The Tractor: X-Y Axis System

✅ Still Relevant: This exploration of Cartesian Z-drop and motion system design remains current. For canonical design decisions, see: - ADR-021: Dual-Rod Motion System (X-Y motion architecture) - ADR-022: Linear Bearing Selection (smooth rods vs. alternatives) - ADR-023: Z-Drop Architecture (why bed moves, not gantry) - ADR-005: Triple-Z Kinematic Leveling

This deep-dive provides rationale for why Z-drop + dual rods works well for scavenger builds.

Overview

“The Tractor” uses a Z-Drop (Z-Dropdown) design where the bed moves vertically on Triple-Z while a gantry handles X-Y motion at the top of the frame. This document covers the X-carriage (“The Plough”), Y-axis gantry system, and motion components.

Z-Drop Architecture

What is Z-Drop?

Unlike bed-slingers (where the bed moves in Y), a Z-Drop design:

Bed: Stationary in X-Y, moves only vertically (Z) on its own independent system
Gantry: X-axis assembly moves front-to-back along Y-rods (fixed height)
Motion: The “Plough” (X-carriage) moves left-right, the gantry moves front-back
Z-Axis: Completely separate — bed drops down as layers are printed

Key Point: The X-Y gantry and Z-axis are independent systems. The gantry never moves vertically.

Benefits: - Heavy bed isn’t moving back and forth (reduces “ringing”) - Higher print speeds possible - Better for large, heavy heated beds - Gantry stays at fixed height (simpler belt paths)

Component Layout

Top of Frame (Fixed Height - X-Y Motion):
┌─────────────────────────────────┐
│  [Y-Motor]     Y-Rods    [Y-Motor]  ← Dual Y steppers
│       ════════════════════       ← Y-rods fixed to top frame
│       ↑                  ↑
│    [X-End]            [X-End]    ← Slide on Y-rods (front-back)
│       ├── X-Rods (60mm) ─┤       ← Carried by X-Ends
│       │   [THE PLOUGH]   │       ← Slides on X-rods (left-right)
│       │    Pitan + V6    │
└───────┴──────────────────┴──────┘

Inside Frame (Separate Z Motion):
┌─────────────────────────────────┐
│                                 │
│  Z-Rods (M8/M10 smooth)         │  ← Vertical guide rods
│  ↕ Triple-Z lead screws         │  ← Drive bed up/down
│      [Heated Bed Platform]      │  ← Moves ONLY in Z
│                                 │
└─────────────────────────────────┘

The X-Y gantry and Z-bed are INDEPENDENT systems.

Dual Y-Axis System

Why Dual Motors (Not Sync Shaft)?

The Tractor uses two Y-steppers (one per side) instead of a single motor with sync shaft:

Feature	Single Motor + Sync Shaft	Dual Y Steppers
Parts	1 Motor, Long Rod, 2-4 Bearings, 2 Couplers	2 Motors, 2 Pulleys
Complexity	High (shaft must be parallel)	Low (self-contained units)
Scavenger Factor	Hard to find straight 500mm rod	Easy—second motor from donor
Robustness	Adds rotational slop over distance	Double torque for heavy gantry
Klipper Edge	Manual squaring only	Auto-squaring every home

Auto-Squaring with Klipper

The killer feature of dual Y motors:

Place one endstop on left Y-rail, one on right Y-rail
When printer homes, both motors move toward back
Left side hits switch first → stops
Right side continues until it hits its switch
Result: Gantry is square to frame within 0.01mm

This level of precision is nearly impossible with a manual sync shaft.

Driver Requirements

For Dual-Y + Triple-Z: - 1 driver for X - 2 drivers for Y (Y and Y1) - 3 drivers for Z (Z, Z1, Z2) - 1 driver for Extruder

Total: 7 Drivers

Options: - MKS SKIP + expansion board (SKR Pico) - Two donor mainboards via Klipper Multi-MCU - Hard-wire Y-motors in series (loses auto-square)

Y-Motor Corner Mount

The Y-motors sit at top corners of the box frame:

# Integrated Y-Motor Mount
motor_offset = 45  # Distance from frame center to motor shaft

with BuildPart() as y_motor_mount:
    # 1. Core Tractor Corner (M10/M12 Hub)
    # [Frame rod clamping logic]
    
