I have recently had a need to put a new 25degree edge on chisels and planes. Some were so far off that using a guide and oil-stone (see the Stanley plane video below) was far too much work.
My bench grinder, a 200mm ‘Supermate’ from the 80s had its original grey wheel on it. It was too hard and too hot. The harder a stone is, and the finer its grit, the more likely it is to overheat a job. It is, I am sad to say, easy to discolour a chisel tip with heat from grinding and, thereby, amend its temper making it either too hard or, more likely, too soft. A proper wet wheel sharpening machine, like a Tormek, was too much money for me given my infrequent need to sharpen wood working tools.
A softer aluminium oxide wheel, white in colour, of a not too fine grit was the next best option I could find. I purchased a Norton 38A46KVBE Medium. I think 46 is the grit. Pictures of the finish show it is fine enough, I was always planning to finish the edge on an oil-stone. It is relatively easy to keep the tool getting too hot as well if the tool is kept moving across the face of the wheel and a light pressure is used.
Knowing how dangerous it can be to grind metal on a bench grinder without having the metal sit on the grinder rest I decided to make one that meant the tool touched the wheel at 25degrees. Commercial versions are also available. A poorly designed tool rest could throw a chisel or other tool into the face of the operator, follow my example at your own peril. I am no expert. Remember this is a what I did blog, not a how to guide.
Benjamin's Bench 