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 diameter. 8) Surface finish: The interior surface of the hole needs to have a finish better than 60 microinches. 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 flow disturbances. 10) Hole Array: Arrays of 1000 to 100,000 holes with prescribed geometric features are of interest. 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.
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