Y Axis Auto-parallel
Y axis auto parallel involves adding a half-carriage as an extra piece to the X Axis Universal Axis. This was done from v19.02 onwards (see 3D Printer Genealogy to prevent the y axes from jamming if the 2 axes are not parallel. By adding an extra half-carriage at the end of the X axis, with rods of the X Axis inside this half carriage - we are allowing the X axis to slide in and out of the Y2 axis (Y1 axis attachment point is fixed). This constrains the X axis at the Y2 Axis in the Y direction, but does not constrain the X axis at the Y2 Axis in the X direction - thereby preventing any jamming due to the distance between the Y1 and Y2 axis not being the same.
When the Y1 and Y2 are not parallel, some non-parallel can be allowed as the Y axes flex. This increases motion resistance. Once the resistance is high enough - the stepper motors cannot overcome it and motion jams, resulting in layer shifts. Additional resistance occurs as soon as the non-parallel is as small as the thickness of a sheet of paper - about 100 microns or 4 thousandths of an inch. The point of this is that it's not possible to achieve this level of accuracy using welded frames, and thus performance would always be suboptimal - some other part of the printer has to take up the resistance by deforming or creating more friction. For this reason, we have added the auto-paralleling mechanism.
Download the CAD from D3D_v19.06 to look at the details of the X axis to see how it is constructed with the half-carriage next to the idler side of the axis. If this is not clear in the CAD, the CAD should be updated.