Tenons and Variations

Projects: Thin plate; Plunder bowl.

One of the routine tasks in turning is to form a tenon so that a bowl, platter, or other workpiece  can be mounted in a scroll chuck. These tenons typically extend out from the workpiece and are removed after they have served their purpose.

In many cases it is possible to form a tenon that lies within the overall volume or on a surface that surrounds a part of the workpiece. For example, a tenon formed inside a bowl, before the hollowing is completed, will allow the blank to be reversed in order to work on the bottom surface or the foot of the bowl.

This article describes several possibilities for using a tenon in situations where it might first appear that a different chucking method is required. Many turners use these techniques on a regular basis so the ideas are not new.

Inset Tenon

What I call an “inset tenon” is one that sits below a flat surface. What you do is mark the diameter of the tenon and then form a groove around that diameter so the jaws of the chuck can reach into the groove and grip the resulting tenon.

The objective of forming such a tenon is usually to allow the workpiece to be reversed in the chuck so you can work on the other side of the blank. If you have a tenon on each side, you can switch the blank around to work on one side and then the other as you refine the shape of the piece.

Waste Block

Typically, part of the thickness of a turning blank is “used up” in forming a tenon. This is not desirable if the blank is rather thin and you don’t want to sacrifice a portion of the wood. An option in this case is to attach additional wood to the blank for the sole purpose of forming a tenon

A waste block is a disk glued to a blank so that a tenon can be formed without sacrificing any of the blank’s thickness.

Some effort is required. First of all, a suitable disk must be prepared. The surface onto which it is attached must be smooth and flat. This requirement can sometimes present a bit of a challenge if the blank is very rough and irregular. Finally, the disk must be accurately positioned and clamped in place while the glue cures. Usually medium or thick CA glue or 5-minute epoxy is used to attach the disk.

Here is a shortcut. Instead of turning a disk and then attaching it to the blank, simply attach a rough-cut block to the blank and then shape the waste block and tenon after the glue has set up.

The photo at right shows a thin blank for a plate with a waste block attached to each side. The waste blocks allow the blank to be reversed in the chuck at will as the top and bottom profiles are developed.

An inset tenon can be used in lieu of one of the waste blocks. It works just as well and is easier and quicker to do.

A project: the use of waste blocks and inset tenons is illustrated in the instructions for making a plate from a thin blank, presented at the end of this article.

Truing up a Thin Rough-Sawn Blank

The use of an inset tenon is particularly helpful if the surface of a thin blank is unusually rough, like straight from the cut made by a chainsaw. Otherwise, getting a surface flat enough to attach a waste block at the outset is a bit of a problem.

Because the blank is thin, it is not desirable to use a screw chuck or a faceplate (screw holes) nor to form a traditional tenon. The next option is to jam chuck the blank against a flat surface, but the presence of the tailstock live center complicates the process of flattening the surface.  

This is where an inset tenon is ideal. If the tenon is formed on the face of the blank that will become the top of the bowl or plate, and therefore be hollowed out, no thickness will be sacrificed. Further, the surface of the face of the tenon does not have to be smooth because it will not make contact with the chuck jaws. Therefore, the presence of the tailstock live center does not present a problem.

Here is the sequence of events.

First, jam chuck the blank against a flat surface with its smoothest side against the jam chuck. Begin leveling the exposed surface to remove the major humps and bumps. Then reverse the blank and do the same for the other side. Play this by ear. Depending on the roughness of the surfaces, you might be able to skip the step of reversing the blank.

Jam the side of the blank that will become the bottom of the plate or platter against a flat surface.  Cut an inset tenon on the exposed surface.

Once the inset tenon is formed, you can mount the blank in a scroll chuck and flatten the other surface as required to attach a waste block. Once it is in place, you will have a means for mounting the blank with either side exposed, and you can proceed normally with the rest of the turning.  

This technique also works quite well for a thin blank that is badly cupped.

A Tenon inside a Large-Diameter Foot

An inset tenon can used with a bowl, platter, or any piece with a large foot. This technique is helpful if an effort is being made to retain as much thickness of the blank as possible. It is assumed that the region inside the foot will eventually be hollowed out, which will remove the tenon.  

A Tenon outside a Smaller Foot

If a workpiece tapers down to a fairly small-diameter foot (or other feature), a tenon can be formed that surrounds the foot. This allows you to minimize the thickness or length of the blank lost to a chucking method without having to use a waste block. Put differently, the foot of the piece extends down into the tenon.

The usefulness of this method depends on the maximum diameter that your chuck jaws can grip. The gripping diameter must obviously be larger than the diameter of the piece at the point where the tenon is formed.

The diagram at right is from the article on making rolling pins. It shows a tenon placed on the side of the blank. The tenon will be removed when the blank is turned (between centers) down to its final diameter.

Here’s an example that uses both an inset tenon and a tenon that surrounds part of the piece. It is part of a light fixture, specifically a hub from which three brass tubes extend out and down at an angle. Sockets for the lights are placed at the ends of the tubes.

The outside tenon was formed by jam chucking the trued-up blank against a flat surface. The blank was then mounted in a scroll chuck in order to form the inset tenon on the opposite side. Once the tenons were formed, I could switch the piece back and forth as I nibbled away at the final design.

