Achieving a perfectly curved and polished cutting edge on your plane irons is an essential skill that many new hand plane users struggle with. Getting the right amount of curvature in the right place on the iron takes a fair amount of practice, patience and an observant eye.
Eventually, it is a skill that becomes second nature. And David Charlesworth’s methods outlined in the article “Learning Curves” will give you a tremendous leg up in acquiring this skill. As with everything in woodworking, of course, there are several ways to get the same result. A new type of diamond stone is just now becoming available that sharpens a perfectly curved edge the first time you use it. I’ve sharpened about 25 plane irons with the product and am quite impressed with the results it gives.
The Odate Crowning Plate was developed by woodworker and author Toshio Odate, and David Powell, the founder of Diamond Machining Technology Inc. (DMT). In essence, the stone is a 3 1/4″ x 7 1/4″ piece of cast iron that has been machined so it has a slight and consistent concavity across its width. Then it is plated with diamond particles – it is available with #60, #45, #30 and #9 micron grits. The amount of concavity corresponds to a slight 37 1/2′ radius. In more technical terms, the chord to arc height on a 3″ width is .0025″.
In practical terms, I’ve found the stones produce an edge that’s just slightly more curved than what I achieve using finger pressure on a flat stone. This actually makes setting the curve in the center of the plane’s mouth easier, though it reduces the width of shaving you will make.
Powell says you can reduce the curvature of the iron on the plate by skewing the iron slightly as you sharpen. I’ve also found you can increase the curvature by using finger pressure on the corners as you sharpen the iron on the plates.
Also note that even the finest version of this stone (#9 micron) isn’t fine enough to polish a plane iron’s edge – you’ll still need to finish up the edge on your polishing stone. There are two ways to go about this. You can use finger-point pressure as described in this article, which works very well in my experience. Or you can dress your polishing stone so it also has the same concavity as the diamond stones. Powell is making a convex diamond stone that will dress conventional sharpening stones expressly for this purpose. This dressing stone was not available at press time so we were unable to use it in our trials.
The bottom line is this: Veteran sharpeners probably won’t need these stones (though I find they help me achieve consistency even when I’m having a bad day). But if you struggle with achieving a curved iron, the Odate Crowning Plate will fix your problem immediately and perfectly.
The stones are about $100 each. If you wish to purchase only one, I recommend the #30-micron stone, which shapes the edge quickly and takes you to the point where you can refine the edge with a #1,000-grit stone and then your polishing stone.
The stones are available from Powell Manufacturing Co., 396 Washington St. #114, Wellesley, Mass. 02481 or 781-237-4876.
— Christopher Schwarz