Category: Machine Shop

Contrary to what I thought, the OMTech 60 W laser’s honeycomb platform gridwork isn’t spot-welded. Instead, it’s many corrugated strips aligned with rods inserted parallel to the Y axis through the strips:

The silvery rod over on the right passes through holes punched in the grid strips. The black thing in the middle is one of the knife edge bars below the platform; it supports the honeycomb grid and can also hold chunky objects.

Because the grid isn’t a rigid structure in the XY plane, you simply jam a solid spike into one of the openings and have the grid realign itself just enough to grip it firmly:

The shape is inspired by the spikes in AC Wright’s kit of laser workholding widgets, greatly simplified and resized to fit this honeycomb. In particular, there’s no need for a compression slit in the stem, because the grid resizes itself around the solid stem.

Eventually, the corners wear off the stem and it won’t fit securely in the grid, whereupon you throw it out. They’re easy enough to cut from whatever MDF scraps you have lying around, so you won’t form a deep emotional bond.

Likewise, after only a few uses, the sharp point vanishes; a blunt tip would make more sense.

The bottom of the arms locate the top point a fixed distance over the honeycomb, which seems to be sufficiently flat and well-aligned for most (well, my current) purposes. The slanted top of the arms feels more comfortable to push than a hard right angle, at least to my fingers.

These work well when the fumes from cutting MDF / plastic / whatever would blowtorch a cardboard sheet holding metal spikes:

The cardboard sheets surrounding the victim maintain airflow from the front opening over the platform surface to where it’s needed, rather than letting it vanish through the honeycomb.

Update: Obligatory Sadler video.

The SVG image as a GitHub Gist:

Mash all these together:

• Russ Sadler’s notion of 2 mm steel dowel pins inserted into a hand-drilled acrylic plate atop a cold-rolled steel sheet
• David Johnson’s notion of using punk clothing spikes as standoffs
• A sufficiently aligned honeycomb platform
• My desire to not form deep emotional bonds with a fixture

To get a cardboard spike plate:

The punk spikes stick to a layer of ordinary masking tape across the back surface which lets them sit flat on the honeycomb and support a sheet parallel to the platform. They’re nominally an inch tall, which works out to a very consistent 24 mm, and come with a matching set of 6 mm M3×0.5 truss-head screws now residing in a ziplock bag against future need.

The spike plate works well under thin paper-like sheets requiring high cutting speed and low power, where the defocused beam just scorches tracks across the cardboard without setting it on fire:

Nota bene: a cardboard sheet makes a terrible backing plate under material requiring slow speed and high power, like MDF / plywood / acrylic, where the cloud of combustible gases under the victim forms a very effective flamethrower. You have been warned.

Obviously, you need not fill every hole. Leaving some holes vacant doesn’t (seem to ) allow much smoke removal downward through the honeycomb, perhaps because there’s insufficient perimeter area to get enough wind into the center section.

Eventually, the cardboard becomes sufficiently scarred to justify making another one, which is easy enough:

The motivation for raising the target above the platform is to provide good airflow to remove the smoke / fumes / smog from the area, thereby reducing unsightly deposits on the lower surface. Given decent airflow across the platform, this works surprisingly well:

That picture comes through the laser’s tinted polycarbonate window, so it’s somewhat blurred, but shows smoke streamers emerging from the victim’s corrugations and across its surface.

All in all, it’s quick, easy, and effective.

Russ Sadler points out that Chinese CO2 lasers lack air inlets matching their 6 inch = 150 mm outlet port, so fumes accumulate over the workpiece as air leaks in through various panel / hatch gaps and small openings. The simplest solution, at least for my OMTech 60 W laser, seems to be opening the front passthrough hatch:

The opening is 33×4 inch = 0.9 ft² with an airflow of just under 1 m/s into the exhaust fan at full throttle, so it’s venting at about 180 CFM. That’s half the duct fan’s 400 CFM in more-or-less free air, but the laser cabinet outlet vent has a perforated cover with maybe 50% clear opening:

It’s not exactly a flame arrester.

Directing the air flow across the platform from front to rear requires sealing the gaps along the front of the cabinet:

And the huge openings on either side of the exhaust duct:

Yes, all those are cardboard sheets and, no, they’re not the final implementation. This is all in the nature of figuring out what works, so being able to cut-to-fit is a Good Idea.

The large gap along the rear edge (on the right, above) for the rear feedthrough opening got a cardboard sheet after engraving some MDF.

Early indications are that it works fine, as witness the smoke streaming off the rear of a cardboard test piece:

Cutting MDF produces copious smoke that fills the cabinet, but it clears quickly and doesn’t escape into the Basement Laboratory if I wait a little longer than I really want to after the cutting stops.

Blocking the unused areas of the honeycomb helps direct airflow in the proper direction:

All in all, it works well.

Nearly everybody with a floor-standing laser cutter eventually decides it’s much too low for comfort:

Those are the 5 inch sections of a furniture riser set (the 3 inch sections are visible at the left rear of the picture, ready for deployment). With the legs extended to their full length, they put the laser’s honeycomb platform about 30 inches from the floor, so the complete set will raise it to 33-ish inches.

I went full-frontal Archimedes by levering each end of the cabinet up an inch at a time using one of those maple shelves atop an increasingly tall fulcrum made of various planks, then lowering the ever-lengthening legs atop stacks of plywood. Eventually I could roll a floor jack under the end beam to simplify the rest of the lift.

Protip: lock the casters to prevent movement.

More height makes reaching inside the machine much more comfortable!

At least for me. Mary says it’s now much too high for her …