Review: LinkSprite Mini CNC

It's a great time to be a hobbyist. No matter how you feel about the Arduino/Raspberry Pi effect, the influx of general enthusiasm and demand it has created translates to better availability of components, a broader community, and loads of freely available knowledge. When people have access to knowledge and ideas, great things can happen. Tools that were once restricted to industrial use become open source, and the price of entry-level versions goes into a nosedive.

As we’ve seen over the last several years, the price of cheap 3D printers keeps falling while the bar of quality keeps rising. It's happening with laser cutters and carving tools, too. Strolling through Microcenter a few weeks ago, I spotted a new toy on the back wall next to the 3D printers. It was LinkSprite's desktop mini CNC. They didn't have one out on display, but there were two of them in boxes on the shelf. And boy, those boxes were small. Laughably small. I wondered, could this adorable machine really be any good? To some, the $200 price tag suggests otherwise. To me, the price tag made it justifiable, especially considering that the next price point for a hobby CNC mill is at least twice as much. I took my phone out and stood there frantically looking for reviews, documentation, anything that was available. It seemed that the general, if sparse consensus is that this thing isn't a total waste of money. Oh, and there's a wiki.

According to LinkSprite's wiki, this little machine will engrave wood, plastic, acrylic, PVC, and PCBs. It will specifically not engrave metal (PCB copper notwithstanding). I’m a bit leery of the chemicals used in the PCB etching process, so the idea of engraving them instead was especially tempting. I pulled the trigger.

Financially speaking, I’m a hobbyist who's trying to build up a better home ‘space for my own use. If I’m spending much more than, say, $200 on a tool or a toy, there had better be a pretty good reason for it. I’m not designing and selling kits, but by investing in a tool like this, I’ve made that kind of goal much easier to attain. Right now I just want to take a bit of a chance, have some fun, learn a lot, and hopefully get good use out of this robot without getting too frustrated or having to fiddle with it constantly.

Experientially speaking, I’ve never used a CNC mill before. But I do have experience with 3D printing and design. While these aren't interchangeable (and are in fact technically inverses), the concept of moving in 3D space applies to both, so my experience helps me justify this purchase to myself in terms of what I imagine the learning curve will be like.

Once home, it was time for the unboxing. I don't know what I was expecting, but it's obvious that LinkSprite cares about this product. It was well-packed inside a custom Styrofoam shell and a thick cardboard box. I figure if they’re going to ship it like that, it will be in good shape when I get it even if it's thrown around a bit in transit and/or by store employees.

My first impression was that it just looks cool. It's minimalist and utilitarian, and has a tiny footprint. If I ever wanted to take it somewhere, it would easily fit inside a milk crate. This thing has what it needs and nothing more. The chassis is about 80% extruded aluminium and the rest is basically injection-molded plastic braces, cast aluminium connectors, and a handful of t-slot hardware. Functionally, there are three motors for Cartesian movement along with the requisite rods and rails, a spindle motor for carving, a control board, and some cable ties. Because it's mostly aluminium, it only weighs 12 pounds.

It comes with two 3mm engraving bits, both of which are half-round and pointy and seem to be good for carving acrylic and decent for softer stuff like craft plywood. Since I want to engrave PCBs, I’ll probably buy drill bits for through-hole designs and a couple of flat end mills for different trace widths.

There isn't much to do once it's out of the box. Along with the power brick, it comes with a USB mini cable, two chucks, two bits, clamping hardware, and five Allen wrenches. The one I got comes completely assembled. All I had to do was chuck one of the bits and slap the heat sinks on the motor driver chips. There's a kit version out there too, which looks to be about $50 cheaper and explains all the Allen wrenches. If you get the kit version, here's a slick build video to go with it.

The coolest/scariest thing is that LinkSprite sells a laser head add-on for $39. I haven't gone looking for details about laser-safe honeycombed bed alternatives, but that is one of the first things I will secure before trying the pew-pew-pew. That, and a suitable enclosure.

I have only one concern with this machine so far. Maybe it's just this lone unit, or maybe it's this dry Midwestern winter, but there's a spark at the board when I plug the barrel connector in. This has happened almost every time I’ve used it.

As far as 3D conceptualization goes, I have the most experience designing with OpenSCAD and using Cura to print on a Lulzbot Mini. If you have experience with 3D printing, then this particular machine should not, in my opinion, present a steep learning curve as far as getting it to do anything at all. Getting it to do exactly what you want, though, that's going to take some trial and error.

