There’s this fantastic device we have at home that I’m fairly sure is for giving back rubs, or maybe massages in general, like on your legs or something. I’ve tried it a few times but mostly the kids play with it. However it bothers me that there’s holes in it where the little knobs are supposed to be, so it doesn’t roll properly. I think the kids pulled them out, but maybe they were just never glued in properly.

Anyway, I doubt I could go in a shop and say “please give me tiny wooden knobs to this size” so I decided to attempt to print some. I’ve done some basic 3D modelling work before so with a bit of work and the right measurements, this should be doable, and it’s an interesting shape.

Here’s a hand-drawn sketch I did quickly after measuring the wooden knob with a pair of callipers. It immediately became apparent that this was not going to be as simple as I thought because shaft part isn’t straight, but more like a chopped-off cone, and the head part isn’t round on every axis, it’s an ellipsoid. I Googled that word ? just to write it in this blog post, it’s the 3D version of an oval.

The measurements include oddly precise values like 3.03mm and although the printer has sub-millimetre precision, this is a bit too precise, but I decided to roll with it anyway.

The software I used for the 3D modelling was OpenSCAD, which I would describe as CAD software for programmers. You declare shapes and their attributes, like position and size in different dimensions, and then click a button to render a preview of your code in 3D space. I can use this to digitise my hand-drawn plan into a 3D model which I can slice into printer instruction code and print.

I’ll upload the SCAD source file and the exported STL model to the thingiverse website soon.

Here you can see the result of the first print. It came out well with normal settings and it hasn’t got any issues despite its small size. The only problem is it’s a bit smaller than it was supposed to be, so it falls out of the hole if you put it in. Luckily its easy to scale it up a bit and reprint it, and the second time round it was right. After printing a few more, all that was left was to glue them in place and the back massager is ready for use again! Below is a picture of the final result.

I’ve printed lots of things before using existing designs from the internet, but this was my first time using CAD software, and I learnt a lot about that. Even if OpenSCAD might not be what engineers are used to from CAD software, as a programmer I found it intuitive and I will probably use it in future projects too.