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 post, MakerSlide 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.
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.
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.
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.
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.
It’s a common misconception that low-powered CO2 lasers can’t cut through metal. But in the right circumstances, you can cut thin metals just fine (even with a 40 Watt laser).
Cutting ability isn’t just a question of power available from the laser. It’s more dependent on the power density of the beam, and the ability of the material to dissipate that power. Thin metals that dissipate heat poorly can be cut quite readily. Mild or stainless steel, titanium, or even brass can be coaxed into being cut or etched. On the other hand, good luck trying to put a dent in thin copper or aluminum foil.
But how to contain it?
If lasers can cut metal, what material can you use for making a laser housing? (Some brave folks have tried making the whole thing out of wood, but that’s a little too… innovative for my tastes.) Most housings I’ve seen use a steel cage to keep stray laser emissions from burning or blinding innocent bystanders. While steel is cheap and makes a rugged industrial housing, I believe this is overkill for low-powered DIY CO2 lasers, for a couple of reasons:
Steel may be cheap, but it’s tough for many DIY hackers to work with unless you’ve already got access to a well-equipped metal shop.
The laser can only penetrate where the power density is highest: around the focal point of the lens. Much further away than that, and the laser is far too diffuse to do much damage to most materials.
Aluminum sheet is much lighter and easier to work with, and dissipates heat far faster than steel. Even a perfectly focussed 100W laser won’t cut through a few millimeters of aluminum sheet.
At least, this was my thinking when I decided to try making my first laser housing out of aluminum sheet. My reasoning turned out to be sound, but my execution… could have used some improvement.
Lesson #1: measure twice, cut once.
Unfortunately I was so excited to finish up the laser project that I ordered pre-cut material before I settled on the final design.
I had originally thought that I would make the Z stage from scratch. But as I took apart the old laser cutter, I noticed that the old Z was a separate module. I thought it would save time to transplant the whole module into the new cutter.
Sadly, the Z didn’t quite fit. It stuck out about 15cm below the spot where I had intended to put the bottom of the box. When my pre-cut aluminum showed up, it didn’t quite reach.
Lesson #2: thickness counts
The second problem was more subtle. Thin aluminum is pretty cheap, but the cost adds up quickly as the thickness increases. I had chosen material that was just a little too thin for the job, and it would pucker as I screwed it into the aluminum t-slot. Any spot where the aluminum doesn’t meet flush with the frame is a potential place where light could leak– which is exactly what the housing is supposed to prevent.
Sealing the edges and corners turned out to be pretty simple. I added a little aluminum angle bracket to all of the edges. That helped hold the sides together, and made a laser-proof barrier at all of the possible places where light might leak.
I still wanted to use something more substantial for the skin, but thicker aluminum sheet would add greatly to the cost and the weight of the machine. It was time to try something else.
The solution: composite materials
After asking around and trolling through various DIY laser forums, I hit on the idea of using a composite material called e-panel (the slightly cheaper cousin of DiBond). It consists of a sheet of high density polyethylene (HDPE) sandwiched between two pieces of thin aluminum sheet. It’s used to make durable signage and kiosks. It’s about half the weight of equivalent solid aluminum, and much cheaper.
This e-panel came pre-painted white. I ordered mine from Harbor Sales, who very helpfully cut it to size (I measured it twice this time around…)
I found that I could trim the smaller parts with tin snips and sheet metal shears, and holes were easily made with a hand drill. The aluminum blocks the laser, but the HDPE makes the material thick and rigid. This stuff is a joy to work with.
Tune in next time for DIY Laser Part 3: The Muscle
There’s no such thing as a cheap laser engraver. Though you might think otherwise if you’re like many DIY types, trolling eBay in the hopes of finding something reasonably priced that will get the job done without maiming anybody.
The trouble with cheap laser engravers is that every corner that can be cut, will be cut. The race to the bottom is a sordid path strewn with incorrectly rated components, cheap wire, hand-hewn “precision” parts made of inappropriate materials, and (literally!) shockingly shoddy high voltage supplies.
I’ve spent the last two years taking care of a $4000 50 Watt Special from Hong Kong. In that time it was offline much of the time, always needing some adjustment or emergency repair. One evening the gantry suddenly stopped moving altogether, while the laser blithely continued to burn deeply into the acrylic on the bed. After disassembling the gantry to find that the Y stage had vibrated itself apart due to a complete lack of washers, locking fasteners, or even Loctite, I decided I had had enough.
Rather than apply yet another band-aid, I’ve spent the last six weeks or so working on my first CNC project: a DIY 60 Watt laser engraver.
Design goal: nobody dies. Also, fun.
This project would be my first large robot, and certainly the first that I would arm with a laser that could kill me. I’m no stranger to questionableprojectideas, but I knew that other folks would likely be using this tool, and I wanted it to be bullet proof. Or at least safer than the death trap we had been using.
I also didn’t want to start completely from scratch. I knew I had a lot to learn, but I also knew that CNC gantries are nothing new, and there is very little need to reinvent the wheel (even if it is a trapped v-wheel on bearings). So I decided to start by looking at various other DIY laser builds, like Lasersaur, various boot-strappable designs, and especially Barton Dring’s CNC laser.
MakerSlide is fun stuff, especially for a CNC newbie like me. It elegantly solves the problem of how to keep your axes perpendicular to each other without twisting– a problem that only gets worse as your dimensions increase. Sure there are many ways to solve this problem (some of which involve less weight than MakerSlide), but this stuff makes it cheap and easy. Plus it offers a very solid and yet frictionless rolling bearing for any sized shuttle you care to throw at it.
