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Tenons and Variations, Part 2

A Tenon on a Tenon

Project: Plunder Bowl

For fun, I turned a couple of bowls where the inside hollow was shifted to one side relative to the outside. This produces a rim that varies in thickness all the way around and one side of the bowl is noticably heavier than the other.

My thinking is that these bowls will make ideal containers for pocket plunder (loose change and all that other stuff in your pockets) that must be put somewhere overnight. It is just so not classy to dump the stuff on the top of a dresser. The thin rim on one side together with the added weight on the other makes it easy to retrieve the plunder without tipping the bowl over.  

The way I did it, in very brief terms, was to form two tenons at the foot of the bowl, one on top of the other. However, the second, smaller tenon was offset from the first by about 5/16”. I then used the centered tenon to work the rim and outside of the bowl, but used the offset tenon to do the hollowing. This caused the hollow to be off center relative to the outside of the bowl.

There are ways to produce such a bowl other than what is presented here. For example, the blank may be mounted on a small faceplate, first centered on the blank and then shifted to produce the off-center hollow. This method presents its own set of requirements for the blank, namely that the foot must be large enough to accommodate the faceplate.

Details, details, details . . .

Some planning is required before you jump in and start making shavings. It’s good to use a compass and draw circles on paper to help visualize the shape of the rim. This will help you determine the amount the small tenon should be shifted relative to the center of the larger one.

The difference in wall thickness from maximum to minimum will be twice the offset. This does not depend on the diameter of either the outside of the bowl or the hollow part. A larger offset produces a greater difference and makes the uneven rim more dramatic.

For a given chuck or set of jaws, there is a maximum offset that you can use. This is determined by the range of diameters your chuck jaws can grip. If you use an offset that is too large, the smaller tenon will be truncated on one side where it intersects the larger tenon.  

If the largest diameter tenon your chuck can grip is called “Largest,” and if the smallest diameter your chuck can grip is called “Smallest,” then the maximum offset you can use is one-half the difference between these two:

 Maximum offset  =  (Largest – Smallest) / 2

What this means is that if your chuck opens and closes over a fairly narrow range, you will be limited in the amount of offset the chuck can handle. For example, the Oneway Talon can handle an offset of 3/8”, but the Nova Supernova2 can accommodate an offset of only 1/4”, and that is cutting it close. Of course if you have a larger set of jaws to install for the larger tenon, this will not be a problem.  

Find “Largest” and “Smallest.”

My suggestion is to put a piece of scrap on the lathe and then turn the largest tenon that the jaws can grip when they are fully extended. Measure this very carefully. Next, reduce the tenon to the smallest diameter the jaws can grip, and measure it. With these two numbers, you will be ready to proceed with your design.

Don’t bottom out!

The total width of the two stacked tenons must not be great enough to cause the smaller tenon to bottom out when the jaws are gripping the larger tenon. This means that the width of each tenon must be about 1/4” or slightly smaller because the depth of most standard jaws is about 1/2” or perhaps 12 mm.

Project:  Plunder Bowl

Here is a detailed procedure for making the bowl. But before you jump in, please realize that in some of the steps you will be turning an out-of-balance workpiece. Keep the lathe speed at a low RPM, say 500 or below.

1.  Mount a bowl blank about 6” in diameter and 3” thick on the lathe using a faceplate or a screw chuck. Shape the outside and form the large, centered tenon at the foot.  Be sure to mark the center of the foot. At this point it is a good idea to sand the outside to completion while the blank is still balanced.

2. Reverse the blank in a scroll chuck and flatten the top of the blank.

3.  Remove the blank from the lathe and mark another “center” that is offset from the true center.  This will be the center for the smaller tenon.    

4.  Jam chuck the blank against a flat surface. Use the offset center instead of the true center. Turn the second tenon to a diameter nearly equal to the smallest diameter your chuck jaws can grip. Keep its width to less than 1/4”.  

5. Remount the blank in a scroll chuck using the smaller, offset tenon. Hollow the bowl to the extent you desire. Sand the interior of the bowl to completion.

7.  Reverse chuck the bowl so you can remove the tenons and shape the foot. The easiest and safest way is to use Cole jaws with tailstock support. The issue here is that the bowl will be way out of balance even though the outer profile will run true.

If you don’t have Cole jaws, another “safe” way to do it is to prepare a jam chuck with a hollow that the bowl will fit into. This will trap the bowl and keep it from moving sideways as it will want to do because it is out of balance.

Another less safe method is to jam chuck the bowl against a flat surface with the point of the live center at the true center. Crank the tailstock in “pretty tight” because only the point of the live center will be keeping the unbalanced bowl from being slung off the lathe. Reduce the RPM to as low a value as you deem practical.

Remove the tenons and shape the foot, but leave a rather large nubbin. If you turn the nubbin down too small, it is likely to break because of the unbalanced bowl. This is especially true if you jam chuck against a flat surface. Sand the foot and surrounding area to completion.

8.  Remove the nubbin off the lathe. Apply the finish of your choice and your plunder bowl will be ready to press into service.  

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