Learned something I should have figured out. Mark your belt pulleys to know if you are experiencing Belt OR Pulley slippage. I’ve marked all my pulleys and shafts.
Left pulley might be a little light in the pic, but there is a pencil mark like the right pulley. Mark your shaft and pulley. Run a know project file that slips. Then check the pencil line.
From my test of Pinewood at 160 ipm, 48 ipm plunge with a 1/4" end mill. The X-axis slipped ~1/8". When the project was over the bit was clearly 1/8" off home. Neither of the X-axis pulley lines were misaligned, so I have belt slippage. Time to try 130 ipm. I should note, I use the Triquetra to home my bits with the material.
NOTE: For the belt tighteners on XCP, note you can over tighten and break the X-Axis spindle shaft. Translation, be careful tightening your belts on the XCP.
This information was per Inventables Customer Support.