Reverse chucking, what!
For this post, I’m making a natural rim bowl using big leaf maple (Acer macrophyllum). The first step is to fit a faceplate to the front of the container; these natural rim bowls will always have an uneven surface to one degree or another. Slide a few shims under the faceplate to level the faceplate with the bottom of the rotating block. The big trick here is to use long screws that will go through the wedges and penetrate well into the wood, the picture shows 3 inch wood screws and wedges. As an added safety measure, support turning with the tailstock using a live center, the 60 degree taper works well.
During the rough turning phase, create a shank for the offset 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 face plate, turn the bowl upside down and connect it to the offset chuck; supporting again with the counterpoint and the live center.
Turn out the inside of the bowl leaving about 1-inch thickness around the sides and bottom. The center post can be removed with a Bit Forstner. Clean the bottom and set it aside to let the wood dry.
Next step after the wood has dried to a moisture content of around 12%. Finish giving the bowl the final shape and thickness of your choice, sand to 250 grit. Topcoat the bowl to protect the wood from stains (fingerprints) and discoloration.
Ready to rotate the bottom, I use a suction cup to hold the bowl while the bottom rotates, I use 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 area. 26 inches of vacuum for 0.5 lbs. per inch of vacuum equals 13 pounds per 28 square inches equals approximately 364 pounds of clamping force on the container; this will work unless you go too fast or hook a chisel; this will move the bowl off center.
Replace the chuck in the bottom of the bowl and attach a reverse chuck alignment adapter to the chuck and place it on the tailstock. Advance the bowl approximately 1/4″ from the pad and clamp the tailstock to the lathe bed. Turn on the vacuum to about 10 inches of vacuum and advance the boom until it contacts the pad. Watch the vacuum gauge, The inches of vacuum will increase when the bowl is in contact with the pad.Loosen the chuck and remove the tailstock, adjust the vacuum to what you want, too much vacuum can scar the bowl.
The bowl will always be slightly off-center, maybe this is okay; it will not be noticed. Be sure not to touch the finished surface with a tool. Rotate the background, sand and apply the finish.
The finished bowl, notice how it sits up a bit and not flat on the table, adds a bit of depth to your project.