Chucking - Recess or Spigot?
Russ Fairfield
>There is no right or wrong way to grip the wood. Both the spigot and internal dovetail have their advantages. Which is used is more a matter of personal preference, what you are turning, and the wood that you have, than one of them being "better" than the other. I can describe my preferences. Someone else will have a different opinion.
I prefer a spigot on the bottom of a bowl because the bottom foot can be turned to a smaller diameter than with the recessed dovetail.
Sufficient wood has to be left outside the recessed area to absorb the expansion forces of the chuck, and this means that the base of the bowl will have to be larger than if it were gripped with a spigot. With a spigot the bowl can be turned to the same OD as that of the jaws as they are gripping on the spigot.
The spigot can be stronger because you can squeeze harder on a piece of wood in compression than you can in tension. I like to turn large pieces of wood, and a large and long spigot is the stronger way to hold a heavy piece of wood. Soft wood can be usually held with compression on a spigot with the aid of some CA glue, and there is often no way that it can be held by expansion inside of a recess.
It may appear that a disadvantage for using a spigot is that the wood that is used is usually lost when the bottom of the bowl is finished. This loss of wood can be prevented when a waste-block is glued to the bottom of the bowl.
The only weakness of the spigot is when the wood has a flat grain that runs across the base of the spigot, and the wood is a species that splits easily. In that case it is possible to lose the bowl. Other than that one special case, I have never lost a bowl because of the spigot breaking, and I have lost a lot of bowls from break-out around a recess.
The chucking recess is useful for turning plates and platters because it has the potential for wasting less wood, making it possible to turn a plate from a piece of 4/4 thickness lumber, and it is faster than gluing a chucking-block to the bottom of the plate and then having to remove it. Keeping a large area outside of the recess isn't a problem. If the edges of the dovetail are sharp, the recess can be less than 1/8" deep, and its bottom becomes a part of the larger recess when the bottom of the plate is finished.
An advantage with gripping the wood outward from inside of a recess that is often missed is that the forces of the chuck are pushing outward into the body of the piece of wood being turned. Placing the outer wood in compression will have the effect of reducing vibration. This can be a real advantage when turning things like thin plates and platters because the wood is stiffer over a larger diameter than it would be if we were to squeeze all gripping forces into the center of the plate.
This is the reason why I prefer the recess for large natural edge bowls. Vibration is often a problem and I can use all the help I can get.
Then there are those times when either a spigot or recess is used because that is the only way it can be done.
