Now that the business end of the arms has been finished, it’s time for the tenons that fit through the body. The mortises are to be cut to 1 1/2” square, and the tenons must be sized to fit. Square joints are used to keep the arm from spinning in the body. Also, two of the arms have their tenons offset to one edge, to allow for changes in distance and angles by simply rotating them in their mortise. This mortise and tenon joinery is unusual in that it is to be intentionally loose (sloppy). A certain amount of play is required to give the dummy the proper action when used, and I’ll have to determine this by trial and error. So, I’m cutting the tenons first and will tweak the tenon-to-mortise relationship as I bore and square up the mortises.
I started by making a 1 ½” square template out of thin EUS (Exotic of Undetermined Species) that I had lying around. This was used to lay out the cross-section of the tenon on the end of the stock. The one in this picture is centered, while the other two’s corners touch one side of the circle. After marking the square, I used a straightedge to extend two parallel sides to the edge of the circle.
Once the pieces were marked, I placed the arm back in the lathe, and used the tool rest as a straightedge to continue the marks down the sides of the blank. These lines create the cut lines for the first two sides of the tenon.
Once all the pieces are marked, it’s time to break out the saw. Some people would come up with a jig for the bandsaw, but I decided to just bite the bullet and do it by hand. I’ve got a Henry Wilson & Sons ripsaw from the late 1800’s, and this sort of deep cut is right up its alley. I just clamped the piece in the vise with one side butted up against one of the guide rods for stability, and placed a step block in the other end of the vise to control racking. Then, it was just a matter of staying on course. This was a bit tougher than you might think. Since you’re cutting a chord of the circle, there’s more wood on one side of the cut than the other. This makes the saw want to lead towards the outside. However, I discovered that by pulling the kerf open a bit with my free hand, I could torque the saw a bit in the cut and ease back on track.
After two sides were cut, it was time to lay out the cuts for the remaining two sides. In this case, a marking gauge works fine. Just be sure the fence of the gauge is deep enough to register properly on the side of the blank. Since this is effectively a square cut with equal amounts of wood on both sides, leading wasn’t a problem.
The last step was to even up the shape with a plane and voila! Three arms ready to install. After the mortises are bored and fitted, I still have to cut a mortise and wedge on the back end to hold the arm in the body, but this will have to wait until I know the exact location.
In the next installment, we’ll be back to work on the body for some filling and smoothing, and hopefully to bore the mortises.