There are two reasons behind choosing the 2020 extrusions.
Reason #2 is for mounting accessories on either side (homing switches, drag chain bracket, dust shoe, mist).
Reason #1 is to bring closer the top face of the rail blocks to the top face of the ballnut mount. There’s another 1mm till they become coplanar, which can be achieved either by adding shims on the blocks or by milling a shallow channel on the bottom of the spindle plate. I chose the latter.
…But first things first.
In the previous setup the Z axis was slim and lightweight enough that I could get away with only one pair of rail blocks on the X. Not any more. The new Z weighs around 4.2kg (9lb) without the spindle mount, spindle and motor which add another ~3kg (6.6lb). I also want to have the option of a VFD in the future and that will also be heavier than the Makita.
Let’s add another pair of blocks on the X.
and make a new gantry plate.
We start the actual Z axis by aligning the extrusions.
Then slide the rails in. We will use the precision ground base of each rail (marked with the arrows) facing the inside.
These particular rails are 15mm wide. The 2020 extrusions are, well, 20mm wide. We sandwich the rails between two stacks of 2.5mm feeler gauges and tighten the first screw (not to final torque, we want the rail to be able to pivot slightly).
Before we move any further we need to add end stops. Derailing a block will cause the ball bearings to fall out.
We calibrate the first rail and tighten all its screws.
The inner rail-to-rail distance was designed to be 80mm in order to use a couple of 20-40-80 metric blocks (minimum rail-to-rail distance is 71mm, so a pair of 1-2-3 blocks on their 3" side could work as well). This along with placing the arrows facing each other ensures that the rails end up parallel. This is the most critical step of this mini build.
Tighten the screws of the second rail, add block and end stops.
We put together the ballscrew assembly.
Then place it loosely on the Z plate.
Add the spindle plate. Move the spindle plate all the way to the top bearing block, tighten the top bearing block. Move the spindle plate all the way to the bottom bearing blog, tighten the bottom bearing block. If we tighten both and skip this step, tolerances are so tight that the slightest misalignment will add friction to the ballscrew.
Assembly is pretty much done.