KS 4: How Flat is the Keybed?

KS 4: How Flat is the Keybed?

At the piano, with its action out and keybed clean, apply the flat side of your straightedge to the keybed's front, balance, and back rails for a sense of what you have. Also, check front-to-back at either side and in the middle. This quick gathering of information can serve you well as you set up to your bedding samples on the bench having noted:

  1. a crowned front rail,
  2. a routed middle area,
  3. an odd shape somewhere, or
  4. none of the above.

On the bench, the straightedge may indicate places where the surfaces disagree. If there is some space under the straightedge on the bench where there was none in the piano, this is useful to know. The backrail of a keyframe, even though it's made more rigid by the topstack, may still sag enough to follow the bench. If you know that there is a blue punching's gap between straightedge and bench where the keyframe's backrail will sit and that there was none in the piano, you can start your setup by adding that blue punching where the gap was. This would be like aiding a tuning with a pitchraise, a simple roughing in to reduce complexity later, an easy approximation costing very little.

Well-taken samples will not let you down, but you may have a hard time getting them to work under certain circumstances. Knowing ahead of time where and approximately how much to shim can save the brain fatigue and self-doubt of searching for where a problem lies later. Remember, when the relative internal geometry matches in piano and on bench, the bench regulation will fit in the piano.

A crowned frontrail can be reproduced on the bench by shimming in the middle and holding down at the ends. Frontrail punchings offer increments of shimming and strips of maple or aluminum leaning on the guide pins and held in place by light clamps can hold down the ends.

A routed middle can be accommodated by a general shimming of both backrail and frontrail to provide adequate space under the action as needed. Use the glidebolts to adapt to the more flat benchtop (possibly even if you have a Custom Keybed, since bedding platforms have a limited range). Just remember to re-bed when returning action to piano.

But an odd shape can baffle, if you are not expecting it. I had a job not long ago, where I had removed the return spring, bedded the cheekblocks, and carefully bedded the keyframe. Then, I sampled the bedding with Keysteps and strike with weighted kissing samples (i.e., with measurements taken by the action itself and memorized with Keysteps and kissing hesitation speeds - more about these procedures later). When I set up on the bench, the extreme shimming I needed for the backrail led me to assume that I had made a mistake in my previous work at the piano.

After some soul-searching, I took the trip with the action back to the piano only to find that my shimming had, in fact, been correct. There was an unusual slope in the keybed's backrail from the tenor break down. Possibly, keyframe warpage made planing of the keybed expedient.

In any case, I had not been wrong in my bedding or sampling, but I had failed to recognize the odd fit. For this I paid a fairly large penalty in time and a little extra in scheduling embarrassment. But I did re-learn an important technique: consult an impartial reference to validate (or invalidate) assumptions and to better inform decisions.

Note, as you go, how a number of my step suggestions have validation as their goal. We can miss imbalances, be fooled by false negatives (or positives), and underestimate accumulations of error. Our success on the bench depends on each step being "bug-free", our equivalent of the carpenter's need to "measure twice and cut once". As I'll undoubtedly mention again along the way, though, the benefits of taking this work to the bench are significant and well worth some extra effort and due diligence.

Next week: Bedding the Cheekblocks

(Index of all articles in this series)


1 comment

Mike Dugan

Marvelous information! I really enjoy your emails. Thank you!

Mike Dugan

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