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Re: What causes cracksq

Author: Gordon (NZ)
Date: 2003-03-27 10:37

"...you have to wait till it's big enough to pin shut."

IMHO: ......

Cracks are not pinned shut. Indeed, this would be setting the area up for cracking again in an even more highly-stressed location at the ends of the pins.

Pinning is done to STABILIZE the timber, so that a crack does not have a tendency to keep opening and shutting. Then , and only then, can the crack be reliably 'filled'.

This is particularly important if the crack goes through a tone hole, which is almost the norm. Otherwise, every time the crack opened up wider than the filling material the tone hole would leak.

If an unpinned crack were filled while it was in its most 'open' state, then when the timber dried and the timber shrank, there would be at least a strong theoretical possibility of the crack extending into the bore, greatly complicating the problems.

Even after pinning, the timber in the region of the crack can still move, affecting the width of the crack. This is because where the pins actually cross the crack they are quite deep within the timber, and do not have a complete effect in stabilizing the SURFACE of the timber.

Sometimes a crack is rather too narrow to be filled. It can be widened (to a shallow depth) with a minute circular saw (or other means) so that a filler has a chance of getting in, and so that the filler has sufficient width that there is more chance of it accommodating slight variations to the width of the crack. i.e. it is easier for a glue/filler to cope with a width change from 0.5 mm + or - 0.025 mm (10% change), a width change from 0.1 mm + or - 0.025 mm (50% change).

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