Shimming Flute Pads

I do a lot of flute repads. I don't know if that's because I'm picky about working with old pads, or whether flute pads wear out more frequently than those on other instruments. But it means I spend a lot of time thinking about pads and how they seal.
Flute pads are held in with some sort of retainer that attaches to the middle of the pad cup, or chimney in the case of open hole pads. Because of that, instead of being floated on a bed of glue, they're shimmed up with little paper washers that are known as, uh, "shims." There are people who ask why they're not glued, and the answer to that is that it's always been done with shims and glue won't work and who the hell do you think you are questioning these techniques and shut up because I've been doing this for 40 years and there can't possibly be a better way than what I learned in 1970.
At least, that's the answer I've seen whenever it's brought up. The 1970's must've been a golden age for the instrument repair industry. That's evidently when they came up with the right techniques for every repair.
But I use shims because the tone holes aren't level and neither are the pads, so shimming is necessary to make pad interface properly with the tone hole and seal it. You can get very picky with shimming, and some people do - cutting sections of shims and gluing them to the back of the pad to "boost" a particular area of the pad that may be leaking. Some say this is a waste of time, and others say it's necessary to achieve a firmly level pad. I firmly believe it's necessary, althoughsome people get a little out of hand with it. You can only do so much to correct the imperfections of - let's not put on airs - cow guts stretched over sheep fur.
But to go back three paragraphs to the point of this post - leveling flute pads gives you time to think. And a conclusion I came to recently is that the front-to-back relationship is, without a doubt, the most important part of leveling flute pads. Miss that, and it doesn't matter how carefully you level the rest of the pad, you'll never get either the response or feel you want.
Imagining the pad like a compass, North would be the back, where the key arm is. South, then, would be the area of the pad furthest from the arm - the part that you inevitably get dirty when you polish your flute. If a pad is too thin, it will hit the tone hole at South, but not at North, resulting in a leak at the back. This pad might have a nice firm feel, but it won't fulfill its job. Leaks at the back of the pad can be difficult to find for someone not familiar with this. A pad that's too thick, will, on the other hand, hit the tone hole in the back (North), but leak in the front. These leaks are easier to see because the leaking part of the pad is right out where you can see it. Pads that leak in the front tend to have a mushy feel because of the tendency to "squeeze through" the leak and press on the pad until the front reaches the tone hole, thereby squishing the back. Gemeinhardt flutes seem especially prone to coming from the factory with pads that are too thick and leak in the front. I guess they figure that over time things will settle, and in fairness it probably is easier for a player to overcome a leak in front than a leak in back by simply pressing harder.
This front-to-back relationship is so crucial to the performance and feel of the pad that it's the first thing I address when leveling pads. I try not to worry too much about the side-to-side (East and West) relationship until later, unless the pad is tilted severely to one side. In most cases, the front-to-back relationship is adjusted by installing whole shims behind the pad. More/thicker shims if it's leaking in the back, fewer/thinner shims if it's leaking in the front. It sounds simple, and in practice it should be, but pads compress and shift and need to be ironed and it's just a pain in the ass to get everything to line up right. Only after assuring myself that the pad is hitting the front and back of the tone hole at exactly the same time with minimal finger pressure do I move on to other parts of the pad. I am borderline obsessive about this, checking both front and back repeatedly with my feeler gauge, approaching from different angles. Inevitably, if I haven't checked carefully enough, I'll find the problem later while doing regulation.
After that whole dance is over, I check the seal at East and West. If one is sealing and the other isn't, I'll try tilting the pad cup to try to get both to seal. While doing this, I'm still rechecking North and South occasionally to make sure things are still square. If tilting the pad cup doesn't solve things completely, then I'll move on to gluing partial shims to the back of the East or West side. Once I've got a good seal at all four compass points, I further break the pad down mentally into eighths, so that now I'm checking NE, SE, NW, and SW. Major leaks here demand more partial shims, and depending on the quality of the instrument and pads, even very small leaks can be shimmed. Once I've checked all eight sections of the pad, especially on a student level instrument, the pad should be sealing about as well as is possible. In my experience, trying to be any more precise on beginner flutes just adds time to the job without adding any improved performance. That's not to be taken as an excuse for sloppy work. Done right, a well-made student flute should play damn well, even if it's never going to feel or sound like a Powell or a Muramatsu.
That being said, if I am working on a Powell or Muramatsu (it could happen someday!), I further divide the pad into twelfths and recheck and shim as needed. I rarely work with pads that will respond to shims that are less than a twelfth the circumference of the pad (30 degrees), but with high-end pads you could certainly make a case for further dividing the pad.
The crux of my process, though, is that the front-to-back relationship must be spot on every time. Fail to address that, and it doesn't matter how much time I spend addressing every other section of the pad, the results will be disappointing.