Since I have a 2x42 craftsman belt sander (that I typically just use for handles) and a reversible 9" disc I do most of my work on the disc. Now keep in mind that I have maybe 12 blades ground and 4 completed knives. Some things I have learned.
Having a reversible disc is useful. Most knifemakers grind edge up and if your disc grinder reverses you can do both sides without grinding one edge up and the other side edge down. VFD for changing the rpms would be nice as well but I don't have that.
Slower would generally be better. So my motor runs at 1750 which I think is tolerable.
The outside of the disc will remove material faster than the inside as it is moving faster.
You will still have to have smooth "pulls" on a disc. I don't and I have not heard of anyone leaving the blade on the disc and just grinding the whole thing at once. It would be impossible on longer blades anyway.
Some like a disc with a 1 degree taper so the disc is a slight cone. That way when you cross your blade over the midpoint of the disc it is not caught by the "upward" side of the disc. That's what I have and I don't think anyone has ever said they notice something is not flat because of the taper.
Most will steer you to the 9" discs that can be purchased through several of the knifemaking sites so you can use regular sheet sandpaper cut to size to save $. You can also use any grit you want. The fine Rhynowet Red goes up to at least 2000grit from
http://supergrit.com/products/RedlineRedFlexSheets
Practice a bunch?
Realize that if you come in at a certain "height" for grinding and then alter that, ie. coming in higher or lower than that on the disc you may have difficulty keeping the plunge line clean
-Then (if you don't have one already) you will probably invest in a carbide faced file guide like the one I bought from forum member Bruce Bump to keep your plunges neat when rough grinding. I haven't mastered the technique shown on the vid 12345677890 (The Count) posted but it's on my list of things to practice.
Anyway, most of these things you will learn by just mangling up some steel, fixing mistakes and just practicing.
