Ford and the Birth of the Model T

I don’t really see how the drawings were useless.

As long as parts were mechanically produced in series, they are standard. It doesn’t really matter then if the drawings were made ex post from existing parts. If you produce from a description with a precision equivalent to the original machinery tolerance, you should end up with virtually identical parts.

> I'm pretty sure that P-51s are maintained these days by making replacement parts by hand

This is fairly common for old cars too when spare parts are not mass produced anymore if they were at all. It used to be really common for all repairs not that long ago to be honest.

My grandfather was a mechanics in the 50s and he liked to explain that machining parts was a common occurrence then because getting parts shipped would take considerably longer than just making them and everyone involved knew how to machine parts anyway.

People (usually on the internet) act like you "need" drawings.

You don't. Tolerances are almost never bespoke except in the rarest occasions of cutting edge green field development.

You can literally buy an old book (the info is all on the internet too but not in one place) and skim through thousands and thousands of pages that tell you what different classes of tolerance for different applications are. You don't need to know what they spec'd. You just need to know what the part was and how it was used.

The thought process goes like "This worn the f out coupling is .9443 inches major diameter, so it was a 1" (nominal) coupling, and it must slip under load, so I will use a class-whatever fit, and the tolerances for that are +/-.0gfy"

An imprecise drawing is also a sufficient starting point for the process.

And like 30% of the time then you get half way through figuring out how to set it up and realize that you can literally just buy whatever you're looking for at tractor supply or Alibaba or whatever and modify it slightly.

The drawings, being made after the fact to fulfill a contractual requirement, would leave crucial details out, and were never checked for accuracy.

If you farmed the drawings out to machinists to make the parts, they would not fit together.

One of the things I did in my engineering work at Boeing was to work out the tolerances for each part. The idea was that if the parts were within tolerance, they'd all fit together. If the tolerances are missing or incorrect, then you're in for a lot of very expensive rework.

Some of the engineers were incompetent, and when the first run of parts were received, they could not be fit together. I'd be assigned to fix it. The assemblies I worked on all fit together first try.

The stab trim system had very powerful motors, and if the jackscrew nut was run into the stops too many times, it would tear it apart. So there was a mechanical cutoff system with cables and levers which would turn the motors off between 1/4 and 3/4 turn from the stops. It was not adjustable, since Boeing did not trust mechanics to adjust it properly. So, it relied on accurately machining the parts. I got the job of figuring out all the tolerances.

On the first build, I got a call from the factory saying, since I'd designed it, I get to be there for the first rigging. I arrived, and the seasoned mechanics laughed at me (I was very young) and said the rigging never worked the first time, and would have to be redesigned. We all crawled into the fuselage behind the rear pressure bulkhead. They manually set the cables to have it 1/2 turn from the bottom stop. Then they chortled, said they were going to break it, and turned the motors on. (Those motors make quite a racket.) They watched the stab trim go full travel to the top. The cutoff system shut off at exactly 1/2 turn from the end, dead center in the tolerance zone. I said "bye guys", climbed out, and went back to my desk.

Airplanes aren't like that. Sure, bearings and bolts are standardized. But the rest is full custom.

airplanes too often were machined to fit. Where tollerances were used you got intrechangable parts and so could maintain it in the field - but that took a lot of skilled engineers.

Boeing put in a lot of effort to specify the tolerances so they fit without needing rework.

Engineers are only called in when the rework is not the same as the specification.