Messages Archive

Wood Movement, Large Dowels

Don Stephan
An interesting question was posed yesterday when a person called to asked about 5 1/2" diameter x 18" long wood dowels used to pull through assembled large wire conduit, to check for burrs and similar that might damage the outer jacket of pulled cabling. (It was explained that the dowel would be painted, and any scratches in the paint would indicate the presence of one or more burrs.)

The caller suggested that ideally the dowel would be within 1/16" of the inside diameter of the conduit. But if the "average" amount of total annual seasonal movement is 1/8" per foot of width, a 5 1/2" diameter dowel would have an "average" amount of total annual seasonal movement of almost 1/16", the desired tolerance.

The solution at first seemed to be finding a tree with small radial and tangential rates of shrinkage. Looking through this table in Bruce Hoadley's excellent book Understanding Wood, southern magnolia seemed a good answer, but of course it's not a commonly available kiln dried hardwood, at least in Cincinnati.

Some thought was given to making a glueup to minimize annual seasonal movement. But it doesn't seem likely that gluing several flat, rift, quartered, or even a combination together to get 5 1/2" of total width would reduce annual seasonal movement. Then some thought was given to a glueup of four 3" x 3" x 18" pieces, but again it doesn't seem likely to reduce significantly annual seasonal movement.

So I'm wondering if there is a solid wood option for this? Is a lamination of several pieces of baltic birch plywood, or exterior MDF, the only solution? Each of these would be very unpleasant to turn.

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