Action Geometry

[Farrell]

I gotta problem. Well, actually lots, but one immediate piano related
problem.

After reading Bob Hohf's recent articles on action geometry and reviewing my
Renner action geometry info and Nick Gravagne's info and Journal reprint
info, I have delved into my 7 year old Boston GP178's action.  It appears
that the action is less than optimally efficient. My  analysis shows that my
action stack is set too low. My hammers appear to be over-centering, the
capstan/heel cushion contact lies below the
"magic line" at key rest, and just barely touches it at full key depression
(so it is arguably on the low side), the knuckle/rep lever contact lies
about 3 mm below the line between wip center and hammer center and full key
depression, and the perpendicular from the knuckle wood to the jack center
lies about 4 mm aft of the jack center (on the B+ side) - which is good that
it is on the B+ side, but I wonder if this is too far. Also, sighting down
the action rails, one can clearly see that the middle portion of the stack
sags lower than the far bass and hi treble ends.

I placed hardwood shims under the stack feet to bring the hammer rail up to
a elevation where the shanks would be parallel (or nearly so) to the
strings. I put about 2mm at bass end, 3mm at bass break, 2mm at tenor/treble
break, 1mm at treble/hi-treble break, and no shim at high treble end.

Sight down rails now and all is very straight. Capstan/wippen heel contact
point appears unchanged :-( , perpendicular from the knuckle wood to the
jack center is improved and now lies about 1 or 2 mm aft of the jack center
(on the B+ side). BUT, Downweight  in the most-raised section went up 2
grams (from 45 to 47) and Upweight went down in same section 2 grams (from
26 to 24). Bass and treble had smaller changes, and Up and Down weight were
the unchanged in hi-treble (where no elevation change occurred).

Now what does all this say? I would think my geometry improved, although
maybe I didn't change action geometry much, but rather only the angle at
which the hammer strikes the string. Do I have to pay for better
string-striking angle with action efficiency? My thoughts are now turning
toward considering moving the capstan in toward the center pin to improve
its relationship to the "magic line".

The bottom horizontal piece of wood of the wippen (what is it called?) to
which the heel is attached is nice and horizontal on my Boston. The keys are
nearly horizontal. The capstan is 90 degrees to the key. But my wippen heel
cushion is not horizontal, but rather it slopes upward toward the pianist.
As such, the heel cushion contacts only the back portion of the capstan and
this relationship is exasterbated through the downstroke of the key. Is this
a bad thing? I know that if I decide to move the capstan line I may need to
do something with the wippens. If my heel are all funky because of their
wierd angle, maybe that will help me decide whether to move the capstans if
other info suggests that would be good to do. How important is it for the
capstan/wip heel to cross the magic line at half-keystroke?

Eyegh, yie, yie. What a nightmare - but I sure am learning - I think. ;-)  I
started this crap at 4 am this morning. Maybe I need to get a life.

Terry Farrell
Piano Tuning & Service
Tampa, Florida
mfarrel2@tampabay.rr.com