CAT had the privilege of hosting Stuart Batty for the August 13th demonstration. Attendees saw the magic of the 40-40 Batty grind for turning both bowls and “between center” pieces. Those who attended the one-day workshop on August 16th took it one step further—perfecting their bowl turning technique and learning to free hand grind their own tools with the 40-40 grind.
What is the 40-40 grind you ask? Simply it is a bowl gouge grind that is ground with a 40° nose and flat wings that are 40° to the nose. This grind can only be accomplished by hand grinding on a platform. Although there are a few commercial jigs which come close, they cannot duplicate this profile. Why is the 40-40 grind so special? The 40° nose slices through the material with the 40° wings peel the rougher, leading cut with a smooth finishing cut with an equivalency of at least 220 grit sanding. Now, this gouge is both a roughing and finishing tool for bowls and spindles. Be sure to sharpen before your last pass for best results.
Stuart emphasizes following a few simple rules to develop repeatability and quality of finish. First rule, your right hand is the “gas pedal” your left (don’t squeeze) is the “brake”. Next, the right hand is used to control the cut by (1) push/pull, (2) lift/drop, (3) twist, and (4) swing. Straight cuts only use actions (1) and (2) while curved cuts use all four actions. Finally, your left hand is (a) providing weight (2 pounds of downward force), (b) positioning the start of the cut, (c) preventing the tool from skidding, and (d) braking. With the right and left hand in coordination with the lathe, they control the variables for a perfect cut:
1. Tool cutting rate or feed speed: slower is better.
2. Lathe speed: higher is better, but always within your safety zone.
3. Flute orientation: open flute leads to a poor finish, a closed flute yields a better finish.
4. Sharp tool. Sharp tool. Sharp tool.
To keep your tool sharp, frequent the grinder often during your turning. Set the platform at 40° to the wheel and lock it down tight. Set marks or place tape on the wings of the platform to establish the 40° go/no go boundary for the wings. Holding with your right foot forward and standing to the left of the grinding wheel establish the first wing. Separately, establish the 40° nose, not too blunt or too pointy, but just Goldilocks right. Then grind the second wing and when completed lightly blend the three elements into a continuous, uninterrupted profile. Easy peasy, Bob’s your uncle.
Foot position also plays an important role. For straight, parallel cuts place both feet under your shoulders the same distance from the lathe. For a straight cut, the right foot is placed under the shoulder with the left foot forward but in line with the shoulder. For an oblique angle cut the left foot is placed in front of the right foot which is under the shoulder. Finally for curved cuts, the left foot is under the shoulder and the right foot forward. In all cases your cuts should start in the uncomfortable position and end in comfortable one. Never stop in the middle of a cut to adjust your feet. If your feet are properly placed you should be able to complete a cut in one continuous motion.
According to Stuart begin on the outside profile of your bowl with three heavy cuts rather than five light cuts. The wings are doing the heavy work in stock removal, so on the third pass sharpen the gouge as the wings will dull quickly with a heavy cut. Using five cuts is inefficient, especially if you are a production turner. Finally, turn the inside of the bowl with the same bowl gouge, however, because of the transition geometry you must switch to a 60° bottom feeder (5/8” British traditional grind) to get the best finish without bruising the grain.
In summary, Stuart stated that if you follow seven fundamental rules you can produce clean, almost ready to finish bowls without fatigue time after time if you:
1. Know the grain direction and account for your angle of cut.
2. Chuck both accurately and securely (correct tenon angle and absolutely flat chuck mating surface).
3. Sharpen tools. Sharpen tools. Use sharp tools.
4. Turn as fast you can while still feeling safe.
5. Adjust your tool rest to the correct height, angle, and distance for the cut you are making.
6. Use the correct stance to provide an uninterrupted, smooth cut.
7. Practice good technique and tool control. Be sure to use both right and left hands correctly and in coordination with each other.
Ken Poirier
CAT Program Manager