There are a couple of things to consider when deciding about the hazards of overcentering (short boring). I personally do not consider it such a problem for a variety of reasons. First, you are only talking about a difference of 1/16". The standard bore on a Model D is 1 15/16" in the treble and you report a measured bore of 2". Second, the strings are generally not parallel to the keybed. They run slightly uphill to the bridge. Thus, the strike angle of the hammer, if calculated based on the strings running parallel to the keybed, will not be precisely 90 degrees anyway. In addition, shank deflection will also influence whether the shank strikes at 90 degrees. The harder the blow, the wider the angle. There are many makers (Bechstein comes to mind) who have routinely short bored their hammers and raked the angle out slightly to compensate. These pianos do not seem to suffer from repetition problems. For these reasons, undercentering (boring longer than the measured difference between center and string height) is more likely to produce a hammer that does not strike at 90 degrees. Ironically, many hammer makers who bore for you add an extra 1/16" for wear. This extra amount is more likely to cause problems with non 90 degree striking and weight than erring on the other side. The repetition issues have been outlined well by other contributors to this thread and I won't address them. One additional point, though, repetition can be influenced by the weight of the hammer and its influence on movement of the balancier beyond the effects of the repetition spring. The new hammer you are putting on is probably much heavier than the old one. This can cause the balancier to deflect more on unchecked blows such as are found in the Ravel piece you mention. I have found repetition benefits generally from repinning the balancier so that there is more resistance (I haven't really measured exactly how much friction, but more than comes out of the factory). This is not an area where excess friction will be felt in the normal operation of the key. Some additional resistance in this area can prevent excess deflection of the balancier which allows the hammer to reset a bit more quickly. I perform this operation on performance pianos routinely even when putting in new Renner wippens. Shimming the rest cushion with additional felt to insure that there is minimal (no more than one hammer shank) distance between the bottom of the shank and the top of the rest cushion can also help if the slight short boring raises the shank too high. Keeping the angle of the shank from dipping to low is important as is making sure that your bore distance doesn't force you to have to set the angle too low. Try this experiment: weigh off a note at 1 3/4" blow. Now lower the shank so that the blow is 1 7/8" and reweigh. You'll be surprised at the difference. All in all, I would prefer a slight overcentering to undercentering, though I usually bore exactly at the difference between shank center and string height. I can't think of an instance where boring longer than this would be desirable. I can think of an instance where short boring is necessary. That is when the length of the measured bore will not allow you to get the hammers under the pin block when inserting the action. This can happen on some older Bechsteins and others. Before removing original hammers it is wise to measure the rake angle (as you have done). If it is not 90 degrees, there is usually a reason. Awhile back, I rehammered a Bechstein in which the hammer height in each section was all over the place. The hammer bore, if taken from measurements, would have ranged from 1 15/16" to 2 3/16". That was just in the treble section. The bass was proportionately longer. The original treble hammers were bored consistently at 1 15/16" while the bass hammers were bored at 2 1/8" and the rake angle changed in each section to compensate for string height differences. Even with wear on the hammers, the action functioned extremely well. Boring the treble and bass hammers at the measured distances would not have allowed the action to slide in under the block. I decided to duplicate the original bore and rake and found no problems with getting the action to regulate or function properly. Just a few additional thoughts. David Love davidlovepianos@earthlink.net ----- Original Message ----- From: Bob Hull To: pianotech@ptg.org Sent: 8/15/2003 3:57:53 PM Subject: overcentering justified? List, I have a question about appropriate bore distance to see if overcentering is justified, even designed into the action from the first. Here's the background for the question(s): In determining the bore distance for a new set of hammers on a Hamburg D, I have done the usual math to determine a perfectly level shank when the hammer meets the string. According to this the new hammers should be bored at 2" for the treble hammers which is a considerably longer distance than the old hammers are bored at and also longer than the specs I have from Steinway ( 48mm). The old hammers overcenter due to their bore and even more so of course due to their wear. Was this intended by Hamburg in their design? After boring at the longer distance I thought would be better than their old short bore distance, I see that of course I have to lower the capstan to have a blow distance like the old and of course the let off. The result is that 1. The shank is only 1/8 or less of the cushion and 2. The repetition speed is very mediocre. (I can improve the repetition speed on the long bore distance hammer by decreasing blow distance to about 1 5/8".) The old hammers repeated very easily and as fast as you could want. (The old blow distance is about 1 3/4". I searched the archives and read Ron Overs, Richard Davenport, Newton Hunt and Dave Love about the benefits of a higher shank. Also, Sam Powell's article in the Sept. '93 Journal about the reduced friction that comes from the knuckle not being too far below the line. (Effects of Hammer Bore on Escapement Friction) These seem to give some approval to overcentering. But, will there be an unacceptable loss of power? Won't the hammer be sliding into the string rather than striking it at 90 degrees? If I resort to overcentering like the old hammers on this piano, would it be advisable to hang them with a positive rake angle to compensate? Hamburg Steinway specs call for 0 rake. I realize some of this may be a rehash of an old subject, but any good guidelines would be appreciated. Thanks, Bob Hull Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software
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