Designing a Segmented Turning
Everyone is always asking me what program I use to design my bowls. I have never used a woodturning design program, or any fancy charts to work out my segment sizes. I’ve come up with a method that makes layout and design very easy. All you need are a few simple drawing tools, all of which you probably already have in your shop.
You will need both a framing square and a roofing square, which you could even make out of a scrap of plywood. You will also need something to draw circles with. I prefer to use a beam compass – two adjustable trammel heads attached to a strip of wood – because it allows me to draw large circles. Don’t use a string with a nail and pencil to draw your circles. The string can stretch and create an oval rather than a true circle. You want to be as accurate as possible when laying out the segments. After all, any mistakes could force you to modify the shape of the turning, or worse, force you to trash a partially finished turning all together.
The drawing board
The drawing board is a 12″ square piece of MDF or plywood. This board makes it easy to draw your square and angular lines by referencing your two layout squares off the edges of the board. You will also need multiple pieces of paper, which will be taped to the drawing board every time you need to do a new drawing. Make sure that when you tape each piece of paper to this board, the paper is square to the edges of the board. If you plan on making a larger turning you will need a larger drawing board, because you will be drawing the turning out full size.
The first thing you want to draw is a side profile of the turning you wish to construct.
1. Start by drawing a vertical center line. Now draw the maximum diameter of the bowl, followed by the overall height. You should consider the height of your segments when deciding on the overall height of the turning. Once the width and height are drawn, sketch in the shape of the bowl. It is only necessary to draw one side of the bowl.
2. Once happy with the shape of your bowl, divide it horizontally into layers. Next, draw in an even wall thickness.
3. After you have a consistent wall thickness, draw segments on each layer. The segments are drawn oversize to the actual profile of the bowl. I have drawn my segments ⅛” larger than the widest and narrowest part of each layer. You might consider using a greater tolerance, maybe ¼”, for your first segmented project. The extra width of each segment allows for some freedom to adjust your shape when turning, as well as account for anything that didn’t line up perfectly during the glue up. All segments are drawn; note the different segment widths on the different layers.
4. Now that the side profile is drawn, you can start drawing the top view of each layer to work out the exact lengths of each segment. The top view will be drawn on the drawing board. The top view of each layer will have to be drawn out separately, because each layer will require pieces of different lengths. Start with the top layer and work your way down.
Mark the center point of the drawing board and set your compass to the widest part of the segment in the top layer. From the center point on the drawing board, draw the first circle for the top layer. Reset your compass to the narrowest part of the segment in the top layer.
5. Transfer the dimension of the second circle to the drawing board.
6. Now you will have to draw the octagon around the outside of the larger circle. Start with your carpenter’s square and draw the four sides that are parallel to the drawing board, than use your roofing square to add the other four angled sides.
7. Use a straight edge to connect the opposite points of the octagon. Draw a line from the point to the edge of the inside circle. Do this at all of the points.
8. Use your square to connect the points on the inside of the smaller circle to create the second octagon.
You can now measure the length and width of each segment. It’s a good idea to label each drawing with dimensions and which layer of the bowl it is. For this eight-sided bowl, both ends of each segment will be cut at the same angle, but the length and width of the pieces in each layer will vary. The hard part is complete and now you’re ready to machine those segments.
Are you limited to turnings with eight segments? Not at all. Try making your own layout squares with different angles. For example, make one at 30° instead of 45° and you can now draw six-sided turnings as well as 12-sided.
The concept is here and, with a little creativity, the sky is the limit. Although more difficult, it’s possible to draw turnings with an uneven number of segments, such as nine. Drawings that have an uneven number of segments will require a series of angled squares that can be made. The same basic principals remain the same; the only thing that changes is the angle you use to lay everything out.
Try this drafting technique in your shop and I guarantee you’ll find great satisfaction in the fact that you’ve figured all this out on paper. You’ll also gain a better understanding of how segmented turnings are assembled.