Experimental study on drilling load and hole quality during rotary ultrasonic helical machining of small-diameter CFRP holes

https://doi.org/10.1016/j.jmatprotec.2019.03.001Get rights and content

Abstract

Small-diameter CFRP holes have a number of applications such as aerospace structures, circuit boards and rehabilitation medical equipments. High quality hole-making technology for the small-diameter CFRP holes is urgently needed due to various mechanical damages in twist drilling and chip removal clog in core drilling. In order to improve the hole quality, rotary ultrasonic helical machining (RUHM) was developed for machining small-diameter CFRP holes in this paper. The trajectory of cutting edge in RUHM was modelled and the intermittent cutting mode in RUHM was analyzed. Afterwards, the comparison experiments were conducted between RUHM and conventional grinding (CG). The results shows that compared to CG, both thrust force and transverse force were remarkably reduced in RUHM with a maximum decrement of 71.3% and 61.5%, respectively. Meanwhile, both the hole edge quality and surface integrity were significantly improved in RUHM. Compared to CG, delamination factor at hole exit was reduced by 12.8–25.7% and surface roughness for hole inner surface was reduced by 51.9%–53.2% for Ra value in RUHM. Moreover, the mechanisms of cutting force reduction, delamination formation and suppression, and surface roughness improvement in RUHM were also analyzed. The results suggest that RUHM is a promising processing strategy for machining small-diameter CFRP holes.

Introduction

Carbon fiber-reinforced plastics (CFRPs) possess superior mechanical properties in terms of high levels of strength-to-weight ratio, stiffness-to-weight ratio, corrosion resistance, fracture toughness, etc. Thanks to these advantages, CFRP has been increasingly popular for applications in a wide range of manufacturing sectors such as aeronautics, aerospace, automotive and medical service. For instance, the proportion of composites in the primary structures of Boeing 787 (Dreamer-liner) has reached to approximate 57%, which can save 15–20% fuel compared with any other comparative wide body airplane (Singh et al., 2013). Besides, both the fan cases and blades in compressor’s cooler section of GE’s Aviation GEnx engine fabricate from CFRP, which can successfully provide 15% lower emissions, 180 Kg weight reduction and 20% reduction in operational cost (Shyha et al., 2010).

Small-diameter CFRP holes (usually < 5.0 mm) have a number of applications such as structures in the aerospace industry, circuit boards in the electronics industry and rehabilitation medical equipments in the medical service. Conventional twist drilling is extensively used for producing small-diameter CFRP holes. However, similar to large-diameter hole drilling, various mechanical damages in terms of delamination (Zhang et al., 2003), inner surface cavities (see Fig. 1) (Shyha et al., 2009), fiber pullout, fuzzing, resin depletion, hole-roundness error (Makhdum et al., 2014) usually occur during twist drilling of small-diameter CFRP holes due to the heterogeneous and anisotropic nature of CFRP laminates. These undesirable drilling-induced damages not only directly deteriorate the surface finish and assembly tolerance, but also reduce the hole strength against fatigue, thus degrading the in-service behavior of the assembly parts (Mishra et al., 2010). Hence, machining of high quality small-diameter CFRP holes was still a challenge, which has been an intense topic for research in the past decades.

As an alternative to twist drilling, core drilling can generate much smaller thrust force and torque compared to twist drilling, which can result in less drilling-induced damages. Because of this, it was reported that the strength of composite laminates drilled by abrasive diamond tools is even superior to that by PCD tools (Persson et al., 1997). In core drilling process, a core drill which is a kind of hollow wheel with diamond grains on the periphery and front surfaces is employed. Besides, from the point of view of tool manufacturing, core drills can easily access to small hole diameters with low cost compared to twist drill bits. Hence, the core drilling method has become an established manufacturing process for making of small-diameter holes in CFRP laminates (Butler-Smith et al., 2015). Nevertheless, the typical drawback of core drilling is the chip removal clog in terms of rod chip blockage inside the core drill and power chip accumulation on tool front surface (Tsao, 2012). In order to solve the chip removal clog during core drilling of large-diameter CFRP holes, Hocheng and Tsao (2005) proposed several kinds of compound core special drills, in which the saw drill, twist drill or candlestick is installed in the center of core drill. Later, Tsao (2006) also put forward a method by pre-drilling pilot hole before core drilling to prevent chip removal clog. However, the application of both techniques becomes challenging with the reduction of the tool dimensions. Hence, the core drilling also does not necessarily present the optimal solution for drilling of small-diameter CFRP holes.

