We left off talking about the two factors that create the striking point for a hammer. Assuming that the hammer glue joint is 90 degrees on the shank, which it must be, these two factors are: 1) The distance from the hammershank centre pin to the centre of the hammer molding, and 2) The position of the hole in the hammer molding, more commonly called the hammer bore distance. In measuring factor number one, the measurement is always taken from the centre of the centre pin, to eliminate any error because of larger centre pins.
Looking at the diagram, this distance for factor number one is labeled (L). In this particular instance, (L) = 5 1/8“, which is a common measurement. (L) determines the type of arc that the hammer will take on its upward movement towards the string. The distance that the hammer travels in this arc is labeled (S). (S) is correctly termed the hammer travel distance. Note that this is different from (6) the hammer blow distance. These two words should not be used interchangeably.
As the hammer is moved in or out on the shank, this distance (S) changes. Minute alterations of (L) by the technician as he glues on a set of new hammers is acceptable in order to achieve the correct striking point. However, keep in mind that the piano was designed with a specific distance for (S). The work that the knuckle does in raising the hammer is in a ratio which is dependent upon (L) and the placement of the knuckle. The closer that these two placements are to what was designed, the better the action will perform.
Of greater importance than factor no1 is factor no2, the hammer bore distance. I regularly find good make grand pianos where the bore distance is wrong from the factory! When manufacturer installs the hammers, he should measure the distance (A) on the diagram. This is the distance between the bottom of the string and the centre of the centre pin on the hammershank. Unfortunately, many pianos are manufactured without regard to this measurement. (A) does fluctuate a little from what the piano was designed to be, mostly because of differences in the thicknesses of the plate castings. Sometimes the string height varies because of improperly installed agraffes, or in the case of a rebuilt piano, the string heights will vary because the plate was lowered in order to achieve proper downbearings.
When rebuilding, always wait until the new strings are installed before sending off for new hammers. Never buy stock hammers. Insist that the hammers be bored to match the piano. Either send to the hammer duplicator the measurements for (A) for each section of the action, or else bore your own. Anyone who has regulated many actions will have noticed that the string heights vary not only between similar pianos of the same make and model, but will also vary between the different sections of one instrument. Ever wonder why the hammer rest rail never fits the bass hammershanks like it does the tenor? One cause can be improper hammer boring. Why? Most grands are made with the hammers bored at a stock distance, to supposedly fit all of the pianos made of that model(s). No interest is paid to what (A) is, or whether (A) is the same for each given section of the action.
Why is this so important? There is more at stake than just the shanks being different heights off of the rest rail. Again looking at the diagram, there is a distance (C) which is the measurement taken from the centre of the hole in the hammer molding to the top of the hammer. If the hole in the molding is properly placed, (C) should be greater than (A) for a new hammer.
In the event that (C) = (A), a new hammer will strike the string at exactly 90 degrees, which is ideal. It produces the best wave shape in the string and will result in the best possible tone, all else being correct. However, we all know that a new hammer will compact as soon as it is played. As the hammer wears, (C) becomes smaller. If (C) becomes less than (A) the hammer overcentres. That is, the very tip of the hammer no longer strikes the string, but rather the back (soundboard) side of the hammer.
Before jumping to conclusions, (C) can be a little less than (A) and still not have the hammer overcentering. Another factor, that of the “whip” of the shank enters into the picture as well. Depending upon the size of the shank, the type of wood used, and the weight of the hammer, the shank actually bends a little as the note is played. This causes the hammer to be greater than 90” on the shank. The strike point changes and keeps the hammer from overcentering.
In order to allow for normal hammer wear, (C) should be drilled to be greater than (A). How much greater? It would depend upon two things. A soft hammer will wear faster than a harder one, and should be given more allowance. If you like to bore your own hammers, here are some guidelines.
For an action where (L) is 5 1/8”, for each degree that the hammer leans back towards the bridge, the striking point is moved 1/32”. If a one degree angle is desired, the bore should equal (A) plus 3/32”. For two degrees, (C) = (A) + 11/64”. Probably a one degree angle is sufficient to allow for wear on a good hard hammer.
What do all of these measurements have to do with setting the blow? Remember that the blow can be decreased to compensate for action wear. Earlier I stated that l would never alter the blow more than 1/8” to 3/16” from the factory specifications. Now I will reword that statement to read if you really want good tone after regulation, check the (A) and (C) distances before regulation to see if the hammers would be overcentering after reshaping them. Also, watch for differences in (A) between the sections. Were the hammers bored to match? If not, for good concert level work in trying to obtain eveness of touch and tone, install a new set of hammers which are matched to the piano. Never wait until the underfelt shows through the grooves on the hammers before replacing them. To give the customer the most for their money, install a new set of hammers while the action is being regulated. This is far better than compromising while regulating and voicing, only to have to replace the hammers and reregulate the action later.
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