Hammer boring fit

[John Delacour]

At 8:23 am -0400 14/5/02, Farrell wrote:

| I always thought that with wood glue AND with hide glue, a well-fitting
| joint is important, and that hide glue does NOT have good gap filling
| properties. No?

Yes and no.  In hammer-head fitting, a very tight joint is essential;
that's why the shank should be compressed by knurling to enable the head to
be rotated vertical after gluing.  As soon as the glue is applied, the
shank begins to swell back to its _natural_ diameter and to close up tight
against the boring.  That's why it should be impossible to rotate the
hammer any more after 20 -30 seconds -- not because the glue has stuck but
because the joint is a compression joint.  Water alone would give exactly
the same effect.  Once the glue is hard and dry, there is a certain amount
of residual elasticity in the joint, so even if the assembly is subjected
to prolonged low humidity, the joint will not come loose.

This is not the case if you are trusting to the glue to fill gaps.  Glue of
whatever quality is hygroscopic and will expand in the damp and shrink in
the dry.  After a while, unless humidity is perfectly stable, the head will
begin to click and that's guaranteed.  It is therefore important, as it is
with centring, to make sure that all the action parts have been kept for a
good while in proper conditions of humidity before the work is commenced.
It's no good keeping the action in a cellar and centring and fitting
hammers there and then expecting things to stay right after a month in the
concert hall.

Yes, thick hide glue has good enough "gap filling properties", but only if
it's the sort of gap that can shrink as the glue shrinks, which it can't in
the case of a dowel in a hole where those conditions that cause the glue to
shrink cause the gap to widen.

If the shanks have been properly cut to length and the heads precisely
bored, there is no need for any fore and aft radial movement.

JD