So how much resistance should I be feeling when manually sliding the gantry along the y-axis? I've gotten through Step 16: "Adding X Slider" but when I tighten everything up, it doesn't want to move along the y axis very easily. For example, when I'm trying to move the gantry along the y-axis, the whole frame wants to slide on the table. I can loosen the screws on the y-sliders (A15 attached to the plates) and it slides pretty smooth.
It should be pretty smooth but it doesn't have to be "friction-less." The Y gantry will be set once you tighten down both sides of one of the X axis rods, so it's important you have everything aligned before tightening down the first rod. Then after that, the Y bushing spacing should be set and you can then work on the other X axis rod to set the x bushing spacing so that is smooth. Hopefully this makes sense. I hope to make a video tutorial soon to explain the process a little better.
I've figured out what was wrong. When I tipped the printer frame over on it's face, the gantry wouldn't budge. I noticed that if I applied a little pressure to the printed brackets that hold the y-rods, the gantry would drop smoothly. Looks like my rods were a touch too far apart. When I initially installed the rods, they pressed in very tight. I'd had the same problem with the bushings originally (too tight) and used a small needle file to remove some material on those printed parts so I pulled everything back apart and did the same for each corner rod holder. Rods went back in. Still snug but not nearly as hard to install. Now the gantry slides smoothly. It shudders sometimes when I slide it back and forth but I'm guessing that's because the belt hasn't been installed so my push-pull forces aren't equalized from side to side when I slide it by hand.