A question left in reference to one of my videos on hanging grand hammers finally receives a response after 2 years languishing unseen on YouTube. Come on, Chris...

Q. With an uneven string height, did you then change the bore distance incrementally as you drilled the hammers?  

A. An important question. String heights are often not parallel to the keybed over the length of the strikeline. Sometimes the discrepancy can be considerable, interfering with backchecking, strike characteristics, and regulation. I prefer a regulation that can be consistent, with as little compromise to dip, blow, letoff, backchecking, damper lift, and especially aftertouch.

So, yes I do vary the bore. I accomplish this by measuring the discrepancy at ends of sections, with a mid-tenor measurement, as well. I draw the outline of this discrepancy on extra-length tails, calculating in the tail length I want, and cut the tails carefully to these lines. Then, I bore all the hammers from their tails - one setup fits all. When I'm done, tenor hammers are a bit longer (where more weight adds projection), high treble are the shortest (where tone benefits from a lighter hammer), and the low bass are somewhat shorter (where less weight means less lead will be needed as weigh-off compensation). All the tails will be the same length, all the backchecks can be the same height, all shanks will be the same distance above their rest cushions, and the blow distance, dip, letoff, and drop produce the same aftertouch. This will give the player more control of power, tone quality, and subtleties from soft pedal to fast passages, to chord synchronicity. It does make hammer travel and strike verticality and good centerpinning even more crucial, while making clearance issues somewhat tighter in the middle. But all-in-all, I love the benefits to playability and tone and have found that I can solve the clearance difficulties. The time-energy costs of custom boring, I find, are way more than paid for by lack of the voicing and fine regulation time-energy drains two-tier boring incurs.

And a related question from a video on keyframe bedding with a WNG Bedding Tool.

Q. So am I understanding correctly that as long as the 3 rails are properly bedded, then the action as a whole is the proper distance from the string line? Is that the purpose of bedding the action?

A. There are two issues here: elevations and solid foundation. First, the action as a whole could have elevation issues, independent of the bedding. But if it was set up correctly to begin with and not interfered with, it will be the correct distance from the string line.

There is a hidden but notable exception. If the string line (strike line) varies in height from the keybed (which to one degree or another, it often does), traditional two-tiered boring (one bore length for the bass and one for the treble) forces action-distance-from-strings compromises.For instance, if blow distance is consistent throughout, a desirable feature, the shanks will be heights varying from their rest cushions the same amount as the string heights vary from the keybed/plane of the action. If the discrepancy is significant, the tails may be too short and/or the backchecks not tall enough for good backchecking. Also, the rear under shoulder of hammers whose shanks are too far from their rest cushions, may collide with the tops of their backchecks - not a catastrophic failure, probably, but not great. If the blow distance is increased, more dip will be needed or less aftertouch suffered. A bore distance choice will be forced, between overstriking and understriking hammers. And so forth. Actually, we can potentially worsen the situation by parts and materials choices and hammer wear will worsen the situation for those that overstrike. And the under- or over-striking has tonal implications that don't help our voicing challenges.

Finally, the purposes of bedding an action. Good bedding provides solid support for the action, minimizing loss of energy between force applied to the keys and output applied to the strings. This also has a time/response component that a player will have try to overcome as they play, i.e. they will have less control of what they are trying to do musically. And, of course, good bedding minimizes unwanted noise in the system.