Marking out a screw thread by hand
Wrapping a divided, diagonally-ruled paper onto the rod
Leonardo gives a method for laying out a screw so it can be cut by hand: cover the rod n exactly with a plain paper, divide the paper into as many equal parallel parts as the threads wanted (a b c d), rule diagonals across them (p q), then wrap and paste the paper on the rod and cut the thread along the diagonals. The sketches at the top show the rod, the banded rectangle a b c d, and the ruled strip p q. Further untranscribed notes also appear on the page.
On this page
Cutting a screw thread by hand
Take the rod n to be threaded and cover it with a plain paper; then wrap and paste the marked paper back onto it and cut the screw along the ruled lines so the thread comes out true. The number of turns is set by how many parts the paper is divided into.
Dividing the wrapper into parallels a b c d and diagonals p q
The paper is opened out and divided into equal parallel parts, as shown in the figure a b c d. Diagonals are then ruled across those parallels, as shown in p q, so that when the paper is wound on the rod the diagonals trace the helix of the thread.