Cherry Hardwood Electronics Enclosure — Design Summary¶
1. Design Overview¶
Enclosure concept¶
Two-piece enclosure, approximately 6" × 4" × 1.25" total.
- Wall plate (dock): solid cherry, 0.250" thick, wall-mounted. Matching 45° dovetail rail on both 4" vertical edges and top 6" horizontal edge. Same species and grain orientation as housing.
- Housing (PCB piece): thicker visible cherry piece (~1.25" finished). Pocketed internally for PCB. 45° dovetail undercut at top inner corner of pocket, running along top 6" edge and down both 4" vertical edges.
Sliding interface¶
Housing drops straight down onto wall plate. Wall plate rails slide upward into housing dovetail undercuts. Housing self-seats by bottoming out against top dovetail. No fasteners between halves. Retention via joint geometry, gravity, and wiring harness preload.
Captured edges¶
- Top 6" edge: primary seating and peel-resistance feature. Only taper engaged at nominal position — all load transfer and seating occurs here.
- Two 4" vertical edges: lateral guidance during installation; secondary peel resistance at corners. Not engaged at nominal — 0.028" normal clearance per side. These engage before the top edge can develop a full wedge, controlling the failure sequence.
- Bottom edge: not captured. Bottom tips of side rails are the critical section under peel loading.
Removal¶
Straight up.
Wall standoff — felt tab system¶
Two felt tabs, 0.020" thick, adhered to the bottom corners of the housing back face. These tabs contact the wall surface and establish the nominal standoff position. At this position:
- Back face of housing: 0.020" gap to wall (felt tabs bridging)
- Top edge of housing: nominally 0.020" off the wall (air gap)
- Dovetail joint: at nominal clearances described in Section 3
This approach accommodates wavy boat bulkhead surfaces without requiring precise wall-to-housing fit. The felt tabs act as compliant datums that establish consistent nominal position regardless of wall irregularity.
Under peel loading, the felt tabs compress against the wall and react the rotational moment — the wall itself carries the moment rather than the dovetail geometry. This is the intended load path for abuse loads (see Section 3, Seating and Load Path).
2. Materials¶
Species selection¶
Black cherry (Prunus serotina) and black walnut (Juglans nigra) are the standard North American furniture hardwoods — the most common of each species.
Tension perpendicular to grain — the critical failure mode:
Textbook clear-specimen values (USDA Forest Products Laboratory, Wood Handbook FPL-GTR-282, Table 5-3a, at 12% MC): cherry ~560 psi, walnut ~540 psi. Cherry is ~4% stronger on paper — negligible in practice. These numbers should be used with caution: tension perpendicular to grain is extremely variable, dominated by small defects, and reduced dramatically by machining marks and grain runout. Effective design values are closer to 200–400 psi for real parts with typical surface quality. Use 200–400 psi for any structural calculation in this application.
Cherry is preferred: slightly stronger, cheaper, and more widely available in thin stock. Both are defensible choices.
Grain quality matters more than species choice. For the dovetail geometry, specify and select boards with:
- Straight grain, no runout near any dovetail region (≤5° maximum runout angle)
- No cathedral grain crossing the dovetail zone
- No knots, checks, or defects within 1" of any dovetail feature
Grain runout will reduce perpendicular-to-grain strength more than any geometric optimization. A perfect dovetail in a runout board is weaker than a slightly suboptimal dovetail in a clean straight-grain board. This should be a procurement and visual inspection requirement with a go/no-go standard, not just a design assumption.
Grain orientation and yield¶
- 6" axis = along grain = horizontal — ~1–3 mil seasonal movement; negligible
- 4" axis = across grain = vertical = slide direction — 60–120 mil seasonal movement
Both housing and wall plate oriented the same way — relative movement at dovetail interface near zero.
Orientation benefits: board width efficiency (only 4" across grain), movement in slide direction (already free axis), minimum movement in constrained horizontal direction.
