Summary

This is a modification for the Z axis of the Piper 2. It retains the four lead screws (they are necessary for the gantry's overall stability) but forgoes two of the steppers, plus their associated hardware (Drivers, Couplers, etc). The two steppers can be easily driven in parallel by a single driver, eliminating the need for a CNC shield, reducing the overall complexity of the build. It also makes the gantry more stable, as each side is tethered and is more difficult to slide/skip when the motors are powered off.

It makes use of two GT2 6mm closed loop belts (I suggest 852mm belts, they should fit everything, unless if you are building a really small version). The belt forms a triangle, with the stepper mounted at the front or rear. The side triangles brace two lead screws each.

Note that this design will steal a few mm of Z travel and force you to use longer spacers for your print surface.

If you are asking yourselves "why not use a third belt, tether everything together and use a single stepper instead", I already built one of my printers using such a setup. After significant testing, I came to the conclusion that using a single Z-axis stepper on the Piper 2 was unreliable, as the high torque required would cause common GT2 belts to jump. It is possible to use a single stepper if you go with belts that can handle greater loads but that would make everything more complicated and expensive than using four individual stepper motors to begin with.

You will need to print the following parts:

2 × Motor Mount (for the steppers)
4 × Locks (these lock the bottom side of the lead screws under the bearings)
4 × Bearing Holders (use them instead of the Z-axis motor mounts - the bearing locking plates are optional, the bearings do not come out of their sockets on my printer).

I also added files of optional idler tensioners. I did not use them but someone might find them useful. The floating part uses a normal idler (with or without teeth) and connects to the stationary part via a long M5 hex screw. Tightening the nut pulls the floating part towards the base, increasing tension.

Parts you will need:

8 × M3 ~20mm screws
8 × M3 nuts
4 × M4 countersunk screws and nuts (length depends on the thickness of your printer's base but 40mm should be plenty).
4 × 608ZZ bearings
2 × GT2 6mm belts (852mm ought to work fine for any printer's size, just play with the position of the motors).
2 × GT2 20T 5mm bore pulleys for the steppers
4 × GT2 20T 8mm bore pulleys for the lead screws