2.2 Fasteners, Torque & Thread Management
Proper tools, locking methods, and verification.
Fasteners aremay smallbe parts with outsized consequences: they set the feelsome of the product,smallest guardparts seals,in a build, but their influence is enormous. They dictate how the product feels in the hand, whether seals stay watertight, and keepif groundsgrounding reliablepaths longremain afterdependable itover ships.years Theof coreservice. aimAchieving isthe right clamp load—the squeezing force that holds the stack—delivered by torque (controlled twisting force) without crushing plastics or chewing up threads. Because clampload depends not just on friction,torque, but on surface finishes,finish, lubricants,lubrication, and threadlockersmaterial pairing—variables that can swing results wildly;dramatically theif samenot torquecontrolled. onWith careful fastener selection, calibrated tools, and traceable checks, assembly lines can turn what is often a dryhidden zincsource screwof and an oiled one does not create the same joint. Mixed materials are standard in box build—steel, aluminum, and molded bosses—so picking the right thread form or insert is as much about serviceability as strength. Consistent technique and verified tools turn thisdefects into a repeatablerepeatable, process,reliable while simple checks and traceable logs make it defensible when a rattle or leak shows up in the field. Nail these fundamentals and everything downstream—gaskets, EMC (electromagnetic compatibility) bonds, and customer perception—stays solid.process.
2.2.1 The idea (in one line)
Get the right clamp load into the joint—every time—without stripping threads, crushing plastics, or leaving anything loose.
2.2.2 Picking fasteners (cheat sheet for box build)
Engagement rule of thumb
- Into steel: ≥ 1.0×D thread engagement.
- Into aluminum: ≥ 1.5×D or use steel insert.
- Into plastics: follow screw vendor chart (often 2–3×D equivalent).
2.2.3 Tools & bits (make the driver do the thinking)
Bits
Prefer Torx (TX) over Phillips—less cam-out. Keep bits labeled & rotated; worn bits drive rejects.
2.2.4 Torque basics without the headache
Clamp load F comes from torque T and friction: T = K × F × D
- D = nominal diameter; K (nut factor) ≈ 0.20 dry, 0.18 zinc-plated, 0.12–0.15 lubricated.
- Lubrication or threadlockers change K → same torque ≠ same clamp. Always spec torque with condition (“dry,” “oiled,” “Loctite 243”).
Practical knobs
- If you change finish/locker, re-validate torque.
- For soft stacks (gaskets/TIMs), consider torque + angle or sequence patterns to avoid squeeze-out.
2.2.5 Starter torque map (sanity ranges—tune to your joint)
(For machine screws into steel/aluminum inserts, “dry” zinc-plated; always validate on your parts.)
Plastics (thread-forming): typically 0.2–1.2 N·m depending on boss and screw; set by strip-to-drive ratio (target ≥ 1.5×).
2.2.6 Locking methods (choose by vibration & service)
2.2.7 Thread health & prep
- Go/No-Go gauge critical holes (23.1). Chase paint with taps; don’t upsize without MRB.
- Blow out chips; no oily taps near label or gasket lands.
- For plastics: confirm pilot diameter/depth; run at low speed and no impact.
2.2.8 Running the screw (repeatable moves)
- Start by hand for 2–3 turns—prevents cross-thread.
- Seat all screws lightly in a pattern, then final torque in criss-cross on lids/frames.
- For gaskets/TIMs, walk torque in two passes (50% then 100%).
- Record OK/NOK and apply a witness mark (paint pen) on critical fasteners.
2.2.9 Verification & audits (prove the numbers)
- Setup: verify driver on a torque analyzer with the right rundown adapter; record setpoint & tool ID.
- In-process audits: per cell/hour, do residual torque check (break-loose method) on a sample (target 80–100% of spec).
- Smart drivers: log torque + angle curves; use OK windows.
- Earth bond joints: measure < 0.1 Ω after tightening (23.1/22.1).
2.2.10 Plastics & inserts (keep bosses alive)
- Use thread-forming screws; avoid machine screws directly into raw plastic unless specified.
- Set torque by strip test: find strip torque on scrap, then run production at ~60–70% of strip.
- Prefer heat-set brass inserts for serviceable joints; spec ultrasonic/heat install parameters.
- If you hear cracking, stop—boss is failing.
2.2.11 Rework & repair rules
Allowed (record it):
- Replace stripped metal threads with helicoil/keensert per drawing.
- Replace failed plastic bosses with threaded insert + epoxy sleeve only if engineering allows.
Not allowed without MRB:
- Upsizing screws ad hoc; mixing thread standards; grinding washers to fit; “just add more threadlocker.”
2.2.12 Acceptance cues (quick visual)
- Head flush; no tilt; washer fully bearing.
- Witness mark aligned across head and base.
- No paint shear or spider cracks around holes.
- Earth washer teeth visible bite into metal.
- No ooze of excess threadlocker onto seals/labels.
2.2.13 Common traps → smallest reliable fix
2.2.14 Pocket checklists
Before the build
- Torque map posted; tools in cal with IDs
- Bits correct (Torx preferred), spares at station
- Threads checked/clean; earth lands bare & clean
- Locking method & condition (“dry/243/etc.”) on SWI
During
- Hand-start; seat, then pattern to final torque
- Two-pass torque for gaskets/TIMs
- Witness marks on critical fasteners
- Earth joints measured <0.1 Ω where required
Audit
- Tool verified on analyzer at start of shift
- Residual torque spot-checks pass (80–100% spec)
- Any strip/cross-thread → stop, MRB, not “heavier hand”