ADR-008: Spider Trident Bed Support

Status

Accepted

Context

The bed support system must: 1. Support a 250mm²+ heated bed 2. Connect to 3 independent Z-motors/leadscrews (Triple-Z) 3. Be printable on small donor printers (A1 Mini, Ender 3) 4. Provide rigidity to prevent bed sag 5. Maintain parallelism to the gantry

In 2026, there are multiple bed support options: - Solid plate: One-piece printed (too large for small printers) - Aluminum frame: Machined plate (expensive) - Modular spider: Interlocking pieces (printable, rigid)

The Amalgam’s bootstrapping philosophy requires using small donor printers to build the larger Amalgam machine.

Decision

We choose the Modular Spider Trident Bed Support - an interlocking three-arm design printed in sections and bolted together.

Why Spider Trident?

  1. Bootstrapping capability: Can be printed on 220mm³ donor printers (A1 Mini, Ender 3)
  2. Rigid triangle: Three arms + hub create stiff structural triangle
  3. Modular assembly: Print 4 parts (hub + 3 arms), bolt together
  4. Interlocking design: Arm-to-arm and arm-to-hub connections for rigidity
  5. Kinematic support: Three-point support matches Triple-Z system
  6. Scalable: Can be scaled for 300mm³ builds with longer arms
  7. Printable: No large parts, all fit within 220mm build volume

Design Concept

       [Arm 2]
          |
          v
    [Arm 1]--[Hub]--[Arm 3]

- Hub: Central triangular piece
- Arms: Three interlocking sections
- Bed Mounting: Bolts to arm outer edges
- Leadscrew Mounts: Three positions at arm endpoints

Consequences

Benefits

  • Small printer compatible: All parts fit 220mm³ build area
  • Rigid structure: Interlocking design prevents flex
  • Replaceable: Broken arm can be reprinted individually
  • Scalable: Longer arms = larger bed volume
  • Lightweight: Mostly hollow, print-efficient
  • Kinematic alignment: Matches Triple-Z motor layout

Trade-offs

  • Assembly required: Must bolt 4 parts together (10-15 min)
  • Alignment critical: Must ensure arm-to-hub is perpendicular during assembly
  • Bolt pattern: Requires precise bolt hole alignment
  • Print time: Multiple parts = longer total print time

Why NOT Solid Plate?

  1. Too large: 250mm+ plate won’t fit on most donor printers
  2. Difficult to print: Large flat plates warp easily
  3. Transport: Hard to ship or store if fully assembled

Why NOT Aluminum Frame?

  1. Cost: Machined aluminum ~$50-80 AUD
  2. Machining required: Can’t 3D print, must buy or CNC
  3. No bootstrapping: Defeats purpose of building from donor printer

BOM Implications (Generic)

Scenario B: Salvaged Bed from Donor

  • Parts A: Donor bed fits 3-point layout (Prusa MK3, Voron)
    • Parts needed: Print Spider + reuse donor bed
    • Cost implication: Very Low (~$5-10 AUD)
    • Donor compatibility: Prusa MK3, Voron, etc.
    • Note: May need to adapt arm mounting points to donor bed
  • Parts B: Donor bed is rectangular (Ender 3, CR-10)
    • Parts needed: Print Spider + new bed aluminum plate
    • Cost implication: Low (~$25-35 AUD for bed)
    • Donor compatibility: All donors
    • Note: Most donor beds too small, need new bed

Scenario C: Upscaled 300mm³ Build

  • Parts needed:
    • 1x Central Hub (printed)
    • 3x Extended Spider Arms (printed - 50mm longer)
    • 24x longer bolts (for larger triangle)
    • 300mm² aluminum bed plate
  • Cost implication: Medium (~$35-45 AUD for bed + hardware)
  • Donor compatibility: All donors
  • Build volume: 300mm³
  • Note: Arms still fit in 220mm³ printer (just longer)

Scenario D: Aluminum Bed Plate (Reference Spec)

  • Parts needed:
    • Printed Spider assembly (as Scenario A)
    • 250mm × 250mm × 3mm aluminum bed plate
    • MK52 magnetic PEI sheet (optional, ~$25 AUD)
    • Bed heater (220V 240W or 24V 300W)
    • Bed thermistor (NTC 100K)
    • 4x M4 or M5 bed clamps (bed-to-spider)
  • Cost implication: Medium (~$45-65 AUD)
  • Donor compatibility: Salvage heater/thermistor if possible
  • Note: MKS SKIPR reference spec recommends MK52 magnetic PEI

