Tales from the Lo̊o̊p

Meanwhile distracting myself from the lack of components due to the world pandemic, I started working on a novel approach to cable driven joints for robots.

One thing in particular is that it requires a continuous loop of steel cable (no fancy dineema string avaliable for tests either), but achieving that is no easy task. The device can’t handle knots in the cable because it loops over itself to allow more grip in the pulley. Given those requirements, and current material avaliability, I decided to try butt welding the ends of the 0,45mm cable I have around.

To do that, I definitely needed more hands than the ones that mother nature decided I should have. So, I designed a support that would hold the cable, bottom copper anvil for spot welding and would support it under a digital microscope to be able to see what the hell I’m doing, this isn’t something you do without magnification.

None of the features that make it mass efficient/lighter where added other than to make it pretty, because, why not?.
Printed it at 0,1mm layer, 0,4mm nozzle, 100% infill.

Cables in the photo are NOT soldered yet, just as a measure of scale and visual guide.

Detail from the holder:

Now. How am I supposeed to weld this, actually?
This is not a huge amount of mass to be welded, so the amount of energy is small compared to usual spot welder requirements. For a trial, I used my bench power supply, set at 20V and 3.5A (max current), and the discharge it can produce before the CC kicks in. Didn’t even bother into shaping the bottom anvil, so it would embrace the cables together. As top anvil I just bent a sheet of copper to use the small radius as contact point. Also added a smidge of soldering flux paste, because it couldn’t hurt, I guess.

I got totally surprised by this:

It’s not the best weld in the world, and after inspection, a single wire that had bent, was totally not soldered, but as first tests go, it was surprisingly good.

So, after all this, how was the weld? I’m not afraid to say fine*. Quickly tied the ends of the test in a loop and pulled with my fingers until my skin started to hurt a little(no force gauge avaliable, sorry), and this thing didn’t break.
Also rolled it on 10mm pulley bearings (9mm diameter bend) AND looped it over itself on a bigger pulley while mantaining tension. It struggled slightly on the underpass, but didn’t break either. Definitely good enough for tests on my new robot joint Drive.

Apart from that, the plastic covering the weld should be taken care off, either by adding a drop of resin in there, so it sits flush against everything else, or, the best solution, knowing the plastic, making a small injection mold to reform the coating around the weld.

BUT…

One step at a time, for now, I can practice more, and when the welds are nice, worry about other details.

*Of course not an ideal weld, but come on, I’m improvising here. Now all this requires is practice and testing for a while, but I’m sure I’ll get proper cable welds in no time.

Until the next time, techno-cowboys!

————— UPDATE —————

After a bit of practice, I am starting to get “full strenght” welds. They’re not pretty, but I can’t break them, my skin hurts too much with the force applied: