Casefinity Liner Interior & Accessory Specification

Status: Proposed specification v0.2 · 2026-07-20 Changed in v0.2: Registration geometry is decoupled from wall thickness: rib width is its own interface constant w (RIB_WIDTH), and WALL_THICK is now a free structural parameter (see INV-2, REQ-4.4). In v0.1 both were the single symbol t, so thickening a wall silently resized every rib, socket, and groove. Scope: The geometric interface between Casefinity hard-case liners (perimeter frames), bins, and accessories (lids, dividers, solid stock). This document specifies the module grid, the interlock/registration features, the fit and clearance scheme, and the tolerance stack-up behaviour so that independently designed parts interoperate.

This is a reference specification: every dimension traces to a named Fusion 360 user parameter (see f3d-extracted-parameters.md) as realized in the generator (hardcase-gridfinity-generator/src/models/). Where the spec and the code disagree, the code is authoritative and this document is a bug.

Requirement language: MUST / SHOULD / MAY per RFC 2119.

1. Terms and coordinate system

Parts are modelled Z-up, centred on XY, with z = 0 at the part's bottom (print-bed) plane. +X = length, +Y = width, +Z = up (case mouth).

2. Nominal datum values

These are the interface constants. All parts claiming Casefinity conformance MUST use these values (or a documented superset).

The interface constants are P, w, b, c (bold values). t, f, and H are structural parameters: a conforming part may choose them freely (walls of 1–6 mm are the reasonable range) without affecting interoperability.

INV-1 (unified bump). WALL_BUMP (bins) and GRID_BUMP (liner) are the same value b = 1.50 mm, and every rib (male feature), on a bin or on a liner, is w = 3.00 mm wide and stands b proud. The female features are near-identical: a bin socket and a liner groove are both a b-deep slot in the wall, kept a blind pocket (backed by an interior boss when the wall is thin — see REQ-4.4), differing only in the slot's tangential clearance (see §4). All share b and w. A bin registers against a neighbouring bin and against the liner with compatible geometry. Any change to b or w MUST be applied to both families together.

INV-2 (wall thickness is orthogonal to registration). WALL_THICK is not an interface dimension. Changing t MUST NOT change any registration geometry — rib width w, bump height b, slot widths, module pitch P, feature centring, or the liner's cavity size (INV-7). This is realized by construction:

• every rib, slot, and boss width derives from w (and c), never from t;
• a bin's walls thicken inward (its interior shrinks; its footprint F(n) and exterior features are unchanged);
• the liner's cavity wall thickens outward into the border (the cavity stays exactly N·P; the border hollow shrinks). t appears in the liner only as the rib's structural embed depth and the boss depth — both behind the wall face, never in a mating dimension.

Historical note: the original Fusion designs drove rib width off WALL_THICK (they were numerically equal at 1.20 mm). RIB_WIDTH is a generator-introduced parameter that freezes that width as an interface constant.

Deviation from the original Casefinity models. w = 3.00 mm is a deliberate change from the originals' 1.20 mm, made for printability: at 1.20 mm a rib is a single extrusion wide on a 0.40 mm nozzle, so it prints fragile and is prone to under-extrusion, and the matching slot is a single-pass gap that closes up on any over-extrusion. 3.00 mm gives both features real cross-section. This is a conforming change under INV-1 (it moves both families together — they read one parameter), but parts built to the original 1.20 mm do NOT interlock with parts built to this spec. Set RIB_WIDTH = 1.20 to reproduce the original geometry; that is the value the generator's ground-truth fidelity checks use.

Upper bound. w MUST satisfy 3w + 2c ≤ P − 2·CLEAR — the socket's backing boss (s + 2w = 3w + 2c wide, §4.1) sits on the outermost module centre, which leaves P/2 − CLEAR to the footprint edge. Above that the boss overhangs and the bin stops tiling at pitch: w ≤ 4.87 mm at the standard P = 15, so the generator caps the parameter at 4.50 mm.

3. The module grid

REQ-3.1. The grid is a square lattice of pitch P. Feature and cavity positions are expressed in module coordinates and converted with P.

REQ-3.2 (bin footprint). A bin spanning n modules in an axis has outer wall faces at

F(n) = n·P − 2c        (footprint, wall-face to wall-face)

i.e. the bin is shrunk by c = 0.10 mm on each face inside its nominal n·P envelope. For P = 15: F(1)=14.80, F(3)=44.80, F(n)=15n−0.20. The −2c is per-bin and independent of n (a 5-module bin is 74.80, not 74.60). This is the single most important fact for the stack-up analysis (§8).

