Determination of the optimal feed rate for step drill bit drilling CFRP pipe based on exit damage analysis

Abstract

The non-axially distributed fibers in carbon fiber reinforced polymer (CFRP) pipes have buckling and bear the initial bending stress. Defects such as delamination, tear and burr at the hole exit are easy to occur in the drilling process. The feed rate is an important factor affecting the formation of defects. In this paper, the influence law of feed rate on exit delamination and tear is analyzed theoretically to prove the existence of the optimal feed rate. The optimal feed rate for step drill bit drilling CFRP pipe is obtained through experiments. Using step drill bit with a step ratio of 0.7 to drilling CFRP pipe, the influence of feed rates (0.01, 0.02, 0.03, 0.04, 0.06, 0.08, 0.1, 0.12 and 0.14 mm/r) on hole exit damage is studied to analyze the formation process of III-delamination and tear at a fixed spindle speed (3500 r/min). The results show that when the feed rate is less than 0.04 mm/r, the hole exit III-delamination mainly occurs in the secondary cutting stage of the step drill. The III-delamination range decreases with the increase of feed rate. When the feed rate is greater than 0.06 mm/r, the primary cutting stage is the main stage that leads to tear. In this stage, the thrust force is greater than 85.01 N, resulting in tear of mode I fracture. And the tear area increases with the increase of feed rate. Therefore, the hole with the least exit damage can be obtained by selecting the feed rate of 0.04 mm/r.