Hollow Custom Screws: Wall Thickness, Bore Location and Load-Carrying Section

Hollow custom screws need more than a bore diameter. The critical checks are the remaining load-carrying section, the minimum local wall and the bore’s relationship to the exterior geometry. A hole can be the correct size but still leave one side too thin. Start with the passage’s function, then define the geometry and inspection method that protect it.

1. Identify what the bore must do

A central opening may carry a cable, provide a gas path, reduce mass or accommodate another component. These functions are not interchangeable. A cable passage needs suitable edge treatment; a vent needs an unobstructed route; a pressure-carrying passage needs sealing and pressure qualification. A hole shown on a screw drawing establishes none of those performance requirements by itself.

For a compact sensor assembly, a sharp exit can damage insulation even when the cable passes freely. For a lightweight linkage, reducing mass may be less important than retaining stiffness or fatigue resistance. Define the interface at the head and at the tip, the required internal surface and any restriction on debris or lubricant.

2. Calculate the remaining smooth section

For a concentric circular bore through a smooth round section, A = π(D² − d²)/4. The remaining-area ratio to a solid section is 1 − (d/D)². The following geometry calculation uses D = 6.0 mm and compares bore sizes at the same outer diameter.

Stress here is F/A for a centred axial load on the stated smooth section. The values are not allowable loads or material strength data. At a threaded section, nominal major diameter is not the appropriate smooth outside boundary; thread-root geometry, stress concentration and the head transition require separate assessment. Material condition and fatigue loading can change the governing failure mode.

3. Control bore location and minimum wall

For circular sections with parallel axes, a geometric minimum-wall estimate is tmin = (Dmin − dmax)/2 − emax, where e is the radial axis offset. With Dmin = 5.90 mm, dmax = 3.10 mm and emax = 0.15 mm, tmin = 1.25 mm. Ignoring the offset would predict 1.40 mm and overstate the thinnest wall by 0.15 mm.

Do not treat a diametral position tolerance, a runout reading and radial offset as interchangeable numbers. Their relationship depends on the drawing’s geometric definition, datum setup and actual form. The calculation above explicitly uses radial axis offset. If the bore wanders or the outside surface is not straight, check local sections throughout the critical length.

4. Choose a process around the hardest feature

A long small bore raises tool deflection, chip evacuation and inspection challenges. Machining from both ends can reduce drilling reach but introduces a possible mismatch where the holes meet. A specified “through hole” should not conceal an internal step that obstructs the cable or creates a local restriction.

Review the sequence of forming, turning, drilling, threading and finishing with the manufacturer. A practical route may use a formed blank and secondary machining, or machining from stock, depending on material, quantity and geometry. The right comparison is finished-part conformity and total process cost, not an assumption that one method is always cheaper. Our cold-heading and machining comparison explains that decision.

5. Check passage quality in the complete assembly

For cable routing, specify the cable envelope, bend restrictions and edge condition rather than only a nominal cable diameter. For a gas passage, evaluate the minimum restriction, path length and outlet. For pressure service, hole continuity is not proof of sealing or pressure capacity; those need a separate design and test specification.

Record bore size, location and accessible internal condition after finishing. An entry plug gauge confirms only what it can physically reach. A functional probe can check passage, but it cannot alone quantify minimum wall. Agree on optical, dimensional or section-based verification appropriate to the part, and identify any destructive qualification samples separately from routine batch checks.

6. Technical FAQ