Hello, I don't understand why this method is still used, and why not in conjunction with crowned ribs. Is it because it is more difficult then (with crowned ribs and 4% moist when gluing) to ascertain a regular , even predictable crown ? I guess that gluing on crowned ribs only does not build enough internal tension to the board to be very efficient is not it ? Just (very) curious. Regards. Isaac OLEG > -----Message d'origine----- > De : owner-pianotech@ptg.org [mailto:owner-pianotech@ptg.org]De la part > de Ron Nossaman > Envoyé : dimanche 31 mars 2002 17:41 > À : pianotech@ptg.org > Objet : Re: Case separation or delamination question > > > > > > > Crowning method was compression, I believe. I didn't see the > piano when it > > first came back, nor before it went out for rebuilding, as this > preceded my > > period of employment at the University. Piano was rebuilt > during the summer > > time and, as previously stated, the humidity in this venue does > hover in the > > 20% and below region for some part of the winter, at least it > did so before > > GWSR (Global Warming Seasonal Recalibration). I installed a > series of > > crown indicator strings between ribs 3&4, 5&6, 7&8, 9&10, and > 11&12 (piano > > has total of 13 ribs). They are held in place with masking tape, and I > > retension them and note bearing whenever I tune. Most > recently, the numbers > > I got were: 3&4 - 1/16"; 5&6 - 1/16"; 7&8 - 1/16" or Flat; 9&10 > - Flat; 11&12 > > - Flat. > > > Compression crowning usually involves force drying the panel to > something in > the 4%MC range (assuming accurate measurement), and gluing on > flat ribs. The > crown comes from the panel expanding with rehydration. In summer > humidity, it's > harder to get them dry, and easy to get crown when they > rehydrate. A soundboard > panel in a room at 70°F and 20%RH (again, assuming accurate > measurement), is at > about 4.5%MC, dropping to 4%MC at 17.5%RH at the same > temperature. The panel is > at a moisture content very near that at which it was assembled, > so it's not > surprising there isn't much, if any, measurable crown. This is > one of the major > problems with this crowning method, and tonal problems in dry > winter conditions > are common with this type of board. The tone should improve some > just by doing > nothing but waiting for the humidity to go up with warmer weather. > > > > > > > With regard to bearing angles, I am currently revising the > method I use for > > measuring downbearing, and so I don't have precise numbers at > the moment, > > however, I am virtually positive that the bearing isn't. Why, is that a > > problem???!! As far as positive on both sides of the > bridge...only in my > > dreams...No, not those, the other ones. > > > That the bearing isn't what? Ideally, you will want both positive > crown and > positive overall bearing. Positive bearing on the front of the > bridge is fairly > critical, less critical on the back, but overall should be > positive - but not > excessive. Yes, knowing the bearing and it's relationship to crown is > important. > > > > > > What do you consider to be "some measurable crown"? 1/32"? 1/16"? What? > > > No numbers, just more than none. If gap = true, then crown - else > not crown. > With a string stretched rim to rim (or cutoff bar) between ribs, > If you can see > a gap between the string and board on both sides of the bridge, > the crown is > positive. If you have no gap, or a gap behind the bridge, but > none in front, > you may have a problem. > > Ron N
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