed Nisley - KE4ZNU - Mar 2011

// Build with...
//	extrusion parameters matching the values below
//	+2 extra shells
//	3 solid surfaces at top + bottom

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

// Extrusion parameters for successful building

ThreadWidth = 1.0;					// should match extrusion width
ThreadZ = 0.33;						// should match extrusion thickness

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

// Plate dimensions

PlateX = 70.0;
PlateY = 66.0;
PlateZ = 5.0;

FrameHoleSpace = 50.0;				// mounting holes in frame
FrameHoleXOffset = 10.0;			//  ... offset from front left
FrameHoleYOffset = 10.0;			//  ... which are *not* symmetrical!
FrameHoleDia = 3.0 + HoleWindage;	// from frame holes
FrameHoleRadius = FrameHoleDia/2;

FanHoleSpace = 40.0;				// fan hole separation in X & Y
FanHoleDia = FanHoleSpace * sqrt(2);	// diameter of hole circle
FanHoleRadius = FanHoleDia / 2;

FanAngle = acos(FrameHoleSpace / FanHoleDia) - 45;

FanDuctDia = 48.0;
FanDuctRadius = FanDuctDia/2;

FanCenterX = PlateX/2;
FanCenterY = PlateY/2;

FanScrewDia = 4.0 + HoleWindage;	// from fan frame holes
FanScrewRadius = FanScrewDia/2;

// Convenience settings

BuildOffsetX = 3.0 + PlateX/2;		// build X spacing between top & bottom plates
BuildOffsetY = 3.0 + PlateY/2;		//	... Y

Protrusion = 0.1;					// extend holes beyond surfaces for visibility
HoleZ = PlateZ + 2*Protrusion;

//-- Build it!

  difference() {
	cube([PlateX,PlateY,PlateZ],center=true);

	translate([(FrameHoleXOffset - FanCenterX),(FrameHoleYOffset + FrameHoleSpace - FanCenterY),0])
	  cylinder(r=FrameHoleRadius,h=HoleZ,center=true,$fn=6);
	translate([(FrameHoleXOffset + FrameHoleSpace - FanCenterX),(FrameHoleYOffset - FanCenterY),0])
	  cylinder(r=FrameHoleRadius,h=HoleZ,center=true,$fn=6);

	cylinder(r=FanDuctRadius,h=HoleZ,center=true,$fn=48);

	rotate(a=[0,0,FanAngle]) {
	  for(x=[-FanHoleSpace/2,FanHoleSpace/2]) {
		for(y=[-FanHoleSpace/2,FanHoleSpace/2]) {
		  translate([x,y,0])
			cylinder(r=FanScrewRadius,h=HoleZ,center=true,$fn=6);
		}
	  }
	}
  }

The original as-it-was-being-machined heat spreader dimensions:

With a pair of low-resistance and (relatively) high-torque NEMA 17 stepper motors driving the X and Y axes, I re-ran that torture test.

At 750 mA with just the aluminum sub-plate:

• X can traverse at 6000 mm/min
• Y begins losing steps at 4000 mm/min

Increasing the Y driver to 900 mA, the Y motor can traverse at 6500 mm/min. It began losing steps at 6600 mm/min.

Increasing the X driver to 900 mA, the X motor can traverse at 6000 mm/min with both aluminum build plates.

At 900 mA with both aluminum build plates, both axes can dependably traverse at 6000 mm/min = 100 mm/sec.

Large moves shake the printer, small patterns rattle it like a castanet, and you (probably) can’t print plastic at that speed, but the XY axes can traverse at 100 mm/s. That’s with about 200 g of aluminum plates atop an HBP, making it far heavier than even the ABP.

Because the firmware does not apply velocity ramping (aka control the motor acceleration), the stages must start or stop within two full step positions to avoid stalling the motors.There’s some belt deformation (not stretch), the Y idler pulley isn’t rigidly mounted, and the stages themselves have plenty of slightly bendy parts.

At 200 full step/rev, a 17 tooth pulley, and a 2 mm pitch belt, 1 full step = 0.17 mm and 2 step = 0.34 mm. I don’t have a good measurement for the other factors, but let’s assume a little bit of slop and go for 0.5 mm of total start / stop distance.

The acceleration required for an abrupt 100 mm/s speed change is therefore:

(0.12)/(2 * 5x10-4) = 10 m/s2

The entire XY stage assembly weighs about 1 kgf, so F = m·a tells us that the force is 10 N. There’s another 1 kg = 10 N from belt / pulley friction, so the total force is 20 N.

The pulley radius is 5.5 mm, so torque = F·r tells us the required motor torque is 20 * 0.0055 = 110 mN·m.

Although I don’t have the exact motor specs at hand (a peril of using eBay as my parts locker), NEMA 17 motors with 38 mm case lengths produce around 200-300 mN·m at 1 A. That matches the Y axis numbers pretty well, given the one-significant-figure accuracy of the measurements.

The X stage motor has a 34 mm case length and is likely good for 150-ish mN·m at 1 A. The 0.5 kgf of X axis belt friction dominates the acceleration force, so that result is in the right ballpark, too.

The motor have winding resistances around 2 Ω, so they’re dissipating something under 2 W at 0.9 A. The cases and driver chips get barely warm to the touch; there’s no need for heatsinks or active cooling!

Because the motors run well within their rated currents and the winding voltage is far lower than 12 V, microstepping is in full effect. The motors hum quietly and the stages move with authority. This is the quietest motion I’ve ever heard from the Thing-O-Matic; isolating the bolt heads from the panels probably improved that, too.

Remember to increase the per-axis speed limits in machines.xml to allow higher speeds; it’s all too easy to fool yourself that changing F6000 to F7000 in the G-Code file makes a difference, even while the XML file caps the speed at F5000.

Now that RepG has a setting for the homing speeds, I can set the maximum XY speed to 6000 mm/s, home at 2500 mm/s, and be done with it.

I might just saw the leadscrew off the damn Z axis motor and replace that puppy, too, although that’s not in the critical path right now.

Note: Don’t replace the motors on your TOM without going through the whole mechanical and electrical upgrade process I’ve described over the last few months. The X axis rod follower is extremely important, but the rest of the mods I’ve described help make the printer work smoothly and reliably. Adding more power won’t make those problems Go Away; you’ll just break something else!