How the rings stack
A segmented bowl is a stack of rings glued one on another. Ring 1 is the base; each ring above it is a little wider or narrower to make the curve of the bowl. Enter each ring's outside diameter, its height in the stack, its wall thickness and how many segments it has. The profile draws the bowl cut in half, so you can see the shape before you cut anything.
The designer starts with Tailstock's default bowl: a 4 1/2" base, a 5" ring 2" tall and a 4 1/2" rim, all with 3/4" walls in 12 segments. Add a ring and it starts at the size of the one below it.
New to segmented turning? How to Make a Segmented Bowl walks through it from the drawing to the lathe: cutting, gluing, stacking and turning.
Why rings must overlap
When you turn the bowl, the inside and outside of every ring are cut back to one smooth wall. That only works if each ring's wall sits on the wall of the ring below it: where the walls don't overlap, turning cuts through the glue joint and leaves a gap or a step.
The designer works out how far each pair of walls overlaps: the narrower outer radius minus the wider inner one. Under 3/8" (1 cm), not enough for a 1/4" finished wall plus cleanup, it outlines both rings in red and tells you which to change. A ring flaring out needs to be smaller or have a thicker wall; a ring closing in needs to be larger, or the ring below it a thicker wall.
Reading the cut list
Each ring gets its own cuts, from the base up. Miter angle is the saw setting for both ends of every segment. Outer segment length is the long side, long point to long point, the length to set your stop block to. Inner segment length is the short side, and segment width the strip to rip before cutting: the wall plus the allowance on both sides.
The lengths use the polygon the segments make, not the circumference, so each ring turns down to its diameter. The Segmented Ring Calculator explains the formula and works out a single ring. Before you mount the bowl, check the Lathe Speed Calculator.
Taller bowls
The free designer here and in Tailstock takes four rings. In the app you can also save a design and share its profile and cut list as an image or PDF. Tailstock Pro adds as many rings as you like, as many saved designs as you like, and shares without the watermark.