Specifications for Rapid Hole Drilling
William P. Latham
Air Force Research Laboratory/Directed Energy Directorate
The features and surface quality of small holes can determine the characteristics
of fluid flow passing through the holes and the flow field created by the holes. Many
applications exist in fluid mechanics where densely packed arrays of small holes are
required. For instance, Laval nozzles for gas dynamic lasers and shower head chemical
reactors and spray dryers. For such applications, the holes may require tight length and
diameter constraints, contouring of entrance, bore or exit, close geometrical spacing, and
a high quality surface finish. The manufacturing process typically must also result in
minimal distortion of the base material. Techniques are needed that will quickly and
inexpensively produce holes in quantities of millions. Below are features that could be
typical of holes for various modern applications. Hole Geometry: Circular cross section
holes consisting of a cylindrical section at the minimum hole diameter, an entrance region
which could be a right angle, a radiused region, or other contour; an internally contoured
section which could be conical or have a more complex contour; and an exit which needs
a sharp edge with respect to the surface that it penetrates.
1) Diameter of cylindrical section (minimum diameter): 0.10 to 0.50 mm
2) Circularity: deviations from circular cross section <0.01 of diameter.
3) Cylindricity: deviations from cylindrical cross section <0.01 of diameter
4) Tolerances on hole features: 0.01 to 0.05 of the hole diameter.
5) Thickness of material: Holes are to be formed through materials of thickness of 2 to
10 times the hole minimum diameter (2 < t/d < 10).
6) Contours: Radially symmetric contouring of some portions of holes is necessary for
come applications. Capabilities to manufacture such features should be defined.
7) Edge conditions: The interfaces between the hole and the material surfaces through
which it passes may need to be sharp (feature radius equal zero) i.e. not rounded edges
where required, or alternatively may need to have a prescribed radius up to the hole
8) Surface finish: The interior surface of the hole needs to have a finish better than 60
9) Irregularities: Desire for no burrs, melt fragments, dross, slag, etc. or other
extraneous features within the hole or at the inlet or outlet that would create undesired
10) Hole Array: Arrays of 1000 to 100,000 holes with prescribed geometric features are
11) Spacing: Holes in an array are to be spaced in both lateral directions within ~3 to 15
minimum hole diameters.
12) Perpendicularity: The hole axes need to be perpendicular to surfaces through which
the holes pass within 0.1 to 0.5 degrees (or alternatively may need to be at repeatable
angular orientation to the surfaces within 0.1 to 0.5 degrees).
13) Parallelism: The axes of holes in an array are to be parallel within 0.1 to 0.5 degrees.
14) Repeatability: Missed holes, i.e. not completely drilled, must number less than 0.5%
of the holes if randomly located. Defective holes that pass through the plate may need to
be much lower in number depending on the application.
15) Flatness of array: The plate through which the array of holes is “drilled” is to be
constrained to be flat within ~.010” per 5” width with a restraining force of 10 lb or less
after drilling and post processing.
16) Types of materials: Holes are to be machined in high strength materials that are
resistant to adverse thermal and chemical environments. Such materials include some
stainless steels, Inconel and Hastelloy like alloys, and similar materials.
17) Lifetime/wear resistance: Holes should be able to maintain a constant configuration
and flow performance through 5,000 to 20,000 hours of high velocity fluid flow. The
manufacturing process should not create geometric features or leave the material in a
condition that would lead to wear over shorter periods.
18) Manufacturing time: The time to manufacture each hole complete in the array
should average less than one minute, significantly less if possible.