Lumber thickness — NHLA S2S minimums¶
| Nominal | Rough min | S2S minimum | Typical actual S2S |
|---|---|---|---|
| 4/4 | 1.0" | 13/16" (0.8125") | 0.8125"–0.875" |
| 5/4 | 1.25" | 1-1/16" (1.0625") | 1.0625"–1.1" |
| 6/4 | 1.5" | 1-5/16" (1.3125") | 1.3125"–1.375" |
| 8/4 | 2.0" | 1-13/16" (1.8125") | 1.8125"–1.875" |
Housing requires 6/4. Needs ~1.25" finished thickness. 5/4 does not get there reliably. 6/4 lands at 1.25" comfortably after a cleanup pass.
Wall plate stock — options and economics¶
Wall plate must be same species, same grain orientation as housing. Nominal thickness 0.250".
Option A — Buy 1/4" cherry direct (best): Stocked by many hardwood dealers for cabinet backs, raised panels. Check supplier first — no resawing needed.
Option B — Resaw from 1/2" cherry: Widely stocked. Resawn to 0.25" with minimal kerf loss.
Option C — Resaw from 4/4 cherry: Works but more wasteful. ~0.5" usable offcut remains.
Priority: A → B → C.
Moisture content and seasonal wood movement¶
Specification and measurement¶
Specify kiln-dried, 6–8% MC to supplier. Measure with pin-type moisture meter (Orion 910 or equivalent) before delivery. Machine within one week of receiving stock.
Machining dry is correct — parts at 7% MC are at their smallest dimension. In boat use they only expand, making the dovetail fit progressively looser, never tighter. Machining wet and drying afterward causes groove shrinkage onto rail — binding or cracking risk.
In-service MC range by environment¶
| Environment | MC range |
|---|---|
| Fresh from kiln — machine here | 6–8% |
| Heated boat cabin, winter | 8–10% |
| Temperate coastal summer | 13–15% |
| Tropical sustained (90%+ RH) | 18–20% |
Dimensional change coefficients¶
Wood Handbook dimensional change coefficients:
- Black cherry tangential: C_T = 0.00248 / %MC
- Black cherry radial: C_R = 0.00148 / %MC
- Black walnut tangential: C_T = 0.00274 / %MC
Movement over 2.6" span (PCB screw separation), tangential direction¶
| Material | Dry extreme | Humid extreme | ΔL |
|---|---|---|---|
| Cherry | 6% MC | 12% MC | 0.039" |
| Cherry | 6% MC | 16% MC | 0.064" |
| Walnut | 6% MC | 12% MC | 0.043" |
| Walnut | 6% MC | 16% MC | 0.071" |
| FR-4 PCB | −10°C | +50°C | 0.003–0.005" |
4" across-grain movement (slide direction)¶
| Species | ΔMC = 6% | ΔMC = 10% | ΔMC = 12% |
|---|---|---|---|
| Cherry | 60 mil | 99 mil | 119 mil |
| Walnut | 66 mil | 110 mil | 132 mil |
Along-grain movement over 6"¶
| Material | ΔL over 6" |
|---|---|
| Cherry (along grain) | 1.2–2.4 mil |
| Steel (60°C swing) | 4.3 mil |
| FR-4 (60°C swing) | 5.8 mil |
| Aluminum (60°C swing) | 8.3 mil |
Swelling gradients and cupping¶
When a boat's exterior humidity rises faster than interior, the outside face of the housing absorbs moisture faster than the inside face. This creates a temporary swelling gradient across the thickness, resulting in slight cupping of the back face — potentially 5–15 mil across the 4" width.
Magnitude estimate: Cherry radial coefficient 0.00148/%MC. Across 1.25" thickness with a 3–5% MC differential between faces during a rapid humidity swing:
ΔL_differential = 1.25 × 0.00148 × 4 = 0.0074" (7.4 mil) worst case temporary cup.
Mitigations:
- Finish both faces of the housing — slows moisture ingress and reduces the gradient. A coat of satin urethane on the back face (even if not visible) meaningfully reduces the differential swelling rate.
- The 45° joint geometry is tolerant of small rail angle changes — it will still seat and engage with a few mil of cup present.
- Species matching on the wall plate means both parts cup similarly, partially canceling the effect at the joint interface.
This is a transient effect during rapid humidity swings, not a permanent condition. Design for it with back-face finishing rather than increased clearance.