Toward the end, after the holes were drilled, etc., the piece was mounted with the chuck jaws gripping the outside tenon so the inside could be hollowed and the inset tenon removed. It was then reversed by expanding the chuck jaws into the hollow, which allowed the outside tenon to be removed.

A Tenon inside a Roughed-Out Bowl or Platter

If a blank for a bowl or platter is roughed out from green wood, you can bet that the blank will warp and “go oval” as it dries. When you come back to finish turn the blank, the first requirement is to true up the tenon so the blank can be mounted reliably in a scroll chuck. This is often done by jam chucking the blank against a flat surface.  

In anticipation of having to true the tenon, I frequently leave a tenon on the inside of the bowl for use when the tenon on the outside is being turned true. Instead of using a jam chuck, I install the inside tenon in the jaws of a scroll chuck and at the same time use the live center of the tailstock as if the blank were being jam chucked in the usual manner.

I see two advantages in doing it this way. First, a stronger grip is applied to the blank than would be the case with simple jam chucking. This might be important if the blank is big, considerably out of balance, and if it has warped significantly.  

Second, during the drying process, the rim may warp, sometimes dramatically, and the body is almost certain to go oval. Both of these can complicate jam chucking and cause the trued-up tenon to be at an angle to the axis of the piece. However, neither of these is an issue if you use the inside tenon because neither the rim nor the body is used.  

Leaving and using the inside tenon does not guarantee a trouble-free re-chucking of the piece. Sometimes the inside tenon will tilt off to the side, especially if it is rather tall. This almost forces a return to jam chucking. Because of this possibility, I always turn the inside tenon down to a level below the rim of the bowl so it will not interfer with jam chucking against a flat surface if that becomes necessary.

The inside tenon will go oval along with the rest of the bowl and therefore make it impossible for the chuck jaws to grip it securely. However, because the tailstock is also used as in pure jam chucking, all that is required is for the chuck to be able to spin the bowl. This is not a problem.

Conventional wisdom says to rough turn to a uniform wall thickness, including the bottom of the bowl, in order to reduce the likelyhood that the blank will split or crack during the drying process. However, I have never observed any checks, cracks, or warping that I believed to be caused by the presence of the inside tenon.  

Once I get the outside tenon trued up, I can turn the bowl around and true up the inside tenon as well.  At this point I will have a tenon on both sides which makes it possible to flip the bowl around as I wish in order to work on either the outside or on the inside.  

At some point the inside tenon has to be removed. I usually do this after the outside profile is shaped and sanded to completion. I then turn the bowl around and work on the inside.

A Tenon Surrounding a Recess

This is just an application of the method described above, but in this case, the recess will be intended for a matching tenon on another piece where the two are to be joined together. (Think mortise and tenon.)  

An example that comes to mind is that of an elevated vessel where something like a shallow bowl or tray is placed on a pedestal. In fact, this technique is used in one option for making the cake stand described in a different article on this site.  

The idea is simple: create the recess, perhaps by drilling with a Forstner bit, and then form a tenon that surrounds it. Another way to look at it is to consider the recess to be formed at the center of a tenon. Of course, the tenon has to be large enough so that it can eventually be removed without interferring with the design of the piece.

A Tenon on a Tenon

An interesting exercise in off-center turning involves placing a small tenon on top of one that is larger, and having the smaller tenon offset from the center of the larger one. One application of this technique is to turn a bowl and have the inside hollow offset from the center so that the wall thickness is uneven around the circumference of the bowl.

This method is described in detail in Part 2 of this article.  

Project:  Plate from a Thin Blank

The main consideration in this case is not to use up part of the thickness of the blank just to form a tenon. The traditional approach is to attach waste blocks to both the top and bottom of the blank so it can be mounted on the lathe with either the top or bottom facing the tailstock. Using an inset tenon avoids having to use one of the waste blocks.  

Step by step:

1.  Jam chuck the blank against a flat surface with what will be the bottom of the plate against the jam chuck. The live center will contact the center of the top of the plate.

2.  Mark the diameter of the tenon. Cut a groove outside the diameter from 1/4” to 3/8” deep.  Square up the side of the resulting tenon.  If the tenon is for dovetail jaws, cut the angle that forms the dovetail on the side of the tenon.  

And that’s it. You now have a tenon which will allow you to mount the blank in a chuck. Observe that you did not have to flatten the surface as you might have had to do for a waste block. This is a good thing because the presence of the tailstock would complicate the flattening process.

3.  Mount the blank in a scroll chuck using the newly-formed tenon. The bottom of the plate will now be facing the tailstock.

4.  Attach a waste block to the bottom of the blank. First, level the surface. This is now a straightforward process using turning tools and perhaps a sanding board because the tailstock is no longer in the way. Then prepare and attach the waste block.  

5.  Shape the profile of the bottom of the plate, working around the waste block, of course. Typically I will sand the bottom to completion and may apply the first coat of finish.

6.  Reverse the blank in the chuck, using the waste block. Hollow out the top of the plate, which will result in the removal of the inset tenon. Sand to completion and apply a finish if you wish.

7.  To remove the waste block from the bottom, reverse chuck the piece using a jam chuck, a vacuum chuck, Cole jaws, or whatever method you prefer.  And that’s it.  

 [Back to top]             [Go to Part 2, the Plunder Bowl.]  

t