REQ-3.3 (bounding envelope). The male ribs on the −X and +Y faces stand b proud, so a bin's true bounding box on those axes is

E(n) = F(n) + b = n·P − 2c + b = 15n + 1.30 mm     (standard grid)

e.g. a 3×3 bin measures 46.30 × 46.30 mm. Slicers and nesting logic MUST use E(n), not F(n).

REQ-3.4 (feature centring). Bump centres along an edge are the module centres (i − (k−1)/2)·P, i = 0…k−1, for a k-module edge — i.e. one bump per module, symmetric about the part centre. The liner uses the same centring over its cavity span (gridCenters), so bin module centres and liner grid centres coincide when a bin sits on-grid.

4. Interlock / registration features

4.1 Bin exterior (the socketed interlock)

A bin carries a complementary feature set so it registers "either way round":

REQ-4.1. Socket slot width MUST equal w + 2c so a mating w-wide rib enters with exactly c = 0.10 mm clearance per flank. Neither dimension involves t (INV-2).

4.2 Liner interior wall (the grid bumps)

The liner presents the same bump geometry on its cavity walls, split so it mates with a bin's exterior in any of the two registrations:

REQ-4.2. The liner groove is w wide — the same width as a rib, with no added tangential clearance (line-to-line). Like the bin socket, the groove MUST be a blind pocket that does not open the cavity into the U-channel hollow: on a thin wall the b-deep slot alone cuts clean through the inner wall and severs it, so it MUST be backed by a boss (REQ-4.4 gives the exact rule). The 0.30 mm mouth chamfer is a first-layer / elephant-foot allowance at the wall face, not registration clearance. Tangentially the groove is a relief, not a precision locator (see REQ-4.3).

4.3 Fit summary — two distinct interfaces

The joint is asymmetric: only the socket pairs are clearanced locators; the groove pairs are line-to-line reliefs.

   LOCATING:  rib w=3.00 ─►│ │◄─ socket s=3.20   →  0.10 mm/flank  (the clean hand-fit)
                           └─┘
   RELIEF:    rib w=3.00 ─►│ │◄─ groove w=3.00    →  0 nominal; seats on process tolerance
                           └─┘   (a blind pocket; line-to-line width, not a locator)

REQ-4.3. A part locates on its socket interfaces (the clearanced 0.10 mm/flank fit — this is what assembles cleanly by hand). The opposing rib-in-groove pair is a blind pocket (closing the cavity like the socket does) but with a line-to-line slot width, so tangentially it is a relief, not a clearanced locator: its practical fit is set by process tolerance and the CLEAR knob (§8.4). Designers MUST NOT treat the groove as a tight datum, and MUST NOT over-constrain a part by relying on both interfaces for precision simultaneously.

4.4 Female features on thick walls

REQ-4.4 (boss rule). Every female feature (bin socket, liner groove) MUST be a blind pocket: the b-deep slot must never open the part's cavity (or the liner's U-channel hollow). How that is achieved depends only on the wall:

• t < 2b (thin wall): the slot severs the wall, or leaves less than b of material behind it, so it MUST be backed by an interior boss (§4.1/§4.2 give each family's boss dimensions — all derived from w and b, except the liner boss depth t + b, which must span whatever wall it backs).
• t ≥ 2b (thick wall): the wall itself swallows the slot with at least b of solid material behind it. The boss MUST be omitted and the slot cut directly into the flat wall.

The male rib and the slot's mouth geometry are identical in both regimes, so a mating part cannot tell them apart — the boss is an internal construction detail, never part of the interface. At the default t = 1.20 mm every part is in the thin-wall regime; a 6 mm liner wall (t = 4b) is in the thick-wall regime.

5. Bin ↔ bin engagement

REQ-5.1. Adjacent bins meet wall-face-to-wall-face: one bin's −X/−Y rib seats fully into the neighbour's +X/+Y socket (b proud into b deep, with the §4.3 locating clearance), so the wall faces become nominally coincident and the rib occupies the socket volume — the joint adds no length to the pack. Each bin therefore contributes exactly F(nᵢ) to a row, and the centre-to-centre pitch of two bins of n_A, n_B modules is

pitch = F(n_A)/2 + F(n_B)/2 = (n_A + n_B)·P/2 − 2c.

REQ-5.2. Because the ribs are handed (−X/+Y male, +X/−Y female), a full tiling has consistent polarity: every interior seam is one rib into one socket. Rotating a bin 180° about Z swaps which faces are male, and it still registers (INV-1) — but its pull tab and any lid features rotate with it.

6. Bin ↔ liner engagement

REQ-6.1. A bin on the cavity perimeter registers to the liner grid bumps with the §4.3 fit. A bin in the interior registers to its neighbours (§5). The liner grid and bin modules share pitch and centring (REQ-3.4), so both hold simultaneously.

