Simon, It's a real pain to machine, saw, file, or scrape. The only operation I've done with it that goes nicely, is tapping, for some reason. Boring holes is the worst. You need carbide for sand casting machining.
Now I know that hard machining qualities goes against the experience of other people, and maybe the alloy variety I've been making up is different than the variety they use. I dunno. But I believe my ZA-12 is correct. I'm using purchased virgin zinc ingots, and mixing in the right amount and type of aluminum (see the recipe, somewhere way back in this thread.)
I was using purchased ZA-2 earlier in the lathe construction (ie. years ago!) -- not mixing that myself -- and it seems to me that it was better/easier to work with than ZA-12. But I can't remember for sure.
The other negative thing is that it goes dull with age. I don't find that such a problem, as I intended to paint the lathe all along -- steel and iron rust, especially in the winter environment here, unless constantly used and oiled. Condensation does it. So it's just a different but similar problem.
And the ZA-12, at least, has huge shrinkage, when casting in sand. You really have to plan for that. It'll suck a hole right down the full length of a sprue. I'm thinking ZA-2 didn't do that as much. I'm going to have to try to do a comparison -- I think I have one ingot of ZA-2 left.
On the positive side, it is tremendously strong, solid, heavy, melts at the lowest temperature of the common structural metals, it's inexpensive, easy to cast (other then the shrinkage problem). I imagine it has excellent vibration damping qualities. It's very dense, but somehow, I dunno, "fluid" seeming
Oh also it's an excellent bearing material, unlike aluminum, or steel. So making lathe slides out of it is a really good choice, in my opinion. And the headstock feels massive and sturdy. My lathe is hard for me to even slide on the bench now, it's so heavy. The opposite of aluminum.
Well those are my experiences, anyway.