On friction: a shaping template and its grinding apparatus
How wide a template must be, and single-edge templates for finishing
Headed 'On friction' (confregazione), the leaf analyses the rubbing of a profile template (sagoma) against the piece it shapes. Leonardo argues that when the worked piece is being polished the template must be no less than half its width: a single-edge template, standing perpendicular, bears more weight and wears the piece unevenly, so template and piece are best made equal. A closing note adds that a single-edge template serves only to give the piece its figure in three or four passes. Three mechanical devices for this shaping and grinding work are drawn down the right margin, and letters a b, e f, c d, m n label the illustrated template.
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A template must be at least half the width of its work
The rubbing of the template against its worked piece should use the template's cutting edge only when first creating the piece. For polishing, the template must be no less than half the width of the worked piece, or it will wear unevenly.
Why a single edge wears the piece unevenly
If the template a b d c had a single cutting edge, standing perpendicular on the part d c it would bear far greater weight than lying on the site f e, and so would consume the rubbed parts more when upright than when oblique, making the hollow unequal. Better that template and piece be equal.
The single-edge template as a finishing tool
The single-edge template is needed only to give the worked piece its figure with three or four motions that bring it to complete perfection.
Three shaping-and-grinding devices
Down the right margin are drawn three mechanical assemblies on bases: an upper apparatus with an upright frame and handle, a similar trestle beneath it, and a bent angular tool profile at the foot, the apparatus for running the template over its work.