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Sandpaper, as well as sanding blocks and detailing sponges, are essential for all CNC projects. They let us remove flaws, smooth surfaces and prepare materials for finishing with a variety of coatings. For many projects though, especially the small ones, they can be too big and bulky and can indiscriminately remove small, often fragile, details, as well as changing shaped edges away from what we so carefully designed.
So, for newcomers to CNC work, I thought I’d add this short post to quickly mention my experiences with spring-loaded detail/finger sanders. Mine is shown in the photo above and it’s simply a plastic finger with a continuous band of replaceable sandpaper around the outside. They come in many sizes, the one here being 20mm wide, although 10mm and 30mm ones are common. They also cost just a few pounds: try searching for something like ‘finger sander spring loaded’ on eBay or Amazon to see what I mean.
One of the most exciting things about finger sanders is the variety of sanding points. To start with there’s a long flat zone at the bottom, allowing sanding of large areas and straightening of cut edges. There’s also a narrow area at the front which allows for getting the sandpaper into tight places and around curves. Plus the rest of it has compound curves that can be very useful too. And when the paper starts to get worn where you need it, just push the pointy front end inward to slacken the sandpaper so you can rotate it around the edge. Personally I find them very useful as you can see in my photo of a Darth Vader routing project below.
Recently I bought some 1.5mm thick Walnut sheet as I wanted to try it out with my CNC router and see how good the results would be. Walnut is quite a dark wood with a nice dense grain, so it seemed like an excellent opportunity to take my CNC intergallactic by making a small inlayed Darth Vader from Star Wars. Hopefully George Lucas will approve if he happens by, so below is my completed Darth Vader head (at around 10% water content, in case you’d like to know).
I was quite pleased with the results, the details being routed to 0.75 mm deep in two passes, and the edge being cut in three passes, using the 1.2mm diameter end-cutting bit shown above (which gave finished grooves almost 2 mm wide). As the image at the top of this page shows, the Walnut sheet cut quite well and cleanly with the spindle at 1000 rpm and a feed rate of 50 mm/min to minimise potential breakage of the bit. The above photo is then after I’d given it some sanding with a fine grit paper, which took little work. Even sanding out the recesses with folded over sandpaper was quite easy, with a little patience. So overall I’m looking forward to some more small projects using this lovely wood.
CNC routers are wonderful machines for cutting and shaping wood and other materials but sometimes there’s no choice but to use a router without computer control. For example, when trimming the edges of wood to size, or for using the router as a finisher with sanding drums. When that kind of work is needed I don’t want another machine: rather I want to use my CNC router for occasional manual feeding. To do that a fence is very useful, as it allows pieces of feedstock to be pushed past the tool/sander in a straight line. Not only can that help ensure edges are kept parallel, but it also allows creation of small mouldings with edge-shaping bits.
This 3D printing project is my attempt at creating a very simple fence system for my cheap Chinese CNC. The fence part simply slides into the groove along the top of the CNC bed, after light sanding to get a good fit that doesn’t wobble. However, for manual feeding we don’t want the fence to be able to move along the bed groove, so a part is included for a simple stop that can be used to prevent that happening. The stop connects to the side of the bed using a winged hold-down nut as shown in the photo below. You can download the 3D printing files by clicking here, including the OpenSCAD file for any adjustments you’d like to make.
Hopefully you won’t mind if I finish with a word of caution. But, using a CNC router for manual feeding of materials brings risks you may not be experienced with. Probably two of the main ones are the dangers of having hands near your tool/cutter (which can be avoided using a proper push/feed stick) and extra dangers of pieces of tools and feedstock breaking and flying away (which can be largely avoided by feeding slowly and wearing sensible clothes and eye protection).
Another rookie error is to feed material with the rotation of the tool, which means it could unexpectedly get pulled through and shoot out like a javelin (I did that on a router table years ago and it’s amazing how fast the expelled material can be). Not only is that dangerous: it can even ruin your work. You may also want to wear ear protection as the sound level will be higher working close to the motor and tool. Basically, please be very careful and don’t take chances using the 3D models provided here 🙂
The water content of wood we use in CNC work is obviously important for many reasons. One is that the quality of router cuts, and the depth of LASER cuts, will be adversely affected if the wood is wet. Also, and maybe more important, is that wood will change length as the water content changes, and as different woods shrink and expand at different rates that can lead to a nicely finished project coming apart over time (or even a lovely finish cracking). And it can have some bad effects on some finishes causing slow drying or cloudy clear-coats. For those reasons I decided to invest in a moisture-content meter so I can get an idea of how dry my wood is before using it, as well as to let me monitor changes as the wood acclimatises to indoor life before I cut it.
