Last summer I scored an 8" Baldor bench grinder for $35 at a garage sale. (gloatgloatgloat!) It had a worn rock and a wire brush, so when I freshened it up, I swapped on a new rock in place of the brush, and used a diamond point to true everything up.
It ran pretty smooth to start with, but eventually the vibration got worse and worse, eventually getting to the point it'd walk the stand around.
Turns out, the previous owner had done a lot of side-wheel grinding, and since the wheel wobbled somewhat, it eventually wore a slight hollow on one half. So now, no matter how much I true the wheel, unless I shave the whole side down, it'll always be off-balance.
So I swapped out the old rock for a new Norton, and that improved things considerably, but I noticed that even the new wheel wobbled side-to side. After some investigating, I discovered that the wheel mounting hardware wasn't itself square:
Neither the nuts nor the flanges had been machined, and all of them were crooked- only slightly, but definitely off.
And since the hex faces of the nut aren't necessarily square to the thread axis, the trick to fixing them is to turn a stub in the lathe...
And simply face the ends.
I've found flipping them and doing each face twice makes them very close to perfectly square to the bore.
The Baldor flanges are just die-cast and unmachined, so they were a bit off too. In this case, a cheap expanding mandrel was an easy way to fixture 'em for a light skim cut.
After facing both nuts (had to make a left-hand stub too) and both flanges (and both sides of everything) both wheels now spin with considerably less wobble, and once properly dressed, the whole grinder is much quieter and runs very smoothly. Not quite as smooth as my big Queen City, on which you can damn near balance a nickel while it's running (the grinder, not the nickel) but still very nice.