Ray Stubbs
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WR Rocks!
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Columbia, Mo. USA
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Segmenting Simplified
Since there are numerous questions about segmenting for bowl making, I have outlined the steps below to simplify the process.
The first step is to draw a sketch of the bowl. This sketch is drawn to true size of the bowl. See sketch below, S1. Notice the centerline on the left side of the page with a cross section of the bowl. If this cross section is rotated about the centerline, then you have a bowl.
Now make a decision on how many segments there are going to be in each ring. This is important because of cutting the segments on the correct angle. As an example, I am choosing 8 pieces to the ring. All the rings in this bowl will be constructed of 8 pieces. I chose 8 pieces because on the weggie sled, it is easy to set these angles. There are triangles you can buy that makes this set up easy to achieve. I highly recommend using a weggie sled on the table saw for cutting the segments. It is accurate and easy to set up.
Now let’s look at the third ring, counting from the bottom up. If the segment rings are drawn as shown in S1, then you have the information needed for cutting the segments. This ring will be at a height according to the thickness of the wood in previous rings as shown in S1. There are 4 items of information needed.
1: T see S3 2: R is measured see S1. Simply measure from the centerline to the outside of the ring. 3: W see S1 and S2, W can be measured in S1. 4: L needs to be calculated but T, R, and W are all measured. (Notice S2 is a view of the segment looking down from the top of the page as indicated by V1 in S2.)
Now it is time for a little math: There are 360 degrees to a circle, so divide 360 by 8 (no. of segments in a ring) and it is 45 degrees. The 45 degrees is the inclusive angle of the pieces to be cut. (See sketch S2). Note: Take that 45 degrees and cut each end of the desired piece at 22.5 degrees as shown in S2. Different numbers of pieces in a ring such as 6, 8, 12 etc. can be used and triangles needed can be found to set the weggie sled. Other numbers may be more difficult to set on the weggie sled because of finding the triangles needed to set the sled. When doing a vessel use the same number of pieces in each ring.
The only thing left is L which is calculated. Notice in S2 that R and W are the same R and W shown in S1 and this represents the radius and width of the segment. L/2=Rx(Tan22.5 or .414). Then L=2x(R x .414) Notice Tan 22.5 is for 8 segments to a ring. If there are 6 segments to a ring, then Tan 30 degrees is .577 and 12 segments to a ring, then Tan 15 degrees is .268. Notice that Tan no. goes down with the increase number of segments. This is high school trigonometry. I didn’t put numbers in for T, R or W because my drawing for the bowl wasn’t to scale. You would want to draw your sketch to scale so that you can measure those features.
Once T, W and L for each ring are measured or calculated, the cutting is ready to begin. Remember the placement of the rings on the sketch depends on the thickness of the wood being used. Each ring can be a different thickness. When gluing the pieces, do two pieces at a time then the pairs can be glued. On each pair, assuming it is a 8 segment ring, check the angle of two pieces with a square since it is 90 degrees or 2 x the inclusive angle. When the 4 segment pieces are glued together, it is a 180 angle, or a straight line. If it is not a straight line then adjustments need to be made by sanding on a disk sander. When the rings are glued, then the top and bottom of the ring is parallel to each other and flat on each side.
When gluing the different rings together, they need to be concentric to each other and the joints need to be staggered, so 4 joints do not meet at the same place. Every other ring the joints should line up with each other.
From S1, notice that the bottom ring has a hole at the center. That is because to get the segments to come to a point is difficult, so the bottom can have a plug.
There is a discussion on this Forum about how to do the bottom. For this discussion I won’t go into that.
Comments and critiques are welcome.
