At 05:53 -0600 8/1/09, Porritt, David wrote:
>I think that among the technicians on this side of the pond people
>usually refer to torque readings in grams as measured from the screw
>hole. That gives a uniform place to measure for a particular piano.
>While one could measure the distance from the bushing to the screw
>hole and say torque is 4 gram/flangelengths it's usually referred to
>as simply grams/....
Fine, but this is now way to teach novices or people who are not
constantly re-centring, and encourages the sloppy thinking that
infests our whole trade. Experienced people don't need these tricks
at all and do it by feel, as I'm sure you do, but I well remember the
pains I went to to get things just right when I was a beginner, using
a special adjustable spring thing used by the Post Office engineers.
What may be right for a Steinway flange is not going to be right for
a Schwander flange, with or without washer. A Steinway flange has
its centre of gravity about 12 mm from the centre and weighs about
3.05 grams, whereas a Schwander flange with washer has the COG 20 mm
from the centre and weighs 5.20 grams. That makes a huge difference
in the initial turning force already acting at the centre before you
add your weight at the screw hole, which will be 23-24 mm from the
centre in both cases. And then you can make things even vague by
saying the flange should just move when you put the screw in it.
Which screw? The oversize Steinway screw weighing 3.8 grams or the
Bechstein screw that weighs only 2.85 grams.
>On the swings, some people believe that the friction should be
>graduated from bass to treble so they measure the swings the hammer
>makes when dropped from horizontal. Consequently if they are aiming
>for 4 swings the actual friction in the bass will be considerably
>higher than in the treble. No two technicians agree if this is a
>valid way to pin. :-)
Swings may be a fair indicator _after_ the fact, but only if you get
things right. Otherwise you'll be pulling the pin in and out until
you get the right number of swings. I occasionally use swings to
demonstrate stiffness or looseness on an existing action but I'd be
wasting my time to use swings to monitor my work.
I get the feel of the friction when I test the pin in the bushing
befor it gets anywhere near the flange. If the lengthwise friction
is right, then the radial friction will be correspondingly right and
if it isn't it will only be because there is friction from the sides,
which can easily be eliminated. If you're going by swings alone,
side friction will give you a false reading.
For people who are learning or who are out of practice or think
they've lost the knack a simple and very exact way of testing this
longitudinal resistance can be made in about 15 minutes. Get two
strips of wood and drill holes for 3 12mm key leads at the end of the
top strip. Drill oversize holes in the top strip to allow free
passage of the shanks of two (say) Schwander repetition screws that
are screwed into the bottom strip, one at the fulcrum and one
somewhere else. The thing should be so designed that the target
force, which is, say, 120 grams, equivalent to 8 15gm key leads,
balances the counterweights when the pin is pushed against the
overhang at about the middle of its length. Here make a mark. If
the pin moves through the bushings when you hold it nearer the end of
the overhang, it's too loose. If you need to push it nearer the
fulcrum, you need further passes with the broach.
In the days when I had more hair, I would always run the pin through
the hair to lubricate it, as many do. I now push the pin into a
little tub of deer tallow then into the bushings from one side, then
out, tallow, and in the other side, so that this minimal but
necessary lubrication is equal on both sides. I rarely use Protek,
which I regard as a tuners' makeshift.
To sum up, it is both faster and far more precise to measure the
force required to move the pin through the bushings than to use
inexact methods to guess at a force 40 times smaller that is required
to turn the pin.
Try it -- you'll like it.
JD
--
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Delacour Pianos * Silo * Deverel Farm * Milborne St. Andrew
Dorset DT11 0HX * England
Phone: +44 1202 731031
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