Reverse chucking, what!
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 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 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 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 faceplate, turn the container upside down and connect it to the scroll chuck; supporting again with the counterpoint and the live center.
Roll 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 for the wood to dry.
Next step after the wood has dried to around 12% moisture content. Finish the cup to the final shape and thickness of your choice, sand to 250 grit. Put a finish coat on the bowl to protect the wood from staining (fingerprints) and discoloration.
Ready to spin the bottom, I use a suction cup to hold the bowl while the bottom is spinning, 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 per 0.5 lbs. per inch of vacuum equals 13 pounds per 28 square inches equals approximately 364 pounds of holding 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 to about 1/4″ from the suction cup and set the tailstock to the lathe bed. Run the vacuum to about 10 inches of vacuum and advance the quill until it contacts the suction cup. Observe the vacuum gauge, the inches of vacuum will increase when the pot is in contact with the suction cup.Loosen the chuck and remove the tailstock, adjust the vacuum to what you want, too much vacuum can mark the pot.
The bowl will always be slightly off center, perhaps this is fine; it will not be noticed. Be sure not to touch the finished surface with a tool. Turn bottom over, sand and apply finish.
The finished bowl, notice how it sits a bit and not flat on the table, adds a bit of depth to your project.