hammer boring.

[Newton Hunt]

Hi Peter,

Boring hammers is not difficult unless you don't have the
tools for it.  I used to do all my boring myself but no
longer, I don't have the tools for doing so any longer.

If you supply the proper information to a hammer maker or
supply house they can do the job well.  Getting the
information is critical.

1.	Carefully measure the distance between the key bed and
the underside of the first and last string in each section. 
If there is more than 5 mm difference between the ends of
the section you may request the hammers be bored with a
tapered bore distance.
2.	Carefully measure the height of the hammer center pin
from keybed at both ends.  Average these two distances and
subtract this measure from the string height at each section
end.  This is the Bore Distance.  You may average
measurements within each section.  Add one millimeter for
future filing but no more than that.
3.	Bore angles are determined by measuring the angle of the
first and last string in the bass to the stretcher or other
member parallel with the front of the piano, average the two
and divide by two.  Usually in the neighborhood of five to
six or seven degrees.  Using greater angles can cause hammer
passing problems.
4.	Measure the angle of the first string in the middle
section and the last string.  Use an angle of no more than
15 degrees in the bottom of the middle section and usually
the top string and the rest of the set is bored at zero
degrees.
5.	Rake angle is determined be measuring the angle of the
center line of the hammer molding to the center line of the
shank length.  In the vast majority of pianos this is 90
degrees but some pianos used a rake angle.  The basic
criteria is that the hammer is 90 degrees to the string at
contact point.  Anything other will cause a loss of power.

The bore size should be 0.08 to 0.10 mm larger than the
diameter of the shank.  They should slip onto the shank but
not have much excess movement that the glue cannot gap fill.

Other important factors is the weight of the original set

You will need to know the distance of the molding center
line to the shank center pin and determining that the mass
of the hammer is perfectly centered over the center of the
shank.

We here in the USA taper the hammers to remove mass.  If
doing this the taper should run from the strike line to the
tail and from 1. 1.5 mm should be removed from the tip of
the tail to almost zero removed from the strike line, on
both sides.  If this was not done on the original set and
the weight remains proper for the action then tapering is
not needed.

The tail needs to be shaped so it interacts properly with
the backcheck and this should be shaped with a radius of one
half of the distance from centerpin to molding center line. 
About 2.25 to 2.50 inches (sorry I don't have a conversion
handy).

Gluing the hammers on is a bit tricky.  First set a sample
at the top, #87 or #84 on older keyboards.  I use saliva to
wet the shank then put the hammer on the shank.  The wood
should swell enough to hold the hammer on firmly.  Install
the action in the piano and move it in and out until the new
hammer produces the loudest and clearest sound.  Install the
keyblock to determine if the action will move to a new
location.  Change the hammer location to accommodate the
keyblocks.  When the action does not move from the optimum
sound location then the sample is set properly.  I make sure
the hammer is perfectly vertical and the shank to molding
angle is perfect then I use a little CA glue to set it
permenately in place without removing it from the shank.

Do this for the bottom of that section and for the top and
bottom of the next section down.  Check for a nice straight
line.  Do not worry if it is not perfectly straight, it is
so on few pianos but if it is off by more than 3 mm then
re-evaluate and double check locations.  If you are not
certain move the shank so the hammer is only striking two
strings.  This often will help determine location.  I
usually glue the rest of the set on in a straight line from
the bottom sample parallel with the hammer rail.

Do not stray too far from the original hammer line unless
needful.  There is very little space to play with on some
pianos.

If you wish to do this yourself then the Renner boring
fixture is the best on the market.  Expensive but it truly
is a fine machine.

The best tools for tapering and tail shaping are those sold
by Bill Spurlock, bar none.

It takes time and experience to do this well and if you
don't want to buy the tools then have someone else do this
for you.  I just like to do things myself.  I am a bit
stubborn that way.
-- 
		Newton Hunt
		Highland Park, NJ
		mailto:nhunt@jagat.com