Peck Drilling of Composite/Metal Assemblies

<div class="htmlview paragraph">Under Contract No. N00019-93-C-0006 EMD with the Department of the Navy, Bell Helicopter Textron Inc. (BHTI) conducted a Manufacturing Process Verification Test as part of the V-22 Engineering Manufacturing Development process. The objective of this test was to develop a “One Step” drilling solution for peck drilling close tolerance holes in V-22 major assemblies. These assemblies consist of Titanium / Graphite / Titanium and Aluminum / Graphite compositions.</div> <div class="htmlview paragraph">The V-22 is a graphite / epoxy composite structure that has metal detail parts mechanically attached to the basic structure. Attachment of these detail parts is accomplished with two piece titanium fasteners that require a close tolerance of .003″ per hole. To achieve this tolerance a drill / ream process is currently used. Implementing a One Step drilling process eliminates one-half of the drill motors, cutting tools, and bushings required to support a drill / ream process and reduces substantially the direct and indirect labor hours associated with the drilling process. There are approximately 954 holes ranging in size from .1905″ to .4385″ currently peck-drilled in the major assembly of the V-22 wing and nacelles.</div> <div class="htmlview paragraph">To meet the goal of a One Step drilling process a test was developed. Test plates of Ti / Gr / Ti and Al / Gr were fabricated. A Gardner Denver model HT-4 power feed drill was used. The HT-4 provides drilling of the hole while periodically withdrawing from the hole (peck cycle). It is this pecking motion from which the drill has been appropriately named “Peck Drill”.</div> <div class="htmlview paragraph">Baseline data was gathered using the drill and ream process initially adopted by BHTI for V-22. In addition, various drill geometries were tested in attempt to achieve a One Step process while meeting the .003″ tolerance requirement. Drill designs such a thin web, thick web, single margin and double margin along with various drill settings were tested with only limited success.</div> <div class="htmlview paragraph">Finally, tests using a dreamer were performed and the results were outstanding. To further investigate, several different dreamer geometries were evaluated. Optimum performance proved to be a specially designed dreamer with a reverse helix on the reaming feature. This dreamer design yielded a 3 sigma range of .000466″ and proved successful at drilling both material compositions. The HT-4 peck drill settings proven the most optimum were a slow feed setting and a short peck cycle at 300 rpm. The same settings and motor speed worked well for both materials which has proven a fortuitous benefit requiring only one tool and settings for all material compositions, a substantial reduction in tool requirements.</div>