Inconel Tube Sheet Drilling

Hamill Manufacturing Company, Trafford, Pennsylvania, was contracted to drill several hundred close-tolerance through-holes in an Inconel tube sheet component approximately 4 inches thick. Hamill Manufacturing had experience drilling Inconel as thick as 2 inches, but the added depth, large number of holes, and close tolerances involved presented a new challenge. Hamill Manufacturing considered drilling and reaming the holes using the same parameters employed when drilling thinner Inconel work pieces. In that case, experience indicated that each component would require over 200 hours to produce. Additionally, if even a single hole was drilled out of specification, the part would have to be scrapped and the process started over again. Prior to commencing production, Hamill asked the National Center for Defense Manufacturing and Machining NCDMM to suggest ways to reduce drilling time, meet required tolerances, and ensure consistent quality. Through existing process evaluation, the NCDMM determined that increased productivity and top quality could be achieved by applying advanced gun drilling methods. Gun drills are specialized hole making tools designed to produce deep, straight, and accurate holes. Analysis of test results determined the best gun drill Star Cutter and drill diameter for the application. The best results occurred when the gun drill entered the pilot hole rotating counterclockwise then changed to clockwise rotation when the actual drilling began. Retracting the drill from the hole with the machine spindle off also was beneficial. The tests additionally determined the best coolant concentration and pressure level for the gun drills internal through-coolant system.