5.7 Fasteners, Torque and Thread Management

Fastener installation is the most frequent and most vulnerable mechanical operation in Box Build. The primary function of a fastener is to create a specific clamping force that ensures electrical continuity (grounding), thermal transfer, and mechanical integrity against vibration. Failure to control this force leads directly to stripped threads, component damage, and latent failures, making strict thread management and calibrated tooling a mandatory requirement.

5.7.1 Fastener Selection and Application

Fastener choice is determined by the material being joined and the need for reworkability.

Use	Go-to Choice	Why	DFM Mandate
Steel/Aluminum	Metric machine screws (M2–M6, Torx).	Predictable torque, high reuse life.	Use Torx (TX) bits over Phillips to minimize cam-out and resulting thread damage.
Plastics (Bosses)	Plastic thread-formers (PT/Delta PT).	Cuts threads that resist pull-out.	Pilot hole size is critical; use low-speed, no-impact drivers.
Sheet Metal (No Nut)	Thread-forming screws (Tri-lobular).	Eliminates nut installation; quick assembly.	Use controlled speed to prevent stripping thin sheets; inspect for burrs.
Reusable Connection	Prevailing-torque nuts (nylock insert).	Built-in anti-loosening for high vibration/serviceable joints.	Note the temperature limit (≈ 100˚C) for nylon inserts.
Ground/Earth Points	Serrated/Star Washer + Machine Screw.	Reliable bite through paint/oxide for low-ohm electrical continuity.	Do not over-torque thin chassis sheets.

Engagement Rule of Thumb

The thread engagement depth is critical to prevent stripping:

• Into steel: ≥ 1.0 x Nominal Diameter (D).
• Into aluminum: ≥ 1.5 x D or use steel inserts.
• Into plastics: Follow screw vendor chart (often 2 to 3 x D equivalent).

5.7.2 Torque Control: Physics and Mandates

Torque (T) creates the necessary Clamping Load (F) to hold the joint together. This relationship is defined by friction (K): T = K x F x D.

Practical Torque Ranges (N·m)

Torque specifications must be validated against the joint, but these sanity ranges provide a starting point for machine screws into metal:

Size	Typical Range (N⋅m)	Primary Application
M2.5	0.35 – 0.6	Common PCB standoffs, light covers.
M3	0.6 – 1.0	Fan guards, light chassis, internal brackets.
M4	1.5 – 2.5	Heavier brackets, lid screws.

• Plastics: Thread-forming screws typically use 0.2 – 1.2 N⋅m; torque is set by the strip-to-drive ratio (target ≥ 1.5 times).

Lubrication and Condition

• Mandate: Always specify torque with the condition ("dry," "oiled," "Loctite 243"). Lubrication or the use of threadlockers changes the K-factor significantly, meaning the same torque does not yield the same clamp load.
• Complex Stacks: For soft materials (gaskets/TIMs), torque application requires sequencing or angle control to prevent material squeeze-out.

5.7.3 Locking Methods and Thread Management

Thread lockers and mechanical washers maintain clamp load against vibration and thermal cycling.

Chemical Locking (Threadlockers)

Method	Use When	Rework?	Notes
Medium Strength (Blue 243)	General vibration; serviceable joints (e.g., maintenance access).	Yes	Mandate: Requires clean threads; note cure time.
Low Strength (Purple 222)	Small screws (M2–M3), plastics.	Yes	Used where blue is too strong and might damage threads upon removal.
High Strength (Red 271)	Permanent structural joints (rare in Box Build).	No (Requires heat)	Avoid near plastics/gaskets.

Tooling and Thread Health

• Mandate: Use Go/No-Go gauges on critical holes to verify thread health, especially after powder coating.
• Prep: Prohibited: Oily taps used near gasket lands or label zones. Blow out chips; use filtered air.
• Plastics: Confirm pilot diameter/depth before running production.

5.7.4 Tooling, Audit, and Rework Control

Accuracy in torque control depends on the tooling used and the logging of its data.

Tooling and Verification

Tool	Best for	Pros	Audit Requirement
Shut-off Driver	Medium torque, high-volume.	Auto stop at set torque; fast.	Requires scheduled PM; reaction control.
DC Smart Driver	Critical joints / Traceability.	Logs torque/angle curve; provides OK/NOK logic.	Mandatory for Class 3; verifies quality.
Bits	Prefer Torx (TX) over Phillips.	Less cam-out; reduces rejects.	Bits must be labeled and regularly replaced.

Audit and Verification

• Running Sequence: Start every screw by hand for 2–3 turns to prevent cross-threading. Final torque in a criss-cross pattern on lids/frames.
• Verification: Set up: Verify driver on a Torque Analyzer with the correct rundown adapter. In-process audit: Perform a residual torque check (break-loose method) on a sample (target 80% – 100% of spec).
• Witness Mark: Apply a witness mark (paint pen) across the screw head and base for quick visual verification that the torque was applied and the screw has not backed out.

Rework and Repair Rules

• Allowed (Record It): Replace stripped metal threads with Helicoil/Keensert per engineering drawing. Minor edge deburr.
• Prohibited (MRB Required): Upsizing screws ad hoc; mixing thread standards; adding more threadlocker (must clean and reapply); Warped panels or spun inserts.

Final Checklist

Mandate	Criteria	Verification Action
Torque Control	All critical fasteners driven by calibrated torque tools.	Tool calibration status verified; use of non-calibrated tools prohibited.
Ground Bonding	Star washers mandatory at all chassis ground points.	Audit confirms permanent metal-to-metal contact is achieved.
Thread Integrity	Go/No-Go gauges used; manual start eliminates cross-threading.	Threads confirmed clear of paint/powder before tool engagement.
Rework Restriction	No ad hoc upsizing; stripped threads repaired with Helicoil per drawing.	Spun inserts and leaning studs are prohibited failures.
Audit Requirement	Critical fasteners checked with a Witness Mark and logged.	MES records torque value/angle via DC Smart Driver for Class 3.
Plastics Rule	Torque set by strip-to-drive ratio (1.5 x target).	Avoids damage to plastic bosses and ensures pull-out resistance.