Scenario E: Budget Build (Tier 1)

  • Parts needed:
    • Printed Spider assembly
    • Salvaged bed from donor
    • Salvaged bed heater
    • Salvaged thermistor (may need repotting)
  • Cost implication: Very Low (~$0-10 AUD)
  • Donor compatibility: Ender 3, Prusa i3, Anet A8
  • Note: May be smaller bed (220mm vs 250mm)

Implementation Notes

Hub Design

Shape: Equilateral triangle
Size: ~120mm sides
Mounting: 3 bolt holes per arm (9 total)
Center: Cutout for cable routing

Arm Design

Shape: Elongated triangle (trapezoid)
Length: 150mm from center to bed edge
Width: 80mm at bed end, 50mm at hub end
Interlock: Tab-and-slot design (arm-to-arm)
Connection: 3 bolts to hub
Bed mount: 2 bolts per arm (6 total)

Assembly Process

  1. Print all parts: Hub + 3 arms
  2. Dry fit: Ensure parts fit together correctly
  3. Tighten finger-tight: All bolts loosely
  4. Align arms: Use square to ensure 90° to hub
  5. Tighten progressively: One bolt per arm, repeat until all tight
  6. Test flatness: Place on flat surface, check wobble
  7. Mount bed: Bolt bed to outer arm edges
  8. Mount to leadscrews: Attach spider to Z-nut mounts

Bed Mounting Pattern

       [Leadscrew 3]
             |
             v
   [Bed Plate 250mm²]
    /              \
 [Arm 1]---[Hub]---[Arm 2]
      |             |      |
      v             v      v
[Leadscrew 1] [Leadscrew 2]

Bed attached to outer edges of 3 arms

Material Considerations

  • Hub: PETG for heat resistance (near bed heater)
  • Arms: PETG or PLA+ for stiffness
  • Infill: 30-40% (grid pattern) for rigidity
  • Walls: 3-4 perimeters for strength
  • Supports: Minimal (interlocking design needs few supports)

Bed Options (Reference Spec)

Tier 3 (MKS SKIPR Reference):
- Bed: 250mm² aluminum plate (3mm)
- Heater: 24V 300W silicone heater
- Surface: Prusa MK52 magnetic PEI sheet
- Cost: ~$50-60 AUD total

Tier 1-2 (Salvage):
- Bed: Salvage from donor
- Heater: Salvage from donor
- Surface: Glue PEI sheet, use painter's tape
- Cost: $0-10 AUD

MK52 Magnetic PEI (Prusa Reference)

  • What: 250mm² magnetic PEI sheet
  • Why: Easy removal, excellent adhesion, removable
  • Compatibility: Works with BLTouch AND PINDA (metal particles)
  • Cost: ~$25 AUD
  • Note: Requires aluminum bed plate underneath

Bed Heater Options

24V 300W (Recommended for MKS SKIPR):
- Efficient, low current draw
- Good for 250mm²
- ~$20-25 AUD

220V 240W (Salvage path):
- Found in donor printers
- Requires AC relay/mains switching
- Dangerous if not insulated
- Free if salvaged

Silicone vs PCB:
Silicone: Better heat distribution, more expensive
PCB: Cheaper, can delaminate, hotter spots

Thermistor Potting (Critical)

  • Why: Potted thermistor provides permanent thermal contact
  • How: High-Temp Red RTV Silicone in thermistor hole
  • Process:
    1. Drill/cut hole in bed plate (if not present)
    2. Insert thermistor
    3. Fill void with RTV silicone
    4. Cure 24 hours
    5. Verify temperature accuracy
  • Benefit: No tape failures, no thermal drift, “set-and-forget”

Maintenance

  • Check bolt tightness: Every 100 hours
  • Inspect arms: Check for cracks or deformation
  • Level calibration: After moving printer or replacing arms
  • Bed flatness: Check periodically with straightedge

Common Issues

  • Bed not flat: Re-check arm-hub alignment, verify bolts tight
  • Sagging under weight: Increase arm infill, add corner reinforcements
  • Wobbly after assembly: Loosen bolts, realign, retighten progressively
  • Arms don’t fit: Check print dimensions, may need scaling for donor

References