3. Dovetail Joint Design¶
Joint geometry¶
Housing (female undercut):
Starting at the pocket wall surface, a 45° taper cuts inward and upward for 0.150" — this is the full bearing face. At the root of the undercut, a 0.150" horizontal flat (the blunted tip). This root flat sits 0.010" away from the wall plate at nominal position — it is the anti-wedge stop that arrests wedge travel before full bilateral engagement can develop.
Wall plate (male rail):
A matching 45° taper face, 0.160" tall (10 mil taller than the housing taper — this dimensional difference creates the 0.010" gap at the housing root flat). Beyond the taper, at the tip of the rail, a 0.090" horizontal flat. This flat is past the end of the taper and does not shorten or affect the taper bearing face.
Why blunted tips:
- Shifts root failure mode from tension-perpendicular-to-grain to shear-along-grain (3–4× stronger allowable stress)
- Reduces stress concentration vs. sharp corner
- Simple modification to a standard 45° bit
Ligament t = 0.250" — material from root flat to outside face of housing. Do not reduce.
Key parameters¶
| Parameter | Value | Notes |
|---|---|---|
| Dovetail angle | 45° | From horizontal |
| Housing taper face height | 0.150" | Full bearing surface |
| Housing root flat | 0.150" | Anti-wedge stop; 0.010" gap at nominal |
| Wall plate taper face height | 0.160" | 10 mil taller than housing — creates 0.010" root flat gap |
| Wall plate tip flat | 0.090" | Beyond taper; does not affect bearing face |
| Ligament t | 0.250" | Do not reduce |
| Wall plate thickness | 0.250" | 1/4" cherry |
Nominal clearances — all values at nominal assembled position¶
At nominal position (felt tabs contacting wall), the clearances at each interface are:
Top dovetail (primary seating — 6" horizontal edge):
| Interface | Gap | Notes |
|---|---|---|
| 45° taper faces | Engaged / bearing | Primary seating surface — only taper in contact at nominal |
| Root flat (ceiling of undercut) | 0.010" | Anti-wedge stop — housing bottoms out here before full bilateral engagement |
| Throat (inner face of wall dock to housing) | 0.070" | Must never contact under any foreseeable load |
Side dovetails (4" vertical edges — per side):
| Interface | Gap | Notes |
|---|---|---|
| 45° taper faces | 0.028" normal | Not bearing at nominal; guidance and peel catch only |
| Side flat | 0.050" | Well clear |
Seating and load path¶
Gravity and wiring harness preload seat the housing — both forces pull it toward the wall and downward, engaging the top 45° taper faces. This is the nominal resting state.
The 0.010" root flat gap is the critical anti-wedge feature. A fully-wedged bilateral taper under a peel or rotation load would generate enormous normal forces on both taper faces, resolving into a twisting couple on the ligament — the failure mode this design avoids. By ensuring the housing bottoms out on the root flat after only 0.010" of additional wedge travel, this mechanism is arrested before significant force multiplication occurs.
Under peel loading (e.g., a cable yanked at the bottom of the housing), the housing tends to rotate away from the wall about the top edge. The intended reaction sequence is:
- Side dovetail corners engage — 0.028" normal gap closes during rotation, catching the housing at the corners before the top edge can develop a full moment
- Felt tabs load against wall — react the rotational moment as distributed compression; the wall itself carries the moment rather than the dovetail ligament
- Root flat bottoms out at 0.010" — wedge travel arrested, preventing force multiplication
- Throat face at 0.070" — never contacts under any foreseeable load
This sequence is preferred because it reacts the moment through the largest available area (felt tabs against wall) rather than concentrating it in the dovetail ligament.
Side dovetails are guidance and anti-peel features only — they are not engaged under normal gravity loading. Their 0.028" normal clearance accommodates CNC tolerances and wavy wall surfaces without accidental engagement during installation. The clearance is small enough that corners engage meaningfully before a peel rotation can fully develop at the top edge.
Clearance philosophy¶
This device works by depending on wood sliding on itself when forces are applied. Clearances are designed to accommodate wavy wall surfaces, CNC router tolerances, and seasonal dimensional changes within reason. The numbers above are nominal and may require adjustment based on field testing.
