Appearance
A manufacturer of wind tunnel models (17-4PH H900, 40 HRC, wing 24 in x 14 in x 3 in, 250 pressure taps, 0.040 in x 2-5 in router tunnels) used a 5-axis CNC micro-drilling centre with a carbide gun drill (0.040 in, Vc = 30 m/min, f = 0.001 in/rev, oil at 100 bar). Each tunnel was drilled to 0.020 in below the surface, then a 0.020 in surface-normal tap hole intersected the tunnel at 90 degrees. Adjacent tunnels as close as 0.035 in. X-ray verified positions within +/-0.002 in.
Wind Tunnel Model Router Tunnel Drilling
Wind tunnel model internal router tunnels are micro-deep-holes that connect the model's central instrumentation cavity to the surface pressure taps. The router tunnel diameter is 0.020-0.060 in (typically 0.040 in for standard pressure taps), and the tunnel length is 2-6 in, depending on the distance from the instrumentation cavity to the tap location on the model surface. The tunnels are drilled at compound angles (the tunnel axis is rotated in two planes to reach the tap location from the cavity), requiring a 5-axis CNC micro-drilling centre with a programmable B-axis and a C-axis.
The drilling sequence for each tap is: (1) The drill is positioned at the cavity entry point and aligned to the compound angle calculated from the CAD model. (2) The gun drill (0.040 in diameter, carbide, 130-degree point angle) drills the router tunnel from the cavity to a depth 0.020 in below the model surface. (3) A shorter, smaller-diameter drill (0.020 in, 120-degree point angle) drills the surface-normal pressure tap hole from the model surface inward (0.020 in deep) to intersect the end of the router tunnel. (4) The tunnel is cleaned by flushing with compressed air and solvent. (5) A stainless steel hypodermic tube (0.040 in OD x 0.020 in ID) is inserted into the tunnel and bonded with low-viscosity epoxy. (6) The surface tap hole is deburred (0.001-0.002 in chamfer) using a carbide hand scraper under a microscope at 20x. (7) The tube at the cavity end is connected to a pressure scanner module. The complexity of the drilling makes the wind tunnel model one of the most challenging components manufactured by deep hole drilling.
Gun Drilling Parameters for Wind Tunnel Model Materials
The table below compares drilling parameters for the most common wind tunnel model materials.
| Parameter | 17-4PH H900 (40 HRC) | Maraging Steel (50 HRC) | 6061-T6 Aluminium |
|---|---|---|---|
| Cutting speed Vc | 25-35 m/min | 15-25 m/min | 60-100 m/min |
| Feed rate f | 0.0008-0.0015 in/rev | 0.0005-0.0010 in/rev | 0.002-0.004 in/rev |
| Gun drill material | Micrograin carbide | Micrograin carbide | Carbide or PCD |
| Coolant pressure | 80-120 bar | 100-150 bar | 40-60 bar |
| Coolant type | High-EP oil | High-EP oil | Emulsified oil |
| Surface finish in tunnel | 8-12 Ra | 6-10 Ra | 12-20 Ra |
| Max tunnel length | 6-8 in | 4-6 in | 10-12 in |
Tunnel Inspection and Acceptance Criteria
The table below shows the quality control checks applied to wind tunnel model router tunnels.
| Inspection method | Feature measured | Tolerance | Acceptance criterion |
|---|---|---|---|
| X-ray radiography | Tunnel end position | +/-0.002 in | Within 0.005 in of target |
| X-ray radiography | Intersection with tap hole | 0.005 in | Within tunnel centre axis |
| Borescope (20x) | Tunnel surface finish | 8-12 Ra | No visible tool marks |
| Air flow test | Tunnel blockage | — | Free flow at 5 psi air |
| Gauge pin | Tunnel diameter | 0.040 in | Pin passes full length |
| Microscope (50x) | Tap hole entrance | 0.001-0.002 in chamfer | No burrs or cracks |
FAQ
Why is the router tunnel drilled from the instrumentation cavity rather than from the surface?
Drilling from the internal cavity to the surface allows the router tunnel to be positioned with a smooth entry at the cavity end, where the hypodermic tube can be inserted and bonded without interfering with the aerodynamic surface. If the tunnel were drilled from the surface inward, the drill entry burr would be on the aerodynamic surface, requiring post-drilling blending that would alter the model contour. The cavity-end drilling also allows the compound angle to be set from the cavity, where the entry point is not constrained by the model surface geometry.
How are the compound angles calculated for each router tunnel?
The compound angles are calculated by solving the 3D geometry problem of connecting two points (the cavity entry point and the surface tap location) with a straight line that does not intersect any other tunnels or internal features. The calculation is performed by the CAM software, which reads the CAD model of the model and identifies the cavity entry point, the tap location, and all obstructions. The software calculates the required B-axis tilt angle and C-axis rotation angle for each tunnel and generates the CNC program.
What is the purpose of the 0.020 in standoff below the model surface?
The 0.020 in standoff (the distance between the end of the router tunnel and the model surface) prevents the gun drill from breaking through the model surface during the router tunnel drilling. If the gun drill broke through the surface, it would create a burr on the aerodynamic surface that would require removal and surface blending. The final 0.020 in is drilled from the surface inward with a smaller drill (0.020 in diameter), creating a clean, burr-free tap hole that intersects the router tunnel at 90 degrees.
How are adjacent tunnels spaced as close as 0.030 in without intersecting?
The CAM software performs a collision-avoidance calculation that ensures no two tunnels intersect within the model body. The software checks each new tunnel path against all existing tunnel paths and rejects any path that comes within 0.010 in of another tunnel at any point along its length. If a collision is detected, the software recalculates the compound angle to find a clear path. The adjacency distance of 0.030 in between tunnel entry points at the cavity wall is the spacing at the cavity end; the tunnels diverge as they travel outward through the model.
What is the cost implication of a failed router tunnel?
A failed router tunnel (one that does not meet the position or intersection tolerance) cannot be repaired because the tunnel is filled with the hypodermic tube and epoxy before the position is verified by X-ray. If the X-ray reveals a position error greater than +/-0.005 in, the tunnel is abandoned and a new tunnel is drilled from a revised compound angle. The abandoned tunnel remains in the model as a dead-end hole but does not affect the model's structural integrity. The cost of a failed tunnel includes the machining time (1-3 hours), the X-ray inspection time, and the consumable materials (drill, tubing, epoxy). The failure rate is typically 5-10% for complex models with 300+ tunnels.
Data are based on published research and industry experience as of 2026.