32) The Blow
Step no12 was adjusting the key height and leveling all 88 keys. Last month we discussed step no31, regulating the jack height. These are the only steps that directly affect the blow distance, or hammer line. Indirectly the repetition springs will also affect the hammer line if the springs are weak. However, we had to make sure the springs were strong enough to support the hammers before the jack height was set. Therefore, we should now be ready to set the hammer blow distance.
Once set, there should be no need in doubling back to readjust this distance. Remember that before all of the jack heights were set, a number of sample keys were regulated for jack height, blow, let-off, drop, and then the regulation was proven to be correct with proper after-touch. I always use the end keys in each section as my samples. First, this allows for any irregularities in the string height going from section to section. Second, it gives me a hammer at the end of each section which has been set to the correct blow distance. Doing step no32 The Blow then becomes a simple matter of drawing a line between the tops of these two end hammers and setting the remaining hammers in each section level to this line.
This, of course, is accomplished by raising or lowering the capstans. If the hammers do not return consistently back to the same height when a test blow is given to the keys, check for weak repetition springs, too much lost motion in the jack height, tight hammershank flange centres, or tight key bushings. Any time that the capstan is changed, it is a good rule to give the key a test blow to see where the hammer will settle in its height. During this test, watch the rebound of the hammer as it comes up off the hammer rest rail/felt. All 88 hammers should rebound with the same freedom. If they do not, stop and check for nonuniformity in the centres, as this will make the action play unevenly no matter how well the action was regulated.
When all of the hammers in each section have been made level to the samples, install the action back into the piano and play each note vigorously. At no time should any one hammer be higher or lower than its neighbours in a section. Any unevenness should be corrected. Incidentally, the practice of raising one hammer to give the proper after- touch to that one key is wrong. In order to have perfect evenness of touch, uniformity of tone (i.e. power), and a uniform repetition, all hammers must be the same distance from the strings. More will be said on this subject at a later time.
In selecting the proper blow distance, look at the tangent relationship between the jack and the knuckle. Ideally, the knuckle should be glued on at 90° to the hammershank, and the jack should be in a straight line with the knuckle core. This would give the ideal of a jack at 90° to the hammershank. The shank should be about a shanks height off the rest felt. If it is a great deal more above the rest felt, make sure that this angle between the jack and shank is checked. An angle of more than 90° will result in a loss of power. What we want is the maximum acceleration of the hammer without destroying this tangent relationship. As has been mentioned before, altering the blow distance or altering the key dip are the only two steps which can be changed to compensate for action wear. Never set the blow distance without also considering the dip. If it is unacceptable to change the dip, and if this tangent relationship of the jack to shank has to be compromised, perhaps you should think in terms of installing a new set of hammers/shanks/flanges.
In the past, I have stated that altering the blow distance directly affects the let-off and drop. This is true, but the amount it affects them is very slight. A technician can certainly set the escapement and then alter the blow without noticing much of a change in the escapement. It is only when the painstaking process of trying to get extremely uniform dip with a block is matched with trying to get extremely uniform aftertouch that one needs to nitpick so closely.
On a new piano, where the hammer line has become uneven due to the balance rail and whippen cushion felt compacting along with the flattening of the knuckle, it is relatively easy to get the instrument back into regulation. One only needs to relevel the keys using precut balance rail punchings, check for strength in the repetition springs, check for correct jack height, and then raise the hammer line back to the original blow distance.
On an older piano, I would caution against making quick evaluations by telling the customer that all it needs is a complete action regulation. Do take the time to check the above mentioned 900 angle between the jack and shank. Will that angle be retained? Next try checking the strike point of hammer no88. Is it good or does the action need to be moved to get good tone? Are the hammers really worn? As a guide, remember that if the hammers have worn 3/32” you can expect the hammer strike line to change about 1/32” forward.
This brings up another question. Are the hammers overcentering? Were the hammers properly bored in the first place for this piano? What do all of these things have to do with setting the blow distance? Remember that the blow distance can be decreased to compensate for the action wearing. I stated in a previous article that I would never alter the blow by more than 1/8” to 3/16”. The reason is that good results would be hard to obtain if the blow is that far out of regulation.
Let us talk for a moment about how the manufacturer should have installed the hammers when the piano was made. From there we can talk about SUch things as strike point and overcentering. First of all, the hammer should have been glued on at 90° to the shank. If it isn’t, then poor workmanship is to blame. Perhaps the shanks were “burned” in the top treble at the factory. That is, the shanks were heated and purposely warped to cause the hammers to have a different striking point. Thus, two wrongs were supposed to equal a right. Poor math is to blame here, for it never will.
Two factors create the striking point for a hammer. Assuming that the hammer glue joint is 90° on the shank, the two factors are: 1) the distance from the hammershank centre pin to the centre of the hammer molding (i.e. where on the shank the hammer is glued), and 2) the position of the hole in the hammer molding, more commonly known as the hammer bore distance. These two placements can vary a little to match the specific piano. I don’t mean a standard model size, but that very piano right in front of you! You and I out in the field should be able to do this, and in fact, we need to be able to trouble shoot to be able to find whether these distances were correct when the piano was made.
Nothing can be taken for granted any more. No matter who made the piano, there is always the possibility that an error was made during manufacturing. This is one reason why I believe that a good rebuilder can take an old piano and make it better than it ever used to be! In the next post, we will get into specifics about how to determine what these distances should be.
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