speedfreak24.7
I don't want this thread to degenerate and you to make a fool of yourself more than necessary.
Generally, for lapping and grinding at low speed with fine abrasives electroplated on a lapidary disk you don't need much power and 1/4 hp is already way too much, especially if you're using gear or belt reduction.
For gear motors, gearboxes, etc. Surplus Centers are your best friends, after Google, of course. Perhaps, look for DC motors as well,
And, speaking about Google, probably, five minutes spent searching the web would have solved your big confusion you have with grit (= measure of the diameter of the abrasive particle) vs. hardness.
Here, perhaps, a link that will help understanding grit (first result with google, by the way).
Here is another table going much finer with the grit/mesh.
Is your tool grinder three phase? The Central Machinery one that I have and all the others I have seen are single phase. If it is single phase, it would be extremely hard, if not impossible to control it with a VFD, since the start winding has to get disconnected by the centrifugal switch after a certain speed (otherwise it will burn) and you would also need to find a way to exclude the capacitor (if it has one) from the equation. Bottom line, if it isn't three phase already, it would be a lot of effort/money to control it with a VFD.
I use my Central Machinery grinder to sharpen HSS lathe tooling. at least in mine, the trunnions of the two tables are a royal pain. For shaping a scraping blade, you can use the 150 grit wheel with the table set at 90° (i.e. blade on the same plane as or parallel plane to the axis of the wheel). You can grind the negative rake while finishing at low speed on fine grit.
I understand that you don't have much time and you can spend only once the little money you have. For $200 or less you can find a slow moving wet grinder (look into woodworking tool sharpening) and, given that you don't need much adjustability for the tool rest (for most jobs you can sharpen with a -5° rake angle, if the cast iron is very soft, you can go to -8°, if it is too hard, you can reduce to -3°), you can cut wood blocks at different angles and, if you really want, cover the wood with some sheet metal.
My apologies, but I'm a semi-old fart, grown up before Internet, when it was much more difficult to gather information and I really get upset when I encounter anybody who doesn't do enough homework to document him- or herself at least on the basics, before asking questions. I find also unacceptable and disrespectful when you write "the guy that teaches scraping classes" instead of calling him by name, especially considering that you have corresponded with him (by the way, again, Google would have helped you also in this: if you search "scraping class", almost all the results on the first page point directly to Richard King).
I stand for what I wrote in my earlier post and, as each of my posts in this thread, I try to be as helpful as I can and I talk about only things I know.
Regards,
Paolo
PS To dress diamond wheels (the ones with diamond in a matrix, not the electroplated ones, of course) you use iron, since at high-enough temperature, it eats up carbon from the diamond.