I've been messing with this concept for a couple of years. That is...How does one square a hammer that has been fully tapered on both sides. Reading the geometry is tricky, since one has removed all the reference surfaces in cutting the full tapers. Double full tapers compound potential measuring error by an order of magnitude. Additionally, proving a taper is equal on both sides of a hammer is unbelievably difficult, as all the original reference surfaces have been removed in the taper. We usually assume that both tapers are the same angle. However, that assumption is fraught with uncertainty. I've made this assumption, like everyone else who does full tapers does, but always was frustrated by it, as I knew how unreliable the assumption was.
So I backed up and tried to define what "square" in this instance actually means, given the fact that reliable references have been removed in the full taper. Here's my definition of "square" as it relates to a fully tapered hammer…Square means: a line drawn perpendicular to the keybed, up through the center of the shank, projecting through the center of the hammer.
Its easy to visualize the center of the shank part of this definition. The center of the hammer not so easy. So to visualize this second point, imagine that the hammer is being suspended like a plumb bob, its plumb line running through the rotational center of the hammer. This line is the ideal force line we would like to be perpendicular to the strings…in my opinion.
The line through drawn the center of the shank, is a reference which is unaffected by taper work done to the hammer...so that's an undisturbed accurate reference. This line bisects the shank's mass. The line then projects further up, to a point at the center of the hammer felt, at the strike point of the hammer. This point represents the midpoint of the mass of the hammer. The line therefore bisects both the mass of the shank and mass of the hammer.
So this definition represents "square", because the force line bisects the mass of the of both the tapered hammer and the shank. A hammer striking in this balanced fashion will have the greatest tendency to strike perpendicular to the strings, with the least lateral deflection at strike.
So…how does one measure to the center of things? Here's what I'm been playing around with for a while.This example shows the hammer tilted just a hair to the right in the photo. Note the pencil dot on the strike point just to the right of the 85mm center marking on the rule.
The shank (on a newly hung shank) is automatically centered by the jig…no interpretation, or sighting is necessary. So this first parameter proves itself easily. The jig is oriented perpendicular to the shank ( as shown), whether the hammer has an angled bore or 90 deg bore. One doesn't angle the entire jig to match the bore angle of the hammer. This because the next step, reading a center point at the apex of the hammer, is actually taking a reading from the rotational center of the hammer, as if the hammer were suspended like a plumb bob from a plumb line. The hammer's angle doesn't matter, as I am reading a center point. Its a little hard to think about, but I like this better than anything I've used previously.I have already marked the strike point with a line while prefiling hammers proir to hanging, in a gang prefile clamp. All I do is eyeball a tiny dot at the center of the hammer width at the already marked strike point.I'm liking it.
I absolutely agree with Jim that "there is an advantage to breaking the squaring and traveling into two distinct and independent measurements'. And as Fred said, "Travel and square are inter-related functionally. If you travel a shank (shim one side of the flange), that tilts the hammer. So there is no real point to getting some kind of perfect square of the hammer separately of travel." I totallly agree.
One can learn to observe the travel of a single hammer from rest position to strike position (using the adjacent hammers in the rest position as references) and perceive whether or not the hammer is properly cast; as Fred aptly said, it follows "the arc of the shank". Determining the need to cast a hammer, can be separate and independent from the need to travel the shank. Because it is often a separate and independent step, why would it be necessary to perform casting before traveling the shank? Certainly, when mounting new shanks and flanges, there is benefit to carefully traveling the shanks to make them travel as desired. And, when mounting new hammers, one could certainly use some kind of jig if they wish to check the 'squaring' of the hammers on these newly traveled shanks and flanges.
However, in the field, it is my experience that it is very useful to learn to determine the need of 'casting' separate of any traveling issues. In fact, in my opinion, there is an advantage to casting first and traveling second, at least under certain circumstances.Rick
Rick, Thanks for your reply…nice post. I see your point.I guess one thing we should always be aware of in posting like this is that we each need to be very clear about our assumptions, or about what problem we are trying to address. Its hard to do this, always, with complete clarity. Your post clarified where your point of view was coming from, and from that point of view completely agree…especially dealing with the endemic factory inaccuracies that plague so much of our work.
To clarify what problem I am trying to prioritize in refining my shop process:1-the procedure is shop related. This level of precision, on-site, has previously been a non-starter for me. This is simply too much to do in too little time to create the conditions I'm creating in the shop. Although this tool is so quick, it is something I've wanted on site for a while. So it may end up with a site life too. 2-The precision I'm referring to, is primarily about notes 45-ish-88. Strings are either approaching 90 deg or at 90 deg to the action stack (ie parallel to the shank travel, or approaching parallel). In top level work, the tonal result is the priority, so simplifying impact has a direct positive tonal consequence. This is one of my big-bangs-for-the-buck processes #45-ish and up. It's a high priority for me, even in one day way-not-enough-time band-aid regulations.
3-In the mid to low tenor agraffes I haven't noticed any problem that squaring can solve, so its not a priority in terms of the precision of squaring. As well, since the squaring from #45-ish to #1, does not seem to have the same tonal consequence as higher in the compass, since strings are at a more acute angle relative to the stack, magnifying potential hammer alignment inaccuracies, and since bores are angled more aggressively, I don't fuss this area the way I do further up. It just doesn't pay the same dividends down there. Not only that, with the soft light hammers I use, I find mating is not that picky either. So, making the compromises required by the as-built inaccuracies makes most sense to me here.
However, in my own shop work, even though verticality is not of great tonal consequence down there, I still vastly prefer to maintain proper alignment without tilting hammers at all, even though tilting is customary in the low tenor/high base. This because, its much easier to come back to the action for follow-up regulations and go through the vertical processes if one knows that verticality is the starting point. I know that on my work, verticality is the starting point, so followups are greatly simplified, and I can really move efficiently and precisely through this part of the regulation. When things are out of vertical, as the vast majority of pianos are #45-1, for my brain, the follow-up becomes more ambiguous, requires more strategizing, and in the crush of time, skipped over or performed only cursorily.
I should say, that I mostly service my own instruments, so setting them up for my own future sanity is another important priority for me.
4- Lets face it…even fine pianos, once on-site, will not get regulation really attended to often or thoroughly. So I want it to leave my shop as clean as I can get it, otherwise it may never be set up anywhere near to its potential.