Bandsaw Fixture for Dissecting Bowls (An original design)
The question that comes to mind is, Why would anyone turn a bowl and then proceed to cut it apart? In this case, the bowl itself is not the intended result; it is used to provide components for a sculpture that may contain many other elements in addition to sections of a bowl. The technical problem is how to dissect a bowl, safely. A bandsaw seems to be the tool of choice, but the problem lies in supporting the wood near the point where the cut is being made. This support is very important. Without it, the bowl can rotate sideways, put the blade in a bind, and be pulled downward with great force. The fixture is shown in the photo. It consists simply of a pedestal that surrounds the blade. The pedestal is attached to a base that is clamped to the bandsaw table. The bowl to be dissected is fed into the blade across the top of the pedestal. If the bowl is maneuvered so that it always makes contact with the very top of the pedestal, it will be supported exactly at the cut, which is a stable situation. Making the Fixture. Turn the pedestal. Decide on the height the pedestal should be for the depth of the bowls you plan to cut apart, taking into account the dimensions of your band saw. The bottom should have a diameter large enough to be stable when you make the initial cut to form the slot for the saw blade. For my fixture, the pedestal is about 6” high and has a base about 4” in diameter. Here’s one method for turning the pedestal. Cut a blank of suitable diameter about 1.5” longer than what you want the height of the pedestal to be. Use a spade or Forstner bit to drill a recess in the base end of the blank. Make the recess about 3/8” deep. Mount the blank between centers with the base end toward the tailstock. True it up. Flatten the end near the tailstock. It doesn’t hurt if it is just slightly concave, and it doesn’t matter if part of the recess remains. Begin shaping the profile, but leave enough wood to turn a tenon at the headstock end. Form the tenon, then remove the blank from the lathe. Put a scroll chuck on the lathe and then install the blank with the jaws gripping the tenon. Use the tailstock to help center the blank in the chuck and to provide additional support. The use of a scroll chuck at this stage allows the tailstock pressure to be reduced in the following step. Finish shaping the profile. At the headstock end, turn it down until just a nubbin is left holding the piece on the lathe. Relax the tailstock pressure a bit when the nubbin starts getting small. Remove the piece from the lathe and remove the nubbin by hand. Smooth the end, and it will be done. Attach the pedestal to the base. I made the base out of scrap 1/2” plywood. It is just slightly wider than the base of the pedestal and is as long as my bandsaw table is wide, which is 21”. Four screws are used to attach the pedestal to the base, located so they don’t get in the way of the blade in the following step. Make the cut for the blade. Once the pedestal is attached to the base, it is pushed into the blade to make the slot. It is here that the stability of the base is important. It must be wide enough so that it does not tend to tip into the blade. Arrange a convenient means of clamping it to the bandsaw table. I use a couple of wood clamps. Two small blocks placed underneath the table make the clamps easier to set. Using the fixture. Maneuver the bowl over the pedestal and into the blade in such a manner that the bowl is supported at the point where the cut is being made. The blade will then enter the wood very nearly perpendicular to the surface. This perpendicularity in itself is not important but it serves as an indicator that the wood is being supported near the cut. Be careful not to let the bowl tip sideways and bind the blade. While this is important, it is usually not a problem because of the rather thin wall thickness of a typical bowl. For your first cut, my suggestion is to try a piece of flat stock. Run a board or scrap piece of plywood across the pedestal. You will observe that if you keep the board level, the blade does not tend to tip, turn, or twist the piece at all. This is a great confidence builder. Here is an issue relating to the wall thickness of the bowl you intend to dissect. If it is less than about 1/4”, you may need to change the blade on your bandsaw to one with more teeth per inch than the 3 or 4 tpi most woodturners use for cutting blanks. A general recommendation is that at least three teeth should be engaged in the cut. Otherwise, a very rough surface is likely to result because of the difficulty in maintaining a smooth and steady feed rate.