Dan,
No video (yet) sorry but as it turns out I had a linear rail block lose its balls (no pun intended).
Spent about 4 hours finding and re-installing all but a few but since it's a warranty replacement (still waiting for it) I bought a set from Amazon and will replace all 4 of them for the Y axis which means I have to take the gantry off again and re-do the procedure so I'll try to video or photo the process. Alas, I have to take off all my top surface and spoil boards to get at the bearing block mounting screws and my gantry weighs about 250 kg so I'm not all that anxious to do it because the machine is working well at the moment.
The alignment is not difficult, just a little fiddly.
1. Start with a straight line drawn with a spring loaded pen/pencil in the spindle on the master rail side on Y (and set X to (close to) zero)
2. Around the center of travel for Y, make a 1000mm set of marks (500 mm each side)
3. Measure and mark your 1000 mm box with a good straight edge rule using the diagonals to ensure it's square
4. Align spindle center (pen) to one line of intersections on X=0 side, move X along line to other side (X+)
5. If gantry is square to master rail, it will follow line and match at the end of line intersections, otherwise diddle the mechanicals until it is
6. Repeat to check
7. Move Y to other perpendicular line and re-check.
8. If there is a discrepancy take the difference of the two to maximise (minimise?) squareness between the allowed distance
9. If there is a discrepancy it shows the master rail is not straight and has a bow of sorts
Any measurement is OK, bigger is better, it's just our old friend Pythagoras for the hypotenuse side.
As mentioned in previous post, if meshing of R+P is not snug and easy for both sizes, power down and move slave side motor. The idea is to find the best full step position for meshing without power. After power on, the meshing may cause a slight movement, so again mechanical adjust if needed.
Then under power move the spindle pen around the box, for the perimeter and the diagonals to show the tracking of the lines.