So far with the LinkSprite I’ve used Inkscape to import pictures and create text for carving both 3mm craft plywood and a similarly thick piece of acrylic. I decided to skip "Hello, world!" and went straight for the Jolly Wrencher. The holidays were fast approaching, which was perfect timing for a little show and tell. I made the "Happy Holidays" carving to show the family and because I wanted to see it carve in Edwardian ITC.

In the future I’ll be expanding my horizons by trying different designs and substrates in my ultimate quest to engrave my own PCBs. Software-wise, I already have my eye on FlatCAM when the time comes for that.

The LinkSprite ultimately needs a list of g-code and a specified step depth. Inkscape has native extensions that generate g-code for you and let you set up the bit diameter, feed, and depth step. A tutorial on this process is beyond the scope of this article, but if you’re interested in a future article devoted to that, let me know in the comments.

Inkscape is capable of sending g-code directly to CNC machines, but for now I’ll be sticking with Candle. This is the program the booklet pointed me towards that has since been renamed from ‘grblController’. Candle is like Cura in that it provides a slick interface for manually controlling the XYZ position of the tool head and sending g-code to the machine. The major difference is that Cura also slices. For all I know, Candle may slice, too, but I’ve been using Inkscape for that. To continue the 3D printing analogy, Inkscape also handles setting the step depth, which is the subtractive process’ answer to layer height.

Before homing the tool head it's time for the most important step: clamping down the work piece. The machine comes with enough t-slot hardware and washers to make four clamps, but for everything I’ve done so far, I’ve used two of these in the back and a pair of large binder clips in the front. In the kit video I linked to above, the guy uses flat corner braces made for fortifying joints. Certain Erector set pieces I have lying around look promising, too.

The LinkSprite doesn't automatically home the tool head, so this must be done manually with Candle. That's easy enough, just move the bed in steps to the start point and zero the XY. Then, do the same thing for the tool head and zero the Z in relation to the thickness of the material.

Once the g-code is ready, the work piece is clamped, and the tool head is at (0,0,0), it's time to carve. This is as easy as starting the spindle motor and hitting send in Candle.

Candle shows, among other things, a carve time estimate. Everything I’ve done has taken a little bit longer than anticipated. The estimate for Happy Holidays was 34:20, but it really took 39:17. C’est la vie.

My favorite thing about Candle is that it lets you follow along visually—there's a wire-frame 3D representation of the tool head superimposed over the model of whatever design is being engraved. While it's engraving, the virtual tool head on the screen traces every step of the actual tool head in real-time and you can zoom way in if you want and watch it do each layer.

Clamp the work piece well. Yes, it's a tiny machine, but it's still applying pressure with a sharp tool at many thousands of RPM. If a poorly secured substrate suddenly goes spinning across the room, the bit is likely to hit the bed and break off and do unimaginable damage. (This is not so much a learned lesson as it is a preconceived precaution.)

Don't forget to zero everything. Unlike a 3D-printer and some other hobby CNC mills, this machine doesn't do any homing by itself. It must be done manually through the grbl controller.

Start the spindle first. It's easy to miss this warning in the booklet and assume that the spindle will start spinning as part of the carving process. It doesn't. You have to start the spindle motor and then send the g-code, or else the bit will stab and drag and probably break.

Buy more bits ASAP. Especially if you dull one by not starting the spindle motor first. You’re going to need more eventually, anyway.

Check the design dimensions. Will that Jolly Wrencher be 5cm across or 5mm? Likewise, if you’re planning to engrave words, check your font size so that it doesn't end up really, really small.

Based on my experience so far, I’m really happy with this purchase. This would be a great buy even at twice the price. It's a good, entry-level machine that's relatively easy to use, more so if you have experience with 3D design and printing. If you want to spend a little and learn a lot, this machine can be your cheap education in the subtractive arts.

My ultimate goal is to engrave my own PCBs. I’ll be studying the guide that [Adil Malik's] PCB milling guide which we published a few weeks ago before giving this a try for myself. I’d also like to try milling a bunch of materials, including some non-obvious ones just to see what happens. Here is the shortlist: corrugated cardboard, a glued stack of chipboard (cereal box cardboard), a linoleum carving block, and one of those flat little hotel room soaps. Will you join me?