Same-species same-orientation wall plate eliminates differential seasonal movement at the joint interface. Seasonal movement in the slide direction (across grain, 4" axis) is the free axis by design.
Wear¶
Wood-on-wood contact at the 45° face under boat micromotion (low amplitude, potentially high cycle) is a long-term wear concern. Contact pressure is very low (~2–6 psi under housing weight alone) so wear rate should be slow.
Carnauba paste wax (Johnson's Paste Wax) applied at assembly — hard natural wax, traditional for wood sliding surfaces, effective 1–5 years before wearing off. Cheap and reapplicable.
4. Structural Analysis¶
Load directions¶
- Pull off wall: resisted over 8" engagement. Strongest direction.
- Shear left/right: ~4" contact. Strong.
- Shear downward: direct bearing at top dovetail. Very strong.
- Shear upward: free — removal direction.
- Peel: critical. Wiring at bottom creates moment. Bottom tips of vertical rails react the load.
Peel force calculation¶
The peel scenario: a force at the bottom of the housing pulling it away from the wall. The housing rotates about the top 6" dovetail. The bottom tips of the two vertical dovetail rails resist the peel force in bending.
Section properties (critical section at bottom rail tip, per rail):
- h = 0.250" (ligament)
- w = 3.5" (engaged axial length along one 4" vertical rail)
- I = 3.5 × (0.250)³ / 12 = 0.004557 in⁴
- c = 0.125"
Moment balance: The peel force F acts at the bottom of the housing, ~4" below the top dovetail fulcrum. The dovetail contact reactions R at the bottom rail tips also act ~4" from the fulcrum. Therefore R_total ≈ F, shared between two rails, so each rail sees R = F/2.
Contact arm: The 45° bearing face on the side rails is 0.150" tall (governed by the housing, the shorter mating surface). The moment arm for bending at the ligament root depends on where contact occurs along this face.
- Midpoint of bearing face: arm = 0.075"
- Tip of bearing face: arm = 0.150"
Under peel rotation, the side tapers engage only as the housing rotates (0.028" normal gap closing). Contact drives toward the outer tip of the taper face — the tip scenario is the more realistic one for peel loading.
Effective design strength: 200–400 psi (real parts with machining marks and potential grain defects — not textbook clear-specimen values).
Per-rail failure load — bending, midpoint contact:
R_fail = σ × I / (c × arm) = (200–400) × 0.004557 / (0.125 × 0.075) = 97–194 lbs per rail
Per-rail failure load — bending, tip contact (realistic for peel):
R_fail = (200–400) × 0.004557 / (0.125 × 0.150) = 49–97 lbs per rail
Per-rail failure load — shear at root flat (not governing):
A = 0.150 × 3.5 = 0.525 in²; τ = 1,200–1,500 psi → 630–788 lbs per rail
Assembly peel capacity (two rails):
| Contact location | Strength assumption | Per-rail failure load | Assembly failure load (2 rails) |
|---|---|---|---|
| Midpoint of bearing face | 200–400 psi | 97–194 lbs | 194–388 lbs |
| Tip of bearing face (realistic for peel) | 200–400 psi | 49–97 lbs | 98–194 lbs |
| Shear at root flat (not governing) | 1,200–1,500 psi | 630–788 lbs | 1,260–1,576 lbs |
Note on grain quality: These calculations assume consistent straight-grain material. A board with runout crossing the dovetail zone could fail at half these loads. Grain selection is as important as the geometry.
Failure philosophy¶
The joint is designed to be as strong as possible. The intended weak links, in order of preference, are:
- Ethernet ports or power cable — if a cable is snagged, these connectors are likely to release or fail before the wood joint, which is acceptable
- Wood joint — if the NMEA2000 connector (threaded, high retention) is pulled hard, the wood may fail before the connector releases; this is preferable to PCB damage
PCB damage (torn pads, cracked traces, broken connector footprints) is the worst outcome and the one the design prioritizes avoiding.
5. PCB Mounting¶
Design envelope¶
Assembly MC: 6%. Parts machined at kiln-dried condition.