In the past decades, ultrasonic vibration-assisted cutting has been successfully used to machine various engineering materials, such as steels (Celaya et al., 2010), titanium alloy (Li. et al. 2017), composites (Geng et al., 2015) and so no. Recently, Feng et al. (2012) and Ning et al. (2016) performed rotary ultrasonic machining (RUM) for drilling large-diameter CFRP holes. In RUM, the ultrasonic vibration is applied on the core drill in axial direction and coolant is pumped through the core drill to prevent rod chip blockage and hole surface overheating. The results showed that compared to core drilling, RUM performed better in drilling CFRP including lower cutting force, lower torque and better surface roughness. However, the hole exit delamination risk increases due to the pumped coolant in the core drill and the wet machining is still undesirable in many aerospace applications due to the detrimental effect of moisture absorption on the shear fracture toughness of CFRP (M’Saoubi et al., 2015). Hence, the use of coolant in RUM has limited the wide application of this process in the aerospace industry. In order to solve the chip removal clog during core drilling of small-diameter CFRP holes under dry condition, this paper developed a novel process, i.e. rotary ultrasonic helical machining (RUHM). This novel method is a hybrid machining process which combines the material removal mechanisms of conventional grinding (CG) and ultrasonic machining. In order to avoid the chip removal clog and reduce the tool fabrication complexity for small-diameter core drills, solid wheels were used in RUHM. Moreover, the helical feed mode was chosen in this process, which can not only facilitate chip removal but also give the possibility of hole dimension control by adjusting the eccentricity of tool center to hole center during hole-making. The helical feed mode is also the characteristic of helical milling process, in which an end mill proceeds a helical path while rotates along its own axis (Olvera et al., 2012).With an objective to improve the hole quality for small-diameter CFRP holes, this paper first comprehensively studied the machinability of small-diameter CFRP holes by RUHM method. In the present work, drilling tests were carried out with solid wheels in helical feed mode with and without the assistance of ultrasonic vibration (i.e. RUHM and CG), respectively. The drilling load (i.e. cutting forces) and hole quality including hole edge damage, delamination at hole exit, surface integrity of hole inner surface were observed and compared for the two processes. In addition, the mechanisms of cutting force reduction, delamination formation and suppression, and surface roughness improvement achieved in RUHM were also analyzed.

Section snippets

Kinematics analysis in RUHM

Fig. 2 shows the schematic illustration of RUHM method developed in this work. In RUHM, the hole is produced by using a diamond coated wheel, which rotates and ultrasonically vibrates along its own axis while proceeds a helical/orbital feed path. In order to precisely express the helical feed movement, both the helical feed speed (vf) and axial feed per helical revolution (ap) were chosen in both RUHM and CG. In order to determinate the trajectory of cutting edge in this process, a workpiece

Cutting force

Fig. 5 shows the acquired cutting force curves in both RUHM and CG under the same condition. It can be seen that there are three components of cutting forces (i.e. Fx, Fy, and Fz) during drilling. The thrust force Fz along tool axis is similar to the thrust force during conventional drilling while the cutting forces of Fx and Fy share a similar variation rule due to the helical feed mode of wheel. In the present work, in order to better understand the influence of cutting force on hole quality,

Conclusions

This investigation presents the machinability of small-diameter CFRP holes by developed rotary ultrasonic helical machining (RUHM) process. The drilling load and hole quality in both RUHM and CG processes were compared and analyzed. Based on the experimental results, the following conclusions can be drawn.

  • (1)

    The trajectory of cutting edge in RUHM was modelled and analyzed. The motion path of cutting edge in RUHM is periodically varying curve instead of a standard curve in CG. This indicates that

Acknowledgements

This research was supported by Postdoctoral Science Foundation of China (Grant No.2018M631301) and National Natural Science Foundation of China (Grant No. 51475031).

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