I am a big fan of the Bear upgrade for the Prusa i3 printers (See details here: https://github.com/gregsaun/prusa_i3_bear_upgrade ). When i was assembling the upgrade, I found it quite tedious to get the Y idler and Y motor holder/pulley aligned with the belt center that is determined by the belt holder. The belt holder is out of sight underneath the frame (and the heatbed, if you've already installed that). In addition. to adjust the motor holder requires removing the motor.
This simple tool allows you to center the motor holder and the idler holder accurately and consistently, without need for iterative adjustments or peering under things with a flashlight.
To adjust the idler, first loosen the idler holder frame screws. Next place the rounded portion of the tool on one of the Y rods. You want the shallower of the two sides pointing down. Slide the tool back near the idler, and then adjust the idler holder so that the idler is centered on the tool (see attached picture). Then tighten down the idler holder to the frame.
To adjust the motor pulley, if the motor is already attached, you will first need to remove it. Next loosen the screws on the motor holder. Adjust the pulley on the motor shaft so that the ribbed section is nearest the motor body, but offset slightly (1mm or so) so that it doesn't rub on the motor body. By placing the pulley in this position, you minimize any flexing of the shaft within the motor bearings caused by the tension on the belt. Place the tool on one of the Y rods, this time with the long section down. Hold the motor up against the mount, but do not attach it with screws yet. Now slide the entire motor holder + motor + pulley assembly to center the tool on the pulley. Tighten the motor holder to the frame, and then reinstall the motor using the 3 screws.
Doesn't matter PLA, PETG, or some other dimensionally stable filament