There are two ways to skin this cat. The cat being a segment made of two solid pieces of wood glued together to make a single segment. Let’s say there are 12 of these segments or 24 pieces of wood in total to make a ring. Each of these 12 segments MUST have edges whose angles are a perfect 15 degrees. Before we begin, though, it is important to make sure that the grain orientation is always the same - flat grain with the grain going around the bowl. If you made these spaces the really easy way, you would have vertical grain segments next to flat grain segments and the glue strength is now gone.
The hard way (type 1) to do this is to make two angled segments. The angles of the two segments will be very non-standard. You won’t be able to cut them on a wedgie sled unless you have a CNC to make a wedge for that angle. The same is true for the other angled segments, When you glue these two segments together, they MUST equal 15”. the result is that you will have to make the rings in halves and the greatest invention for segmenters has been the wedgie sled that makes that unnecessary.
The easy way (type 2) is to make a single segment of whatever SEL you like at 15 degrees. To that you add a solid piece of wood of whatever width you want. That piece MUST have parallel sides. When you glue that piece of wood to the segment, It will have a combined SEL of whatever you like and the pair will make a single segment with perfectly angled 15 degrees edges.
If you were to make a bowl that had some rings of type 1 and type 2, you wouldn’t be able to tell which rings were which as they will both have horizontal grain going around the bowl.
However, there is a difference and this could be a big difference. With type 1, the edges of every segment point at the center of the bowl. With type 2, None of the edges will point to the center. If your bowl is a column, it simply won’t matter. With type 2 and a wall profile of any shape, at the elevations where the profile is changing, you can notice a curvature of the visible joint. This could be a distraction or a design element. I would certainly do this on purpose - purely for the interest value. People will wonder how you made segments from curved wood.
To see exactly what this ring will look like, use Woodturner PRO where you can look at the top of individual rings with the Ring View. You can also do this using two different Ring Types. The Flat type simply shows the spacers glued to the segment. If you make the ring using the Wheel Type, you can still specify the segment and the spacer (the spacer is the wheel spoke). The Ring View of this will show you that the spoke is the predominant piece and the segment is simply the pices of wood that allow the spokes to be oriented to the center. Even though the spokes are oriented to the center, the parallel sides of the spoke fall on either side of the center. This is what will cause the curvature of the lines when turned but in a typical profile, the curvature would be very small.
Lloyd