It took me some time to figure out what that means and how to measure it. This is my conclusion:
When you look at your Y-rails from below, you find the "y_min_bumper printed part" that you installed in "04. CoreXY Idler Assemblies" of this instruction on both sides. The Y-rail carriage should bump into those two bumpers simultaneously.
If this is not the case yet you would feel a little play, either on the left or on the right side, when you do the following:
1. Move the X-gantry to the very front
2. place one finger of your left hand on the gap between the left Y-rail carriage and the left y_min_bumper
3. place one finger of your right hand on the gap between the right Y-rail carriage and the right y_min_bumper
4. pull the X-gantry a little more to the front.
If you feel some play on the left finger, the space is not equal and the rotation of the X-gantry is like in picture 3 of this step. If you feel some play in your right hand, the rotation is the other way. If you feel no play at all, of course everything is ok
I suggest to fix the belts in the opposite order: first install them in the belt grabbers (Step 21 & 22) and attach them loosely to the EVA3 back part.
After you have done that, attach them to the EVA3 front part as shown in Step 17.
This way you don't need to cut the belts at this point. You can instead leave them hanging from the EVA3 front part until you are sure how much tension to apply and then cut them off.
With my third rod, I found a very fast method to unpack:
The rods are packeged in a square sheet of plastic foil. If you are lucky enough to find the corner(s) of this plastic sheet (on my rods it was somewhere in the center of the rod), carefully follow the edges and unwrap the foil without ripping it.
Think of it as a kid trying to unpack their chocolate easter bunny conserving the aluminium foil.
If you manage to do that, you will have your rod unpacked in seconds.
It took me some time to figure out what that means and how to measure it. This is my conclusion:
When you look at your Y-rails from below, you find the "y_min_bumper printed part" that you installed in "04. CoreXY Idler Assemblies" of this instruction on both sides. The Y-rail carriage should bump into those two bumpers simultaneously.
If this is not the case yet you would feel a little play, either on the left or on the right side, when you do the following:
1. Move the X-gantry to the very front
2. place one finger of your left hand on the gap between the left Y-rail carriage and the left y_min_bumper
3. place one finger of your right hand on the gap between the right Y-rail carriage and the right y_min_bumper
4. pull the X-gantry a little more to the front.
If you feel some play on the left finger, the space is not equal and the rotation of the X-gantry is like in picture 3 of this step. If you feel some play in your right hand, the rotation is the other way. If you feel no play at all, of course everything is ok
I suggest doint this in the opposite order (see my comment for Step 17)
I suggest to fix the belts in the opposite order: first install them in the belt grabbers (Step 21 & 22) and attach them loosely to the EVA3 back part.
After you have done that, attach them to the EVA3 front part as shown in Step 17.
This way you don't need to cut the belts at this point. You can instead leave them hanging from the EVA3 front part until you are sure how much tension to apply and then cut them off.
Thanks for the hex key trick
With my third rod, I found a very fast method to unpack:
The rods are packeged in a square sheet of plastic foil. If you are lucky enough to find the corner(s) of this plastic sheet (on my rods it was somewhere in the center of the rod), carefully follow the edges and unwrap the foil without ripping it.
Think of it as a kid trying to unpack their chocolate easter bunny conserving the aluminium foil.
If you manage to do that, you will have your rod unpacked in seconds.
I suggest to pick up the 4 M4 Nyloc Nuts from Step 12 at this point and loosely fix them to the screws after you assembled the bearing.
When I followed the next steps, this assembly fell apart more often than I am willing to admit. With the nut in place this can easily be avoided.
I first had a hard time inserting the magnets.
What helped me then was
(1) Make one piece of Step 16 (which was a bit tricky)
(2) Using this piece, put two new magnets on the already inserted magnets and make one piece from Step 17
(3) Put two new magnets on the piece from Step 17 and use the piece to push the magnets inside of a piece from Step 16
(4) Repeat (3) until you are done with Step 16 - then you can use this technique for Step 17