The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

MTD Snowthrower Friction Drive Rebuild

During the last snowstorm of the season, the venerable MTD snowthrower carved a trench out of the garage and across the driveway, then abruptly stopped moving. The motor roared and the auger turned, but the drive clutch handle had no effect, so I dragged its carcass into the garage and we completed the mission by hand.

Popping the belly plate on the next sunny day revealed the problem: the jam nut (part 34) anchoring the Friction Disk Wheel (part 28) to the Friction Wheel Bracket Assembly (part 32) had gone missing:

MTD Snowblower – page 26 – friction drive parts

Worse, the Wheel’s threaded shaft spent some time rattling around in the Bracket while chewing up its thread:

MTD Snowthrower – friction disk wheel – damaged thread

This would ordinarily be No Big Deal, but what you see of the shaft is all you get: it rotates freely in the bearing embedded in the Wheel with no way to hold it while cleaning up its threads.

Having already promised to replace the Wheel, I installed the new Wheel using a castle nut secured with a generous dollop of red Loctite, then tapped two of its castellations into the shaft’s slot as a mechanical anchor:

MTD Snowthrower – friction disk wheel – castle nut

I really wanted to lay a nice hard roll pin along that slot through the nut, but there’s no convincing way to secure such a thing without a second nut. Maybe next time?

While I had the drive train apart, the sad state of the Wheel Shift Rod Assembly (part 29) became apparent:

MTD Snowthrower – wheel shift rod – worn

I scuffed up the shiny wear mark, turned a suitable acetal bushing, filled the trench with epoxy, and squished the bushing in place:

MTD Snowthrower – wheel shift rod – acetal bushing

The flange might hold it in place against the Frame Shift Bracket (part 18), which snugly contains the rest of the bushing against the epoxy, so the whole affair might outlast the next season’s first snowstorm. We shall see.

A nice new R-clip secures the Friction Wheel Bracket Assembly in place against the old washer:

MTD Snowthrower – friction bracket R-pin

You might want to insert it the other way, but the black plastic housing above it extends just far enough to thwart your (well, my) desire.

2022-04-25
Kukoke Outlet Timer: Over-powered Zener Diode

If the title seems familiar, it’s because there’s no visible difference (apart from the “brand name”) between the Enover timer that failed a little over a year ago and the Kuoke timer that recently failed:

Kukoke timer – overview

That’s what it looked like after the repair. Prior to that, it’s just a blank display with no response to any inputs.

Given identical hardware, the overheated phenolic PCB under the Zener diode came as no surprise:

Kukoke timer – zener heat death

As promised, though, this time I epoxied a brass shim heatsink to the new diode in hopes of cooling it enough to live long and prosper:

Kukoke timer – zener heatsink

I suppose I must now preemptively affix heatsinks in the two surviving timers, because we all know how their stories will end.

2022-04-22
Gentec ED-200 Optical Joulemeter: Specs

The Gentec ED-200 optical joulemeter from the Box o’ Optical Stuff is so thoroughly obsolete that no datasheet exists for it anywhere online:

Gentec ED-200 – measurement setup

The best I could come up with, after many dead ends, is a 2001 capture from gentec-eo.com at archive.org with the barest hint of specifications:

Gentec ED-200 specs

The Max Energy Density spec suggests longer pulses are allowed to deposit more energy, probably because more time gives thermal diffusion an opportunity to spread the heat across the target; at CO₂ laser wavelengths that may not apply.

With the platform lowered as far as it goes, the ED-200 is 130 mm below the laser nozzle where the beam diameter is about 6 mm for an area of 0.3 cm². Ignoring the ideal Gaussian beam profile by smearing 60 W uniformly across the circle gives a power density of 200 W/cm², which means the laser pulse must be less than 0.5 W·s / 200 W = 2.5 ms to stay inside the power density limit.

I sincerely hope Gentec overbuilt and underspecified their detector.

