DIY Laser part 3: The Muscle

The gantry is the part of a CNC robot that puts a tool just where it needs to be to get the job done. The tool can be anything: a rotary tool, a plastic extrusion head, a sharpie, a vacuum attachment, or anything else you like. In the case of my laser cutter, the tool is a mirror and lens arrangement that focuses a beam of light onto the work piece.

Let it slide

As I mentioned in my first postMakerSlide makes it easy to put a gantry together in short order. It elegantly solves the problem of how to keep your axes perpendicular to each other without twisting. It uses trapped Delrin v-wheels on a long, ridged piece of t-slot, with steel bearings to keep the movement smooth.

Trapped v-wheels on bearings keep your axes straight and perpendicular.
Trapped v-wheels on bearings keep your axes straight and perpendicular.

Hip Hop Ya Don’t Stop

A nice side effect of using aluminum for all of the gantry pieces meant that I could use magnetic reed switches instead of physical microswitches. With a steel gantry, the steel would eventually become magnetized and interfere with the switches.

Reed switches.
These tiny switches sense a moving magnet, marking the edges of the gantry.

I found that adding 3 layers of marine grade heat shrink tubing not only protected the reed switches, but made the switch the perfect size to fit inside the edge of the t-slot. Heat it up with the heat gun, press it into place, and when it cools, this reed switch is permanently mounted. You can still slide it along the slot to get the edge of the stop in just the right position. Then all of the wiring can run along the slot, and stays held in place with little rubber grommets. No tools, no glue, just rubber friction is all it takes.

Reed switches nestle nicely in the edge of the t-slot
Reed switches nestle nicely in the edge of the t-slot

Magnets harvested from a couple of old laptop hard drives seal the deal. One rides along with the shuttle on the back of the X axis, the other rides on the side of the Y.

I used full stops on the X and Y (one each at the minimum and maximum). You can get away with two (one each at minimum X and Y) and rely on software to keep the gantry from running off the end, but I think full stops are worth the extra effort. They provide an extra sanity check in case something goes wrong with the motor drive. This is especially important when you’re still figuring out your motor speeds and step counts. Which brings us to…

De Hot Stepper

I used one stepper motor for each of the axes. The X axis motor came from a recycled project, and is somewhere in the 1A / 1.8deg/step / 1.9Nm range. This was definitely overkill for the very light X shuttle (it only holds the laser head and a magnet, and weighs very little). But you can’t beat free, so in it went.

The Y axis motor is a dual-shaft (Longs 23HS8610B): another 1.0A, 1.8deg/step, 1.9Nm. This turned out to be slightly underpowered for the Y axis. The Y needs to move a lot more weight than the X (the Y axis MakerSlide, plus all of the weight of the X shuttle AND motor…) I ended up dropping the step count down a bit and slowed it down until it didn’t drop any steps, and it’s quite happy.

Dual-shaft Y axis motor.
Dual-shaft Y axis motor. Those flexible couplings made it easy to fudge the alignment of the motor to the shaft end bearings.

The Z axis used whatever motor came with the Z module from the old laser; it looks a lot like the X motor. A couple of minutes with a volt meter and a cheat sheet helped figure out how to wire it up.

I standardized on MXL pulleys and belts. They’re common enough to be reasonably cheap, and still provide plenty of grip.

Put it all together and away you go: the gantry can auto-home without any visible stop switches. It won’t run off the edge of the gantry, and all of the wiring is safely tucked away.

Tune in next time for DIY Laser Part 4: The Heart

In the meantime, here is the photo album for the completed build.

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