When grinding or sharpening machine lathe tools, in most cases a bench grinder will do the trick. Since there are a variety of configurations that can be ground into a tool I will only cover the basic cutting tool configuration.
The first thing you need to know is whether your tools are “High Speed” steel or Carbide tipped cutting tools. Like everything else there are advantages and disadvantages with both types of material. High Speed will need to be re-sharpened more often than carbide but the carbide can fractured and break more easily. Also the carbide portion of the cutting tool is usually brazed to a piece of mild steel and when grinding you must be sure that you do not heat up your tool while grinding as that will weaken the braze and this can also become a problem, overheating, with High Speed tool bits, also can be problematic, as the excessive friction heat can cause the loss of the temper of the steel.
Now about the bench grinder, you need two wheels the coarse for roughing out the pattern of the tool and to grind relief and a fine wheel to finish the edges smooth and burr free. As with the use of any power tool always observe all safety precautions, do not wear gloves or loose clothing near moving parts, and ALWAYS wear eye protection. The dust created by grinding tools can cause serious eye damage. And keep your hands away from the moving wheels as you can receive serious injuries.
Basically you can grind both types of cutting tool in the same manner, first using the coarse wheel grind back the tool to the following shape (this will cover the basic cutting edge configuration), first addressing a High Speed tool bit. If you look at the piece of High Speed steel you will notice that already a face relief exists. Set the approximate angle of the relief (about 10 degrees) on the guard plate in front of the wheel to maintain this relief area. Also you will see that the face (cutting edge) when looking down at the tool is square. The first grind is to taper back this edge at about a five (5) to ten (10) degree angle from the left front corner. This will assure that ONLY the cutting edge (tip) makes contact with the material to be tooled and the heel of the tool does not rub against the finished surface. On the lead face of the tool (the left hand side of the steel) grind back the excess material under the edge to within 1/8 of an inch (125/1000) to ÂĽ of an inch (250/1000) from the top surface. The main point (idea) of these relief grinds is to assure that only the cutting tip of the tool makes contact with the material to be machined. Then using the fine wheel grind the finish to a smooth burr free surface and set the guard plate at two (2) degrees and make a smooth clean grind on the forward edge and the leading edge (left side), then very slightly radius the tip, a sharp point will only break and then gouge your part in the machine. Frequently dowse tool in water to keep from over heating and loosing its temper.
Carbide tipped tool is easier to grind as the reliefs already exist on most tools ground from factory. You may want to grind a bit of relief under the leading edge, again I repeat that you have to dowse the tool in water frequently to prevent overheating and the weakening of the braze holding the Carbide portion of the tool. When re-sharpening, use the fine wheel only and set your guard so that only the carbide is being dressed. Again you want a small radius to the tip for the same reasons explained in the preceding paragraph the only difference is that you want to make it a very small radius and this is better accomplished by using an Arkansas Stone and hand lapping the tip to a two or three thousands (2/1000 or 3/1000 inch) radius. On either a High Speed or Carbide tool, using an Arkansas Stone, lap the edges of the cutting surface to clean any burrs and slightly break the sharpness of the edge. I repeat, the extreme sharp edge is more likely to hook in the part and cause a break in the tool's cutting surface and also can damage the finish on the work piece.
Finally place the tool in the tool post of the lathe, do not tighten more than to just hold the cutting-tool, and run it up to the work piece with the lathe turned OFF. Visually check to see that only the cutting surfaces meet the work piece and that the tool is on center to the work piece. Assured of this then tighten down and check again. Due to the varying sizes of tools you may have to remove the tool and shim to maintain center and to flush tool to the edge of the tool post.
