Last week I did a demo and show-and-tell on multi-sided bowls for our wood turners club. This was our first meeting in our new location (the Monona Grove HS wood shop), and was well attended. I demonstrated how to turn one type of multi-sided bowl, and did a CD/slide presentation of all three techniques. I won't go into the turning details here, but will post some examples of each style of bowl - they are very different.
The first technique involves turning the multiple sides (three to as many as you can construct) by using off-center axes, and then turning the inside and outside of the bowl proper on the center axis. Here are examples of a 3-sided cherry bowl and a 3-sided walnut plate done this way:
The second type of bowl is a three-sided bowl that is turned from a precisely cut cube of wood. (This technique can be used only to turn a three-sided bowl.) Two opposing corners of the cube are cut off with a band saw sled having very exact angles, in order to mount the blank accurately between centers, and you go from there. Here are three examples: the painted ones are cedar; the natural one is birch.
The third style also can have as many sides as you want, from three on up. In this straight-sided design, the polygon is drawn on the blank (or on a glued-on paper template) and then the sides are cut off with a saw; accurate layout and cuts are essential. The bowl is then turned, preserving the straight sides. In the examples below (both cherry), the straight sides form the rim in the 5-sided version, whereas in the 3-sided bowl the wings have been "dropped" part way down the side of the bowl, and are also curved down.