Platform sharpening is a quick and convenient method of sharpening woodturning tools using a grinder or sander with a tilting platform. With the angle set, the tool simply rests on the platform while the grinder does its stuff. The process is extremely simple, and platform sharpening is the best method I know for scrapers, skew chisels and square-ended gouges. Some people hone the tools to refine the edge, but most probably use them straight from the grinder. You can use the platform for other gouges once you have the knack of swinging and twisting the gouge.
Setting the angle
There are a couple of problems to deal with in platform sharpening. The method often recommended for setting the platform angle is to match it to the existing tool bevel. You can ink the bevel where it touches the wheel for greater precision. I find the sound and feel of the grind when turning the wheel slowly by hand more helpful than the ink. When just the heel or the edge is in contact with the wheel, the sound is harsh and more grating. When the bevel rests properly on the wheel the feel is softer. Either way, copying the existing bevel works well, but is a bit slow, and liable to cumulative error. The platform angle will never be set perfectly. If there is the slightest bias in one direction the bevel angle will gradually change.
It’s more consistent, and quicker, to use a simple setting jig. The one I described in another post will maintain consistency. If the platform angle is always the same, any one tool placed on it will always be ground the same. The bevel angles may not be the same on different tools. This is because the edge of a thicker tool will be higher on the wheel.
Longer tools
The second problem is that long and heavy tools are hard to keep in proper contact with the platform. They are too unwieldy. You need a light touch on the grinding wheel, which is not easy if you are holding the tool firmly in place on the platform, as you must to keep the angle. This means you may need platforms of different sizes. A large one to give stability to large tools, and a small one for short tools, which would have insufficient ‘reach’ to sharpen on a large platform because the handle hits the platform edge.
Platform sharpening with the Robo Rest
A minor inconvenience is that when platform grinding different tools at the same time, you may have to change the angle. The wheel normally has to come to a stop before you can do that. I use a ‘Robo Rest’ adjustable platform that I can set without stopping the wheel. A pin in one of the indexed holes locks the platform at any of the provided settings. Unfortunately this rest is currently unavailable.
Because it doesn’t reference from the wheel itself, the Robo Rest is most useful when using a CBN wheel. These don’t wear down, but other wheels get smaller with wear. With those, you have to move the platform closer to the wheel from time to time. Otherwise the tool angle will change. I found it very quick and easy to set the angle. It does its job well, with a repeatable grind. Its build quality is perhaps not as good as I would like. The platform is not perfectly flat, and there is some looseness in the locked platform due to the tolerance in the diameter of the locking pin. The maker points out that more rigid construction would add to the cost. Pulling the platform forward in use takes up the slack and makes the grind more consistent. The flatness is adequate for most purposes.