Maximum service MC: 20%. Tropical sustained environment (90%+ RH). This is the worst case.
Design ΔMC: 14%. All movement is expansion from the assembly condition — one-directional.
Mounting geometry¶
Three screw locations into cherry posts, ~0.5" thread engagement.
Datum-to-float distances (across grain, the critical dimension):
| Hole | Center-to-center from datum (across grain) |
|---|---|
| Float A (closer) | 0.843" |
| Float B (farther) | 1.0547" |
Differential movement calculation¶
The PCB (FR-4) and cherry wood expand at vastly different rates across grain. The PCB is anchored at the datum hole; the float holes must accommodate the difference.
Worst-case float hole (B), cherry, ΔMC = 14%, L = 1.0547" across grain:
ΔL_wood = 1.0547 × 0.00248 × 14 = 0.0366" ΔL_FR4 ≈ negligible Net differential ≈ 0.037"
Float hole A, cherry, ΔMC = 14%, L = 0.843" across grain:
ΔL_wood = 0.843 × 0.00248 × 14 = 0.0293" ΔL_FR4 ≈ negligible Net differential ≈ 0.029"
All movement is in one direction — the wood posts move away from the datum as MC rises. The screws travel with the posts. The PCB holes stay put.
Float hole sizing¶
Float hole: 5.5mm NPTH (0.2165")
| Dimension | Value |
|---|---|
| Float hole diameter | 5.5mm (0.2165") |
| Screw shank (#6 major) | 0.138" |
| Total diametral clearance | 0.0785" |
| Radial clearance per side (if centered) | 0.0393" |
Float hole offset — biasing for one-directional movement¶
Because assembly always occurs at 6% MC (driest condition) and all movement is one-directional expansion, centering the screw in the hole wastes half the clearance on contraction that will never happen. The float hole centers are offset 0.0196" toward the datum relative to the actual cherry post positions. This places the screw 1/4 of the total clearance on the contraction (safe) side and 3/4 on the expansion side at assembly.
| Per side | |
|---|---|
| Contraction side (toward datum) — insurance only | 0.0785 × 0.25 = 0.0196" |
| Expansion side (away from datum) — working clearance | 0.0785 × 0.75 = 0.0589" |
Offset PCB drill coordinates (center-to-center from datum, across grain):
| Hole | Nominal Y (post position) | PCB drill Y (offset toward datum) |
|---|---|---|
| Float A (closer) | 0.8430" | 0.8234" |
| Float B (farther) | 1.0547" | 1.0351" |
Verification — worst-case float hole B at 20% MC:
Movement = 0.037". Available expansion-side clearance = 0.059". Margin = 0.022". Adequate.
Verification — float hole A at 20% MC:
Movement = 0.029". Available expansion-side clearance = 0.059". Margin = 0.030". Comfortable.
Assembly note: Float hole offset direction must be called out on the PCB drawing relative to a physical reference — e.g., "float holes offset toward datum edge" — so the assembler does not need to reason about grain orientation. At assembly (6% MC), the screw should visibly sit slightly off-center toward the expansion side of the hole, confirming correct orientation.
Fastener hardware and hole sizes¶
Screw: McMaster — #6 × 1/2", 316 SS, rounded head for plywood and OSB. Major diameter 0.138" (3.505mm). 93360A220
Pilot hole: 3/32" (0.094") for #6 into cherry (hardwood).
Washer (float screws only): McMaster — UHMW polyethylene, ID 0.150", OD 0.375". 95649A221
Washer check at 5.5mm hole: The washer (OD 0.375") bearing on the PCB around the 5.5mm hole gives annular contact width = (0.375 − 0.2165) / 2 = 0.079". Adequate bearing area.
PCB holes specified as NPTH (non-plated through-hole) in Gerbers, metric sizes for JLCPCB:
| Hole | JLCPCB drill | Imperial equiv. | Function | Notes |
|---|---|---|---|---|
| Datum | 3.8mm NPTH | 0.1496" | Locating; no washer | Radial clearance 0.0058" — snug fit, anchors PCB |
| Float (×2) | 5.5mm NPTH | 0.2165" | Seasonal slip; washer under head | Centers offset 0.0196" toward datum; 0.059" expansion-side clearance |
Assembly: Datum screw first. Float screws snug — PCB should rotate slightly about datum by hand confirming no over-clamping. Float screws must not be tightened to the point where friction prevents seasonal slip.