Also, there’s a useful overview document from Genetc-eo.com, wherein it is written:

The Voltage Response
The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.
Pulse Width Versus Rise Time
Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.
Gentec Energy Detectors, page 2

Figure 2 shows the overall waveform:

Gentec Energy Detectors – Figure 2

Which looks a lot like this 10 ms pulse at 50% duty cycle:

Gentec ED-200 – 60W 50pct 10ms

The pulse was 10 ms long, much longer than the 1.5 ms ED-200 risetime spec, but the overall shape looks right. Dividing the 3.3 V peak by the detector’s 10.78 J/V calibration value (11 J/V works for me) says the pulse delivered 300 mJ = 300 mW·s. Dividing 300 mJ by 10 ms gives 30 W, a beam power astonishingly close to the expected value.

The OMTech laser has a nominal 60 W output, although the tube life drops dramatically with regular use over 70% = 40 W. Power does not scale linearly with the laser tube current displayed on the power supply milliammeter, with the maximum value presumably preset to the tube’s 20 mA limit producing 60 W. The 20 kHz PWM duty-cycle chopping applied by the controller should linearly scale the average power downward from there.

It looks like the ED-200 might deliver reasonable results for millisecond-scale pulses at low PWM duty cycles, but it was obviously intended for much milder lasers.

On the other paw, it’s fully depreciated …

2022-04-21
Gentec ED-200 Optical Joulemeter: Accessories

The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:

Gentec ED-200 – measurement setup

It’s an optical pyrometer producing, sayeth the dataplate, an output of 10.78 V per joule of energy applied to its matte black absorber. Whether it’s accurate or not, I have no way of knowing, but aiming the business end toward the sun and waving my fingers over it produced a varying voltage, so there was hope.

It has a 1/4-20 socket on one side and my spare magnetic mount expects a 3/8 inch rod, so I drilled a suitable hole in a suitable aluminum rod and cut the head off a suitable bolt:

Gentec ED-200 mounting rod – parts

A dab of Loctite intended to secure bushings completed the assembly:

Gentec ED-200 mounting rod – assembled

I later replaced the nut with a finger-friendly nylon wingnut.

Which allows a measurement setup along these lines:

Gentec ED-200 – measurement setup

The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:

Gentec ED-200 target – scorches

The 1 mm graticule lines give a jogging suggestion to hit the center, assuming you (well, I) manage to hit anywhere on the target at the first shot. The beam is supposed to fill most of the central region, which is obviously not going to happen here, and it must not be focused to a pinpoint. The previous owner (or his minions) put a few scars on the surface and I expect to make similar mistakes.

Early results look encouraging:

Gentec ED-200 Joulemeter – first pulse

The SVG image as a GitHub Gist:

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view raw Joulemeter Targets.svg hosted with ❤ by GitHub

I added the two mounting ears in anticipation of putting the joulemeter in the beamline between the mirrors to measure their loss.

2022-04-20

OMTech 60 W Laser: Laser Power Indicator

Although the OMTech laser controls the laser power supply with a key-lock switch, there’s little visible difference between the OFF and ON positions. Having occasionally mistaken it in both directions, this seemed like a useful addition:

Laser Power Lock Indicator - installed — Laser Power Lock Indicator – installed

The strip of black duct tape below the lock muffles the rattle of the triangle hatch key against the metal cabinet.

Two snippets of foam tape hold the knob to the lock cylinder, making an admittedly tenuous connection, but the knob fits around the outside of the switch housing with minimal clearance and ~~doesn’t~~ shouldn’t suffer any torque or pulling, so it might work.

The solid model looks about like you’d expect:

Laser Power Lock Indicator - solid model — Laser Power Lock Indicator – solid model

Unfortunately, it has no good orientation for printing, so I let PrusaSlicer generate support material inside the knob:

Laser Power Lock Indicator - Support structures — Laser Power Lock Indicator – Support structures

Suffice it to say: removing all that plastic did not go well.

I eventually grabbed the knob in the lathe and bored the interior out to its more-or-less proper dimensions, figuring nobody would ever notice the carnage, and it worked reasonably well. In the unlikely event I need another pointer, I’ll add a support spider to hold up the interior with minimal contact and less plastic.

