Y Gantry Plate Installation
Remove the clear plastic wrap surrounding the linear bearings and ball nut on the left and right Y-axis assemblies. Remove the plastic ball screw nut support packaging by sliding this out forwards or backwards.
Support packaging removal
Take out the Y gantry plates, making note of the differences between the left and right plates. Specifically, the left Y gantry has holes which allow for the stepper motor to be mounted.
Left and right Y gantries
Move the linear guide blocks away from the aluminum ball screw nut block, since we will assemble the gantry on the linear guides first. 
Linear guide blocks away from ball screw nut block
On each Y-axis gantry plate, there are two sets of four (4) holes in a square pattern, aligned in a row. These should match up with the two linear guide blocks.
Grab eight (8) M4-16mm socket head cap screws. Insert one screw through the Y-axis gantry plate into one of the linear guide blocks. Insert a second screw into the second block. Then, install the remaining screws into both blocks, but do not tighten at this stage. 
Loosely fastening screws between Y gantry and linear guide blocks
Y-gantry sitting on linear guide blocks at reference edge
Then, slide the Y-axis gantry plate over top of the aluminum ball screw nut block.
Aligning Y gantry to aluminum ball screw nut block
Install four (4) M5-16mm socket head cap screws, going through the plate into the aluminum block. If you received your AltMill after December 10th 2024, install M5-20mm ‘ball nut screws’ included with your kit for these holes instead. Loosely secure.
Loosely fastening aluminum block screws
Repeat the screw installation on the Y-axis gantry plate for the other side, keeping screws loosely fastened. 
Left Y-gantry screws installation
Push the left and right Y-axis gantry plates into the bump stops at the front of the machine.
Gantries pushed against front Y end plate bump stops
Install the remaining two (2) bump stops onto the Y-axis gantry plates using two (2) M5-16mm socket head cap screws.
Installing bump stops onto Y gantries
Then install one of the inductive sensors onto the left Y-axis gantry. This will be the X-axis sensor.
Add a lock washer, then thread a nut on. Fasten nuts with the wrench until secure, sandwiching the Y gantry as the sensor is fixed in place. Leave the wire free for now.
Similar to the Y-axis, align the end of the sensor 1mm before the bump stop.
X-axis sensor installation
X-axis Assembly Installation
Grab the X-axis rail assembly and place it on the table. Stand up the assembly so that the X gantry plate, and the notch on the plate, are facing upwards.
X rail placed on table, noting orientation of X gantry plate
Insert two (2) M6-35mm flanged button head cap screws onto each end of the X-axis rail, in the holes surrounding the black bearing blocks as shown in the photos. Ensure they stick out of the rail by about ¾ to 1 inch.
Installing screws onto ends of X rail, sticking out about 3/4″ to 1″
Lift the X-axis assembly, making sure the X gantry is facing towards you, then rotate the assembly so the bottom is slightly angled away from you. Mount the assembly onto the Y gantries by seating the front-facing screws on the rail end into the slots of the Y gantry, then seat the rear-facing screws into the second set of slots by pivoting the assembly towards you.
Side view of mounting X-axis rail onto Y gantries
Tilting motion for mounting X-axis
Jay mounting X-axis assembly onto Y gantries
Loosely install the remaining M6-35mm flanged button head cap screws into the two (2) lower holes on the X-axis assembly on each side, totalling (4) screws. Do not tighten the hardware.
Loosely securing screws through X-axis and Y gantries
There is a recessed area on the Y-gantries that aligns the X rail to the correct position, squaring it to the machine. Use a clamp to push the X rail and left Y gantry into each other, so that the rail sits in the locating recess.
Clamping X rail and Y gantry for alignment before securely fastening screws
Cross section showing locating recess on Y gantry
You can now securely tighten the M6 screws on the two ends of the rail, totalling to eight (8) screws. Leave the clamp on until the end of this assembly section.
It is OK for the X rail to protrude past the gantry on the rear. Just make sure the ball screw is aligned to the hole in the gantry.
Tightening screws connecting Y gantry to X rail, 4 on each side
Tighten all the screws for the left and right Y gantries, at the linear guide blocks and aluminum ball screw nut block. There are 12 screws on each side, totalling to 24 screws.
Tightening screws connecting Y gantry to linear guide and ball screw nut block, 12 on each side.
Remove clamp. You have now completed the X and Y gantry assembly, great work! You deserve a small break with a beverage of your choice.