It seems like the manufacturers of these parts aren't producing in bulk, why aren't the tolerances exact and have a min/max? Are they not able to measure the parts out of the factory better? Is the measuring equipment on their end and the gearboxes company difference? Or are these like nanometer tolerances which we just can't manufacture consistently?
Here's an example for something as simple as a hobbyist using a metal lathe in their garage:
For a rough part, you can measure how much material you need to take off, do it in a few passes without stopping to remeasure, and you're within 5 thousandths of an inch or so.
For a shaft that is going to slide or rotate in a bore without too much force, you need to be within a thousandth or two, so for the last pass you will stop, measure carefully, and only take a little bit of material off at a time so that the cutting forces are low and there is less deflection in the tool to throw off the size of the part.
For a press fit (two parts are sized precisely enough that they can be pressed together with a hydraulic press and then never come apart), you need tolerances in the tenths of a thousandth of an inch. For this, you'll dust off your special expensive micrometer, and it's important to let the part cool before measuring, because the heat from machining can cause the part to expand and cause you to take off too much material, which would ruin the part.
So even for the same person on the same machine with the same material, the effort (and therefore cost, if you're doing this commercially) can vary quite a bit based on the tolerance needed.
I don't know aerospace, but I imagine the required tolerances require vary from "looks good from here" (seats in the cabin) to "must ride on a film of oil this many microns thick when spinning at 10,000 rpm." And the manufacturing processes can be anything from "intern with a saw" to "specialized metrology lab with most expensive machines in the world" depending on what engineering specifies.
I don't know much about machining but this was a really cool explanation for the significant/difficulties of tolerances, thanks for explaining mrfredward
> why aren't the tolerances exact and have a min/max?
Because time and money are not infinite and +/-0.005 is a hell of a lot cheaper than +/-0.0005.
This is an assembly of stamped and sheared aluminum and composites. Machine shop level accuracy gets real expensive real quick with those construction methods and this isn't intentional defense industry pork so the cost does need to be kept under control.
