I'd like to briefly look again at where keyframe meets keybed. It's the foundation of our regulating, but because not much of what's at play can actually be seen or directly accessed, we are challenged to apply what we think and see what happens. And thinking evolves. I am familiar with at least four categories of keyframe bedding: the shimmed and securely fastened upright, the full-fitting/no-glidebolt grand, the stiff Steinway/Mason style grand, and the more flexible Yamaha/Kawai style grand. The upright keyframe has the greatest stability but its solution won't work in grands where the action needs to be easily removable and the player benefits from soft pedal shifting. The full-fitting keyframe is removable and movable but lacks easy means of adapting to life-of-piano changes and requires too much skill to bed. The stiff and the flexible grand solutions move and remove easily but approach keyframe/keybed connection a little differently.
Keybeds strive to provide a solid ground from which a player's power can be levered and controlled into musical outcomes. Because pianos must be movable and able to stand on non-industrial flooring, over-built solutions don't work well. So grands have wooden keybeds and, although those are built to be flat and rigid, pedals attach to their undersides and distort them slightly when in use, enough to cause noise and loss of power if not accommodated.
Glidebolts exist to implement this most-sensitive connection but must do it using a least-sensitive screw thread, intentionally coarse to maximize grip in the rail and prevent stripping during fortissimo playing. Getting to the place you want by glidebolt is a tricky, Zen-like activity - a bit like getting a string's tension into exactly the right place using an intermediary tool on a tightly held tuning pin. Both situations require a strategy, applications of the plan, and openness to rubber-meets-road feedback. The WNG Keyframe Bedding Tool's dial indicator thankfully provides a matter-of-fact and easy-to-read arbiter of glidebolt/keybed connection.
Small sections of keyframe rail will crush or break before they bend but full-length rails can sag, even with the added stiffness of a topstack. This slight weakness in keyframe structure complements the levered distortion of keybed to maintain bedding contact if set up well.
The stiff keyframe approach to balancerail bedding starts by retracting all glidebolts until none contact keybed. This suspension of rail puts it under stress and applies whatever sag gravity can induce, both front-to-back and side-to-side. When glidebolts turn down to touch keybed with pedals engaged, they are assured no loss of connection during play and provide a sharing of action weight that will not disrupt either backrail or frontrail fit. Coarse glidebolt threads and inadequate testing sensitivity may challenge the application and accountability of such balance. But employing downward stretch of balancerail and dial indicator accuracy of observation can achieve the needed continuity of connection between the pedaling and non-pedaling parts of play.
Crowning hedges against frontrail disconnection with application of pressure from cheekblocks. This works well when bass and treble gaps between keyframe and keybed are less than .010" but complicates when exceeding .030". Further complications arise if cheekblocks overbear on their guide pins or glidebolts over-extend to cantilever the backrail. And keybed dags bearing on the backrail can complicate things even more. Add to all this friction from return spring pressure and treble end entrapment from indentations created in the keyframe over time.
Another problem can be keyframes collapsing as they age to where there is not enough glidebolt retreat space for bedding. Such flattening, or its equivalent in new keyframe making, may account for the strategy of keybed routing in some pianos. Routing may restore the needed stretch and space but it creates a hazard of extended glidebolts colliding with front of keybed and way complicates shimming for bench setup.
Stiff Style Balancerail Bedding Summary:
Remove return spring, validate back and front rail bedding (action fully-assembled), bed cheekblocks, and retract all glidebolts to clear keybed. Start with #1 glidebolt in the bass. This area has both heaviest hammers and the most compensatory lead in keys (and thereby most contiguous influence). Place WNG Tool on the closest key button that leaves room for adjusting the glidebolt. Then, turn glidebolt #1 down until indicator moves and back up until it just doesn't. Try with pedals. Although they are unlikely to affect this first stud (glidebolts function as studs not bolts), note what does or doesn't happen. When bedding for setting up on a bench, don't depress the pedals. There are no pedals on the bench. But do include pedals when rebedding upon return.
Leave the WNG Tool where it is by stud #1 and adjust stud #2 until the indicator moves. Then, back #2 up until #1 makes contact again. Now, move the WNG Tool to a button near #2 and validate no motion by pressing lightly on stud or nearby balance pin. If indicator moves, turn stud down to where it doesn't and recheck #1. Both studs must be in contact (and if not headed to the bench, both must stay in contact when pedals are depressed). Proceed in this leapfrog fashion, always observing and eliminating motion in the previous stud before validating no motion in the one being adjusted. When the highest stud has been set, validate all studs to confirm the group fit. And then re-validate backrail and frontrail fits. Once internalized, this method is fast and pain-free. The traditional approach in the hands of a master may produce the same result a little faster, but I still would be interested in testing outcome with indicator.
Next time: Where Rubber Meets the Road (Flexible Frame)