REQ-6.2. Perimeter bins are located by the liner at the case datum; interior bins are chained off them. See §8 for how error propagates through the chain.

REQ-6.3 (Z). A bin's floor rests on the liner foot; its rim sits at/near the cavity mouth (H nominal). The pull tab (PULL_TAB_HT = 5 mm) and any lid stand above the rim and MUST NOT be assumed to fit under a closed case lid without checking case_internal_height ≥ H + PULL_TAB_HT.

7. Liner interior sizing — the integer-module invariant

INV-7 (interiors snap to whole modules). A liner's cavity MUST be an integer number of modules on each axis, independent of wall thickness. This is guaranteed by construction by the border formula, not left to the user — note that t appears nowhere in it (the cavity wall thickens outward into the border, INV-2), so a liner rebuilt with any t in the 1–6 mm range presents the exact same N_L × N_W module cavity:

N_L = floor(OVERALL_LENGTH / P) − SIDE_BOARDER_BIN_ADD      (cavity length, modules)
N_W = floor(OVERALL_WIDTH  / P) − FRONT_BOARDER_BIN_ADD     (cavity width,  modules)

cavity_length = N_L · P          cavity_width = N_W · P      (always exact multiples)

The free outer dimension is absorbed entirely by the border, which is never smaller than the whole modules reserved plus the sub-module remainder:

border_total(axis) = OVERALL − N·P = P·BIN_ADD + (OVERALL mod P)
border_per_side    = border_total / 2

Worked defaults (standard grid, OVERALL = 350 × 250, SIDE_ADD = 4, FRONT_ADD = 3):

Gridfinity liner (P = 42, OVERALL = 350 × 250, SIDE_ADD = FRONT_ADD = 1):

REQ-7.1. The 42 mm liner cavity being a whole multiple of 42 means a standard Gridfinity baseplate/bin set (42 mm module) drops into the pocket; the standard Gridfinity ~0.5 mm/module baseplate clearance lives inside that pocket and is not part of this spec.

REQ-7.2 (minimum border). border_per_side MUST remain ≥ t + b + m_struct (wall + bump + a structural margin m_struct ≈ 3 mm) — ≳ 6 mm at the default t = 1.20, rising to ≳ 10.5 mm at t = 6 — or the border cannot carry the U-channel wall and grid bump. Increasing BIN_ADD trades cavity modules for border, and thickening the wall consumes border hollow; neither MUST drive the border below this floor.

SHOULD-7.3 (UX). Because the cavity is always N_L × N_W whole modules, the generator SHOULD display the resulting module count (e.g. "19 × 13 modules") and MAY offer an inverse mode where the user enters N_L, N_W and the outer size is computed as N·P + 2·border.

8. Tolerance and stack-up

This section addresses the practical question: single bins fit beautifully — what happens with many bins side by side across a full liner?

8.1 Two independent error sources

1. Designed clearance 2c = 0.20 mm per bin (0.10 per face), realized as the rib/socket flank gap. It is absolute and per-bin, independent of n (REQ-3.2). A row of K bins summing to N modules leaves total assembly slack

   S = C − Σ F(nᵢ) = N·P − (N·P − 2c·K) = 2c·K = 0.20·K mm.
   

   Slack scales with the number of bins K, not the number of modules N. Many narrow bins → lots of slack (forgiving, but more rattle). One wide bin or pack → little slack (crisp, but unforgiving).

2. Printer dimensional error, which scales with absolute distance: a fractional scale error ε (steps/mm, flow, thermal shrink) displaces a feature at distance d from the part datum by ε·d. Over a full liner span this dominates.

8.2 The stack-up result

Let bins print at scale 1 + ε_bin and the liner at 1 + ε_liner. The pack fits iff

(N·P − 2c·K)(1 + ε_bin) ≤ N·P·(1 + ε_liner)
  ⟹   ε_bin − ε_liner  ≤  2c·K / (N·P)  =  0.0133 · (K/N)      (standard grid)

Key consequences

• Ratiometric cancellation. If bins and the liner are printed on the same machine, material, and profile, ε_bin ≈ ε_liner, the left side is ~0, and the pack always fits — the 0.20·K slack scales along with everything else. This is why your hand-fit today is excellent, and it is the primary recommendation.

