Bedding the Keyframe

By Jim Busby, RPT

Sponsored by Piano Technician Tutorials and published with permission from Jim Busby. 



Tools List:

Long, rigid ruler

Keyframe screw regulator (Pianotek JKR-1: $26) 

Clear, plastic ruler

Masking tape

Stubby flathead screwdriver


Before bedding the keyframe, take measurements of dip and aftertouch on a few sample notes (i.e. one sharp and one natural at each of the glide points). It’s always good to have these “breadcrumbs” to guide you back to where you started if necessary. Work with the fallboard and keyslip removed and cheek blocks installed. If you are working on a Steinway, be sure to screw in the cheek blocks tightly. Steinway keybeds are crowned by 1/32” so that the cheek blocks compress the ends of the keyframe down.


Here is how I bed the keyframe:


Raise all the glides so that you are positive they are NOT in contact with the keybed. I use a small voicing block and press down near each glide as I do this, looking for the keys to dip slightly at the front of the keys as I press on top of the key button area. Only raise the glides slightly. More than “not contacting” is not needed. Now, if all the keys at every glide slightly dip as you push from the balance rail area, the glides are all off of the keybed and you can move on to the next step.


Check the bedding of the back rail by carefully inserting a thin yet rigid tool (a rigid ruler works well for this) from on top, through the strings, between or behind the keys and onto the rail. Once the tool is on the rail, tap gently on the tool and listen for any knocking sounds coming from the backrail. This is best learned by doing it, along with coaching from your mentor or someone experienced at it. 


The good thing about backrail bedding is that it rarely needs to be done. If, however, you bed the front rail and balance rail correctly, but find a loss of power or keys that bounce, you should check the backrail bedding. 


If you do find knocking, find the points on the rail where it doesn’t knock, then sand off the keyframe material (not the keybed!) ever so carefully until the knocking stops. Another way to look at it is this: the knocking comes from space between the keyframe and keybed, so you must take wood off where it DOESN’T knock until the keyframe is lower at that point. In a nutshell: sand where it doesn’t knock until the knocking stops. Now on to the front rail.


The front rail is bedded the same way as the back rail, except it is much easier! In fact, I won’t even try to write that down here but will show you on video. By watching how I do this on the front rail, you should better understand how to do the backrail.


Now that the back rail and front rail are bedded, you are ready to bed the balance rail by setting the glides. I like to start with the middle glide. First, I tape a clear ruler (or something similar) on the front of the keybed and lean it on a key. Next, press down on the glide and watch for the key to bounce slightly through the clear plastic ruler. (Do this just to ensure that the glides are not in contact with the keybed.) 


Now slowly turn the glides clockwise (as looking from above) until you see a slight movement of the key upward. That means the glides have contacted the keybed. Move the glide back and forth a few seconds on the clock and leave it to where the glide is still NOT in contact, but the slightest movement of the tool (two minutes on the clock) makes the key rise. You are now very close, but still not touching.


Starting again at the middle glide and working out, tap/lift until the glide just barely doesn’t knock, then press firmly on the pedals until the glide does knock. Pressing firmly on the pedals causes the keybed to flex downward slightly. You should set the glides so that a heavy-footed player won’t press on the pedal and have the action knock. Learning to tap-lift-press is second nature to me and by using this method I can set glides in a couple minutes.


Some technicians remove some of the keys, then bed the keyframe using strips of paper under the glides. My experience with this is that the action is too light with the keys removed and the action will knock when the stack is replaced.


Editor’s note: The paper method of key bedding is often used during action rebuilds in the shop and can be useful on pianos with hidden glide bolts or buried glide bolts (such as pre-Gibson Baldwins, some Kawais, and Bostons).


Some technicians just use the tap-lift method only, thinking that it is enough. However, by learning the pedal pressing method along with tap-lift you’ll have an additional trick to help you get it right, and you’ll ensure against the knocking caused by pianists that nearly stand on the pedal.

Excerpt From: “Grand Piano Regulation.” Apple Books. For more information or to purchase the book, visit pianotechniciantutorials.com. 

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Most of our photography was done by the talented ChiaYu Lee.

 To see more of his series from Heart One Pianos, visit his website. 

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