Northrop Corporation manufactures body panels for the Boeing 747 aircraft. There are 1259 different stringer configurations used on the three 747 models with an average of 839 stringers per ship set. Until recently, all drain holes and skin coordination pilot holes were drilled manually using plastic application template tools (PATTS). Inventory costs were high and manual drilling errors led to excessive scrap and rework rates. Northrop engineers recognized that automating the stringer drilling process would produce higher quality parts at a lower cost.
Northrop worked with Electroimpact, Inc. to develop the Automatic Stringer Drilling System (ASDS). The ASDS automatically clamps and drills all straight and contoured stringers used on the 747. Stringers are mounted on a rotating platform that provides +/- 90° of motion. Two servo-servo drills are mounted on a cantilevered arm with 25 feet of X-axis travel. An infra-red linear encoder maintains the X-axis accuracy within +/-0.002″ over the 25 foot workzone. A statistical process control (SPC) system monitors the time, date, axis position, spindle rpm, peak spindle current, feed rate, and peak feed current for each hole drilled. Initial studies show cycle times are reduced by 80% with a dramatic drop in scrap and rework rates.