The strange thing is that how a bowl of gong frontier ground can affect the quality of its execution is that Turner tries deep and closes the form. To improve, I grind the groove in a particular way, including a second bevel. To fully understand the secondary, the benefits of the mall, we first need to see how the bowl gong is working, with the passage of time how the mill changes.
We see a traditional bowl gouge of about 1970 vintage. The flute is U-shaped, the edge is square to the axis of the tool, and the bevel is ground to a uniform 45°. It cuts shallow bowls of an open form very well. However, it does not do as well on deep, closed forms. That’s because the geometry of the grind actually gets in the way of the cut.
In the 1980s, Irish turners found that by sweeping the face of the tool back at an angle and grinding the bevel asymmetrically, the gouge cut much better in deep, closed forms. Typically an Irish grind has the nose bevel ground to about 70° with the sides at 30° to 40°. The grind can be adjusted for even deeper cutting.
As the bowl becomes deeper and the sidewall height increases, the transition between the curved wall and the bottom becomes more restrictive. Since the tool must be presented very near the axis of the centers, it is like working in the bottom of a mineshaft. Our traditional gouge would need to be about 70° from the centerline to cut effectively. Even the Irish grind suffers here because the heel of the nose bevel forces the cutting edge up and away.
The problem is two-stage oblique grinding, which shortens the friction ramp and allows the tougher radii to be cut. I grind the secondary grind without the aid of an additional specialty jig by simply setting the pocket of my Wolverine Jig closer to the grinder.
How wide to make the secondary bevel depends on how narrow you want the rubbing, or primary, bevel to be, and this will be dictated entirely by your work. Even for shallow bowls, a secondary grind is helpful. All of my photos in this article illustrate bevel sizes I think are appropriate for average faceplate work. As your skill increases and you tackle deeper vessels, you will benefit from grinding the secondary bevel bigger and the rubbing bevel smaller.
Previously I always refused to teach the second class to grind my basic class logic reason. My feeling was that the biggest struggle for a neophyte is to learn to “ride the bevel,” which is a sense of feel much like riding a bike or snow skiing. My logic was that the bigger the bevel, the easier it is to sense being on it. Now, after introducing the secondary grind idea on the first day of the faceplate section of some of my basic turning classes, I can say that I was wrong! If anything, the neophyte has an easier time with a secondary bevel than without it.
One, Two … Three!
John Michelsen, elegant wood hat, who came up with a tool, he called a vector grinding fixture. It produces a secondary bevel plus yet a third bevel he aptly calls the tertiary. Happily, Johannes’ jig marries into the Wolverine System and represents an improvement over their Veri-Grind Jig. The tertiary grind effectively radiuses the primary bevel’s heel, allowing the gouge to turn tight radiuses with aplomb. There are some other plums in the Vector pie, too.
Firstly, you can create a true 45° rubbing bevel with a curved cutting edge on each side a bevel is created. This means that no catching corner where the bevel meets the flute is possible. The curved edge also has less chance of the turner being overwhelmed by getting the entire side bevel cutting suddenly. While the 30° side bevel that is typically achieved on the Irish grind is OK for green/wet wood, in practice you’ll find that it does not do well when cutting dry wood. The 45° attack angle gives the least tearout, wet or dry, against the end grain — which happens twice a revolution in faceplate work. The Vector Fixture is very well-made and costs about $150.
The Vector Grind Fixture
ontrolled by articulating the strut that fits into the Wolverine Pocket Jig, the Vector Fixture accomplishes the same thing by how far the tool protrudes from the face of the fixture.
The Vector Fixture comes with a nifty device that mounts on your grinding table for correctly locating the gouges. The gouge is inserted flute down, and the thumbscrew locks against the round bottom section of the gouge. This allows the tool to be ground until there is no flute left.
The main grinding is done halfway, and the vector pillars are fixed in the center of the left and right holes. In this photo I grind the nose and the right half of the pillar is dug right in the center of the hole. For the left part of the tool, support the left hole. A slow, continuous scan from the distal end of the nose.
For the secondary grind, the strut is placed in the center hole the farthest from the grinder. Grind until the rubbing bevel (the cutting bevel) is between 1/16” and 1/8 ” wide. Adjust for smaller and larger gouges proportionally.
Three-stage grinding, support placed close to the wheel center hole. This heel of the convex and concave changes into a radius, so that mud can be very difficult to do.
Cut the Chatter
One of the greatest benefits of secondary convexo-concave variation is the creation of a heel-recumbent surface created when the cutting edge is cut with a bowl of ground outside the standard Irish to jointly create an undesirable interaction between the chatter modes. Like corduroy roads, even worse is cutting the progress. The lifting of the tool slightly to other tools often correct the problem. No matter how the secondary grinding is achieved, in fact, this is a chatter problem!