Range of error is nearly 1/100th of an inch

After simply removing my Z-carriage and re-attaching it (re-calibrating it), my X-Carve did appear to work as expected at first, so I tried to carve a simple 1.5 inch by 1.5 inch square. It did function properly, but when I measured the finished product the square was 1.489 by 1.492 inches. I’m cutting 0.205" thick plywood at a depth per pass of 0.015" and a feed rate of 10 inches/minute, so I’m pretty sure it should have managed a range of error less than 1/100th of an inch.

I’d suggest checking the following, as they are the easiest to do (in order) and most likely culprits:

You may also want to check that the little screws in the pulleys are still tight and one is in the middle of the flat on the shaft. If they’re slightly loose, it’ll allow the shaft to turn before the pulley does, which can increase the slop.

By re-calibrating, do you mean that you measured and adjusted the steps/mm? That’s something that’s going to be unique to each machine, so to get maximum accuracy you should do that. There’s a video on how to do that calibration at Steps/mm Calibration

Finally, your belt tension and pots may be slightly off. It’s possible that a loose belt is allowing a little more slop. I don’t think it’s your pots, but that’s not outside the realm of possibility. There’s videos to check them both at X-Carve Maintenance/Troubleshooting Videos - Add Your Own!

Another easy thing to check is the actual bit measurement. It may say 1/8" or 1/4" on the package, but it is highly likely that the bit cutting diameter is different. Measure carefully and then use those numbers for specifying your bit size in your CAM program (Easel).