Deep Hole Drilling for Fastener Manufacturing: Process Parameters for Bolt, Screw, and Stud Bores

A manufacturer of aerospace-grade high-strength bolts (AISI 4340, 38–42 HRC, Ø20 mm × 120 mm, requiring Ø6 mm × 100 mm axial through-bore concentric within 0.1 mm TIR) was using conventional twist drilling from both ends (50 mm per side), achieving concentricity of 0.25–0.40 mm TIR with 18% scrap. Switching to single-pass gun drilling (carbide K15, Vc = 50 m/min, f = 0.025 mm/rev, 80 bar EP oil) achieved concentricity of 0.03–0.08 mm TIR — scrap eliminated, X-ray inspection reduced to first-article sampling, and cycle time reduced from 90 seconds to 35 seconds per bolt.

Fastener Types and Drilling Requirements ​

Axial Bore Requirements by Fastener Type ​

Gun Drilling Parameters for Fastener Axial Bores ​

Concentricity Control: Gun Drilling vs Conventional Methods ​

FAQ ​

What fastener applications require deep hole drilling? ​

Deep hole drilling in fastener manufacturing serves several distinct functional requirements. The primary applications are: (1) Weight reduction bores in aerospace fasteners — aircraft structural bolts, shear bolts, and engine fasteners often have axial bores that remove 20–40% of the fastener weight while maintaining sufficient shear and tensile strength. A typical aerospace bolt (Ø20 mm × 120 mm, AISI 4340) with a Ø8 mm axial bore weighs 45 g instead of 60 g — a 25% weight reduction. For an aircraft with 50,000 fasteners, this saves 750 g total — not large per fastener but significant when accumulated across the airframe. (2) Shear pin bores — some fastener designs incorporate a transverse pin through the axial bore to provide secondary retention or to create a controlled shear plane. The axial bore must be precisely positioned and concentric to ensure the shear pin engages correctly. (3) Lubrication passages — fasteners in high-temperature or high-wear applications (turbine engine bolting, brake caliper bolts) use axial bores as oil passages for controlled lubrication. (4) Wire routing bores — in electronic and avionics applications, hollow fasteners serve as wire harness routing channels through bulkheads and structural members. (5) Thread relief bores — some fastener designs use a reduced-diameter section at the thread root (created by the axial bore) to improve fatigue performance by reducing the stress concentration at the thread root. The bore creates a more favourable stress distribution by shifting the neutral axis inward. (6) Hydraulic/pneumatic passages — hollow bolts used as fluid fittings in hydraulic manifolds and pneumatic systems have axial bores that serve as flow passages. The surface finish requirement for these applications is typically Ra < 1.0 µm to minimise flow resistance. (7) Instrumentation bores — specialised fasteners with axial bores for thermocouple or strain gauge wiring are used in test and measurement fixturing. The bore must be smooth (Ra < 0.8 µm) to prevent damage to the delicate instrumentation wires during assembly.

How is concentricity controlled in axial bore drilling of fasteners? ​

Concentricity control in axial bore drilling of fasteners is the most critical quality parameter and the primary reason gun drilling is preferred over conventional drilling. The mechanisms controlling concentricity are: (1) Starting conditions — the gun drill engages the fastener blank at the entry face. The starting bushing (or drill bushing) must be aligned within 0.01 mm of the fastener centreline. For fastener drilling, the starting bushing is typically integrated into the workholding fixture and is aligned to the fastener blank by referencing the blank's outer diameter (OD). The bushing-to-blank alignment should be within 0.02 mm TIR. (2) Guide pad guidance — the gun drill's guide pads (carbide pads that ride on the bore surface) provide self-guiding action once the drill has entered 2–3× the bore diameter. The guide pads burnish the bore surface and maintain the drill's position concentric to the existing bore. The self-guiding action means that once the drill establishes the bore path, it tends to maintain that path — hence the critical importance of the starting alignment. (3) Workpiece stiffness — for fastener drilling, the workpiece (bolt blank) is short and rigid (L/D typically 3–10:1 for the fastener itself). The short workpiece length means that workpiece deflection is negligible, and concentricity is primarily determined by the drill's entry alignment and its own straightness. (4) Drill tube straightness — the gun drill shank must be straight to within 0.05 mm TIR per 100 mm length. For deep fastener bores (100–200 mm), this requires precision-ground drill shanks with runout < 0.02 mm at the drill tip. (5) Cutting parameters — lower feed rates (0.015–0.030 mm/rev) produce lower cutting forces and less drill deflection, improving concentricity. Higher feed rates increase the radial cutting force component, which can cause the drill to deviate from the centreline. The recommended feed for concentricity-critical fasteners is f = 0.015–0.025 mm/rev. (6) The achievable concentricity for gun-drilled fastener axial bores is 0.03–0.10 mm TIR for standard production and 0.02–0.06 mm TIR for optimised setups with bushing support and precision alignment. When concentricity tolerance is < 0.05 mm TIR, the following are required: precision-ground starting bushing (ID tolerance < 0.005 mm); drill tip runout < 0.01 mm; and CNC-controlled pick-up of the drill into the bushing (no manual alignment). The practical quality control method for concentricity in production is: sample basis measurement by CMM or dedicated concentricity gauge (1 per 50–200 parts for Cpk > 1.67 processes), with real-time process monitoring by spindle power and thrust force for anomaly detection.