    # 2. Motor Cantilever
    with BuildSketch(face.sort_by(Axis.X)[-1]) as s:
        Rectangle(50, 60)  # Massive plate for NEMA 17
    extrude(amount=12)
    
    # 3. NEMA 17 Pattern with tension slots
    # Use 10mm slots for belt tensioning by sliding motor
    for loc in GridLocations(31, 31, 2, 2):
        SlotOverall(10, 4.5, rotation=90)

X-Axis: “The Plough”

Design Philosophy

The X-carriage is called “The Plough” in Tractor terminology. It’s based on the Mendel i2 style but upgraded:

Redesigned: Clamps instead of zip-ties for linear bearings
Structural: Cut-aways with thick walls (not solid infill)
Nested: Pitan/Titan extruder sits between the two X-rods

Rod Spacing: 60mm (Center-to-Center)

For a top-down “nests between the rods” configuration:

Component	Dimension
NEMA 17 Motor Width	42.3mm
Plastic Wall Thickness (each side)	4mm × 2 = 8mm
LM10UU Bearing Clearance	~10mm
Total Spacing	~60mm

Why 60mm: - Wide stance resists “yaw” (twisting left-right) - Fits Pitan motor body with clearance between rods - Provides balanced weight distribution for direct drive

Structural Optimization: “Hollow Bone” Principle

Strength comes from skin tension, not solid plastic:

Use 4-6 perimeters (walls) instead of 100% infill
Design structural “ribs” and “webs”
Cut-away weight-saving pockets leaving 3-5mm thick beams

# The Plough: Skeletonized Carriage
rod_spacing = 60  # Center-to-center
bearing_od = 19.4  # LM10UU with slop
wall_thickness = 4.0

with BuildPart() as plough:
    # 1. Two horizontal Bearing Cylinders
    # Top and bottom bearing housings
    
    # 2. The 'Spine' connecting them
    # Structural plate, not solid block
    
    # 3. Cut-aways (Lumpy optimization)
    # Triangular or slot subtractions leaving X/V truss

Split-Clamp Bearing Housing

Replace zip-ties with proper clamps:

# LM10UU Bearing Clamp
bearing_od = 19.0 + 0.2  # Nominal + slop
bearing_len = 29.0
clamp_bolt_dia = 3.5     # For M3 bolt

with BuildPart() as bearing_mount:
    # 1. Main Block
    with BuildSketch() as s:
        Rectangle(bearing_od + 10, bearing_len)
    extrude(amount=bearing_od / 2 + 5)
    
    # 2. Bearing Bore
    # Semi-circular channel for LM10UU
    
    # 3. The "Split" for clamping
    with BuildSketch(Plane.XY.offset(bearing_od / 2)) as s3:
        Rectangle(2, bearing_len)  # 2mm slot
    extrude(amount=10, mode=Mode.SUBTRACT)
    
    # 4. Bolt ears for M3 pinch bolt

Benefits of Split-Clamp: - Zero slop (accounts for print inaccuracies) - Serviceable (swap bearings without breaking carriage) - Pre-load adjustment (slight tightening grips bearing perfectly)

X-Ends (Where X Meets Y)

Function

The X-Ends are the “sliders” that carry the X-axis rods while moving along the Y-axis:

Hold LM10UU bearings to slide on Y-axis rods (front-back motion)
Clamp M10 smooth rods for X-axis (carry the Plough’s rails)
Provide mounting point for Y-axis belt attachment

Note: The X-Ends have nothing to do with the Z-axis. In Amalgam’s Z-drop design, the Z-axis (bed platform) is a completely separate system with its own rods and lead screws.

Design for 60mm Horizontal Spacing

# Tractor Gantry X-End
with BuildPart() as x_end:
    # 1. Y-Axis Bearing Housing (slides on Y-rods, front-back)
    with BuildSketch() as s1:
        Circle(radius=15)  # Outer housing for LM10UU
    extrude(amount=40)
    
    # 2. X-Axis Rod Sockets (perpendicular, carries The Plough)
    # Two M10 holes at 60mm spacing
    with BuildSketch(Plane.YZ.offset(20)) as s2:
        for loc in [(0, 30), (0, -30)]:  # 60mm total
            with Locations(loc):
                Circle(radius=5.1)
    extrude(amount=30, mode=Mode.ADD)
    
    # 3. Belt Anchor
    # Attachment point for Y-axis belt (pulls X-End front-back)

Belt Path Strategy

For Top-Down (60mm Spacing)

The GT2 belt should run:

Center-line: Exactly at 30mm mark (midpoint between rods)
Underneath the carriage: Avoids interference with motor
Parallel to rods: Prevents “cocking” during fast moves

Single Belt (Recommended): For a 300mm span on M10 rods, one center-line belt provides sufficient force.