Mechanical viability: #6 into 0.5" cherry face grain, pullout ~150–200 lbs. Over-tightening preventing seasonal slip is the only real risk.
Dock-to-wall fasteners¶
Screw: McMaster — #6 × 5/8", 316 SS, extra-wide rounded head (truss) sheet metal screw. Four per assembly. 93406A152
Pilot hole: 3/32" (0.094") into hardwood bulkhead. For plywood or fiberglass bulkhead, predrill ~0.090".
CNC clearance holes in the cherry dock: 6mm (0.236") — screws pass through the dock freely and bite into the bulkhead only. The head bears on the cherry, the threads grip the bulkhead (plywood or fiberglass). This eliminates any splitting risk in the 0.250" dock.
Installation: No need to overtighten these. Ideally wood can float a little because it's going to make clearance for itself one way or the other, possibly by splitting, if seasonal movement is constrained.
6. Supply Chain and Cost Estimates (Qty 50)¶
Wholesale pricing threshold¶
Most dealers require 100–200 BF minimum for wholesale pricing. At qty 50 (~30 BF total), budget at retail pricing.
Material cost per assembly¶
- Housing blank (6/4): ~0.5 BF
- Wall plate blank (1/4"): ~0.08 BF
- Total: ~0.58 BF per assembly
| Species | Housing/piece | Wall plate/piece | Total material/assembly |
|---|---|---|---|
| Black cherry | $5–7 | $0.80–1.20 | $5.80–8.20 |
| Black walnut | $8–11 | $1.10–1.60 | $9.10–12.60 |
Machining cost per assembly¶
CNC router shop rate $75–150/hr. Wood runs 18,000–24,000 RPM at 100–300 IPM vs. aluminum at 8,000–12,000 RPM at 30–60 IPM requiring coolant on a machine costing 3–5× more per hour. Aluminum enclosure machining is roughly 3–5× the cost of wood for equivalent geometry.
| Operation | Time/piece | Cost/piece |
|---|---|---|
| Housing: pocket + dovetail + outer profile | 8–12 min | $10–30 |
| Wall plate: rail profile + outer profile | 3–5 min | $4–12 |
| Total machining per assembly | 11–17 min | $14–42 |
The wide range is driven primarily by shop rate variance ($75–150/hr). At a mid-range shop ($100–110/hr), expect $18–30 per assembly.
Finishing cost per assembly¶
4.5 sides of housing (all exterior plus one coat urethane on back face for moisture control). Wall plate not finish-coated.
| Operation | Cost/piece |
|---|---|
| Light hand sand, 150 then 220 grit | $4–8 |
| 3 coats wipe-on urethane, dry between coats | $8–15 |
| Materials pro-rated | $2–4 |
| Total finishing | $14–27 |
Hardware cost per assembly¶
Based on McMaster pricing at qty 50 assemblies:
| Item | Qty/assembly | Pack size | Pack price | Unit cost | Cost/assembly |
|---|---|---|---|---|---|
| UHMW washer, 0.150" ID | 2 | 25 | $7.61 | $0.3044 | $0.6088 |
| #6 × 1/2" rounded head, PCB mount | 3 | 100 | $21.28 | $0.2128 | $0.6384 |
| #6 × 5/8" truss head sheet metal, dock to wall | 4 | 100 | $13.68 | $0.1368 | $0.5472 |
| Hardware subtotal | $1.7944 | ||||
| Shipping + tax burden (33.05%) | $0.5931 | ||||
| Loaded total per assembly | $2.39 |
Total per-assembly cost — qty 50¶
| Cherry | Walnut | |
|---|---|---|
| Material | $6–9 | $10–14 |
| Machining | $14–42 | $14–42 |
| Finishing | $14–27 | $14–27 |
| Hardware | $2.39 | $2.39 |
| Total | $36–80 | $40–85 |
At a mid-range shop rate, cherry assemblies will likely land in the $47–57 range.