What physically makes up a CD you buy in a store?

The reason that I'm asking is that I just bought an Audio Desk CD lathe. The manual says that I should trim the bottom (or readable side )of the CD in such a way that it will look like this when I'm done:

Bottom
___
/
|___

Top

Well, looking at the edge, even with a magnifying glass, afterwards is darn near impossible.

I'm afraid that if I trim it too much, I will expose the foil inside and it will start to oxidize and eventually the CD will be useless.

I've been buying a lot of HDCD's, gold CD's, XRCD's, imports and remasters lately because they do sound better on my current system. I'd hate to have to replace them just because I mistakenly trimmed off too much.

A big problem is that the foil, or whatever it is, doesn't seem to be the same diameter for all of the CD's. Trying to adjust the Audio Desk for each CD would be a hit or miss proposition at best.

As I was trimming one of my R.E.M. CD's that didn't have a graphic on the top of it, I trimmed the top side by mistake instead of the bottom. I saw my error and trimmed the bottom, and that CD sounded better than any of the other ones.

I trimmed the top a few more and the sound on those improved too. But I'm still concerned with the possible overtrimming and the exposing of the foil.

Does anyone know anything about the makings of a CD and possible harm that can come from this trimming or overtrimming?