Suppose you’re building a clock and you have a gear train in the EarthMoon.zip This is a Kythera file which has the nice property that when the first gear revolves around once, the last gear revolves 1/29.530; this means if the first gear is attached to a 24-hour clock, the second gear revolves once every 29.53 days, a synodic month, or the time between new moons.

(Okay, the gear ratio has an error of 1 degree every 3.2 months, which means the ratio is not exact but will drift from the actual phase of the moon over the years, but it’s close enough for this exercise.)

But we have a problem: the distance from the first gear axis to where we want to put the month dial is 50 millimeters, and our gear train is 71 millimeters in length.

That’s where making a “Gear Ring” comes to the rescue.

This runs an algorithm which arcs the gears along the arc of a circle or ring, positioning each of the gears so that the total distance from the first gear to the last is the target distance you want. You can also indicate if you want the gears to arc clockwise from the first to the last, or counter-clockwise.

So first, let’s pick the two gears we wish to bring closer together. Open the ‘gears’ panel and select the first gear and the last.

Now, under the Gears menu select “Make Gear Ring…”. This will show a dialog allowing you to specify the distance between the first and last gear in your chain, and if you want this to wind clockwise or counter-clockwise.

Enter 50, and press “Done.”

Your gears will be rearranged so that the first and last selected gear centers are now 50 mm apart.