Thursday, December 25, 2008

The Perch Stretchers

One of the persistent difficulties to working green wood is understanding its advantages and the processes that can exploit them. How wet? How dry? When do I shape it? It can be confusing to the newcomer.
This installment of the perch building process gives a great opportunity to clear up a few things about green wood. There are only a few reasons to work green wood, and if you take them into account, it should be pretty clear when and how to use it.

Number One: green wood splits and shaves along the fibers which allows for stronger, yet more flexible parts. This enables the thin turnings and solid bends.

Number Two: green wood cuts easily with handtools, enabling quick shaping without the need for powertools

Number Three: green wood is cheap and easily found, with the only drawback being that trees don't come in squared lumber. Of course, in green wood chairmaking, the parts rarely use the type of flat references that dried lumber provides

When using green wood and making process decisions, I ask myself which of these benefits is essential and which is merely helpful. In making the stretchers for this perch, I have tried to take advantage of the green wood advantages where possible and abandon them where the returns diminish.

There are a few options when making the stretchers for the perch.

One, I could have split the wood green, turned it green, dried it and then sized the tenons to the final dimension. This works fine and has the advantages of the strength of the split wood and the ease of cutting green wood. The drawback is the time it takes to dry down the piece and the need to dramatically reform the tenons before assembly.

Option two is to split the wood into billets, rough shape the stretcher, dry it down and then turn the entire piece once it's dried. Once again, the benefits of the green wood splitting and roughing help out and turning the final shape from a slightly oversize rough wouldn't be too difficult in the dried wood. Plus, I would be able to turn the tenon to the finished dimension at the same time that I turn the rest of the stretcher to shape.

Option three is to saw the blank from a dried piece (being careful to follow the fibers as though it was split), turn the stretcher (not as much fun in dried wood), dry it down, and then turn the tenons to final dimension. This is the option that I chose, mainly because the curly maple that I am using is already in the dried plank form. I am definitely sacrificing the ease of cutting for the benefits of speed in drying (the plank is already air dried) and the ability to cut close to finish dimensions.

Turning is different from shaving, in that the center axis of the turning can be easily made to follow the fibers through either splitting or sawing. I would never substitute sawing for shaving when making spindles or bends.

In the video, you'll see me turn the stretchers with the tenons oversized .025". This is because the wood is already dry and experience tells me that it won't shrink dramatically in the kiln. If the wood was green as in option 1 above, I would oversize the tenons .060" or .080".
I know that talking about thousandths of an inch can be a bit odd at first, but don't be intimidated, it's just a way of talking about the movement of the wood that is tough to describe accurately with out resorting to such tiny increments. My goal is always to turn the tenon to a size that will leave me trimming a minimal amount after drying. The goal is a final tenon of .625" or 5/8". When the piece comes out of the kiln, I want the dimensions of the tenon, as measured along the growth rings to be just a few thousandths oversize, for easy trimming to the final .625". It is of course easier to hit this mark if the wood is dry to begin with, which is why I tend to exercise option 2 above as my general practice.

To sum up, when deciding how and when to use green wood, the considerations of strength, ease of working, availability and flow of process, all come into play. If you find one factor that calls for the sacrifice of another, just make sure that it's one you can live with.



The posting on seat carving is coming up.

2 comments:

Steve in Kansas said...

Pete,

Thanks for the video on turning stretchers. I have a couple questions. . . First, do you make the stretchers a big longer than the measured distance between legs to add tension? (I think I add 5/16 inch when making chairs.)

Second, where did you get that "sizing tool"? I've never seen that before.

Peter Galbert said...

Steve,
I do not add any length to my stretchers to "pre load" them. I can do a posting later to explain my reasons. The sizing tool is available from any turning catalog company. It's sometimes called a bedan sizing tool.