How do you turn the bottom of the bowl? Do you have to leave a rabbet or tenon? What if I don't want to leave a tenon as a foot? or the chuck damaged the foot and I want to clean it up/remove it?
There are 4 ways I am currently aware of to turn the bottom of a bowl.
Turn it first, in this case you need to leave a Rabbet or a tenon for chucks to clamp on/in so that you can turn the bowl around and hollow out the inside. This leaves the tenon or rabbet, and there could be tool marks left from the chuck.(alternately you can glue a piece of 'waste' wood on the bottom and use screws to attach it to a faceplate still might need clean up after removing the waste wood)
After hollowing out the bowl, there will likely be a tenon or rabbet and quite possibly some ugly tool marks, or the tenon might be damaged. (or you just want to remove the tenon/rabbet for aesthetics. That's where the next 3 come in.
Flat jaws - you can buy Flat jaws for many/most chucks out there and these are very useful. They can clamp down or expand out to hold the rim of a bowl. The uprights that hold the bowl are rubber coated screws so they won't mar or damage the edge of the bowl. The screws can be placed at different distances to accommodate different size works. Some of the jaws have some variable slots to help hold oblong bowls (likely from some warping). The larger chucks tend to have angled nubs to help grip the bowls.
- Vacuum chucks - these use an air compressor to create a vacuum holding a piece onto the chuck. It needs a good seal so holes in the work will preclude this as an option, even porous wood might not work, though finishing the inside of bowl can overcome porous material. This is especially good for working on bowl and things with an uneven edge or a 'natural' edge bowl which won't work with flat jaws. In my experience this should mostly be used for finishing work, and not to try and turn the whole outside. Too heavy of cuts and the bowl will pop off, possibly getting damaged in the process.
Jam chucks -
a. This is where a chuck such as the vacuum head of the Vacuum chuck (or other hand made ones) are used to 'push' the bowl against, using the tailstock to hold the piece against it. This lets you turn most of the bottom other than a small piece around the tailstock with can be cleaned up easily with a chisel and some sand paper.
b. The other is to create an insert that fits the piece perfectly and is actually 'jammed' on the chuck like a hat on your head. The friction will keep it there. Once again this is generally for finishing work. Jam chucks are always usable and certainly cover where the other two types fail.
I'll add that the waste block, or glue block, can have a rabbet/tenon turned in it to be gripped by a chuck. Once the bowl is turned, you can start turning down the glue block as you desire, and can go smaller than the chuck diameter depending on thickness of the block. The bowl bottom can be prepped flat and smooth with the wood blank turned with the bottom out before attaching the glue block, not requiring much work to clean up after glue block removal.