Best of all, MakerSlide can be cut with a band saw (or even a hack saw) and it’s compatible with standard 20mm t-slot.
Grown-up tinker toys
If you’ve never used t-slot before, think of it as grown-up tinker toys. You can get aluminum in just about any length or thickness, and it all fits together with simple screws, spacers, and steel tabs. Tighten the screw and you’ve got a very strong joint. Add an appropriate spacer and you can set the angle to whatever you need. The aluminum is a strong, light alloy that easily drills, cuts, and taps. Given a chop saw and a couple of hours, it’s easy to prototype just about anything out of t-slot. Best of all, adjustments are as easy as adjusting a screw, and shifting a piece around leaves no visible tool marks on the aluminum.
I found that Misumi’s online store had everything I could possibly need. They’ve got data sheets on everything, reasonable prices, and packages typically show up in a couple of days. And they will cut their t-slot to order, so getting started was as easy as downloading Barton’s drawings and placing the order for all the t-slot I would ever need.
Or so I thought at first. It didn’t take long before I realized I’d want to deviate a bit from the original design to fit with parts I had ordered or already had on hand. Two or three round-trips to Misumi later, and I had a reasonable first stab at a laser chassis.
The wonderful thing about standards
I also realized early on that I had better get my act together regarding fasteners. I had been lugging around the same bucket of screw-compost since high school: a morass of sheet metal screws, wood screws, and hex caps. Lock nuts, wing nuts, and washers. Zinc, brass, steel, and anodized. Metric and imperial. All mixed together in a little box that I’d occasionally dig through, wondering why I could never find something that matched what I needed.
I performed one of the most liberating acts I’ve ever done in my shop.
I threw the whole thing away.
It seemed obvious that if I was working with t-slot, I’d need the right fasteners in easy reach, in copious quantities. So I placed another order to the fine folks at the Bolt Depot and standardized on:
4mm and 5mm
regular and nylon lock nuts
various lengths (mostly 8mm and 10mm, with a few longer selections for variety)
I also picked up a couple of cheap plastic organizers at Harbor Freight. After a few minutes of organizing my new stainless steel bounty, I’d never have to hunt for the right screw for the rest of the project. This planning probably saved me days of effort, since I would only need two hex keys to completely assemble (and later, maintain) the machine. It also let me banish the false god of fractional inches from the project early on. Plus the hardware looks fantastic and will never rust.
Get it together
Once I got into the rhythm of grabbing the right screw and the right hex key, assembly went much more quickly. I soon had a free-standing chassis with a rolling X and Y gantry on casters (thanks again, Harbor Freight).
But there was still the problem of the Z axis. They’re surprisingly tricky to design, since they need to hold quite a bit of weight perfectly flat while slowly raising and lowering it. That would have to wait until later, when I had a better idea of how this whole thing was going to fit together.
I had made a nice skeleton, but how exactly was I going to keep the dangerous BURNINATING BLINDING PEW PEW PEW LASER inside?
Back when we opened our workshop, one of our members went nuts and bought a $4000 laser engraver for the space. He very graciously offered to let us all use it if we treated it well and replaced the consumables.
With sporadically heavy use over a year and a half, the poor thing has seen a variety of conditions from pretty okay to painfully broken. We’ve run into many of the same problems seen by other folks who have also learned that there’s really no such thing as a cheap laser cutter.
Our cutter started life as a Jinan Artsign JSM3060U. Sadly it didn’t even survive shipping, and needed a fair amount of debugging and back-and-forth with the vendor just to get it going the first time. Over the course of the last 18 months it has had two new tubes, a new HV power supply, fresh optics, a gantry overhaul, two chillers, two pumps, and extensive rewiring. We even built it its very own room. One of our members started calling it the Laser of Theseus. At this point it’s more like a project than a tool… But having access to a laser cutter is so tremendously useful that it still seems worth all of the cash and attention.
If you go down the path of discount Chinese laser cutters, be prepared to learn this lesson: in the quest for rock-bottom prices, margins get razor-thin so every possible corner has been cut. This includes design decisions that make no apparent sense at all, until you realize that it probably shaved a few cents off of the BOM. Fasteners that have no washers. Tension screws that have no fasteners (or even loc-tite). Wiring that is hopelessly too small gauge (probably copied from a design intended for 220V supplies.) Exposed live wiring. Gantry components that appear to have been hand-filed out of scrap metal and bent into approximately the roughest shape that could still be called “close enough”. And on and on.
One very useful upgrade we recently made was to replace two of the optics holders with actual, professional kinematic mounts. Before the upgrade, we used the stock mounts. These appear to have been designed (and possibly manufactured??!?) by five-year-olds. Here, see for yourself:
Loosen any one of the five rough screws, and the whole mount sags. That goop is loc-tite, which I added after a frustrating hour trying to align these things. It helped, a little. Maybe. Also, notice how the tiny mirror has a ridiculously large aluminum piece covering at least 1/8″ all the way around it? This is what cheap looks like.
Alignment time went from over an hour to about ten minutes. The larger mirrors are easy to find, and the fine pitch thumb screws make adjustments trivial. The whole mount is under spring tension, so after you set the position it will not vibrate out of alignment. Little upgrades like this may not seem worth it first, but after the second or third time you find yourself fighting the twisted bits of aluminum scrap that came with your bargain laser, you will wonder why you didn’t install real mounts in the first place.
In short, if you buy a cheap laser expect to spend time and money keeping it eeking along. Having now repaired or redesigned just about every component of this beast I think that my next laser project will likely be a fresh build from the ground-up. At least then I’ll know exactly which corners I cut, and why…