Although without modern computer controlled machinery a great many things were not practical to achieve, this was less difficult than might have been believed and much ingenuity allowed very complex 3D shapes to be machined and cast.
Above, a Junkers-Jumo opposed piston aero engine main crankcase casting being machined in Dessau, Germany.
One way to manufacture very complex 3D shapes was to use the copy-process. An expert craftsman was tasked with constructing a “master” part, probably at twice or three times the scale of the final part to reduce errors. This would then be used as a guide to then machine parts from the master part. Above is a turbine blade being machined on the right, with the larger master-blade visible on the left.
The machine has an automatic control which keeps the stylus in contact with the master part, and slowly decends to finish the new blade.
It is easy to forget that although these engines were intended to be state-of the art, they also had to be produced quickly and in vast numbers. The most popular aero-engines were made in volumes up to 20,000 per year, three per hour! Vast effort had to be expended on clever multi-operation machinery to machine all the parts very fast.
Inspection was also vital, and every big manufacturer had a complete laboratory for quality control, with everything from an eye-glass (above) to X-Ray machines.