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CardBoxGen Fabrication Model

CardBoxGen is a centerline SVG generator. It draws the paths the laser should follow, then uses an explicit kerf and fit model to predict the final material edge after cutting.

References Used

The model follows common laser-cutting practice, but the implementation is original to this MIT project.

  • Boxes.py treats material thickness as a critical measured value and warns that even small thickness changes affect finger-joint stiffness. It also distinguishes its burn value as a radius-style correction and recommends fit tests.
  • LightBurn documents kerf as the material removed by the beam, explains that outside parts shrink and holes grow when a beam follows the centerline, and recommends kerf test cuts.
  • MakerCase-style generators are useful UX references: users enter dimensions and material thickness, then export SVG/DXF with optional kerf adjustment.

Sources:

Parameter Convention

CardBoxGen uses:

kerf_full_width_mm = full width removed by the laser cut
burn_radius_mm     = kerf_full_width_mm / 2

This is different from software that asks for a half-kerf or burn-radius value. If another tool uses a radius-style kerf parameter, do not copy that number directly into CardBoxGen without converting it.

Sign Convention

For a centerline cut:

  • external material tabs shrink after cutting;
  • internal gaps, holes, and slots grow after cutting;
  • open-edge finger recesses are modeled separately from closed rectangular holes;
  • changing kerf changes drawn cut coordinates, not the requested nominal internal cavity.

Width Formulas

For features bounded by two side cuts:

final_tab_width_from_drawn(drawn, kerf)  = drawn - kerf
final_slot_width_from_drawn(drawn, kerf) = drawn + kerf

drawn_tab_width_for_target(target, kerf)  = target + kerf
drawn_slot_width_for_target(target, kerf) = target - kerf

CardBoxGen targets:

target final tab width  = nominal segment width - clearance / 2
target final slot width = nominal segment width + clearance / 2

The drawn widths are normalized across the edge so the shared FingerPlan length remains exact. Very unbalanced kerf/clearance combinations can exceed the valid fit domain for odd shared finger plans; validation returns FIT_COMPENSATION_UNBALANCED instead of silently exporting those files.

Depth Formulas

Open-edge finger-joint depth is a one-direction fit:

target final tab depth  = thickness
target final slot depth = thickness + clearance
drawn tab depth         = thickness + kerf
drawn slot depth        = thickness + clearance - kerf

This assumes the recess is open to a cut edge. Closed holes use the width formulas because two opposing cut sides define the final dimension.

Nominal Mechanical Model

For box-like presets, inner dimensions are the source of truth. The nominal model stores:

  • material thickness;
  • internal width/depth/height;
  • panel specs;
  • shared edge-pair joint plans;
  • cutout specs;
  • layout constraints;
  • validation limits and warnings.

The generated metadata records the internal and external dimensions, the shared edge-pair plan ids, tab masks, drawn widths, final-width estimates, and fit-model limits.

Corner and Edge Composition

Fingered edges do not start at panel corners. Each panel edge reserves a clean no-finger zone from both ends:

corner_clearance = max(thickness, target_finger_width / 2, 2 * kerf) + max(drawn_tab_depth, drawn_slot_depth)
usable_joint_span = edge_length - 2 * corner_clearance

Finger plans are distributed only across usable_joint_span. If that span cannot fit the requested odd minimum finger count at or above the minimum feature width, validation returns JOINT_USABLE_SPAN_TOO_SHORT and export is blocked. This prevents corner artifacts caused by two adjacent finger profiles overlapping at the same panel corner.

For omitted tray front height, CardBoxGen may raise the default lowered front to the minimum safe joint height and reports FRONT_HEIGHT_NORMALIZED. If the user explicitly provides an unsafe front height, generation is blocked instead of silently changing it.

Laser Cut Path Model

The SVG writer emits:

  • mm units and a viewBox;
  • red CUT paths for panel outlines and cutouts;
  • blue SCORE paths where used;
  • gray ENGRAVE labels;
  • JSON metadata embedded in <metadata>.

The default export mode is centerline compensated by CardBoxGen. A future nominal/no-kerf export mode can be added for workflows where laser software applies kerf compensation itself.

Calibration Guidance

Measure actual material thickness with calipers before generating production parts. Nominal 3 mm stock may differ enough to change joint stiffness.

Cut calibration_mating_strips.svg on the target machine and material before cutting a full box. Use the measured result to tune:

  • material thickness;
  • full-width kerf;
  • assembly clearance.

Use only one kerf-compensation system at a time. If CardBoxGen compensates kerf, leave cutter-software kerf offset disabled for the same file.

Known Physical Limits

CardBoxGen rejects impossible or unsafe geometry when it can detect it:

  • nonpositive thickness;
  • negative kerf or clearance;
  • kerf greater than or equal to thickness;
  • too-small internal dimensions;
  • even or tiny finger counts;
  • finger pitch below minimum feature width;
  • joint usable spans too short after reserved corner zones;
  • unsafe window/slot margins;
  • divider bays that are too narrow;
  • screw holes too close to edges;
  • rotary pockets too close to axle or rim;
  • candy-machine wheel layer counts that cannot assemble.

The candy-machine preset is an educational prototype for dry flowing solids. It is not food-safe machinery and must be physically prototyped for bridging, jamming, and friction.