The trolleys are the wooden pieces that move along the gantry rails. They're driven by the turning cable on the pulleys, and they need to have one open tunnel for the cable to pass through and one fixed point attached to the cable. Each gantry has a different trolley on each side in order to achieve this movement.

In the Y axis for example, the fixed points are to the left on both trolleys, meaning these parts have to be constructed differently. This is why on the shop drawings there are two different types- one has the thumb screw closer to the hole on the end (trolley 1), the other has the fixed point further from the hole (trolley 2).

The Stylus has to have fee movement in both axes in order for it to be able to draw smooth curves. You'll notice that the lower hole on the stylus is bigger, this is because my tolerances were too tight for my workmanship and I had to drill out a larger hole.

The pipe holes on the trolleys and stylus have brass inserts I made by hammering the 1/2" brass tubing into the hole, cutting it off, then countersinking the edge of the brass. This made for less friction, and it's a nice detail if you look closely.

The ridge cut around the perimeter of the trolleys is a way to control the tension on the cable. It keeps the cable centered on the hole and the thumb screw, preventing slipping and settling as the piece gets used. I made this groove by cutting a quick, shallow, straight line on each face, then going over it with a square file at a 45º angle, giving me a nice groove.

The hole centered on the groove gets the threaded brass insert. This was a real pain to screw into the hard wood, and I ruined more than 1 trolley this way. It's very hard to keep them flush to the top, and there's no change that the flat head screwdriver recess on top of the insert will take the force needed to screw it in. Eric Forman showed me this ingenious trick for getting the insert to screw in using a cut-off screw (10-24), a couple of nuts, and a drill that made my life a lot easier.

As long as the dimensions are carefully controlled and the parts are tested for ample play as they move past each other, everything should go smoothly. A combination of gluing on paper templates and marking cuts and holes by hand did the trick for this step.