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June 17, 2019 – CNC Maker Zone

Making vibration dampening CNC feet using flexible TPU 3D printing filament

There’s no getting away from it: routing with a CNC machine can cause lots of vibrations to travel into the work surface below, causing quite a bit of annoying noise too. To combat that I separate the machine from the work surface using 25mm thick firm sponge sheet: the blue non-flammable foam sold for furniture. It does an excellent job of isolating machine vibrations, but is really overkill for a small machine. So I decided to play around designing some flexible feet to fit onto the 2020-section aluminium frame that would do a similar job of preventing machine vibrations being transmitted to the hard work surface.

Obviously the best way of doing that would be to use a soft material that will wobble around: not enough though to flop down, and not so little that the rigidity transmits vibrations through it. As I’d recently bought some flexible TPU filament for my 3D printer, which is quite elastic, it seemed a good choice for making a squidgy wobbly thing (other flexible filaments may work well too). Designing a dampening system is obviously a rather complicated thing to do, probably best done by professionals using finite-element modelling and things like that. But that’s no reason not to have a go anyway!

For a DIY approach building a damper that flexes with the frequency of the vibrations, and preferably does that inefficiently, can be a good starting point. That way some of the job is also done simply because some vibrations will be cancelled out by others that are out of phase with them. So I went through a few iterations of small dampers so that I could squash them and get an idea of how rigid or squishy each design would be. It turns out that making squishy models at small scale using TPU isn’t as easy as you might expect: even with 10% infill density shells, creases and corners all increase rigidity too much. In the end I came up with the design below in OpenSCAD.

The OpenSCAD 3D model of a prototype vibration dampening foot.
The OpenSCAD 3D model of a prototype vibration dampening foot.

The design includes a flexible adapter at the top to squeeze into the slot in a standard 2020-section aluminium extrusion, as used on many cheap CNC machines. It’s the best design I’ve managed so far for a TPU damper just 20mm wide, and was designed to be printed without support material. It’s not floppy even though it has only 10% infill density, but has enough ‘squidginess’ to flex slightly under the weight of the CNC machine using a damper at each of the back corners and one under the middle front of the frame. If you want to have a go at printing your own, you can get all the files, including the STL file for 3D printing, by visiting the Thingiverse page.

The good thing is that this prototype allows vibrations to set up flexing in a variety of directions that can help prevent them being transmitted straight through to the surface below. Of course, I realise this is a work in progress, but the design does seem a useful starting point for some more experiments: perhaps tweaking the infill density and dimensions to start with so as to fine-tune flexibility while retaining the ability to support the CNC machine without bouncing around. And as a final note the design was easy to fit to a 2020-extrusion even though the slot ends were closed off: I’ve put a photo below to show what a finished and fitted damper looks like 🙂

A 3D printed TPU foot fitted onto the CNC machines frame.
A 3D printed TPU foot fitted onto the CNC machines frame.
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3D printing with wood-infused PLA filament

Working with real wood is always exciting, but not everyone has a CNC machine for cutting and carving. And given the limitations of small desktop CNC machines, generally with only three axes, there are many times we can find ourselves wanting to make something with wood that isn’t going to be possible. So, for times just like that, I was excited to go and buy some wood-infused 1.75mm PLA plastic filament to use with my 3D printer. Wood-infused filament promises to allow 3D printing of complicated shapes while ending up with a part that looks and feels like wood.

For anyone who hasn’t heard of it, wood-infused PLA is a mixture of normal PLA combined with finely-ground wood powder. Ratios vary but generally around two-thirds PLA to one-third wood. Colours also vary, but mostly you should be able to get at least light and dark varieties, sometimes even ones limited to a specific species. The version I bought was a light-wood shade no-brand Chinese filament I picked up on eBay for fifteen UK pounds for a kilogram reel, so quite inexpensive. It was described as PLA infused with recycled wood powder, making the material doubly environmentally friendly.

To test the filament I needed a 3D model to print that would be virtually impossible to produce with my 3-axis CNC machine. I decided on a 3D scan I did of an eagle carving I bought in the Philippines in the 1990s, which I scanned with an XYZPrinting hand scanner v1.0A. In case you’d like to try printing it yourself I’ve uploaded the STL file to Thingiverse (click here to go there). The filament instructions stated a temperature of 180-200°C so I was able to slice the model in Cura and produce G-Code using the same settings I use for generic PLA. The results, before and after removing support material, can be seen in the photo below.

The eagle carving 3D printed with and without support material
The eagle carving 3D printed before and after removing support material.

Probably the first thing you’ll have noticed is that the print is quite ‘stringy’ where the nozzle hops between different areas. Probably that means I should tweak the retraction settings when slicing, but actually (compared to generic PLA and especially TPU anyway) it was very easy to clean off using just a small metal nail file. In fact, this was the thing that struck me most when cleaning up the print: the wood-infused plastic is very easy to sand and file to a nice finish. If you’re used to hours of effort filing, sanding and priming PLA prints to get them smooth you’ll likely be overjoyed to sand wood-infused filament! However, I should caveat that by mentioning that this filament was also more brittle than ‘normal’ PLA: in fact one of the prints cracked across the legs (where infill, set at 25%, was probably too low to give enough strength) during sanding, so care in slicing and finishing is required.

As for the print being faux-wood, well it does have the look of a very fine-grained wood. And, after sanding the surface does feel a lot like you’re holding a soft wood. The layer-lines in the print do add a little of the look of wood, but for the eagle carving I think lots of parallel lines throughout the whole height is a giveaway that this isn’t ‘normal’ wood. But if you’re not inspecting it closely the print could blend in with other wooden items on a shelf. So I wondered, would using a wood-infused filament with an oiled or stained surface add to the effect. To find out I tried oiling an eagle using natural-colour Danish Oil and another with a Brown Mahogany water-based stain, as you can see in the photo below.

An eagle carving after finishing with Danish Oil (left) and water-based stain (right).
An eagle carving after finishing with Danish Oil (left) and after applying water-based wood stain (right).

Compared to my other tests using Danish Oil, I think the results were a little subdued. Because the wood particles are suspended in a larger amount of plastic I suppose I shouldn’t expect the oil to soak in very much or have much effect on colour. And the oil did seal the surface and give a little of the effect of an oiled wood. It even felt a little like handling wood afterwards too. The stain was also quite effective and looks quite good from a distance, but close up you can see the layer-lines where the stain has soaked in more. And the stain was difficult to apply evenly as it just runs off the surface, so some time in the future I’ll try this with an oil based stain too for comparison.

So, after the above do I think wood-infused PLA is a useful alternative to using real wood? Well, in retrospect I was a little disappointed, but only because the prints lack the look of wood grain and, because of that, lack the ability to be finished using normal woodworking techniques intended to bring out the grain. That’s more a reflection on my expectations though, rather than on the material itself. In fact, for making complex objects the filament did a good job of making the best of a very difficult job I wouldn’t be able to do any other way. It was very easy to clean-up and sand the surface to a smooth finish, and without looking closely it gives a nice ‘wood-esque’ effect. So, yes, I think wood-infused PLA is a very useful tool for some interesting projects, either on its own or as a part of larger wood-plastic composite objects 🙂

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