The Thingomatic uses timing belts to move its various parts. Timing belts are belts made of reinforced rubber with little teeth molded onto the inside, which mesh with slots in pulleys. Thingomatic uses continuous belts 6mm in width, with 2mm between teeth (the GT2 drive system).
The belts we use have silkscreened part numbers which include the number of teeth on the belt. For example, this belt has the code: GATES GT 2MR193 2655MC
The number after the "R" indicates the number of teeth on the belt. In this example, for instance, the belt has 193 teeth.
We mount our idler pulleys in slots to allow you to tension the belt properly, but even when the pulleys are at their loosest, it can be tough to get the belt around them. Often, it is simplest to run the belt around all the pulleys except for the small metal drive pulley first. Then get the belt partway around the drive pulley, and then rotate the pulley to pull the belt onto it.
In general, belts should be tight enough to minimize slack, but not so tight that they start placing a lot of stress on the motor shaft or pulleys. Once a belt is on, turn the motor pulley with your fingers to gauge if there's too much resistance. In general the belts should not be tight enough to make a distinct note when plucked with your finger. Also, if your motor makes a humming or buzzing noise when operating, loosen the belts, operation should be nearly silent.
The two belts used on the Thingomatic can both be fairly tight, but do leave enough slack for smooth operation of both the X and Y stages.
Next step: Thingomatic X Stage Assembly