Scottie Builds things some times. Eric the destructor - first beetleweight robot

[Scottie]

Surprise time! I was waiting to post about it when my robot was fully operational, but i think i just do it now anyway. For me, part of making a beetle weight robot is to learn something new. I can safely say, interfacing bldc motors with gear boxes not designed for them, working with hdpe and assembling a weapon system that was not just a motor and a 20g blade has been a pretty valid experience! But i wanted something more, what could i do? Tbh there is only so many ways you can make a vertical spinner, but there are loads of ways you can make it look different aseptically. So i though perhaps i could anodise the metal bits. And that is what i am doing.

I have two types of metal on my robot, Titanium (front armour plate) and Aluminium (blade and internal structure). Both can be anodised. This post will just be about anodising Titanium as they both require slightly different processes, and i haven't started anodising my aluminium parts yet.

Preparation.

Before i anodised the titanium I wanted to polish them. I bought the plate from ebay and it had a rough finish on the surface, which was pretty ugly. Whatever surface finish you have will also be present post anodising. So i bought a buffing kit off ebay. This had aluminium and steel buffing compounds, 3 types of buffing discs for drills and some vienna lime. I couldn't find buffing compounds optimised for titanium so i just used the steel buffing compounds. buffing is pretty simple so wont document how i did it (I didnt actually document it in the first place DOH!), lots of tutorials on youtube though. Here is a side by side comparison of before buffing (below) and after (top).
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The basics

By using a weak acid electrolyte and passing a DC voltage through it via a titanium anode and aluminium cathode, An oxide layer forms on the anode. This oxide layer on the titanium is transparent, allowing light to pass through it and reflect off of the titanium surface. Some light also reflects off the top of the oxide layer. These two reflections can add constructively to make a certain colour to be more visible, making the surface of the titanium appear coloured. By varying the oxide layer thickness different colours can be produced. This is achieved by varying the DC voltage. Typical voltages are anywhere between 12V-120V. Caveat, you can only increase the voltage to produce different colours as a greater voltage creates a thicker oxide layer.

Anodising equipment

Weak acid - I used distilled vinegar for pickling. (You can use coke for this)
A plastic tub big enough for your part to be anodised and the cathode to be submerged in
some titanium wire
A DC supply (I required up to 74V)
Alumium cathode - I used some take away lasagna trays
crocodile clips and power cables
Volt meter
And importantly your titanium part, which is your anode

My requirements

I would like to have my plates coloured purple and to produce this I could use to voltages ranges, 18V or 70-74V. At 18V you get a sort of dark purple, it looked kind of washed out to me and i wanted something more vibrant. So I decided to use 74V.

my setup
I looped some titanium wire around some screw holes in my titanium armour plate. I connected the titanium wire to the positive of the voltage supply via some crocodile clip terminated wire. I connected the aluminium cathode to the negative of the supply.
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Problems

My initial setup used a 64V DC supply in conjunction with a 1Ah 11.1V Lipo battery. I didn't think about how long this would take and the process draws around 500mA initially due to the size of my plates. Yeah my battery went flat in 30 minutes. So I ended up with this really weird colour that I could never replicate, let alone understand the colours that it produced (below).
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This is was rather frustrating as you cannot re anodise it without removing the oxide layer, which I did by rebuffing the surface. I solved the voltage issue by using a variable DC-DC boost converter that could output up to 80V from a 12V supply.

Processing and end results

It took about an hour to fully anodise the surface of each plate to the colour I wanted. Initially I started at a low voltage and then turned the power off around 20V. Here is what it looked like half way through the process.
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I then increased the voltage to 74V and let the process complete. Here are some side by side comparisons from before and after anodising with pre anodising plates in rough and shiny finishes.
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and here is the end result.
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The plate on the left is the plate i had to re anodise. Its slightly greenish on the bottom, probably due to me leaving some oxide layer from the previous attempt. It also looks slightly duller due to being buffed twice and thus is more reflective.