What Leonardo da Vinci machine was designed to roll and flatten metal into sheets?

Leonardo da Vinci designed a rolling mill to flatten metal into sheets.

A rolling mill passes metal between rotating cylinders. The pressure reduces the material’s thickness and produces a more even surface. Leonardo’s mechanical studies include arrangements of rollers and gearing that show how power could be transmitted to the cylinders in a controlled way.

The idea was significant because metalworking required both force and consistency. A mill could make sheet metal more uniformly than repeated hammer blows, while gearing could help coordinate the rollers. Such equipment would have been relevant to armorers, toolmakers, and other Renaissance metalworkers.

Leonardo did not invent the entire concept of rolling metal from nothing; earlier metalworking traditions used rollers in various forms. His contribution was a detailed investigation of machinery, power transmission, and practical workshop operation. The design is therefore best understood as an early rolling-mill concept, not a modern industrial mill.