Chinese Death Ray

December 28, 2020

I got my wife a cool tool for Christmas. Hopefully it doesn’t burn the house down. The “K40” laser cutter/engraver has been ubiquitous for several years; it’s a fairly powerful (nominally 40W) CO2 laser, with power supply, X-Y motor drive, and controller board in a metal chassis, all for under $500 – which sounds like a real bargain, considering the several-thousand-dollar price of commercial-grade systems. However, it’s best thought of as a starter kit for the technically competent, rather than a ready-to-use consumer product… These things are remarkably sketchy, even in the low-expectation context of lowest-possible-price Chinese factory output (see: my horizontal bandsaw from Harbor Freight). You’ve got high voltage, funky wiring, mystery electronics, water, and a freakin’ laser all in one box; what could possibly go wrong?

There are no safety interlocks whatsoever, so it’s entirely up to the user to not blind, burn or electrocute themselves.

The purpose of the laser is, of course, to burn things. That’s what it’s doing when it is engraving or cutting; if that process starts running away, you could quickly have some significant flames on your hands. Did I mention the large fan for forced-air ventilation / magnification of unintended flames? Fortunately, there’s not a lot of plastic in the immediate vicinity of the laser.

Fortunately, there’s a substantial online community working with (and improving) these machines, and it’s a great resource for anyone new to the K40 laser. An excellent starting point, and wealth of real information, is Another good entry point is All the points I’ve made here, and some more, are reinforced by Joshua Guthrie as well.

So far, we’ve run some basic tests on cardboard and paper, and worked on getting some basic safety measures in place. I think we actually got pretty lucky with the particular machine we received, as the laser focus was dead-on right out of the box, and while the expected Chinese-factory-sketchiness was there, the power supply was in fact grounded and the wiring was not horrible.

The only gripe I really have at this point, is that the “enable” line on the power supply seems to be permanently grounded. As that’s where you’d normally connect any safety interlocks, it’s pretty annoying. I’ve put the interlocks in the path to the laser firing control line, between the control board and the power supply, and that will work fine to prevent the control board from inadvertently firing the laser - but it does nothing to keep the laser from going on when, say, the “test” button is pushed. I plan to install a guarded momentary switch on that function, so at least you can’t accidentally lean on the button (as you could in the original design.)

Chinese Death Ray - December 28, 2020 - chad r. frost