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.
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.
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”.
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.
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.