ADR-024: Heated Bed Size Selection

Status

Accepted

Context

Amalgam’s scavenger philosophy requires that the heated bed come from donor printers. The dual-rod motion system (ADR-021) provides flexibility in build volume, but the bed size should align with what’s actually scavengeable.

Previous Specification

The original config specified 250×250×250mm build volume, which is larger than any standard donor bed. This was aspirational rather than practical for scavenger builds.

The Dual-Donor Reality

ADR-021 (Dual-Rod Motion) and ADR-005 (Triple-Z) establish that Amalgam requires two donor printers for a practical scavenger build: - 6-7 NEMA17 motors needed (most donors have 4-5) - 8 smooth rods needed for Dual-8 configuration - Single donor cannot provide enough parts

Given two donors, the scavenger approach is: use the smaller of the two beds.

Donor Printer Bed Survey

Donors WITH 8mm Smooth Rods

These donors provide both beds AND rods for the Dual-8 build:

Donor Printer Bed Size Smooth Rods Notes
Prusa i3 clones 200×200mm 6× 8mm Original RepRap lineage
Anet A8 220×220mm 6× 8mm Massively popular, cheap
Anet A6 220×220mm 6× 8mm Similar to A8
Prusa MK2 250×210mm 6× 8mm Rectangular bed
Prusa MK3/MK3S+ 250×210mm 6× 8mm + 3× LM8LUU Premium donor
Tronxy X3 220×220mm 6× 8mm Budget option
Geeetech i3 200×200mm 6× 8mm Prusa clone

Donors WITHOUT Smooth Rods (V-Slot Rollers)

These donors provide beds, motors, and electronics but no smooth rods:

Donor Printer Bed Size Motion System Notes
Ender 3 / 3 Pro / 3 V2 235×235mm V-slot rollers Very common
Ender 5 235×235mm V-slot rollers Cube frame
CR-10 300×300mm V-slot rollers Large format
Voxelab Aquila 235×235mm V-slot rollers Ender clone
Artillery Sidewinder 300×300mm V-slot rollers Large format

Compact Printers

Donor Printer Bed Size Notes
Prusa Mini 180×180mm Small bed, cantilever design
Ender 2 150×150mm Very small

Key Insight: 235×235mm is an “Ender Size”

The 235×235mm bed is primarily associated with Creality Ender 3 series printers. However, Ender 3 uses V-slot rollers, not smooth rods.

To use a 235×235mm bed in a Dual-8 Amalgam: - One donor must be Ender 3 (provides bed, motors, board) - Other donor must be rod-bearing (provides rods, bearings) - This is a valid but mixed-donor configuration

For a pure rod-donor build (2× Prusa clones or 2× Anet A8), the 235×235mm bed is not available from scavenging.

Decision

We adopt 220×220mm as the reference specification heated bed size.

Why 220×220mm?

1. Most Common Rod-Donor Bed Size

The Anet A8 was one of the most popular budget 3D printers ever sold, with millions of units. Its 220×220mm bed is: - Widely available on the secondhand market - Common in “broken printer” lots - Standard replacement part (cheap if needed)

2. Aligns with Dual-8 Motion Capability

For 220×220mm build volume with Dual-8 rods:

X-rod span = BUILD_X + CARRIAGE_OVERHANG + CLEARANCE
          = 220 + 80 + 30 = 330mm

Deflection analysis (from ADR-021):
- Dual 8mm at 330mm span, 280g toolhead: 0.026mm (GOOD)
- Well within acceptable tolerance

220mm build volume is comfortably within Dual-8 capability without pushing limits.

3. Practical Build Volume

220×220mm (48,400 mm²) provides: - 21% more area than 200×200mm (40,000 mm²) - Handles most practical prints (phone cases, enclosures, brackets) - Large enough for meaningful projects - Not so large that print times become excessive

4. Clean Number

220mm is easier to remember and communicate than 235mm. It’s also a metric-friendly dimension.

5. Not Ender-Dependent

Choosing 220×220mm means the reference build doesn’t require an Ender 3 donor (which provides no rods). Two Anet A8 donors give you everything needed.

Why NOT Other Sizes?

200×200mm (Prusa Clone Size)

Pros: - Smallest common size = maximum scavengeability - Original RepRap heritage - Most conservative choice

Cons: - 17% less build area than 220mm - Anet A8 beds are equally common and larger - Leaves capability on the table

Verdict: Supported as minimum, but not reference.

235×235mm (Ender 3 Size)

Pros: - Very common bed size - Slightly larger build volume

Cons: - Primarily from V-slot printers (no rods) - Forces mixed-donor builds - Awkward number (not metric-round) - Only 7% more area than 220mm

Verdict: Supported if Ender donor available, but not reference.

