KS 11: Bedding and Action Elevations

KS 11: Bedding and Action Elevations

Two requests for further thoughts came in this week: David Skolnick on "special problems created by a ‘collapsed’ rigid frame" and Alan Eder on "how to determine whether or not the hidden glidebolts protrude the amount that they should before proceeding to bed the other glidebolts?"

David actually noted two scenarios (perhaps related): 1 "front to back ‘crown’ has disappeared (or nearly so) making the traditional ‘knocking’ method virtually impossible, i.e., no gap to create knock" and a "high likelihood frame will slap with aggressive playing" and 2 "backrail contact with keybed is at front edge of rail rather than rear – somewhat cantilevered".

Each of these points turns attention to action elevations, an under-discussed aspect of piano design. Last week's article suggested leaving hidden glidebolts where they are, barring a clear need for reset. If there are two and they are unbalanced, an adjustment to one or both would be needed. But what height should they be corrected to and why?

The Asian, flexible keyframe collapses on purpose as a strategy for helping its foundation be well-rooted. This Tai Chi method of bedding, relaxing the rail to sink with keys and action, achieves a firm, balanced connection with the keybed, where collapsing is our friend, making bedding easier as long as there is room. From the keyframe's point of view, adequate clearance works. But from the overall action's point of view, best height lies somewhere above that minimum.

Since wippen and hammer heights are set in relation to strike, and since capstan adjustment will adapt to support these, elevation of the key is the variable to be optimized. Keys must fit case parts: no gap above keyslip and adequate clearance under fallboard. And keys must relate to their key pins correctly: enough balance pin above bushings and enough front pin inside bushings.

In a flexible keyframe, hidden glidebolts can set the key level to both comply with these constraints and to limit the instabilities of overly tall punching stacks at either rail.

Another consideration should be the point of half-travel contact between capstan and wippen cushion. The slide paths in and out of that spot balance when landing on the imaginary line between balance hole and wippen center. If the action was designed for this to happen, tweaking backrail cloth thickness (and rail shimming) can help restore it. But only if the case part and key pin compliance is met. If not, trying to achieve that would mean a redesign of action parts and geometry. It's a nicety that must defer, in most cases, to what the piano was born with - like hidden studs. Sometimes, there is a compelling reason to change what's there, but otherwise, trying to return to the original intent makes sense.

For the collapsed stiff keyframe, choose one or two visible glidebolts (borrowing the Asian elevation strategy) to raise the balancerail back up to its original height. This would be the ideal solution, restoring the keyframe's intended shape, as long as there is enough flexibility to not disturb the front or back rail bedding. In David's comments, he further noted that "planing to relieve the condition would likely compromise the keyframe's remaining structure", but that weakening might actually provide the needed flexibility if it's stiffly refusing to go back.

Another reason for a stiff keyframe's lack of clearance can be the removal of part of the backrail for a player system. The trimming will likely take away the rear lip, reducing or removing front-to-back keyframe crown. Adding a strip of hardwood to the back underside of this shortened backrail can restore best height but will require re-bedding from scratch.

To correct what part of backrail bears on keybed without altering key elevation, a strip can be added to the existing lip in cooperation with changing to a new, less thick backrail cloth. 

For a keyframe with no glidebolts, a hardwood strip can be glued along the front underside of the frontrail instead, reducing impact on key elevation. The frontrail of the keyframe shown here was cantilevered, forcing this approach since there was no way to back up the balancerail. A quick dry fit was needed first, of course, to validate that case and key pin relationships would permit this change.

The strip could have been applied with traditional glue, but difficulties in clamping and then having to re-fit the whole frontrail led to a more radical solution. With backrail fit, keys off, topstack on, and keybed protected with wax paper, I glued the strip to the frontrail using thick viscosity CA glue (applied to the strip) and weight of the keyframe for clamping. This served to bed the frontrail and created space under the balancerail. 

Then, taking the same approach to the balancerail, I cut squares of hardwood and pre-sized them to be just less in thickness than needed. Positioned where glidebolts would want to be (if there were enough room for access from above), the squares were installed using this same gluing technique. Finally, testing backrail and frontrail, I tweaked the fit with a little 220 sanding (mostly rounding edges of the squares).

This was not a standard solution, I admit, but considering the potential budget and scope creep of alternatives, it was cost-effective. And it worked!

Next week: Solving Excessive Friction

(Index of all articles in this series)

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