ADR-001: M10 Threaded Rod Skeleton

Status

Accepted (Updated: M12 → M10)

Context

The Amalgam requires a rigid frame that maintains dimensional accuracy over years of operation. In 2026, there are multiple frame options available: - Aluminum extrusions (2020, 2040) - V-slot profiles - M8/M10/M10 threaded rods (RepRap heritage) - 2020 extrusion with printed corners

Modern “appliance” printers often use aluminum extrusions for lightweight frames and high-speed operation. However, this approach creates machines that can be expensive, proprietary, or require complex assembly.

Original Decision (M12)

The original ADR specified M10 threaded rods, prioritizing maximum rigidity. After deeper analysis documented in docs/decisions/tractor_01_frame_and_assembly.md, we reconsidered this choice.

Decision

We choose M10 threaded rods as the reference specification for the Amalgam frame skeleton.

Why M10 Over M12?

Feature M8 M10 M12
Cross-section ~50 mm² ~78 mm² ~113 mm²
Rigidity vs M8 1× 3× 5× (overkill)
Weight/Meter ~0.3 kg ~0.5 kg ~0.7 kg
Nut Size (Hex) 13mm 17mm 19mm
Printability Easy Easy Can make brackets bulky
  1. Standard 17mm Wrench: Universally available (vs 19mm for M12 which is less common)
  2. Easier to Cut: Hacksaws handle M10 more easily than M12
  3. Smaller Printed Brackets: M10 fits in more compact corner designs
  4. Scavenger-Friendly: Common in automotive applications—excellent for salvaging
  5. Adequate Rigidity: 3× the rigidity of M8 is plenty for a 235×235 build volume
  6. M12 is Overkill: 5× M8 rigidity provides no practical benefit at our target speeds

Why Not M8?

M8 remains viable for ultra-budget scavenger builds, but: - Requires very careful corner design to maintain rigidity - Less margin for error in assembly - May flex at larger build volumes (>250mm)

The design remains parametric—users can build in M8 or M12 if desired.

Consequences

Benefits

  • Long-term stability: Frame doesn’t flex or deform over time
  • No bed tramming: Once square, the frame stays square
  • Software-compatible: Heavy frame works with Klipper Input Shaping to cancel remaining resonances
  • Repairable: Any broken rod can be replaced at a hardware store in minutes
  • Salvage-friendly: No proprietary extrusions or special hardware required

Trade-offs

  • Heavy: The frame is significantly heavier than aluminum alternatives (~15-20kg total)
  • Slower max speeds: Mass limits max velocity to 70-120mm/s (not 600mm/s like Voron)
  • Longer setup time: Requires careful alignment and torque of jam nuts
  • Less “pretty”: Threaded rods have an industrial, exposed look

BOM Implications (Generic)

Scenario B: Salvaging from Photocopier/Large Equipment

  • Parts needed:
    • Scavenged M10 rods (verify straightness)
    • May need to clean threads
    • 3D-printed corner brackets
  • Cost implication: Very Low ($0-10 AUD for nuts/washers)
  • Donor compatibility: Only if large equipment has M10 rods
  • Build volume: Limited to scavenged lengths
  • Note: May have higher lumpy_factor due to worn threads

Scenario C: Converting from Donor with Aluminum Frame (Ender 3, etc.)

  • Parts needed:
    • Must buy new M10 rods (aluminum not compatible)
    • Corner brackets replace existing frame parts
    • Some donor hardware may be incompatible
  • Cost implication: Medium (~$45-55 AUD)
  • Donor compatibility: All donors, but requires replacing frame
  • Build volume: Unlimited (cut to length)

Implementation Notes

Thread Types

  • Bright Zinc Plated: Smoother threads, lumpy_factor = 0.2
  • Hot-Dip Galvanized: Industrial rust-proof, lumpy_factor = 0.5+
  • Black Oxide: Intermediate, lumpy_factor = 0.3-0.4

Frame Geometry (from config.py)

SKELETON_X = BUILD_VOLUME["X"] + (X_OVERHANG * 2) + 40
SKELETON_Y = BUILD_VOLUME["Y"] + 120
SKELETON_Z = BUILD_VOLUME["Z"] + 80

For 250mm³ build volume: - 8x horizontal rods: ~550mm length - 4x vertical rods: ~330mm length - Total: 12 rods

Assembly Considerations

  • Use two jam nuts per joint to prevent vibration loosening
  • Verify squareness at each corner during assembly
  • Torque nuts to 15-20 ft-lbs (moderate, not extreme)

References