Skip to content
Ch 04Before you start

Before you start Chapter 04

Before you start · Chapter 04 — A/B drive units and front idlers

Builds the four CoreXY sub-assemblies that carry the A and B belts — two motor-carrying drive units and two front idlers with their tension arms. Unlocks Ch 05 (gantry), which bolts all four onto the Y extrusions.

What you're building in this chapter: the four corners the two long belts run around. In CoreXY two motors fixed at the back pull two crossed belts, so nothing heavy rides with the toolhead (glossary). A is the rear-right corner, B the rear-left. Each drive unit is a two-half printed frame with flanged bearing stacks on two standing bolts and a stepper motor underneath. Each front idler is the same bearing sandwich without a motor, plus a blue tension arm. The two belts run at two heights, and most of this chapter sets those heights exactly. Everything is built on the bench and bagged.

Revali pointing
Two drive units, two idlers, all handed. Lay them out left and right. Chapter seven notices.

Time: 3.5–5.0 h hands-on, first build (survey §5.1 P04).

Sessions: 7 × ~30 min — the Pause: lines below break the chapter into 7 segments; every minute figure is a first-build estimate.

Prerequisites:

  • Ch 01 — Frame. Nothing from Ch 02 or Ch 03 is needed; this is bench work and can run in parallel with them (survey §5.1: P04 ← P01).
  • Print batch B03 — A/B drive units + front idlers, 1 plate, 8.5 h, 119 g black (print plan). front_idler_right_* go with the A drive, front_idler_left_* with B (Step 04.2).
  • Print batch B02 — the accent day, plate B02-P2, for [a]_tensioner_left and [a]_tensioner_right.
  • Print batch B00 — for pulley_jig.stl. Without it you are setting two different pulley heights with calipers.

Tools

  • Hex drivers 2 mm, 2.5 mm, 3 mm, 4 mm — a 2.5 mm ball-end helps at the M3×40
  • 1.5 mm hex — the pulley set screws (verify on bench against the kit's pulleys)
  • Temperature-controlled soldering iron + LDO brass M3 insert tip (4 heat-set inserts, Steps 04.3–04.4)
  • Printed pulley_jig.stl (Voron-2 STLs/Tools/)
  • Digital caliper — pulley height fallback, and to confirm both drives match
  • The free end of the kit's 6 mm A/B belt reel, uncut — the belt-path straightness check. Cutting belt waits for Ch 07's matched cut
  • Tweezers or a small screwdriver to place spacers into the stacks without dropping them
  • Masking tape + marker — label every assembly A or B as it is finished

Consumables: Loctite 243 (blue), for the pulley set screws if they are not already pre-applied.

Printed parts

Parts arrive in the bins named below (see the bin map in print/README.md#bins); check the bin label's list before you start.

Looks like STL Bin Qty Colour
a_drive_frame_upper.stl 04-A 1 Black
a_drive_frame_lower.stl 04-A 1 Black
b_drive_frame_upper.stl 04-B 1 Black
b_drive_frame_lower.stl 04-B 1 Black
front_idler_right_lower.stl 04-A 1 Black
front_idler_right_upper.stl 04-A 1 Black
front_idler_left_lower.stl 04-B 1 Black
front_idler_left_upper.stl 04-B 1 Black
[a]_tensioner_right.stl 04-A 1 Blue
[a]_tensioner_left.stl 04-B 1 Blue

All ten are in Voron-2 STLs/Gantry/AB_Drive_Units/ and STLs/Gantry/Front_Idlers/. Two more parts sit in those same folders and are not used here: [a]_cable_cover (Ch 07) and [a]_z_chain_retainer_bracket_x2 (Ch 06). Keep them bagged.

Hardware (chapter totals)

Fastener / part Qty Where
Stepper motor, 0.9°, LDO-42STH48-2004MAH(VRN) 2 one per drive (p.75, p.79)
GT2 pulley, 20 T, 5 mm bore, 6 mm wide (Pulley, 2GT, 20T, 5mm ID 6mm W) 2 one per motor shaft (p.75, p.79). The kit has exactly two; the four 9 mm 20T pulleys are the Z-drive pulleys used in Ch 02
F695 flanged bearing (5×13×4 mm) 16 6 per drive (p.74, p.78) + 2 per front idler (p.65, p.69). The A/B side uses F695 only — 625-2RS is the Z-drive bearing (Ch 02)
M5 precision spacer, brass — the manual's "M5 shim" 16 one above and one below every bearing pair
M5×30 BHCS 4 2 per drive, upper frame → lower frame (p.73, p.77)
M5×40 SHCS 2 1 per front idler — the idler axle: first the stack-building aid, then refitted from the top through the tension arm into its M5 nut and clamped firm (p.65–66, p.69–70)
M3×30 SHCS 6 3 per motor (p.76, p.80)
M3×40 SHCS 2 1 per front idler — the belt tensioner screw, through the frame's front wall into the arm's heat-set insert (p.67, p.71); set with the belt on in Ch 07 Step 07.5
M3 washer 2 under each M3×40 head (p.67, p.71)
Heat-set insert, brass, M3×5×4 4 2 in a_drive_frame_upper, 1 in each tension arm (p.64)
M5 hex nut 2 1 in each tension arm foot (p.64)

Read first

  • A and B parts are mirrored and swap silently. The A drive's printed frames carry a cutout the B frames do not have (p.73, circled). Only a_drive_frame_upper has the two heat-set insert bosses — b_drive_frame_upper has none. Build one complete assembly at a time; never have both sets of frames loose on the bench together.
  • Where each assembly ends up, standing in front of the printer: A = rear right, B = rear left. The official manual shows it (p.63, p.83 — A on the right, B on the left, seen from the front) but never writes it; the LDO wiring guide does, and the LDO Klipper config repeats it (## B Stepper - Left, ## A Stepper - Right). It follows that the A idler is the front right pair and the B idler the front left pair. src
  • The bearing stacks are the mistake everyone makes. Every F695 pair goes flange-out — the two plain faces touch, the two flanges face away from each other — with one spacer above the pair and one below. The drive's far post takes two such pairs (4 bearings, 4 spacers); the near post and each front idler take one (2 bearings, 2 spacers).
  • The A and B pulleys sit at different heights and face opposite ways: A is hub-down at 16.5 mm (p.75), B is hub-up at 6.5 mm (p.79). That difference is what stacks the two belt planes. No amount of belt tension in Ch 07 will fix it if you set them the same.
  • Short a bearing, spacer or pulley? This chapter needs exactly 16 F695, 16 spacers and the two 6 mm pulleys. Recount against your Ch 00 inventory sheet, then contact Fabreeko support. Never borrow a 9 mm Z pulley.
  • LDO's Build Notes have no entry for p.62–81. The only kit deviation in this whole chapter is the standing substitution from their p.19 note: brass M5 precision spacer everywhere the manual writes "M5 shim". src

Sources for this chapter:

Video coverage (Steve Builds, pre-release LDO kit — differs from Rev D+): Part 3 @0:58:20 (+3m), Part 3 @1:04:20 (+14m), Part 3 @1:07:20 (+10m), Part 3 @1:18:03 (+2m), Part 3 @1:23:20 (+9m), Part 5 @1:12:20 (+8m)

helperHelper steps: 14 · 04.1, 04.2, 04.9, 04.12, 04.16, 04.19, 04.22, 04.24, 04.28, 04.30, 04.33, 04.35, 04.37, 04.38

← PreviousCh 04 — chapter overview

Next →Step 04.1 — Lay the chapter out as four separate kits