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From: Lawrence Zeitlin (no email)
Date: Sat Jul 02 2005 - 09:53:32 EDT
John and Judy write:
<<
I saw a rather sad occurrence earlier this week when a local chap, who is
also doing a refit on his wooden boat, started to refasten the hull below
the water line with silicon bronze screws that he had imported from on of
the well known eastern US suppliers - about every 3rd screw would just about
come up tight and then the head came off. Now we are not talking about
trying to drive the screw blind into oak or such hardwood, we are talking
about a guy who had the correct tapered drill / counter bore drill and was
driving the screws with an old style brace and driver bit. In looking at the
screws that failed we could not see anything wrong like cracks or such, they
just did not seem to have enough strength and these were # 12 X 2 1/2 screws
not little by any means..
Sure glad that did not happen to me as I used a cordless drill / driver set
up and drive the screws (same size) until the drill stalled. In all the
screws I used I only damaged about 20 and that was due to the driver>>
===============
My boat carpenter father-in-law would have been appalled. A brace and bit can apply enough torque to twist the head off any screw. You have to learn to be delicate. Using the stall point of a cordless drill to set the screw is almost as bad for the screw and life shortening for the drill. Most good drills/drivers have an adjustable clutch that limits the torque. Each size screw requires a different torque setting.
Screws are designed to be strong in tension and have sufficient thread area to prevent them from being pulled out of the wood. They are not intended to pull ill fitting planks together. The proper technique for driving a screw is to drill and counter sink an appropriate pilot hole, then LUBRICATE the screw (Ivory Soap works well in wood), and drive it in until the head bottoms in the countersink. Turn 1/4 rotation more. Then STOP. Expansion of the wood when wet will provide additional tension and sufficient friction to prevent the screw from backing out.
And if your wallet can bear the expense, Monel fastenings are better than silicon bronze or SS.
Larry Z
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