The term motion work comes from clock-making or watch-making, and refers to the mechanism which controls the hour hand and minute hand on a clock or watch. It is the mechanism which allows an hour hand to turn 1/12th (or 1/24th) the speed of the minute hand.

For Kythera, a “motion work” is any mechanism which drives two gears on the same axis at a known fixed ratio, built around four gears.

This tutorial will cover the steps in Kythera to construct a motion work for an hour and minute hand–but it can be extended to control two gears rotating at any known fixed ratio. For example, one could conceivably build a gear system which steps down one gear by a factor of 27.5, representing the speed the moon orbits around the earth as the earth rotates on its axis.

The Hacking Den contains a much more detailed article which also covers editing the gear 3D models in OpenSCAD so as to build a complete motion work, including axes which can be used to drive arms of a clock.

Let’s get started, shall we?

First, as before, open Kythera, and create a new project. This will create a file with a single 12-toothed gear.

Because our motion work system relies on four gears, stacked in two layers, you will next need to create a second layer. So, under “Gears” select “Add Layer.”

If you switch to the “Layers” tab, you can see your second layer added to your model: The convention, by the way, is that when you select a gear, all gears on the same layer are shown in yellow. The selected gear in blue, and gears not on the same layer (and not selected) are transparent gray.

For our exercise, we will assume we are creating a motion work that is attached to this gear. So, select the gear as before: Now under “Gears” select “Create Motion Work…”

This will run you through a series of dialog boxes which allow you to specify the motion. This first screen allows you to specify the motion work you are attempting to create. You can specify the gear step up or step down ratio as a fraction or as a decimal number, and you can specify the smallest gear you want the system to propose when calculating gear ratios. This is important if you have gears with a large axis hole and a small diametral pitch; this keeps the system from generating gears whose center hole is bigger than the outer ring of teeth.

Because we want to create an hour-hand/minute-hand mechanism, we leave the target ratio as 12:1.

Press Next. This screen shows a list of proposed gear ratios which will work. That is, this table shows proposed gear sizes for the four gear mechanism which will allow you to create a motion work that has two properties:

1. The four gears neatly stack on two axes.
2. The total step down ratio meets our requirement of a total 12:1 step-down in motion.

Take, for example, the first line:

`8:32 10:30`

What this means is that the proposed mechanism starts with an gear of 8 teeth which meshes with a gear of 32 teeth. The 32-tooth gear then attaches on the second axis to a 10-tooth gear, which meshes with a 30-tooth gear that is on the same axis as our 8-tooth gear.

Doing the math, you can see the width of the axes of the 8 and 32-tooth gear is the same as the 10 to 30 gear (8+32 = 40 = 10+30). Further, the ratio of motion of the total mechanism is the ratio of the 8 to 32-tooth gear (4:1) and the 10 to 30-tooth gear (3:1) for a total ratio of 12:1.

The same is true for all the proposed motion-works.

So let’s pick the first, and press “Done.”

Oh, noes! An error!

All the gears light up in red, which indicates a constraint violation. If you select the “Error” tab, you can see a list of those constraint violations: The error should be apparent: the first gear we started with overlaps one of the gears in our motion work. We can deal with this in a number of ways; we’ll deal with it here by simply making the first gear big enough so the axis doesn’t overlap. So, select the Gears panel and pick the first gear. Now set the number of teeth to 30. Of course this shows one of the interesting limitations of building a motion work: you don’t want the axes the gears revolve around to overlap. This is part of the art of building a gear train.

Now that we’re done, we can view the entire system by pressing “start” in the lower-right corner to start animating our system.