Now that the action is positioned on the bench, shim the backrail to be stable. Do this with front punchings, placed every ten to twelve inches. These will support backrail and frontrail the way glidebolts support the balancerail. Share the weight evenly between these points of contact. And when done, draw placement markings on rail, punchings, and bench so that if the keyframe is moved, when it is returned, the shims can be placed back in their original locations.
For the frontrail, a gentle clamping of guide pins may be needed. This depends on bench shape vs keybed shape, including whether or not there is crowning. Rough in glidebolts to have end leveling sample keys just touch their Keysteps with frontrail touching down (you will have to redo balancerail bedding when the action goes back in the piano). Then add your straightedge and check the mid-range leveling sample (#44). If there is no space between Keystep and straightedge, and you get no tap back when tapping it, and the ends don't tap or skate, perfect. If there is a tap, shim under the frontrail in front of the Keystep. If #44 is too high (producing symptoms), shim the ends of the frontrail to lift sample keys, Keysteps, and straightedge higher and fine-tune with punchings under #44 until there is no tap. Do not touch the Keysteps except to make sure they are back against their keypins and centered.
Do the three-point tap test (described last week) to validate equal contact with keys #1,#44, and #88, and position-mark that shimming. A felt knife can help lever the frontrail for punching insertion. Then go halfway to #1 and halfway to #88, adding shims that will share frontrail weight but not change the three-way validation.
Finally, adjust the glidebolts so each bedding sample key just taps on its Keystep. And adjust again so each just doesn’t.
In the piano, we connect weight of action with balancerail by lowering glidebolts to make contact. On the bench, we work the inverse: where glidebolts already contact and bear weight, we adjust them to shape the action. In the piano, action shape fits to keybed to obtain is support. On the bench, bench supports action to the shape it was in the piano. The bench tries to become the shape of the keybed. In both cases, we strive for balancing the shared weight, but on the bench, we may need to touch up backrail or frontrail shimming. Regardless of whether all details are exactly the same, the on-bench bedding will match the in-piano bedding when no validation points tap back, including those of the three leveling samples.
It helps to have initially checked the piano's keybed with a straightedge. Knowing, for instance, that the keybed was routed to be slightly concave (as found in some Steinways) red-flags the possibility that overall levels of backrail and/or frontrail on the bench will need to raised to allow enough room for adjustment. In such cases, the action probably rocks on the bench.
Next week: Strike Setup on the Bench