Railway semaphore and lathe: what they have in common

1. Step 1. Railway semaphore — is made of — Cast iron. On a cast-iron mast.
2. Step 2. Cast iron — is material for — Screw-cutting lathe. The bed of Maudslay’s lathe is cast iron.
3. Step 3. Screw-cutting lathe — evolved from — Lathe. Maudslay added a lead screw and a slide rest.

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