Conventional Helical Milling Research Articles

Investigation on cutting performance in ultrasonic assisted helical milling of Ti6Al4V alloy by various parameters and cooling strategies

Ti6Al4V alloy is one of the typical difficult-to-machine materials and often results in rapid tool wear, leading to poor machining quality in aircraft assembling. Compared to conventional helical milling, the ultrasonic assistant helical milling (UAHM) process has indicated its superior performance; however, it is still a great challenge to improve the hole surface quality and accuracy. In addition, few studies have been conducted on the effect of different variables and cooling strategies on the hole-making performance in longitudinal-torsional ultrasonic assisted helical milling (LT-UAHM). This paper, for the first time, reports effects of machining variables on geometric precision and surface roughness in LT-UAHM of Ti6Al4V. In addition, the lubrication/cooling mechanism on the simultaneous application of LT-UAHM and MQL is theoretically analyzed. The design approach of Taguchi experiment was employed to study how major variables such as the cutting speed, tangential feed, axial feed, and the workpiece hardness influence the dimensional and geometrical tolerances and surface roughness. This paper also discussed the effect of three cooling strategies, i.e., dry condition, air coolant, and minimum quantity lubrication (MQL) in LT-UAHM. Theoretical analysis demonstrated that the MQL coolant can be nebulized into hyper-fine droplets owing to the resonant cavitation phenomenon. Combined with the penetrating action caused by the separate-cutting principles of LT-UAHM, the cooling and lubrication performance of MQL was further enhanced. As a result, LT-UAHM with MQL had the most positive effect on circularity, cylindricity, nominal diameter, and surface roughness, contributing to 34%, 32%, 39%, and 40%, respectively. The second important machining factor was the cutting speed, contributing to 31%, 29%, 36%, and 22%, respectively. The tangential feed and workpiece hardness have the negative effect on geometrical accuracy, respectively.

Experimental research on new hole-making method assisted with asynchronous mixed frequency vibration in TiBw/TC4 composites

With the gradual promotion and the application of difficult-to-machine materials such as titanium matrix composites in the aerospace field, high-quality hole-making technology has become a major demand in aviation manufacturing. In order to improve the hole-making quality of TiBw/TC4 composites, asynchronous mixed frequency vibration-assisted hole-making (AMFVAHM) method is proposed. The process consists of two steps: ultrasonic vibration-assisted drilling (UVAD) base hole and low-frequency torsional vibration-assisted helical milling (LFTVAHM) target hole. Based on this process, the cutting trajectory modeling is established, and the hole-making experiment on TiBw/TC4 composites is conducted. The experimental data show that during the hole expansion stage, the maximum XY-plane average milling force decreases by 30.96% and the maximum axial average milling force decreases by 24.49% compared with conventional helical milling (HM) when the torsional vibration frequency and the milling frequency are the same in LFTVAHM. The hole-making experiment shows that AMFVAHM can reduce the chip size, tool wear, and some other defects such as entrance/exit burrs, scratches, and fractures of the hole wall. Comparing with HM and UVAD, the verticality of hole wall increases by 71.43% and 86.21%, the inlet damage decreases by 27.98% and 31.60%, the outlet damage decreases by 2.80% and 14.47%, the hole wall roughness (Ra) decreases by 36.29% and 63.43%, and the maximum white layer thickness decreases by 19.99% and 67.66%. Meanwhile, AMFVAHM process not only reduces the cutting force and cutting temperature but also improves the hole-making quality due to the fretting friction effect of LFTVAHM in secondary hole expansion, which meets the need for high-quality hole-making of difficult-to-machine materials in practical engineering applications.

