The grand work vision that set these tools and protocols in motion came out of the notion that "there must be a simpler way".
This photo from the Home Page shows the taking of a key measurement in the setup of a grand action: the difference between strike and the hammer center, a distance that renders optimal bore distance for preparing new hammers. Custom boring from accurate measurements represents a significant opportunity when hammers are being changed or a significant insight, if they are not.
So how and where is this measurement taken? The most "accessible" procedure is to take string height measurements at #1 and #88 and subtract from them the hammer center heights of #1 and #88 respectively (measured awkwardly out of the piano from the outside hammer centers down to the bench, which may not be a match with the keybed). Two distances are derived, one for the bass bore and one for the tenor/treble bore. Typically, this two-tier boring method produces bass hammers about 5/16" longer than tenor/treble hammers. In this system, the hammer strike is certainly optimal for #1 and #88, but what of what's in between?
Many high quality grand pianos exhibit what I call "bowed plate syndrome", with the middle of the plate higher than the ends. It may happen as the casting of the iron plate cools and it seems worse in some, less severe in others. (And since smart and resourceful makers don't "correct" for it, it probably is retained for structural reasons, avoiding a catastrophic failure of plate breakage, for instance.)
Recently, I worked on a Steinway D, which displayed a 5/16" discrepancy between the middle of the plate and the outer edges. The string height for #1 equaled the string height for #53! Two very different lengthed hammers were being asked to play strings of the same height. Another view of this might conclude that if the bore was correct for #1 and #88, everything in between was, to one degree or another, overstriking.
Also, consider how the backchecking works out with this discrepancy. Can the backchecks in the middle be tall enough and/or the tails long enough to have good checking and good clearance at the same time? If the rest cushions at #1 and #88 are a suitable distance under their shanks at rest, how suitable can that distance be in the middle with 5/16" added? This instrument had a thick layer of backcheck buckskin on the under side of the rear shoulders of the hammers, the worst found where the strings were the highest. Imagine trying to perform with an action whose hammers are colliding regularly with the tops of their backchecks because the stop designed to prevent that is out of the range for such prevention.
As a regulator, these are hard discrepancies to reconcile, with parts as they come and specs as we'd like to have them. The two-tiered boring system (almost universally adopted by manufacturers and rebuilders trying to replicate manufacturers) guarantees the need for extra thought, extra work, and extra (this can make us grumpy) compromise: another subject worthy of more detail.
But obviously compromises can be made and strategies employed (like incrementally shimming the rest cushions, lifting backchecks, lengthening tails if new hammers are being installed, and generally splitting differences). Anyway, the complicated relationships are made more complicated and complexity is intensified with this traditional collision between the competing interests of prepping hammers and making stable plates.
The benefits of simplicity, ease, and efficiency are manifold and not served at all well by this conflict. However, by custom boring the hammers, fierce adversaries team up to produce an equal string of strong benefits. With accurate measurements of the discrepancies varying overall hammer length, i.e., by cutting tail lengths to correspond with the discrepancies, boring can be calibrated (and executed) from the tail, with all tail lengths the same - one simple setup bores all hammers, possibly for all makes and models. And the hammers are lightest in the high treble, where less weight enhances tone, meatier in the middle, where projection is enhanced by more hammer weight, and lighter again in the low bass, where less compensation needed to balance the action is a benefit - three wins, plus one for simple boring. But also, blow distances, backchecking, aftertouch, and the whole chain of regulation adjustments resolve into being consistent, even, and from the player's point of view, controllable. More wins. And how does this affect voicing?
So, back to the Home Page photo. The under sides of the templates of the Grandwork™ Regulating Rack are set to string heights along the strike line. The templates are precisely 3" tall. With digital calipers perched on the top of the templates and measuring down with their depth gauge, each sample center can easily produce the correct bore for its hammer by subtracting 3". Strike sliding directly over the hammer centers in the Grandwork™ Regulation Station makes custom boring grand hammers simple, easy, and very efficient. And custom boring unties an unfriendly knot of hard choices and unwished-for compromises that otherwise undermine energy and strength needed for the final laps of tuning and voicing.