david weaver
I haven't ever measured / checked the flat surface of my radius wheel for flatness. Since I only have the 80 grit, maybe that tempers the following opinion, but here's what I think - any amount of hollow is so minute that it really doesn't matter.

I think the reason that it's hard to get something flat on the wheel from side to side when you're grinding something long and flat is that you're literally trying to place the center of a wide blade or chisel (which probably is flat to within a thousandth or two) on a surface that's moving. Hitting that surface on one side or another is much easier.

I have actually gotten comments from people on youtube videos (I don't know if I have a grinding video up) because they thought I was going to wear out my cbn wheel by biasing toward the left. It's simply easier for me to remove just the amount of metal in the hollow that way and not worry about placing something flat on the grinder. It's the hollow I'm removing, not the squareness or straightness of the tool that I'm establishing.

I wouldn't change the wheel I have - it's just dandy. One of the cornered wheels like Roger is talking about might be nice for toolmaking (but the reality is, I can file all of those things that need that brash corner). I've I've said the center of my wheel is out of flat (it's possible that I said that), I can't remember checking it, but I also never bother about it.

I have placed chisels on the rest before and grind the entire chisel without moving it - no harm is done by that, either, and whatever out of flatness there may be, I can't perceive any harm once I take those chisels to the stones.

You can ultimately make heat with these wheels if you really want to. I can't remember the heat transmission properties of CBN vs. Alumina - someone can look that up. Perhaps it's 80% CBN and 40% alumina or something. It's very easily detected, plus the CBN is miles sharper than a broken in al-ox wheel that needs to be dressed, or even one that has been dressed but is in the middle of use. To get an idea on durability, the first thing I ground in earnest on my wheel was a 1/2 inch mortise chisel that Mel Fulks gave me. He actually gave me a pair in exchange for something or help on something - can't remember what - good guy he is, though. He had found them and knew they were the kind of thing i liked. The back of one of them was hollow (socket chisels in this case, like framing mortise chisels) and I realized the bevel was cut upside down on it. It's a thick chisel.

I literally held the chisel in one place on the wheel and (picture this) cut the bevel from 25 or 30 degrees upside down to 25 or 30 degrees right side up in the other direction (on a chisel that's probably 3/8ths thick). It was a LOT, and other than settling in a little bit, the wheel seems no worse for the wear. There was a pile of burning red metal between the wheel and the tool rest. No clue how hot the tool get, but it didn't blue significantly from the edge because I took a few breaks - I'm sure I could've burned it, though, just as I've noticed certain things will still get hot (laminated irons - the soft steel still generates a whole lot of heat) - they just get uniformly hot and not at the edge.

At any rate, the wheel is a dandy, and I wouldn't exchange it for anything at this point. I never grind with water nearby, and for day to day maintenance, it's really uncommon for me to get an iron that I can't cool just by pulling it flat side slowly across my palm (including the irons that have mild steel or iron backing). I did that with the pink wheels, though, too, but you can just go a little faster with the CBN and the irons are a little cooler when all is said and done.