250×210mm (Prusa MK2/MK3 Size)

Pros: - Premium donor quality - Rectangular allows longer prints in X

Cons: - Prusa MK2/MK3 are less common as scrap - Rectangular complicates some designs - Overkill for typical prints

Verdict: Supported for those with MK2/MK3 donors.

250×250mm (Original Spec)

Cons: - No standard donor provides this size - Would require purchasing a bed - Contradicts scavenger philosophy

Verdict: Removed from reference spec.

Consequences

Benefits

  1. True scavengeability: 220×220mm beds available from Anet A8 donors
  2. Aligned with Dual-8: Well within rod capability at 330mm span
  3. Practical volume: Large enough for real projects
  4. No mixed donors required: Two rod-bearing donors provide everything
  5. Common replacement: If bed damaged, 220×220mm beds are $15-25 AUD

Trade-offs

  1. Smaller than Ender 3: 220mm vs 235mm (15mm difference)
  2. Not maximum possible: Could go larger with purchased bed
  3. May have leftover bed: If one donor has 250×210mm Prusa bed

Parametric Support

The build system supports multiple bed sizes via config.py:

# Reference specification
BUILD_VOLUME = {"X": 220, "Y": 220, "Z": 220}

# Alternatives (parametric CAD handles all)
# BUILD_VOLUME = {"X": 200, "Y": 200, "Z": 200}  # Prusa clone minimum
# BUILD_VOLUME = {"X": 235, "Y": 235, "Z": 235}  # Ender 3 size
# BUILD_VOLUME = {"X": 250, "Y": 210, "Z": 210}  # Prusa MK3 rectangular

BOM Implications

Reference Build: Two Anet A8 Donors

Component Source Size
Heated bed Donor (smaller of two) 220×220mm
Bed heater Donor 12V or 24V (rewire if needed)
Build surface Donor or purchase PEI/glass/BuildTak
Thermistor Donor 100K NTC

Cost if bed damaged: ~$15-25 AUD for replacement 220×220mm MK3 aluminum bed

Mixed Donor Build (Prusa Clone + Ender 3)

Component Source Size
Heated bed Ender 3 donor 235×235mm
Smooth rods Prusa clone donor 8mm
Build volume Limited by config Up to 235×235mm

This configuration is supported but requires updating config.py to match the larger bed.

Frame Sizing Update

With 220×220mm build volume, frame dimensions (from ADR-001 formula):

X_OVERHANG = 35.0  # Pitan extruder
SKELETON_X = BUILD_X + (X_OVERHANG * 2) + 40 = 220 + 70 + 40 = 330mm
SKELETON_Y = BUILD_Y + 120 = 220 + 120 = 340mm
SKELETON_Z = BUILD_Z + 50 = 220 + 50 = 270mm

This is a more compact frame than the previous 250mm specification, reducing: - M10 threaded rod usage - Total frame weight - Material cost

Rod Span Analysis

X-Axis (Critical for Dual-8)

X-rod length ≈ BUILD_X + 2×CARRIAGE_OVERHANG + CLEARANCE
            = 220 + 80 + 30 = 330mm

Deflection at 330mm span, dual 8mm, 280g toolhead:
δ = 0.026mm (GOOD - well under 0.035mm threshold)

Y-Axis

Y-rod length ≈ BUILD_Y + GANTRY_DEPTH + CLEARANCE
            = 220 + 80 + 30 = 330mm

Load: X-gantry assembly (~1.5kg distributed across 4 rods per side)
Verdict: Well within capacity

Supported Bed Sizes Summary

Size Status Donor Source Notes
200×200mm Supported Prusa i3 clones Minimum size
220×220mm Reference Anet A8 Recommended
235×235mm Supported Ender 3 (no rods) Mixed donor build
250×210mm Supported Prusa MK2/MK3 Rectangular
180×180mm Supported Prusa Mini Compact build

Implementation Notes

Config.py Update

# Reference specification (updated from 250×250×250)
BUILD_VOLUME = {"X": 220, "Y": 220, "Z": 220}

Heated Bed Compatibility

Most 220×220mm beds use: - Mounting: 3-point or 4-point M3 screws, ~209mm spacing - Power: 12V (~120W) or 24V (~200W) - Thermistor: 100K NTC (standard) - Connector: 2-pin or screw terminal

The Spider Bed support (ADR-008) accommodates standard mounting patterns.

Z-Height Consideration

With 220mm XY build volume, Z-height options: - 220mm (cube): Balanced, reference spec - 250mm: Taller prints, uses full Z travel - 200mm: Shorter frame, faster builds

Reference spec uses 220×220×220mm (cube) for simplicity.

References