A Pictorial Article by Bob Hamilton
This is an old technique whereby you can turn a fairly large bowl from flat boards with almost no waste. Normally the rings are simply glued together with a butt joint, but to add an interesting detail I have chosen to machine a sliding dovetail joint on the edges of the boards before cutting the rings.
Photo 1: Stock preparation
These boards are about 5 ½” wide and about an inch thick. The faces and the joining edge have been jointed and planed. A sliding dovetail joint has been machined on the edge.
Photo 2: Laying out the rings
Here I have slid the joint together and I have laid out concentric circles on the face of the stock. The centre point for the circles is at the centre of the dovetail joint, not where the boards meet. The pencil lines on the walnut board do not show up very well in the photo, but they are there.
Photo 3: Carry the layout lines down the edge
Photo 4: Finding the cutting angle
Now I have separated the boards again and carried the layout lines down the edge of the board. I connect the diagonal between two of the lines. This is the angle I want to set my band saw for cutting out the rings. In this particular case I made the concentric circles ¾” apart, and this makes the cutting angle fairly steep. If I had spaced the circles by an amount equal to the thickness of the boards the angle would be approximately 45°.
Photo 5: Setting the angle
I set a sliding T-bevel to the cutting angle for use in setting up my band saw. This is not actually the same board as in the previous two pictures. I have inserted a couple of pictures from my second attempt at this to better illustrate the method.
Photo 6: Setting up the saw
I have used the sliding T-bevel to set the table tilt on the band saw. The upper blade guard was removed for the photograph. With the angle set, the half rings are cut from the boards freehand, following the layout lines. I used a ¼” x 6 tpi blade to cut the rings. There are two possible ways to cut the half rings. One way is to place the stock between the band saw column and the blade, starting the cut at the right hand end of the ring and following the layout line by rotating the piece in a counterclockwise direction. Doing that will result in the face of the stock that is down against the table of the saw becoming the top of the ring. That is fine if you planned it that way when doing your layout and left the largest ring a little extra wide. Unfortunately, if you lay out your rings right to the edge of the board the way I did, then cutting it that way will leave a very narrow rim on the bowl.
Photo 7: Cutting between the column and the blade
A better way to make the cut is have the stock on the outside of the blade, starting the cut at the left hand end of the ring and rotating the stock in a clockwise direction to cut the ring. That will result in the face of the board with the layout marks on it being the top of the ring. The extreme outer circle is cut with the table returned to a horizontal position, so the top ring has a square edge and there is lots of material there to form a rim on the bowl. I will try to remember that next time.
Photo 8: Cutting to maximize rim
Photo 9: The half rings have been cut.
Here the rings have all been cut, and you can see there is very little material for a rim on the largest ring.
Photo 7: Ring overlap
Here I have stacked the first half ring on to the base disk to show the overlap that results from cutting the half rings on an angle. This overlap provides a glue surface for stacking and gluing the rings together. The angle of the cut determines the angle of the slope of the bowl walls, and there is not a lot of material there to get fancy, so bowls made this way will be pretty plain as a rule.
Photo 8: Half rings stacked for a preview
Here I have dry stacked the half rings to give a preview of how the finished bowl will look. The next step is to glue together the half rings into full rings. Once the glue had had a chance to cure a bit, I mounted the base disk on some auxiliary plywood jaws on my 4 jaw scroll chuck on the lathe and turned a chucking recess into the bottom face of it.
Photo 9: Base disk mounted to cut chucking recess
Photo 10: Chucking recess cut
Then I stacked and glued the rings together, using some concrete blocks on top of the stack to provide clamping pressure. Once the glue had cured I mounted the bowl on the lathe and turned the outside and inside. I did not take any pictures of the turning of that first bowl, so I am inserting some pictures of the second bowl on the lathe.
Photo 11: Bowl mounted on chuck
Photo 12: Outside rough turned
Photo 13: Inside rough turned
Photo 14: Bowl ready for sanding
Photo 15: Completed original bowl side view
Photo 16: Completed original bowl top view
When stacking the rings for the final glue up you need to keep them centred as well as you can. The better job you do in keeping them centred the more wall thickness you will have to play with during the turning operation. With this dovetail variation of the “bowl from a board” technique, you also need to keep the joint lines aligned between layers or there will be a tell-tale step in the joint on the finished bowl. I used regular woodworking glue on this first bowl, and it seemed to tack up fairly quickly which helped keep the lower layers from shifting while I was aligning the upper layers. On the second bowl I used polyurethane glue, and then clamped the stack down to a plank rather than just putting concrete blocks on top for pressure. The poly glue seemed to be a bit “slipperier” and sometime during the process of applying the clamps I had some slight shifting that I failed to notice, so there are a few joints with a step in them.
Also, on the second bowl I cut the rings in the “proper” direction so I had lots of material to form a rim. I also changed the spacing of the concentric rings from ¾” to 7/8”, which reduces the slope of the bowl sides. What I failed to take into account was that if you start with the same width boards and cut the same number of rings using a wider spacing then you are reducing the diameter of the base of the bowl. What I wound up with was a bowl that is badly out of proportion to my eye, with the massive rim overpowering the narrow base.
Photo 17: Badly Proportioned
Photo 18: Both bowls together
I much prefer the overall form of the original bowl, but I would like to have had a bit of a rim on it. In this last photo I have simply stacked the second bowl inside the first one. I think that it illustrates a better form than either of the two on their own.
Photo 19: The two bowls nested.