What are the advantages of gun drilling over conventional twist drilling for fastener axial bores? ​

Gun drilling offers clear advantages over conventional twist drilling for fastener axial bores, particularly in concentricity, surface finish, and single-pass capability. The comparison for a typical aerospace bolt (Ø6 mm × 100 mm bore in Ø20 mm AISI 4340) shows: (1) Concentricity — gun drilling achieves 0.03–0.08 mm TIR versus 0.15–0.50 mm for twist drilling. The 3–5× improvement in concentricity is the primary driver for adopting gun drilling in fastener manufacturing. (2) Single-pass capability — gun drilling produces the full 100 mm depth in one pass from one side, eliminating the alignment errors inherent in two-ended drilling. Conventional twist drilling from two ends requires the bolt blank to be reversed and re-fixtured, introducing indexing error. Even with a single long-series twist drill, the maximum L/D ratio for a Ø6 mm twist drill is approximately 10:1 — insufficient for a 100 mm bore (L/D 16.7:1). (3) Surface finish — gun drilling produces Ra 0.3–0.6 µm compared to Ra 1.5–4.0 µm for twist drilling. The smoother surface is important for fatigue performance: a smoother bore surface reduces stress concentration at the bore wall, improving the fastener's fatigue life by 15–30% in high-cycle applications. (4) Hole diameter consistency — gun drilling maintains IT7–IT8 diameter tolerance (±0.009–0.015 mm for Ø6 mm) along the full bore length. Twist drilling produces tapered holes with diameter variation of 0.02–0.05 mm along 100 mm depth. (5) Burr formation — gun drilling produces minimal entry and exit burrs (typically < 0.05 mm), while twist drilling produces significant burrs (0.1–0.5 mm) at both the entry and exit, requiring a separate deburring operation. The elimination of the deburring operation saves 10–30 seconds per fastener. (6) Tool life — gun drill life in alloy steel fastener drilling is 150–400 bores per tool (for Ø4–8 mm), compared to 200–600 bores for twist drills. While the twist drill has slightly longer absolute tool life, the gun drill produces a superior bore with tighter tolerances. The total cost per bore is typically lower for gun drilling ($0.05–0.08 vs $0.08–0.15 for twist drilling) due to the elimination of secondary operations (deburring, inspection) and reduced scrap.

What quality control methods are specific to fastener axial bores? ​

Quality control for fastener axial bores addresses the specific functional requirements of hollow fasteners. The key methods are: (1) Concentricity measurement — the most critical quality parameter. Measurement is performed by: mounting the fastener between centres (referencing the bolt OD or thread pitch diameter); inserting a dial indicator or non-contact probe (laser or air gauge) into the bore; and rotating the fastener to measure the bore runout relative to the OD. The TIR (total indicated runout) is recorded at 2–3 positions along the bore length. For aerospace fasteners, the concentricity requirement is typically 0.05–0.15 mm TIR. (2) Bore diameter — measured with air gauging or pin gauges at both ends and mid-length. Tolerance is typically H8–H9 for fastener bores. (3) Surface finish — measured with a contact profilometer (small probe, 2 µm radius) inserted into the bore. For fatigue-critical fasteners, Ra < 0.8 µm is typically required. For lubrication passage bores, Ra < 1.0 µm is acceptable. (4) Wall thickness — measured by ultrasonic gauging or mechanical calliper. The minimum wall thickness between the bore and the fastener OD must be maintained to ensure the fastener's load-carrying capacity. For a Ø20 mm bolt with a Ø6 mm bore, the nominal wall thickness is 7.0 mm; the minimum acceptable wall thickness might be 6.5 mm (accounting for eccentricity). (5) Fatigue testing — for new fastener designs or process qualifications, a sample of hollow fasteners is subjected to axial fatigue testing (typically R-ratio = 0.1, 10⁶ cycles) to verify that the axial bore does not reduce the fatigue life below the design requirement. The fatigue test frequency should be annual or with each material/process change. (6) Process verification — for production monitoring, the gun drilling process parameters (spindle speed, feed rate, coolant pressure, spindle power, thrust force) should be recorded for each fastener or at regular intervals. Statistical Process Control (SPC) charts for concentricity and bore diameter should be maintained, with Cpk > 1.33 required for critical aerospace fasteners. (7) Traceability — aerospace fasteners require full traceability of each production lot. The bore drilling records (date, machine, operator, tool serial number, parameters) must be linked to the fastener lot number.

This article provides an overview of deep hole drilling for fastener manufacturing. Process parameters, tool selection, and quality control methods depend on the specific fastener material, geometry, and application requirements. The technical data presented here reflects industry standards and documented case studies as of 2026.