The one I got was only a few pounds on eBay so I’m not expecting really accurate readings, but testing some thin wood sheets at home I’ve been getting sensible readings between around 7% and 12% so hopefully it’ll be useful. Using it is very easy once set to the wood mode: the two metal pins are pushed into a non-vital area (end-grain is a good place too) like in the photo below and, after a moment, the reading appears on the small LCD display. Personally I’m quite happy with it and with a little luck it’ll provide me with a good idea whether my wood is too damp to want to use and let me monitor it to make sure I know when it’s ready for a project.
CNC machines have plenty of moving parts and if we want our routing and LASER cutting to be as good as possible the movements need to be as smooth and unrestricted as possible. When I built my cheap Chinese CNC it came with rigid shaft couplers for connecting the threaded lead screws to the stepper motors. That meant that the motor spindle and lead screw couldn’t be adjusted to be perfectly in-line, just because of tolerances in parts and mountings. So, for part of a full rotation the lead-screw was binding (i.e. becoming hard to turn) and the rest of the turn was fine.
If you find you get the same problem, you could just do what I did: change the rigid couplers for slightly flexible ones. In the photo below I’ve shown the difference between the two. The bottom one is the original rigid one and changing to the new one involved no more than loosening the grub-screws, moving the lead-screw to the side, then inserting the new coupler, pushing in the end of the lead-screw, and tightening the grub-screws. For just a few minutes work it made X and Y axis motions so much smoother. And, for only a few pounds/dollars per coupler it was a cheap job too.
I tend to prefer to minimise safety risks when I’m using my CNC, and that includes making sure I don’t injure myself touching fast-rotating cutting bits. Obviously it’s unlikely that the motor will start up when I’m removing or attaching a bit, although it’s a little more possible when the G-Code file includes pauses for tool changing. But, to be completely sure nothing can go wrong, I made the 3D printed switch box below to allow me to isolate the spindle motor from its electrical supply. It prints as two parts which need to be glued together to fit the top of a 20mm extrusion, like in the photo above. The hole in the front is made to house a standard 18x11mm rocker switch. Click here to go to Thingiverse to get the STL files for your 3D printer.
Once the two parts of the box are stuck together, with the switch inserted, the positive wire from the spindle motor controller to the motor itself, which should be red, needs to be cut. The position where you do that needs to be planned so that the wires can be run to the box and the full movement of the motor along the CNC’s x-axis isn’t compromised. Personally I found the best place is near the controller board as on my CNC the red wire ran straight past the box position. You can use standard spade connectors crimped onto the ends of the wire, after stripping some insulation from the end. Below is a photo of the back of my switch box so you can see what I mean.
20mm square extruded aluminium section is commonly used for cheap CNC routers, especially for kit versions. So I designed this very simple 3D printed part that clips over three edges of the section. I did that for two reasons, the first being that I wanted to use one with two of my frame-end feet 3D prints (click here to read more) to make a stable tri-point support to reduce vibrations when routing. Secondly though, it’s just a useful thing to have to glue to anything I want to fix onto the frame while still being able to remove it later (e.g. bit holders or cable tie down pads). If you’d like to 3D print your own, just click here to get the STL files on Thingiverse.
Mahogany is a wonderful wood with fine, beautiful looking, grain. It’s ideal for any CNC routing project where you want to give your work a professional, and high-quality, look. So I thought it a good idea to write a short article about my experience of working with it on my cheap Chinese 1610 CNC machine. I decided I’d use a small (c. 50mm or 2″ long) fish design because it includes some shallow work for inlay, as well as a full-depth cut around the outside. The sheet was cut from a 1.5mm thick sustainably sourced plank and, according to my meter, had a water content around 9%. Below is a photo of the wood on the CNC during cutting.
As this was a small piece I chose to use a 1mm end-milling bit, so that the shallow details would look tidy. Bits/tools of that size are easily broken through overly deep or fast cutting. For that reason I cut in two passes for the shallow details and three for the outside cut-through, which equates to between 0.5mm and 0.6mm depth per pass. Also, I kept the feed rate down at 50 mm/min which, although a little slow to watch, usually keeps my small diameter bits in one piece. The spindle speed was the maximum of 1000 rpm which, together with the low feed rate, provides a nice clean cut in my experience.
As you can see in the photo below, after just a light sanding, including using a folded piece of sandpaper to run along the shallow grooves, the fish looked quite nice. The only issue I’ve found, if it can be called an ‘issue’, is that the cutting tolerance around the bit is a little more than I’d like: with a 1mm diameter bit I ended up with 1.5mm wide grooves. That’s a 50% overcut which, given the simplicity of the spindle system, and the amount of vibration, is probably technically not bad for a small tool, although it does cause some difficulties cutting thin grooves for things like hammered wire inlaying. Overall though, my experience is that Mahogany can give inspiring and impressive results.