Getting Started with Kythera

So you want to string a bunch of gears together, and you don’t know where to start?

Well, let’s get you going!

Kythera allows you to design complex gear trains using spur gears. The idea is that all your gears are strung together in a chain, from the first (parent) gear to one or more child gears. Logically we can represent the relationship between gears as a tree structure:


Now Kythera is not a full CAD system; it’s primarily for designing gear trains.

And the tree-like structure of Kythera means when you design your gear train, you start with the first parent gear (1 in the diagram above), and you add new gears relative to it’s “parent” gear.

Before we go further, it’s worth a word as to how gears are specified.

Basically, you specify each gear by the number of teeth:


The pressure angle, which gives the angle of the teeth on the gear where pressure is applied to the adjacent gear:


You also specify the “Diametrical Pitch”. The “Diametrical Pitch” is the pitch diameter of the gear:


that is, this is the diameter of the gear if you replaced all your gears with wheels, and divided the number of teeth N by the pitch diameter D.

This last value, the “Diametrical Pitch” (DP, or often P) as well as the pressure angle PA must be the same for two adjacent spur gears to work together properly. That means if you have, for example, a gear from SparkFun which has a “32P gear pitch”, this means the gear’s diameter is the number of teeth N / (32 teeth per inch). Thus, the linked gear with 32 teeth has a pitch diameter of 1 inch. A 16-tooth gear would be roughly 1/2 inch in diameter, and a 48-tooth gear would be about 1 1/2 inch in diameter. (“About”, because the teeth stick out beyond it’s pitch circle.)

So now let’s string together our first mechanism. For our mechanism we want a gear chain that will reduce the rotation by half.

Start up Kythera.

When you do, Kythera will create a new document for you.

Initial Screen

The window shows an initial gear, but it is not selected. On the left is a 3D presentation of your gear, along with two dotted lines: one showing the outer diameter of the gear, the other showing the pitch diameter of the gear as defined above.

On the left, you have four panels; one allowing you to edit the layers of your mechanism, the selected one showing your gears, then document settings and finally errors (such as where gears appear to overlap). For now we’ll concentrate on the gears panel.

The 3D view.

You can navigate in the 3D view with the mouse.

  • To pan, left-click and drag.
  • To rotate, right-click and drag. Or left-click with the control button down and drag.
  • To zoom in or out, slide your finger up and down on your mouse’s rotate button (if it has one), or hold down the option key and left-click and drag up and down.

There are several icons which can aid in viewing your model:

  • Show Rings  This shows or hides the outer diameter and pitch diameter rings around the gears.
  • Perspective  This switches the display between perspective and orthographic projections.
  • Recenter  This resets the display to the default display of the model. If you get lost browsing your model, press this button.

You can also select gears by left-clicking on them.

The Gear Panel

You set all attributes of your gears (including their relative position to connecting gears) by updating the values in the right gear panel.

Select your gear by left-clicking on it, or by picking it in the gear list at the top:

Selected Gear

Notice all of the attributes for the gear fill in for this gear. We can see the number of teeth (12), the diametrical pitch (by default, 1 tooth/mm), the pitch diameter for this gear (12 mm), and the pressure angle (14.5°).

By default the system uses metric units. You can change this setting (and a bunch of others) by selecting the “Document” tab, but we wont’ worry about that for now.

Let’s go ahead and add a second gear.

While the first gear is selected, under the “Gears” menu, select “Add Gear.” This will create a new gear attached to the selected gear, and select the new gear.

Two Gears

Notice the connection properties for this gear: it’s attached to the first gear (the parent gear is “Gear 1”), and its connection angle is 0°.

Now we want to step down the rotational velocity of our system by half–so select the number of teeth and type “24” and return.


Notice a few things.

First, in the gear information table, the gear ratio is given as “1:2 CCW”; this this gear rotates stepped down to 1/2 the rotational velocity of the first gear, and if the first gear turns clockwise, this gear turns counter-clockwise. The RPM for the gear is also displayed; if the first gear’s input RPM is 6, this will turn at 3 RPM.

Now press the “Start” button at the bottom right of the screen, and watch the gears rotate. This is useful if you need a sense of how your model will move in real life.

Exporting your model.

The power of Kythera is the ability to export a representation of your mechanism which you can load into a separate CAD system and modify, or send to a 3D printer for printing.

Let’s export your model now.

Under “File” select “Export BOM…”; this will export the “bill of materials” report, a directory containing a number of files that you can use for 3D printing.


The key file here is the ‘readme.txt’ file, which gives you a quick overview of all of the other files here, and assembly instructions for assembling the model.

We have a number of other tutorials available to help you understand how to use Kythera. Please visit our product page for more information.

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