Refer to the "Y axis" page on the parts list to gather your parts for this part.
Refer to the photo below for these steps. Take one of the 7/16 (or M12 if you're using metric bits) rods. Spin a nut into about the center of the rod, then put a nut on either end and spin them in a bit. Put one of the rectangular feet onto each end, making sure that the slot on top has the stops to the outside (so that the smooth rod can be inserted and the stops will keep them from sliding out the end). Secure each end with another nut. Do this again for the other rod and two ends. Get your ruler and make sure the corners on each rod are the same distance apart as close as you can make it - this will make your axis as square as possible to start with.
Put a nut into the nut trap in the belt tensioner and thread a #6-32 x 3/4" screw into it until the screw just barely protrudes into the larger opening as shown:
Take one of the large threaded rod/corners assemblies and put these small threaded rods in, putting a washer on the small rods first, then securing the other side with a washer and another nut for each rod. The motor mount side mounts with the motor mount bit pointing towards the inside, with the longer bit up (towards the top of the corners). The idler side has the belt tensioner/idler on top, the plain rod on the bottom. Refer to the following two photos:
Also as seen in the above photo, press three linear bearings into the three holders, put them onto the two Y axis smooth rods (the medium length ones) and snap them into the top as shown.
NOTE: be careful to put the two on top and one on the bottom with the idler on the left and the motor on the right. We are putting the motor in the FRONT of the printer, unlike many assembly guides, because it works better that way with the plywood frame.
Now you will want to start tightening things up. On ONE end of the two large rods, screw the end nut to flush with the end of the rod, then clamp the other nut down tight (pretty tight but don't crush the plastic) onto that corner for both on that end. Then do the same at the other end but you can bring that side in a little to give a little room for adjustment later.
Now tighten one side of the short, small threaded rods in the same way. Then when you move to the other side, use a ruler to make the assembly be 165mm exactly between the two rails (center to center, easier to measure left to left or right to right) as shown below.
Now use four large zip ties to secure the smooth rod to the axis assembly as shown (do this in each corner):
Once you've tightened that all up, get the square plywood print surface. Mark a line along one side 20mm in from the side. Though most assembly guides have you assembling the print surface centered, with the box frame I've found that you need to offset about 25mm towards the front, otherwise you will not be able to achieve the full 200mm of travel. So with the line you just marked at the top, make a mark along the line at 33mm from the right, another 83mm from the left. Then make a mark at the BOTTOM of the plate 25mm right of center (140mm from the left).
These photos combine a view of the marks with the next step, attaching the plywood to the axis.
Also make a mark along the center line of the board to mark where the belt holder will go.
Lay the platform on your work surface and place the Y axis assembly over the top, with the side with two bearings towards the side with the line 20mm from the edge. The two marks that you made mark the location of the OUTSIDE edge of each bushing holder. Secure each with a single #6 x 3/4" wood screw (if your 3/4" screws are long and stick out the top, you may have to use 1/2" screws). The mark in the center on the other side marks the location of the third bushing holder. Secure that with another screw, then put in the second screw into each bearing holder.
Once you have put in all the wood screws, you can put the 6-32 x 1" screws and nuts in on top of the linear bearings and snug them down, securing the linear bearings in place.
Get your belt holder and clip out the support section as shown - ONLY the part shown. The middle supports are needed for strength. This one just gets in the way.
When you are done it looks like this:
Get one of your stepper motors and a pulley, install the pulley on the motor (with the collar facing out to keep the belt as close to the motor as possible - this gives the belt less leverage to twist the motor out of square). If your motor shaft has a flat side, put a set screw on it. Then use two M3x10 screws to attach the motor to the holder. Adjust the nuts holding the motor holder to the frame so that the pulley is centered in the frame and the motor is level. Center the idler on the other side too so that it is in line with the pulley. Now screw the belt holder to the plywood, centered along the line between the bearings.
Get a length of timing belt. You need to fish the end of it through the belt holder attached to the plywood - I find the easiest way to do that is to use the end of a zip tie as a tool as shown. Be sure the teeth are facing up.
Secure that end of the belt to the holder using a small zip tie as shown
Finally, clip the excess belt and zip tie end - don't cut the belt SUPER close to the zip tie, in case you have to remove it and redo this procedure again later.
Get your long endstop switch holder and secure a switch to it using two #4 screws and nuts. If the leaf on the switch is long, bend it over so that when it is mounted on the smooth rod, it comes close to the rod but does not touch it. The linear bearing will be pushing on this. Then mount the switch to the smooth rod on the side with just one linear bearing, between the bearing and the side of the base away from the motor. Secure it with a #6 screw and nut. Don't tighten it up quite yet, we need to adjust it later.
Be sure to mount it exactly as shown, with the screws facing down like that. The holder will not clear the print platform in other positions.
Y axis done! Good job.