A piano key frame is an integral structure that independently houses the piano keys and is what the action mounts upon to create what is collectively called the Action. The design of the key frame establishes fixed key positioning according to the overall design of the piano. Piano key frames are not trans-model interchangeable and each key rest in a dedicated position. The piano keys are held in position by key pins at the fulcrum (balance rail) and front (front rail) while the backs of the keys rest on the cushioned back rail. All contact points of the keys are cushioned with specialized pieces of woven felt with distinct characteristic. The balance rail and front rail pins are felted with key punchings while the back rail is lined with felt.
The key frame itself is comprised of integrated wooden sections and is designed for stability and functionality. It is built to fit precisely to the keybed, which is slightly crowned so the keyframe, once anchored at the back and ends, can not change position with climated changes, or rattle and move while the instrument is being played. Most vertical piano key frames are held in fixed position while grand piano key frames are design to shift to either end for use of the unichorda (soft) pedal. During this movement keyframes move upon specific bearing points as illustrated below.
Reconditioning of a keyframe is a crucial aspect to piano rebuilding. In most cases, back rail cloth, balance rail punchings, and front rail punchings are replaced with new materials. In addition, all key pins are cleaned (in some cases replace), glides are polished, and all bearing points are dry lubricated.
Piano Key Frame with keys removed
Piano key frame felt deteriorates for various reasons. Commonly it is dry rot and moth damage that is largely responsible. As well, the felt compresses and becomes deformed altering its relationship with the key. This makes precision regulation impossible as well as increasing action noise.
Removal of old back rail cloth
Accurate duplication of type of felt, thickness of felt, and width of felt is important to maintain the geometrics of the piano action. Original felt is used to verify dimension of new felt. When a key frame appears to have been previously refelted samples may not be reliable so measurements are taken to establish new felt dimensions.
Cleaning Key Pins with 4/0 steel wool
The balance rail and front rail pins are cleaned to insure smooth noiseless movement of the key. In some cases, the pins have excessive wear or corrosion requiring the pins to be replaced.
Measuring out new back rail cloth
The back rail cloth is cut from strips, were as the front rail and back rail punching are individual pieces.
Preparing to glue new back rail felt
Back rail cloth is glued to the key frame either on both edges or only the front edge. This allows for a cushion to form as the felt is clamped and glued. Notice how on this frame there is a shim layer of felt in the center to help form a crown.
Clamping new back rail cloth
Uniform clamping compresses the felt at the glue joint forming a specific contact point for the key as illustrated in the following pictures
Checking key alignment of back check to crown of back rail cloth
The apex of the back rail cloth crown must be in vertical alignment with the back check at the point that the hammer head tail will contact it.
Installing new front rail and balance rail punchings
Balance and front rail punchings are installed individually. Since the keys are regulated by the insertion of card and paper punching under the felt it is advisable to install a certain amount that will rough-in the regulation, leaving finer adjustments for later.
Key Frame Refelted and Newly Recovered Keys Installed
Keys are installed after frame is refleted. The keyboard is now ready for regulation of key height and key dip.