Mesh wooden gears on a board, turn the crank, and watch every arbor turn at its true ratio. When the numbers come out right, print the gears at actual size and go and cut them.
Gears only mesh when their teeth are the same size, so the whole board shares one.
Keyboard: focus the board. Up and Down pick a gear. Left and Right turn the crank when the crank gear is picked, and swing any other gear round the one driving it (Shift for bigger steps). Plus and minus change the tooth count, Enter adds a meshing gear, Delete removes one, Space starts or stops the weight.
Speeds are exact, as fractions of the crank. Each mesh reverses the direction; two gears on one arbor turn together.
| Arbor | Gears | Turns per crank turn | Way round |
|---|
Every gear at actual size, with a true involute tooth: 20 degree pressure angle, addendum 1 module, dedendum 1.25 module. Each one is labeled with its tooth count, module and arbor hole, and every page has a 1 inch square and a 25 mm bar to check the scale before you cut.
The SVG is in millimetres. Cut lines are black hairlines; the labels and the check square are blue, for engraving or deleting. For a scroll saw, print the pages, glue them to 1/4 in plywood with a glue stick, and cut just outside the line. Below the base circle the tooth flank is drawn as a straight radial line.
A gear train trades speed for turning force, or the other way round, in amounts that are fixed and exact. Two gears mesh when their teeth are the same size, and their centers sit half their combined pitch diameters apart, which here means the module times the total number of teeth, over two. Put a small gear and a big one on the same arbor and the ratios stack. The clock train gets its 60 to 1 from 64 over 8, times 60 over 8, which is 8 times 7.5.
The motion work under it, 10 into 30 and then 8 into 32, is 3 times 4, which makes 12. Both of those pairs add up to 40 teeth, so their centers are the same distance apart, and that is what lets a real clock put the hour wheel on a pipe around the minute arbor. Here they are laid out apart so you can watch them. The pendulum is only for looks.
Cut a test pair before the whole train. Wood moves, blades wander, and a little extra room in the arbor holes forgives a lot. Nothing you build here leaves the page.