Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The honeycomb grid panel doesn’t have a perfectly flat surface, but the bench block kinda sorta averages things out:
OMTech platform alignment setup
I jogged the platform down until the nozzle just cleared the block, then measured the space at various spots across the grid. Somewhat to my surprise, it’s just about as good as you can expect:
OMTech platform alignment – 2022-03-17
You could argue that the left side is lower by half a millimeter.
The four stout single-start leadscrews moving the platform have a 4 mm lead (= pitch for single-start), driven by a belt with a 0.2 inch (!!) tooth spacing around 14 tooth sprockets, so moving the belt by one tooth produces 0.286 mm of vertical motion. I could loosen the belt and turn the left rear leadscrew one tooth to raise that corner, but not right now.
The laser lens has enough depth of field to cover a millimeter without too much spot size variation, unless you’re being very fussy. A quick focus check:
OMTech 60W laser – focus check – detail
The center hole happened pretty close to the lens focal point, with the adjacent holes 1 mm above / below that point. An accurate initial focus setting is obviously important, but varying by half a millimeter on either side seems survivable.
The OMTech laser cutter has six access hatches, each with one or two latches. These are not locks, although you do need a triangular “key” to turn the latch plug:
OMTech laser – latch – cylinder point up
Being that type of guy, I want all the latches to have the same plug orientation when they’re closed, so that I can hold the key one way, poke it into any latch without thinking too hard, and have it fit onto the plug:
OMTech laser – latch key – latched position
A quarter-turn clockwise (remember clocks with hands?) then releases the latch:
OMTech laser – latch key – unlatched position
Inside the hatch, the closed position corresponds to a tongue capturing a flange around the cabinet opening (not shown):
OMTech laser – latch – latched position
After the quarter-turn, the tongue releases the flange:
OMTech laser – latch – unlatched position
So, we’re not talking high security here.
As delivered, the plugs had more-or-less random orientations when they were closed and some required a counterclockwise quarter-turn to release.
It turns out the latches aren’t a complete unit that simply drops into a hole in the hatch:
OMTech laser – latch parts
I sympathize with whoever must assemble ten handfuls of parts into ten latches on a production line and I also understand why orienting the plug wasn’t on that person’s to-do / QC checklist. I further understand why two cylinders lacked the big toothed washer under the nut; it’s not essential to the function and nobody will ever miss it.
The plug has a triangle on one end (for the key) and a square on the other (for the tongue), with one triangle point aligned to a side of the square:
OMTech laser – latch plug
To my way of thinking, that point must be upward, as shown in the first picture, when the latch is secured.
The cylinder can fit into the square(-ish) hatch hole in four possible ways, but its symmetry allows only two unique orientations. It must look like this in order to put that point upward when the plug is maximally counterclockwise (my finger is pointing upward):
OMTech laser – latch cylinder
So I devoted a pleasant half-hour to reducing the latch entropy.
The screw attaching the tongue to the plug also controls the friction of that spring against the plug as you (well, I) turn it. All the screws now sport a dab of Loctite to ensure the tension remains mostly constant (at least for a while), as do the two large nuts lacking corresponding toothed washers.
The “key” has no marking to indicate its “point-up” orientation, so I stuck a snippet of label on one side, with a jaunty red highlight marking the point. Something better will surely occur to me, but it’s no longer in the critical path.
The best place for the OMTech laser cutter seems to be snuggled at base of the chimney, venting into the long-disused fireplace through the steel plate adapting a long-gone wood stove to the opening:
Duct fan installed
The short run of flexible tubing allows some give-and-take at the cutter’s vent outlet. The elbow on the duct fan’s output terminates in a blast gate to cut off the draft blowing up (or down!) the flue with the fan off.
The cutter arrived with a huge high-speed axial blower screwed to its output baffle:
OMTech 60W laser – OEM vent fan
The noise from that fan had to be heard to be believed.
The cylindrical exhaust duct attached directly to the motor with four screws, only two of which matched holes in the baffle plate:
OMTech 60W laser – modified vent
A trial fit revealed the assembly rattled something awful: those two screws let the duct vibrate against the baffle. Match-drilling two more holes into the baffle let me mount the duct with three screws and, in combination with the foam gasket, it is now solid and quiet.
A quick check shows the duct fan draws 10 to 11 m/s through the baffle at full throttle, roughly 400 CFM. That’s pretty close to the flow measured through a long pipe and, with only 6 ft³ of stink inside the laser’s cabinet, ought to exhaust the fumes just fine.
Lacking a loading dock, I built a level unloading platform in the driveway:
OMTech 60W Laser Cutter – unloading platform
The OMTech 60 W laser cutter arrived inside a generous supply of plywood obviously intended for practice cutting and engraving:
OMTech 60W Laser Cutter – crate
Knowing the crate wouldn’t fit through our “36 inch” basement door, we stripped the cutter down to the crate’s steel-framed baseplate:
OMTech 60W Laser Cutter – uncrated
I raised the cutter (using the foot-pad screws) enough to slide 3/4 inch planks under the casters so we could roll it over the lip of the crate base.