• The enemy is differential scale, not absolute size: bins and liner printed on different printers/materials, or anisotropic X-vs-Y error. The tolerable differential is 0.0133·(K/N):

  Pack composition	K/N	Max differential scale |ε_bin−ε_liner|
  All 1-module bins	1	1.33 % (very forgiving)
  Mixed, ~2 modules/bin	0.5	0.67 %
  A few wide bins, ~5 modules/bin	0.2	0.27 %
  One bin fills the row (K=1, N=19)	0.053	0.070 % (tight)

• Per-joint alignment (a second, tighter check). Total fit is necessary but not sufficient: each rib must still land in its socket/groove within the c = 0.10 mm flank gap. A bin's far rib is ≤ n·P/2 from its registration datum, so its misalignment is ≈ |ε_bin − ε_datum|·(n·P/2). Requiring < c:

  wide bins amplify drift:  a 10-module bin (150 mm) at 0.1 % differential
  drifts its end rib by 0.075 mm — inside the 0.10 mm gap, but only just.
  

8.3 Absolute-accuracy budget (single element, worst case K=1)

For a single bin/pack of n modules that must drop into an n-module pocket with only its own 2c = 0.20 mm:

8.4 Requirements & guidance

• SHOULD-8.1. Print a liner and the bins that fill it on the same printer, material, and profile. This makes error ratiometric and removes stack-up as a concern for any realistic ε.
• SHOULD-8.2. For large single elements (a bin or lid spanning ≳ 8 modules), calibrate XY scale (flow / steps-per-mm / measured calibration cube) to keep absolute error < 2c = 0.20 mm over the span (≈ 0.07 % at the full default cavity).
• MAY-8.3. On an uncalibrated or high-shrink process, raise CLEAR (bins) from 0.10 toward 0.15–0.20 mm to widen every flank gap, accepting more play. This is the intended tuning knob; RIB_WIDTH, WALL_BUMP, and P MUST stay fixed (they are interface constants — INV-1). WALL_THICK is free (INV-2) — thicken walls for strength at will; it changes no fit anywhere in this section.
• MAY-8.4. Prefer more, narrower bins over one wide bin when print accuracy is marginal: each extra bin adds 2c = 0.20 mm of total slack and shortens the drift lever n·P/2.
• NOTE-8.5. The liner's own perimeter is split into bed-fitting dovetail pieces (splitPieces); those seams are a separate, generously-cleared joint (BOARDER_DOVETAIL_CLEAR = 0.20 mm) and do not interact with the module grid clearance analysed here.
• NOTE-8.6. The single tightest interface is the perimeter bin-rib↔liner-groove relief (§4.3, line-to-line width). It is the first place an over-extruded / oversized print binds. If perimeter bins feel tight while interior (bin↔bin) joints are fine, tune extrusion width / flow, or raise CLEAR, before touching any interface constant.

9. Accessories

9.1 Lids

Lids seat on a bin's rim with their own clearance LID_CLEAR = 0.10 mm and a sliding-lock scheme (LID_LOCK_*, LID_PULL_*). A lid's footprint tracks the bin footprint F(n); a lid MUST use the same P, w, c so it shares the bin's tolerance behaviour (§8). Lids stand above the rim (REQ-6.3).

9.2 Dividers

Liner dividers are full-height cross-ribs on the long walls at evenly spaced grid centres (BOARDER_DIVIDERS). They subdivide the cavity on module boundaries; a divider MUST fall on a grid line so it does not steal a bin's module, and MUST be w wide (like every module-boundary feature, its width does not follow t — a thick-walled liner with t-wide dividers would eat into the adjacent modules).

9.3 Solid Block (module stock)

The Solid Block is a bin footprint with no cavity (keeps F(n), ribs, sockets, pull tab). It is module-conforming stock: it registers exactly like a bin, so a tool holder machined out of it drops into the grid with the §4.3 fit. Custom accessories SHOULD start from F(n) + the §4.1 feature set to inherit conformance.

9.4 Conformance checklist (new part)

A part is Casefinity-conformant iff:

1. Its module envelope is n·P with footprint F(n) = n·P − 2c (REQ-3.2).
2. It carries the §4.1 handed rib/socket set at module centres, w wide, b proud/deep, sockets w + 2c wide (REQ-4.1), blind per REQ-4.4.
3. It uses the interface constants of §2 unchanged except CLEAR (the only tuning knob); structural parameters (WALL_THICK, FLOOR_THICK, height) are free.
4. If it defines an interior pocket, that pocket snaps to whole modules (INV-7).

Appendix A — Symbol → parameter → source map

Provenance of every default: f3d-extracted-parameters.md. Geometry realization: hardcase-gridfinity-generator/src/models/bin-common.ts (bins) and .../perimeter.ts (liner); the shared registration interface — rib/slot/boss widths, module centring, the REQ-4.4 boss rule — lives in one place, .../registration.ts. Invariants INV-1, INV-2, INV-7, the fit REQ-4.1 and the boss rule REQ-4.4 are exercised by npm run scaling (including a WALL_THICK sweep over 1–6 mm on the liner).