An experimental study on the effects of various drill types on drilling performance of GFRP composite pipes and damage formation

Drilling is the most commonly used machining operation among others for assembly applications. The necessity of this operation has come to the forefront in composite materials that are becoming widespread nowadays. In this study, drilling performance of glass fiber reinforced plastic (GFRP) composite pipes used in many sectors such as natural gas transmission lines, pressurized sewer lines, industrial waste transmission, defense industry, and construction industry was investigated. The GFRP pipe was produced by filament winding method with a winding angle of +/-55 degrees. Different drill types (conventional twist drill, brad and spur drill, and brad center drill) with a drill diameter of 4 mm were utilized for drilling the pipe and the effect of drill type on the drilling performance was investigated. Drilling tests were performed at a constant 5000 rpm spindle speed and six different feed rate parameters (25, 75, 125, 175, 225, and 275 mm/min). Thrust forces generated during drilling were recorded and after the drilling operations, hole exit surface damage, and borehole surface damage were examined by a digital microscope and scanning electron microscope (SEM). Results showed that the brad center drill produced lower thrust forces while the twist drill generated higher thrust forces. The severity of damages could vary depending on the tool geometry and feed rate. Especially, at lower feed rates, conventional twist drill results with increased delamination and uncut fibers as compared with other drills. The brad center drill presents better performance since it generates fewer damages. Also, it is observed that these damages formed in the winding angle direction (orientation).