The specs say it’s 34 inches wide, but, not at all to our surprise, that’s just the cabinet. The hinges on the access hatches and the lid handle make it just over 35 inches wide, which we slowly and carefully verified would not fit through the 34 inch door opening:
OMTech 60W Laser Cutter – slow fit check
Raising the lid to get the handle out of the way, then pushing gently inward on the sides, eased it through without damage to either the cabinet or the door frame:
OMTech 60W Laser Cutter – door fit
Standing on the plank let me raise the outer end enough to roll it forward and lower the casters onto the planks inside the door.
The supplied aquarium pump circulates five gallons of distilled water to cool the laser tube. My simple test patterns so far haven’t dumped much heat into the water:
Dot Mode – 15 pct power – 1 2 3 ms on – 0.25 mm spacing
The doily on the left shows 9% power cuts right through paper. Dot Mode fires the laser every 0.25 mm (in this case) for a specified number of milliseconds to reduce the total energy; 3 ms produces dark dots, 1 ms is a pale brown, and 2 ms looks pretty good.
Concerted rummaging in the Basement Warehouse produced some rather old acrylic sheets:
Acrylic Stockpile
Washing with detergent and denatured alcohol cleaned off a lot of grunge, but the yellow tint says it’s been around for a while. In fact, It Came With The House™ when we bought it three decades ago.
One sheet was a status board in an automobile machine shop:
It seems suitable for a semi-permanent laser cooling water monitor, particularly because it can perform arithmetic to show the difference between the inlet and outlet temperatures. The minuscule clock face at the center top of the display shows it’s in auto-power-off mode, which can be defeated by a Vulcan Nerve Pinch while turning it on.
Having a large backlit display was a selling (well, buying) point and the instructions have this to say about its operation:
Dual Thermocouple meter – backlight instructions
The instructions say nothing about defeating the backlight timeout. The description is technically correct, because the two seconds before it goes dark is “within 30 seconds”, but I’d rather have a nicely lit display that’s on all the time.
Five screws hold the back cover in place, with no nasty prying required to pull it apart, and the build quality is about what you’d expect for a cheap meter. The circuitry fits on a single PCB and perhaps the thermistor over on the right serves as the cold junction compensation:
Doodling the backlight circuit layout suggests it’s pretty simple, even without filling in the component values:
I replaced the transistor base resistor with a somewhat larger 4.7 kΩ SMD part and added a flying wire to jam the transistor on all the time:
The IC is a serial EEPROM with its VCC and ground pins in the usual places, so, when the power to the EEPROM goes on, the backlight turns on and stays on.
The meter draws a bit over 8 mA with the backlight running, which means the trio of AAA cells won’t last all that long. When things settle down, I’ll conjure a simpleminded power supply running from a convenient voltage inside the laser cabinet.
A crude test setup to measure the duct fan’s air flow against resistance from plausible lengths of 6 inch duct and fittings:
Duct fan test setup
The orange stripe (upper left corner) marks the blast gate mounted on the steel plate closing off the fireplace: when the stripe is visible, the gate is open. It’s hot-melt glued into a plywood square reducing the 8 inch hole in the plate.
I won’t be using five feet of steel duct, but [handwaving] it’s what I have on hand and should produce results similar to a shorter length of flexible duct [/handwaving].
A useful conversion factor from the anemometer’s air flow in meter/sec to the corresponding volume flow in ft³/min (colloquially CFM), based on a 6 inch diameter opening with uniform airflow:
The air flow up the chimney depends strongly on basement temperature, outdoor temperature, and wind speed. On a midwinter’s calm-but-freezing evening it ran around 1.5 m/s → 57 CFM and the next day I measured 0.7 m/s → 27 CFM with wind gusts pooting old-fireplace smell into my face.
One might reasonably conclude something has gone horribly wrong, as the ductwork seems to contribute negative resistance and increased airflow. I think it’s a combination of the natural flow up the chimney, combined with a bit of flow straightening through the pipe directing air into the fan’s blades and measuring the (mostly uniform) inlet stream instead of the (somewhat segmented) outlet stream.
Anyhow, the controller has eight speeds with surprisingly linear output. I doubt the upper line’s slope of 50 CFM/click means anything, but the consistency of both suggests a 4:1 flow range, from which I can pick the lowest speed that provides enough fume extraction.
The basement has enough air leaking in (and out) that opening the exterior door had no discernible effect on the flow through the fan and up the chimney. At top speed the fan will produce two air changes per hour, chilling the basement something awful in the winter and introducing too much warm+moist air in the summer. This may call for a separate duct for outdoor makeup air, but that’s a problem for another season.
