Sharpening is one of the essential skills for woodturning. Most turners use a grinder to sharpen tools, but each type of tool needs a different grinding angle. For example, a skew chisel will usually have a more acute angle at its edge than a roughing gouge, which in turn will have a keener edge than a scraper.
Each of these tools (though not bowl and spindle gouges) can be ground while flat on either the grinder’s own tool rest or a separate platform rest, which just has to be set to the correct angle. One way to do this is to blacken the existing tool bevel with a marker pen then adjust the platform until the bevel comes into full contact with the wheel, which is inched round by hand so that the ink removed will show the accuracy of contact. Or you can just match the bevel to the wheel by eye, looking from the side. But in either case, any error can be cumulative. It will be repeated and possibly amplified next time the tool is ground. This can lead to a gradual change in the bevel angle.
I use an easy to make grinder platform angle setting jig that gives a consistent, repeatable result very quickly. It consists of a scrap of plywood with two projecting screw heads that align it to the wheel rim, and a straight edge at the bottom to which the platform can be set. The jig is very easy to make and use. A different one is needed for each tool type and thickness (thicker blades contact higher on the wheel rim and because of its curvature will be ground to a sharper angle than thinner ones). Use plywood thick enough to take the screws in its edge without splitting, but no dimensions are critical. All you have to do is set the platform angle as you wish, perhaps copying an existing tool angle, then cut the plywood so it rests on the platform with its edge fairly close to the wheel rim. Insert the two woodscrews (as far apart as the wheel guard and platform will allow, and angled so they are radial to the wheel axis. Drill pilot holes to prevent splitting.) The screws can be turned in or out to fine tune the fit. If necessary, the inner edge of the plywood can be cut away to follow the wheel rim more closely.
To use the jig, loosen the platform, place the two screw heads in contact with the stationary wheel rim, bring the platform into contact with the bottom of the jig and tighten up again. Every time you sharpen the same tool, or another of the same blade thickness, the angle will be just the same, making for quick results and saving tool life.