boring hammers

[Stéphane Collin]

Hi beloved list.

I'm preparing to do another hammer change on an old piano, and I wanted to
update my (poor limited) knowledge about good hammer bore practice.
Up till now, I decided bore length from difference between strings height
and center pin height in each section.  I was surprised to see how far you
can go off by doing those measurements, and how much one millimeter does
count here. So I was happy to read about this wonderful shortcut which is to
try in each section a dry fit of a hammer with known bore length, and see if
it is perpendicular to string at contact time (really clever).  This gave me
also opportunity to ascertain that certainly in the treble, the string
height can differ by one or even two millimeters depending on where
precisely you measure it.  Of course it should be at strike line, but again,
how do you do that accurately ?
Now, I have a few questions.  How do you set up your boring jig in order to
make sure that the hammer molding is perpendicular to the shank (that is, if
you aim for that) ?  The molding doesn't seem to have any parallel sides,
and is asymmetrical of shape.  Also, for those tenor and bass hammers who
are bore with an angle, how do you calculate where the bit enters the
molding, in order for it to come out at the back approximately in the middle
of the molding (that is if you aim for that) ?
Do you really adapt bore length at every section, or do you allow say one
millimeter offset in uniformity's sake ?
I read John D comment about the hammer shank length being fixed by design
and so tweaking the hammer rake in order to get the perpendicular to string
hit (if I got that right, John). In the ballpark that we play, is this
really preferable to tweaking the hammer shank length and maintaining the 90
° rake ?
I also experience problems in doing a dry fit : if it is tight enough, it is
so difficult to move the hammer into its right position, while if it is easy
to move the hammer, it won't stay in its right position after the first hit.
Has someone found a solution for this dilemma ?  I have considered gluing
the hammer simply like if it was to stay there, and then ungluing the usual
way.  But maybe there is another less demanding trick ?
All advise welcome, of course.  I'm mad about learning.

Best regards.

Stéphane Collin.