re:[ATM] my 20"

["Ellen Mackenzie" <mackenzieconsulting@sympatico.ca>]

An alternative to the wood with a low profile is to epoxy a warm ( 120 F )
wire on the outside of the tube. When the wire cools you have a pre-stressed
support for your tube. This is an old trick I learned from a carpenter who
increased the load bearing properties of joists by running 10 ga wire along
the bottom side of the joist with a take-up nut at the end of the joist to
tension it. I make crosshairs this way with 32 ga wire at 250 F, they never
sag. You need not epoxy the whole length of the wire, I work on a shear of
1000 psi for epoxy to calculate the length of wire capture.
	 Choosing the right wire you could also thread the ends for 4/40 or
6/32 threads and with suitable metal ends on your Sonotube you would
increase its' rigidity considerable without the bulk of wood. A note of
caution, go easy with the preload, or you could conceivably collapse the
Sonotube with too much tension. With an epoxy seal on the bearing surface of
the Sonotube, I wouldn't pre-tension over a design load of 200 psi. An
aluminium bicycle rim comes to mind to fit over the end of your Sonotube. It
is a simple sheet metal operation to create a rim of a style of your own
design, form it in the flat, and with a disposable filler piece the size of
the thickness of the Sonotube, roll it into the desired circle and TIG weld
to suit.


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