Sharpening a bowl gouge: 40 degrees, 55 degrees, and a jig I made
For the first year I sharpened the bowl gouge freehand on the grinder and got a different edge every time, which meant I never knew whether a bad cut was the tool or me. In March I built a jig from scrap plywood and a bolt, and the tool has been consistent since. This is what I settled on and the numbers behind it.
The grinder is a slow-speed bench grinder with an 80-grit aluminum oxide wheel on one side and a 180-grit wheel on the other. The jig is a simple pivot: a plywood arm with a cup at the end that holds the gouge handle, pivoting from a block clamped to the bench at a fixed distance from the wheel. The distance from the pivot to the wheel face, together with how far the tool projects from the cup, sets the bevel angle. For my 12 mm gouge, a projection of 50 mm and a pivot distance of 180 mm gives a nose bevel of about 40 degrees, which I checked with a protractor against the ground bevel. A projection of 65 mm gives about 55 degrees for a second gouge I keep for the steep inside curve near the bottom of bowls, where the 40-degree tool cannot ride its bevel.
The swept-back wings on the gouge come from rolling the tool in the cup as it moves across the wheel, from one wing through the nose to the other, in one continuous motion of about two seconds. The jig makes the roll repeatable because the cup constrains the handle to a cone. Freehand, my wings were always uneven, one longer than the other, and the tool would catch on the long side.
The procedure now takes about forty seconds per gouge: a light touch on the 80-grit wheel only if the edge has been damaged, then two or three passes on the 180-grit wheel with the tool barely kissing the stone, feeling for the burr on the inside of the flute with a thumbnail, then one stroke of a fine diamond card inside the flute to take the burr off. No honing of the bevel; a freshly ground edge cuts bowl wood fine. I grind far more often now that it is quick and reliable, perhaps every fifteen minutes of actual cutting on hard dry wood, and the surface off the tool is noticeably better, which means less sanding, which was the whole point.