Belt Gripper: Use screw-tensioned gripper instead of just a slot: - M3 bolt pinches belt into toothed cavity - Much more secure than friction-fit slots

Belt as “Weakest Link”

On a 300mm span with M10 rods, belts are the compliance point:

Tractor Tip: Keep belt as close to smooth rods as possible. The further belt is from rod, the more “leverage” to twist carriage during fast moves.

Smooth Rod Specifications

M10 Smooth Rods (Reference Spec)

For X and Y axes, M10 smooth rods provide:

~2.4× stiffer than M8
Higher natural frequency (reduces resonance)
Cleaner Input Shaping results
Can push to higher accelerations

Scavenging Smooth Rods

Best Source: Large office photocopiers

The Challenge: Photocopier rods are often induction hardened (HRC 60+)

Cutting Methods: 1. Angle Grinder (Best): 1mm thin “Inox” cutting disc 2. Dremel: Reinforced cutoff wheel (slower but works) 3. Hacksaw (Pro Hack): Grind away hardened skin first, then cut soft core

Safety: Always wear eye protection—hardened steel shards are very sharp.

Rod Quality Check

Roll the rod across your MDF baseboard: - If it “wobbles” or light gaps appear underneath → it’s bent - For frame: slight bend is okay (nuts pull it straight) - For Z-axis movement: must be perfectly straight

Hybrid Mounting: Threaded + Smooth

Your corner brackets hold both:

M10 Threaded: For the structural frame
M10 Smooth: For the motion rails

“MK2/MK3 Style” Cradles

The smooth rods sit “on top of” the threaded rods:

# Threaded-to-Smooth Rod Adapter
with BuildPart() as adapter:
    # Lower half: Clamps onto M10 Threaded
    with BuildSketch() as s:
        Circle(radius=15)  # Outer shell
    extrude(amount=30)
    
    # Subtract Threaded Rod hole
    Circle(radius=5.5)  # M10 clearance
    
    # Upper half: Cradles M10 Smooth (20mm above)
    with BuildSketch(Plane.XZ.offset(15)) as s3:
        Circle(radius=5.1)  # Tight fit for smooth rod
    extrude(amount=30, both=True, mode=Mode.SUBTRACT)

Securing Smooth Rods: - Blind socket (30mm deep hole) - Blue-tack at bottom, or - Small M3 grub screw to pinch rod

Direct Drive: Pitan + E3D V6

Configuration

Extruder: Pitan (Titan clone) direct drive
Hotend: E3D V6
Fans: Heatsink fan + part cooling fan

Carriage Integration

The Pitan motor nests between the X-rods:

Motor body centered at 30mm mark
Nozzle exactly centered between rods (balanced weight)
Belt grippers included in carriage design

Weight Considerations

Pitan + V6 = ~400-500g concentrated mass

The “Nod” Problem: As carriage accelerates, front-heavy mass wants to twist gantry.

Solution: 60mm rod spacing creates large “I-beam” effect, drastically reducing tendency to nod or twist.

Key Parameters Summary

Parameter	Value	Notes
X-Rod Spacing	60mm (center-to-center)	Horizontal, top-down
Y-Rod Spacing	Determined by frame	Fixed to top of box
Rod Diameter	M10 smooth	Reference spec
Bearing	LM10UU	19mm OD, 29mm length
Belt	GT2 6mm	Single, center-line
Motor Mounting	31mm pattern	NEMA 17 standard

Assembly Sequence

Build Frame First: Complete the M10 threaded box
Install Y-Rods: Fixed to top corners via cradles
Assemble X-Ends: With bearings for Y-rods
Install X-Rods: Through X-Ends, maintaining 60mm spacing
Mount The Plough: Slide onto X-rods with bearings
Install Belts: Through grippers, tension via motor slots
Wire Endstops: Left Y, Right Y, X-min

Squaring Check

After assembly: 1. Home Y-axis (both motors) 2. Verify gantry is parallel to frame 3. Home X-axis 4. Verify Plough moves smoothly full travel