A machine for grinding and polishing a curved metal surface
The sagoma, its lead polisher, the metals of template and workpiece, and the swivel
Two figures develop Leonardo's machine for grinding and polishing a curved metal surface with a profile 'sagoma' (template). The notes specify an interchangeable lead template set in the polisher and swapped as it wears, worked with fine emery to bring the workpiece to perfection before the copper is impressed upon it. Using his planetary code for metals, Leonardo prescribes the template of copper (Venus) or of tin (Jupiter) and lead (Saturn), often recast and amalgamated, and explains that the device should stand upside down so the template presses by its own perpendicular weight, sparing the centre of the swivel (circunvolubile) from wear.
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Interchangeable lead template in the polisher
In the polisher (pulitore) room is left to insert the lead template so it can be exchanged one after another as it wears. With emery the 'male' of the piece is brought to perfection, upon which the copper (rame) is then impressed once perfectly levelled. Points m, a (water) and surface n are labelled.
Metals of the template and the shaped piece
Using his metal code, Leonardo directs that the template be of Venus (copper) or of Jupiter (tin) and Saturn (lead), often recast 'in the lap of its mother' and used with fine emery. The shaped piece is Venus and Jupiter kneaded together; first Venus and Mercury are mixed with Jupiter and the Mercury made to escape, then all is amalgamated so copper and tin unite very finely.
Inverting the device to spare the swivel's centre
The apparatus should stand upside down so the template rests on the shaped piece by its perpendicular weight. Then the centre of the swivel (circunvolubile) will not wear, having no weight upon it, and the polishing surface is received and supported most comfortably.