| 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. |