Slow Speed Grinders
High speed vs. slow speed is not the important question here. What are the critical issues are 1) the type of grinding wheels you use, 2) how much pressure you apply when grinding, and 3) keeping the wheels clean.
1) Virtually all lower price grinders come with gray silicon carbide wheels. These wheels are designed to remove large amounts of metal quickly! Great for taking large nicks out of lawn mower blades or cleaning up weld joints. However, this is exactly what you don't want for turning tools. Turning tools are too expense to grind away quickly. Besides, even the finer grit gray silicone carbide wheels do not give a good edge. What are needed are friable aluminum oxide wheels, most are white and some are pink, does not matter. "Friable" means that the grains of aluminum oxide fracture readily exposing fresh, sharp edges of aluminum oxide which do the sharpening. Good grit choices are approximately 60 grit for shaping the tool to the grind you want and a 120 grit for getting and maintining a sharp edge.
2) What "burns" the tool steel is much more a function of how much pressure you apply to the tool against the wheel then the speed of the wheel. I can "burn" a tool on a slow speed grinder almost as quickly as I can on a high speed one. Just apply enough pressure, it'll burn. The only pressure required is the weight of the tool itself. Use your fingers just to guide and rotate it (for curved edge tools) gently on the wheel. You want to see very few sparks. Lots of sparks...too much pressure, back off. This light touch also does not burn, or "blue" the steel. True, HSS will not loose its temper if blued, but the blue appearence just bothers me and says that too much pressure was applied when sharpening. Also, since you are removing extremely small amounts of steel with the light touch approach, your tools will last for a very, very long time.
3) It is very important to keep you wheels clean. Dirty wheels are clogged with metal and prevent clean removal of steel during sharpening. This leads overheating and not a sharp edge. When the wheels get heavy black streak on them, it's time clean. WARNING!!! Do not use a star wheel type wheel desser, they tear up and pit the surface of the wheel something fearce. Use a stone (cheap) or diamond (expensive) type wheel dressed, both of which work well on aluminum oxide wheels. When cleaning your wheels be carefull not to inhale the aluminum oxide dust that is thrown off in the cleaning process, not good for the lungs (same is true for silicon carbide dust). Wear a breathing mask of some sort when cleaning. It is also a good idea to frequently vacuum up the grinding dust around your grinder.
ECONOMICS:
Let's first make the assumption that we are talking about an 8" grider as opposed to a 6" one. The 8" is a better choice for two main reasons. First is that you get less of a hollow grind on the 8" because of the large diameter. Second is that you get far more sharpenings for the useful life of the wheel before it has to be replaced due to wear.
If you were to purchase an inexpensive grinder, say around $50 dollars, it will most assureadly be a high speed one that comes with gray silicon carbide wheels. New 8" aluminum oxide wheels will cost you about $33 each (one 60 grit and one 120 grit), that's $66. Total investment is by this route is approximately $116. Woodcraft has an 8" slow speed grinder which comes with the proper two aluminum oxide wheels (60 grit and 120 grit). The regular selling price of $95 and it is frequenly on sale for $75. Much better deal than above.
Bottom line. If need be, save up and get the proper sharpening equipment. It will give better edges and make your tools last a lot longer.
Peter Toch
Just let it air cool gradually.