Tuesday, June 26, 2012

Copper Bellows. A lesson for me.

I'm trying to eliminate leaks in my(still not working) laser so I decided to go for copper bellows instead of my current O-ring seal for mirror adjustment.
But I couldn't find any copper bellows, so I decided to give it a shot and see if I could make a pair.
After first considering soldering them up from parts, I decided to electroform.

Here is a picture of my wax mandrel. It is made from injection moulding wax. It turns ok if you don't try to take too much off in one go.
I then cut the recesses or corrugations or what ever you would call it. With the piece still spinning, I hit it with a touch of a gas flame to soften the corners and give me a nice smooth finish.


I parted off and heated a bit of copper wire and stuck it in to the end. Then I was ready for conductive coating.
There are a few options here. Sputter, chemical silver, electroless copper, conductive paint, graphite.
I decided to go with graphite because it was the simplest and I had it at hand.

 So I dipped the wax in "white spirit" for 20 seconds and drained it. Then the surface of the wax was slightly sticky.
Then I bunged it in graphite powder. I made sure it was all covered before the white spirit flashed off.
After a minute I brushed the excess powder off with a makeup brush.
A very important note here is that burnished graphite powder is significantly more conductive than non burnished.
Just rubbing with the bristles of an artist paint brush is enough to burnish it.
I need to reach into the interior ring to burnish the graphite there before it would plate properly.

 Here it is beginning to get its initial copper coating. Nice!

I electroformed for 8 hours giving me 8*24microns so 0.2mm in theory!
 Measuring the bare wire versus the electroformed part confirmed the thickness.
 I drilled and melted out the wax.

I then very carefully annealed the tube, in the process evaporating the remaining carbon. I pickled
and ultrasoniced the remaining crud off.

And then disaster!! The inner rings were so thin that they cracked in the ultrasonic. They came out with some small holes and
crack lines.
I had always planned on re electroforming inside the tube as I thought they would be a little thinner. But they are significantly thinner and quite brittle.

So I placed a copper pipe electrode into my copper plating solution and began plating from the inside.
After another 4 hours...
 I ran the ends on the belt sander to clean them up...
 And tin/lead soldered it together. Ta da, one flexible mirror mount!

In retrospect I would do this differently. I had a huge problem with the shape of the mandrel causing different plating thickness
I bet anyone with plating experience could have seen that coming. So for the next electroformed copper bellows I will follow these guides:
  • Interior corrugations must be wider than they are deep.
  • Returns must be angled to face the electrode for copper plating reasons.