Replaceable drill bit with compound step and sawtooth structures for damages and drilling-cost reduction of CFRP composite

Abstract

Frequently-occurred drill-exit damages and short tool life are challenging issues for drilling CFRP in aerospace industries. High-performance and cost-effective drill bits are always desired. This paper proposes a replaceable drill bit with compound step and sawtooth structures for drilling CFRP to reduce damages as well as cost. A multiple-stage damage reduction concept of reducing thrust force and performing upward cutting on the drill-exit materials is proposed and introduced for drill bit design by combining step and sawtooth structures. Compound geometry of step and sawtooth structures is further integrated on a replaceable drill tip with a reusable body for cost-saving improvement. Drilling performance and cost were evaluated among an ordinary step drill bit, a compound drill bit, and a replaceable drill bit with the compound geometry. Results indicated that the compound and replaceable drill bits could produce better drill-exit and hole wall qualities as expected and the hole sizes are also more stable based on Cpk value; in addition, the tool life of both is over two times than the ordinary one. Drilling costs of the compound and replaceable drill bits are formulated with the consideration of manufacturing cost and tool life, and a case study is conducted for cost-saving comparison. In the specific case, it indicated that the replaceable drill bit is preferable in cost if over 58 CFRP holes are required to be drilled, the expected saving of overall cost would reach 9 % under 116 CFRP holes, and the saving ratio will finally reached a fixed level of 20 % for a large amount of drilling. This saving ratio would be affected by the manufacturing costs of drill bits. The cost-saving calculation method would be referenced for replaceable drill bit with compound step and sawtooth structures for industrial partners in mass drilling task for CFRP.