Why Weigh Off?:
Let’s start with a designer’s point of view. Longer, heavier gauge piano strings need larger, heavier hammers to sound good and have sufficient power. Shorter, lighter gauge strings need smaller, lighter hammers. To make the full range player-friendly, the mechanics can be designed to work well without compensation at the light end and with key weights added elsewhere as needed.
The basic parameters we know. Too much compensation hampers key return and system reset (repetition). Too little feels harder to play and worsens the mismatch between bass and treble. Choices in action geometry, hammer weight, friction, downweight, and upweight push and pull individual action characteristics but all need to exceed a player’s ability to repeat, enhance their ability to control, and remove as many impediments as possible from the flow of their artistic energy.
Additionally, the manufacturer needs to stay within a carefully calculated budget. The most high-end, expensive pianos receive the most well-articulated recipes and the rest live with greater compromise. Designers and manufacturers weigh off because they have to – an unfriendly action will not sell.
For the piano technician, why weigh off? One answer is that we’re asked to, not as such, generally, but we’re asked to do work that will require it. A rebuild gone wrong that we're hired to correct. Existing hammers that are too heavy. Hammers filed to where they’re too light. One well-known manufacturer has gone through periods of accepting extremely loose pinning. Adding friction can improve stability, control, and tone, all worthy benefits to the customer. But increases in friction will raise downweight and lower upweight. The weighoff that worked well with their loose pinning may no longer be acceptable when repinned.
Many pianos, including some heyday Steinways and Masons, were pattern-leaded. The designer called for more lead in the bass to no lead in the high treble, following an arbitrary progression estimated to come out within tolerance. But variations, acceptable in the original, can be problematic when things change. Undertaking responsibility for a good outcome, the original weighoff can be a hazard, pattern-leaded or not, particularly if it was executed solely on downweight, which many were.
Another answer is that you’re forced to weigh off in response to changes you made. This can be expensive, if not included in your proposal. And embarrassing or disheartening, if you didn’t see it coming. But it is a legitimate way to learn. And making use of what we learn ups our game.
The best answer is that you want to weigh off. For an action that you have restored or rebuilt, weighoff adds an appropriate refinement that improves the chances of a best result. For a full regulation, it lessens vulnerability to inherited variables. And well done, it will make any action a degree more responsive and more even – two big pluses for your customer.