Bowls that have a bottom larger than 6″ sometimes split or come loose from the next ring with the movement of the wood. All wood shrinks and expands over time.
If you make a floating bottom you will not have that issue. The center piece of wood is not glued in and floats inside of a ring of wood.
The bottom starts with a 3/4″ or thicker segmented ring. I usually start it on a glue block with a paper joint. You can make the ring as large as you want and the ring should be about 1.5″ or more wide. This usually gets turned smaller as you finish the bottom of the bowl.
Turn it round and sand flat. Turn the inside of the ring round.
Using a thin cutoff tool, cut the ring in thirds, leaving 2/3s on the glue block. Keep the cut off piece (1/3rd) for later, this will become part of the inside of the bowl.
Sand what is left of the ring on the glue block flat.
Turn a dado on the inside of this segmented ring about 1/2″ wide and 3/16″ deep.
On another glue block, Glue a solid piece of wood (it can be glued up to get it the correct size) wider than the inside of the dado on segmented ring.
Turn this round to fit inside the dado. Make it a little smaller than the dado, this leaves room for the wood to move, and thicker than the dado is deep. I like to make this about 3/8″ thick or more for now. Sand this flat and remove it from the glue block. The sanded side will be the bottom of the bowl.
Place a small piece of tape on each end of the disc on the end grain. There is less movement in this direction. This will keep the finish off the wood.
Finish the sanded side, it will become the bottom of your bowl.
The finish is to keep glue from sticking to this side of the disc.
Remove the tape. Using a small amount of glue on the previously taped area, glue the disc in the dado of the segmented ring.
Only glue the taped off sections, do not glue the complete ring. You want it to be able move with changes in humidity.
Once this has dried and using tail stock support, turn this floating piece flush with the segmented ring.
Use a push cut toward the headstock to cut this down till it is flush with the ring. If you push toward the center it will probably break the small glue joints. Once you have most of the wood removed, take the tailstock away and finish the floating plate.
Sand the ring and floating bottom flat. This will be the inside, bottom of your bowl.
Using the ring you cut off earlier (1/3 piece), glue it in place over the segmented base ring on the glue block. Taking care not to let the glue get in your dado or on the floating bottom.
Sand flat. Turn the inside lip of the ring smooth.
The floating bottom is complete and you are ready to start adding layers to the bowl.