Yeah, the laser really needs a stack light showing its condition and safety status …

The OpenSCAD source code as a GitHub Gist:

	// Indicator for OMTech laser power lock
	// Ed Nisley KE4ZNU 2022-04-09

	KnobOD = 35.0;
	KnobHeight = 22.0;
	KnobTaper = 4.0;

	PointerLength = 45.0;
	PointerThick = 3.0;

	TipOD = 2.0;

	/* [Hidden] */

	//——

	Protrusion = 0.1; // make holes end cleanly
	HoleWindage = 0.2;

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
	FixDia = Dia / cos(180/Sides);
	cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides);
	}



	//———-
	// Create part
	// Plenty of magic numbers from actual measurements

	module Pointer() {

	difference() {
	union() {
	linear_extrude(height=PointerThick)
	hull() {
	circle(d=KnobOD,$fn=24);
	translate([PointerLength – TipOD/2,0])
	circle(d=TipOD,$fn=12);
	}

	cylinder(d=KnobOD,h=KnobHeight – KnobTaper,$fn=24);
	translate([0,0,KnobHeight – KnobTaper – Protrusion])
	cylinder(d1=KnobOD,d2=KnobOD – 3.0,h=KnobTaper + Protrusion,$fn=24);
	}

	translate([0,0,-Protrusion]) {
	PolyCyl(29.0,14.0 + Protrusion,24);
	PolyCyl(24.0,14.0 + 5.0 + Protrusion,24); // leaves clearance under pointer
	}

	translate([0,0,KnobHeight])
	cube([12.0,2.0,2*KnobHeight],center=true);

	}
	}

	//———-
	// Build it

	Pointer();

view raw Laser Power Lock Indicator.scad hosted with ❤ by GitHub

And doodles giving the dimensions of the key lock, not all of which can be true at the same time:

Laser Power Lock Indicator - Dimension Doodles — Laser Power Lock Indicator – Dimension Doodles

2022-04-19

Figaro TGS5042 CO Sensor

The hallway fire detector recently told us it scented carbon monoxide, but we hadn’t been doing any cooking or baking (in the kitchen two rooms away), the furnace (in the basement) hadn’t run for a few hours, and nothing else looked like it was on fire. I had recently replaced the alkaline batteries after a similar false alarm a few weeks earlier; it seems the detector failed after half a dozen years or so.

Tearing it apart revealed something resembling an 18650 lithium cell:

Figaro TGS5042 CO sensor – overview

Which made no sense, given the circuitry.

A casual search shows a Figaro TGS5042 is actually a carbon monoxide sensor. I’m mildly surprised enough gas gets through the vents fast enough to produce an early alert:

Figaro TGS5042 CO sensor – vent detail

I tore it apart to reveal a few droplets of whatever the electrolyte might be, so it hadn’t completely dried out.

The Product Information flyer doesn’t define what “long life” might be, but another page says “10 years”, so apparently the rest of the circuitry failed around a not-quite-dead-yet sensor.

2022-04-18
OMTech 60 W Laser: Air Assist Flowmeter

With the solid state relay switching the air assist pump, an air flowmeter seemed like it would come in handy:

OMTech Laser – air flowmeter installed

It’s stuck to the lip inside the top hatch on the right side of the cabinet, which may not be the most convenient location, but keeps it out of the way and doesn’t require much additional tubing.

The 6 mm tube kit included some (1/8 NPT?) push fittings that came heartbreakingly close to matching the flowmeter’s internal threads:

OMTech Laser – air flowmeter – push tube fittings

Given that the air pump doesn’t produce much pressure, two snippets of 1/4 inch silicone tubing suffice to couple the blue 6 mm tubing to the flowmeter’s barbs:

OMTech Laser – air flowmeter – silicone tube adapter

The run from the air pump to the flowmeter is now new blue tubing, with the original black tubing running through the drag chain to the laser nozzle:

OMTech Laser – air flowmeter – tube layout

Replacing a number of overly tight cable ties along the way may remove enough restrictions to counterbalance the additional tubing.

Opening the flowmeter’s valve all the way puts 14 l/m = 0.5 CFM through the nozzle. I have no idea of the proper rate, other than more is better while cutting and less is better for engraving.

Four years ago, Russ Sadler laid out the plumbing required to automatically select high and low flow air assist, which seems like a worthwhile project.

2022-04-15