Quotethe_digital_dentist The X axis guide rail uses a bearing block to attach it to one of the Y axis bearing blocks, so that when the frame expands and the Y axis guide rails move apart, the X axis rail can slide without creating large side loads that might cause the Y axis to bind. I had a question regarding this, I've been following your build for a while (and I may have missed this, apologieby caffeine_addict - CoreXY Machines