As I mentioned earlier, a couple of years ago, I got a 4" contact wheel from a Guild regular, although I have to admit I can't honestly recall if he gave it to me, which i think is the case, or if I bought or traded for it. I do a lot of buying and trading like that, and to that Guilder, thank you either way. :)
Anyway, this one mounts a little different from the others, as it has this "axle" sticking out to one side. The end is threaded for a bolt, and I suspect you're just suppose to drill a hole in the mounting arm and run a bolt through. I didn't like that, as it seemed awfully flimsy. It'd probably be fine for a knifemaker who doesn't put a great deal of pressure on his blades, but for my general-use needs, I wanted something stronger.
Checking it against the existing belts (note the witness marks on the tool rest) I find out it needs basically exactly one in of offset.
Rooting through the remnant pile and a bit of bandsaw work gets me these- a chunk of 1"x1-1/2", the 1-1/2" square tooling arm, and both pieces cut off at 45 degrees.
Why cut them off at 45 degrees? 'Cause that's how I roll. Any idiot can leave it square. :)
Anyway, after a quick check for clearances, I drilled, tapped and countersunk (not in that order) a 3/8"-16" hole in the end of the smaller piece.
And then drilled a slightly-less-than-5/8" hole where the axle will go.
Put the grinder back together with the other wheel and dress it up a bit...
Then TIG weld the two parts together.
Now, the reason for the undersized axle hole: I wasn't sure how square I could keep everything as I welded it, or what sort of distortion I might wind up with, so once it was fully welded and allowed to fully cool, i squared it up in the mill and bored the hole with an endmill.
A quick test-fit, and it works just fine.
And as a last step, a light cut in the bandsaw opens up a pinch slot to the axle hole.
Tap the wheel into place, eyeball an alignment, and tighten the pinch bolt. Holds solid as a rock.
It's just that easy.
This one, too, tracks perfectly with the same tracking wheel setting as the other two so I'm happy. :) That gives me a 4" wheel, an 8" wheel, and the "square wheel"- and, one of the idlers on the square wheel is rubber-covered, so technically I have a 2" contact wheel as well. I think that all should serve me pretty well for the time being, though I'll likely eventually end up with either a 10" or a 12" wheel as well, maybe both.