For this post, I am making a natural rim bowl using Big Leaf Maple (Acer macrophyllum). The first step is to attach a faceplate to the front of the bowl; These natural rim bowls will always have an uneven surface to one degree or another. Slide some shims under the faceplate to level the faceplate with the bottom of the swivel block. The big trick here is to use long screws that go through the shims and into the wood well, the picture shows 3 inch wood screws and shims. As an added safety measure, support the turn with the tail using a live center, the 60 degree cone works well.
During the rough turning phase, create a spigot for the shift chuck; I like a recessed dovetail about 3/8 inch deep. Before removing the faceplate, make sure the offset chuck is tightened into the dovetail. Remove the faceplate, turn the bowl and fix it to the scroll chuck; again supporting with the tail and the living center.
Rotate the inside of the bowl leaving about 1 inch thick around the sides and bottom. The center post can be removed with a Bit Forstner. Clean the bottom and set aside for the wood to dry.
Next step after the wood has dried to a moisture content of approximately 12%. Finish shaping and final thickness of bowl of choice, sand to 250 grit. Topcoat bowl to protect wood from stains (fingerprints) and discoloration.
Ready to rotate the bottom, I use a suction cup to hold the container while the bottom is rotated, I use a suction cup with a 6 inch suction cup with an extension. You will need a vacuum generator, a pressure gauge, and a valve to control the amount of vacuum. In round numbers, 1 inch of vacuum equals 0.5 pounds. pressure. A 6-inch suction cup has approximately 28 square inches of surface. 26 inches of vacuum per 0.5 lb. per inch of vacuum equals 13 pounds by 28 square inches equals approximately 364 pounds of clamping force on the container; this will work unless you go fast or hook a chisel; this will move the bowl off center.
Put the chuck back into the bottom of the container and attach a reverse chuck alignment adapter to the chuck and place it on the tail. Advance the bowl to about 1/4 “from the suction cup and lock the tail into the base of the lathe, run the vacuum to about 10 inches of vacuum and advance the pen to make contact with the suction cup, observe the vacuum gauge, the Los inches of vacuum will increase when the bowl is in contact with the suction cup, loosen the chuck and remove the tail, adjust the vacuum to what you want, too much vacuum can mark the bowl.
The bowl will always be slightly centered, maybe this is fine; it will not be noticed. Make sure not to touch the finished surface with a tool. Turn the bottom over, sand and apply the finish.
The finished bowl, notice how it lays a bit and not flat on the table, it adds a bit of depth to your project.
