Well it was hard. And once the method was worked out, hard turned into slow.
This is the end result:
It all began with a commercial acrylic ice cube. They were beautifully finshed but a crappy shape in my opinion. So I went at it with the belt sander and reduced it to this shape.
Very rough and dirty. The next stage is to sculpt the vague surface features I wanted on the cube. It was to be nice and melty so the surface needed to undulate rather than change angle too quickly. I used a 120 grit sanding cylinder in the flexshaft:
Now that all six sides were sculpted morealess to the desired shape its time to reduce the surface texture. Its worth noting that the surface features even out alot in the following steps. So on to medium weight steel wool. This is a bummer as it must be done by hand. I ended up with a good method where I twisted it up into a rope and folded ot over. Giving me a dome shaped abrasive surface, good for getting into those hard to reach low points. You need to remove all evedence of the cutting marks of the sanding cylinder...its slow.
Then on to the fine steel wool. Same procedure here. The objective is to remove all the scratches of the medium steel wool. Again slow and labour intensive. Hand finishing like this really allows you to control where material is being removed. The high points will always be finished first and want to wear down before the lows will be smooth. It is nessasary to keep an eye on this and adjust where you are working to compensate:
On to the lapping, I hope I'm using the term correctly here. It is a really bad idea to let solvents anywhere near acrylic like this as it will craze and crack badly. Even from the vapour. So all lapping and polish mediums must be water based. If it smells don't use it.
Now you could go and buy some or...
use toothpaste and grit mixed together! I made a milky consistancy here of 400grit carborundum and the cheapest toothpaste I could find and water. It worked a treat!
I worked it with a felt mop. Its a bit messy if you don't watch the speed. Also dont let it dry out as you work it on the surface of the acrylic.
I was at the dental hygenist the other day and my mind wandered as the nice lady scraped below the gumline...and it hit me. Teeth are just like acrylic ice cubes!
So I spat out the blood and asked her how she managed to polish such difficult shapes as teeth?
She showed me this:
So the next polishing step is cheapo toothpaste with a cup shaped polishing mop. These have plastic bristles much like a toothbrush. But the clever thing is they tend to polish the outside edge of a dome rather than the tip. Just what I needed. At this point the acrylic is really becoming see-through:
The final step is flame polishing:
This is also a critical step. The acrylic will bubble very easily under a flame this hot. I litteraly pass the flame accross the surface in one smooth move and thats it. If you must go again, wait untill it is cool and go again. Donn't be tempted to go back over it again while it's hot, the corners will bubble and then you are back to square one!