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How did people machine metal parts manually before automatic CNC lathes?
- Jun 01, 2018 -

How did people machine metal parts manually before automatic CNC lathes?

Manual machining is still not a terribly uncommon technique today, with both milling machines and lathes.

The general technique is that each adjustable axis of the machine, with a hand crank driving a screw, is designed to produce a repeatable distance of motion. Generally this will be a nice round number, such as “1 revolution of the crank = 0.100 inches of motion”, and the crank will have a graduated marking ring that is scribed and labeled with subsets of that dimension so that you can accurately turn it, for example, $\frac{1}{10}$ of a revolution to get 0.010 inches of travel on the tool.

The other step is to measure the relevant dimension of the part you’re machining, and subtract the target dimension to calculate the amount of material you have left to remove. Measure and check frequently to reduce the impact of errors in the measurement, calculation, and tooling. If you’re patient and have decently-maintained tools, it’s not too hard to repeatably reach the precision of commonly-available measurement tools at 0.001 inches.

There are many subtleties and clever techniques to improve the efficiency and accuracy of this process, often learned over decades of practice in the trade, but fundamentally the above two aspects are the key to manual machining.