What Is Tolerance Part II
This is a good companion video to the previous blog post about high tolerance key fitting. This is a good demonstration of how loose keys, keys fit with very low tolerance, can actually cause leaks or exacerbate what leaks are already there.
If the hinge rod in question in this video fit the Low C key hinge tube with a much higher tolerance it would not rock like it does in this video.
You can see the previous post by scrolling down below or go here.
http://www.cennamowoodwinds.com/blog/december-11th-2017

What is Tolerance?
You'll hear meticulous repair techs talk about tolerance in regard to key fitting. So, what is it? Put simply, tolerance describes how two pieces fit. There's High Tolerance and Low Tolerance. High tolerance is when the mating parts have very little deviation, meaning they fit tightly. Low tolerance is when greater deviation is allowed, meaning they fit loosely.
The goal in saxophone overhauls is to fit the key work as tightly as possible without making it bind. Traditionally this is done by swedging the keys with a three point swedging collet like those in the pictures below. It has proven to be effective for decades and will undoubtedly continue to be used by repair techs everywhere for years.

But very old horns with badly worn out hinge tubes need more attention than that. Just swedging the keys harder becomes ineffective. In order to increase the life of these irreplaceable instruments, new rods need to be made. And, it's preferable to fit them to high tolerances.

The new .108" rod fits into this hinge tube with a tolerance of .0005". The higher the tolerance, the tighter the fit, and the more skill that is required to make it so. Now I'm not making parts for NASA, so .0005" is damn good. There is no concentric slop in the key whatsoever and the key falls free on the rod. The greatest benefit to doing this is the parts mate at all points in the hinge tube, so it feels especially solid, and this will last for several more decades now.
When you run into badly worn out keys like this and you just swedge them harder, they cinch up at the ends only, and if there's as much slop as these keys had, you'll leave them misshapen as well. You can see the swedge marks from the previous overhaul here - which is fine, but going for it again would have made them visibly ovaled at the ends like footballs. This last photo is the new hinge rod in the accurately reamed key.