Segmenting--Making a bowl from a flat board

by Fred Holder

I began doing segmented work several years ago using information found in the Fine WoodWorking on Faceplate Turning book. There are several articles on the subject in various levels of difficulty in this book. Some of my first pieces were pretty sad and several of them flew apart when turning. Gradually, I worked out a way to make them work. I never did get into the fancy, decorative vessels that a number of people are making these days. My bowls were simply functional and made from a flat board. Several were made from mahogany, I had a supply of 1" x 2" left over from some other project and it was good practice. I also made several from pine.

I seldom do segmenting anymore because of the time required to complete a project and, in addition, I also seem to have plenty of wood large enough to make the bowls that I want to make. I believe this comes with time at turning, people hear that you turn wood and ask if you would like some of the tree they just cut down or the tree that they just pruned. Beginners generally have more trouble finding such wood. With experienced turners, I believe the wood tends to find them.

I'm going to briefly outline the process that I go through in converting a piece of flat board into a bowl blank. Once its a bowl blank, you simply turn it like any other bowl blank, except that this blank is already hollowed and you already have a lot of time invested in it.

Method of Layout to Determine Segment Size.

I begin one of these bowls with a full sized drawing of the bowls shape similar to Figure 1. I like to do this on grid paper with 1/4" grids. The base of the bowl should set on one of the lines. Then, I rule lines every 3/4 inch up the bowl. The 3/4 inch sections between two lines will be a segmented ring. I generally use 1-1/2" wide stock to cut the segments from; therefore, I mark a 1-1/2" area covering the bowl wall with approximately the same amount on each side of the bowl wall. I do this for each wall. At this point, I've defined the dimensions for that segmented ring.

I repeat this operation for each 3/4 inch slice of the bowl. On small bowls, an eight segment ring is adequate; however, as the size of the bowl increases, so must the number of segments to cut down on the wasted stock. If eight segments are used on a larger bowl, then the stock must be considerably wider than 1-1/2 inches. On an eight inch bowl, I would use about 20 segments in each ring. A larger bowl would require even more.

One way to get a feel for this is to make another drawing for each ring. This drawing will be a top view of thebowl and a specific ring. Draw two circles to define the wall of the bowl. The outer circle should define the top outside of the bowl wall in the area where the ring will be located. The inner circle should define the bottom inside of the bowl wall at the bottom of the particular segment.

Segmented Bowl made from White Pine. Contains 108 segments. Diameter is approximately 8 inches.

Top view of Pine Bowl. The very bottom is a round plug. Grain orientation is the same as the Segments.

Then draw a circle that defines the inner dimension of segmented ring and another circle that has a diameter that is 1-1/2" larger than the inner circle. Draw a radius line from the center to the outside ring. At the intersection of the radius line and the outside circle draw a tangential line (90 degrees to the radius line that just touches the circle). If you make this a vertical line on grid paper, then the tangential line is easy to dray. The radius line is the center line of a segment. At this point, create another tangential line that just touches the inside line, the line, which marks the bottom of the segment.

This may seem like a lot of work, but if you want things to come out right, it is necessary. Now, decide on the number of segments you want to use. On an eight inch bowl, 20 is a good number.

Divide 360 degrees by the number of segments to obtain and included angle of 18 degrees for a 20 segment ring. This means that the angle each side of the radius line must be 9 degrees. Measure 9 degrees on either side of the radius line and make a mark. Now, draw a line from the center to the outside circle. Where these lines intersect the tangent lines drawn at the inside circle and the outside circle, define the outline of your segment.

If there is not enough material in the ring to allow for adequate turning, then you either need to use wider stock to cut the segments or you must use more segments in each ring to make better use of the stock. Hopefully, the illustration will make this all clear.

You need to make a drawing for each ring that will be used to make up your bowl blank. Be sure and identify these with the drawing so you don't get confused. On each drawing you will have defined the shape and size of the segments that will make up that ring. That becomes your blueprint for making the ring. Mark the drawing with an I.D., such as 1, 2, etc. or A, B, etc. Then when you cut the segments for a particular ring, mark each segment the same way. This keeps you from getting them mixed up at assembly time.

There is a lot of ways to cut the segments. It mostly depends on what type of saw you have available. If you have a bandsaw, you can transfer your segment drawing onto cardboard and make a template to mark the segments with. You can mark the segments and then carefully free hand saw out each segment. Or you can make a jig to cut them at the specified angle or use your miter guide if it is any good. Then you need only to mark the place to cut with your template. You can use a chop saw (i.e., miter saw) to cut the segments if you have one. Again, you have to set this up to get it exact. Your template can help here. Some people also make a sliding jig for their table saw. This is probably the most accurate method once is it properly set up. Over the years, I've used my chop saw, my table saw, and my bandsaw for the job. They all work!

Once the segments are cut, they need to be assembled on a flat surface into a ring to make sure that they are ok. At this point, you can mark one or two, if necessary, that will be modified slightly when sanding. I try to sand each joining surface on a disk sander. It would be best to have a table that was square with the disk; however, I normally do my sanding freehand. When all pieces have been sanded and the fit checked and found to be good. I use a piece of wax paper on top of a flat surface for the glue-up. I place glue on one of the faces and rub the joining face against it to distribute the glue. Then I press them together tightly and lay them down on the wax paper. I continue to make up two's and then do the same thing to make up 4's and so on until all joint areas have been covered with glue and the ring is assembled on the wax paper.

At this point, I apply the clamp made up from as many hose clamps as is necessary to surround the ring. I tighten the ring, wipe off the excess glue and tap down any high points with a mallet. I generally let these dry for 24 hours before I remove the clamps.

Incidentally, I've used a number of different glues, epoxy, Hot Stuff CA glue, Hot Glue, Carpenters Yellow glue, and Tightbond II. I've been most satisfied with Tightbond II, I've never had a properly glued ring fail with it. I have had joints pop with the other glues.

After all of the rings have been glued up, I face off each side of each ring, either on a disk sander or I chuck it up on the lathe and face each side to give a good glue surface for joining the rings together. I also never let the glue line of one ring align with the glue line of an adjoining ring. To do so creates a weak area as the end grain glue job between segments is not as strong and the side grain glue job between the rings.

Small Segmented Bowl made from offcuts from a cabinet shop. The segments are all the same size. The bowl was given an expanding shape by inserting thin slices of wood in between segments. Wood is oak, apple, and walnut. The outside of the bowl was turned to give it a basket shape.

I generally glue two rings together since they are easier to clamp this way. Once I have a group of two ring assemblies, I usually use the lathe as a clamp to assemble all of the pieces together. A number of turners I know have taken old cider presses and modified them into a glue-up jig that works very well. One must remember that segmented work is not production work. You may be able to cut the segments in a production mode, but the production techniques end there.

This is really a much more complicated subject than I had realized. I was a math major in college and have had many classes in mathematics so that the math required to deal with segmented designs are not that complex to me. However, trying to explain this process without extensive use of mathematics has been much more difficult than I had expected. I can only hope that you readers can wade through this dissertation and understand what I'm trying to tell you. I have made a drawing to accompany the explaination to help you understand how to approach the design of each ring.

This has been just the basics of segmented bowl construction. If you want to add designs with different colored woods, etc. You must work those designs into the segments and your drawing job becomes even more complex. Without a drawing, you simply can't make a design come out the way that you want it to. Segmented work requires considerable design on paper before you even begin to cut your wood. Good luck and happy segmenting!

Note: Incidentally, I forgot to mention, that I generally make the bottom piece a solid piece of wood. This adds additional strength to the assembly.