A comparative study on cutting forces and power consumption in plain and ultrasonic vibration helical milling of AISI 1020 steel

The present study aimed to implement a sustainable machining method to improve energy efficiency in helical milling (HM) of AISI 1020. Therefore, ultrasonic vibration is integrated with conventional helical milling to reduce cutting forces. A model was developed for power consumption in terms of cutting forces in x, y and z directions, tangential and axial feed speeds. Series of plain and ultrasonic vibration helical milling (UVHM) experiments are conducted using 10 and 8 mm diameter mill cutters at different working conditions and experimental results for cutting forces are collected. Formation of chip and its geometry are investigated using the cutting trajectories of the bottom cutting edges of the cutter. Cutting forces and power consumption are estimated related to chip geometry in plain and UVHM processes and compared. In UVHM, the axial force is reduced by around 47% as the ultrasonic vibration is applied in the axial direction and the power consumption is reduced by 34%. The results showed that ultrasonic vibration has a significant effect on chip morphology, cutting force and power consumption, indicating that ultrasonic vibration assisted machining has a wide application in manufacturing. The process parameters are optimised as 2000 rpm of cutter rotational speed, 156 rpm of cutter orbital speed and 0.3 mm of axial depth of cut using 8 mm diameter cutter and the chip thickness, chip depth and power consumption are found to be 0.3969 mm, 0.2665 mm and 835.6 W respectively at optimal working condition.

A tilted orbital grinding technique for hole-making of CFRP composite laminates

Carbon fiber reinforced polymer (CFRP) laminates have been widely employed in the manufacturing of airplane. Usually, hole-making is the final machining process to join CFRP laminates with other components. However, CFRP laminates are considered as hard-to-machine materials, which results in some serious failures during hole-making process such as fiber pull-out, fiber break, and matrix delamination. To improve the hole-making quality, a tilted orbital grinding (TOG) method was proposed in this study. In the proposed TOG method, a grinding wheel was used as the machining tool and its axis was set as tilted against the axis of the hole with a small angle. Through replacing the revolving motion of the tool in conventional helical milling with conical pendulum motion of the grinding wheel, a hole could be made by a grinding process. The thrust forces, cutting temperatures, and the quality of the holes in the proposed TOG process were investigated by experimental methods. The results show that the thrust forces and cutting temperatures in TOG process are greatly less than those in the conventional orbital milling process. Scanning electron microscope (SEM) images of the hole that made by TOG method show that there is no crack generated in the entrance side of the hole, and the pull-out delamination in the exit side of the hole is significantly reduced. The experimental results indicate that the proposed TOG method has a potential application in the hole-making of CFRP laminates.

Kinematic view of cutting mechanism in hole-making process of longitude-torsional ultrasonic assisted helical milling

Compared with conventional hole-making process, rotary ultrasonic machining (RUM) is commonly believed to improve the cutting performance, such as low cutting force, good hole quality, long tool life, and high material removal rate. However, this work proposes an interesting observation on hole-making process using longitude-torsional ultrasonic assisted helical milling (LT UAHM). Due to the special kinematic motion in LT UAHM, the trajectory of the cutting tool has complex characteristics which may generate a negative effect on machining performance. In this study, a comparative experiment between LT UAHM and conventional helical milling (HM) was carried out. The results showed that the axial cutting force is not always reduced, the machined hole quality is also not improved consistently in LT UAHM. To comprehend the cutting mechanism of LT UAHM incisively, a kinematic trajectory model was proposed. By the theoretical kinematic trajectory model, LT UAHM can generate the ultrasonic elliptical cutting in the process, and the phase shift is a critical factor that can affect the elliptical trajectory shape even the vibration cutting direction of the tool’s cutting edge. The equivalent cutting angle and direction of friction between the rake face and chip in processing change accordingly. As a result, the magnitude of cutting force and machined hole quality in terms of burr height in this process will be significantly affected.

Experimental Investigation on the Tilt Helical Milling of Carbon Fiber Reinforced Plastics (CFRP)

Carbon fiber reinforced plastics (CFRP) has been widely used in various aircraft structural components. However, it is difficult for conventional methods such as drilling and helical milling to meet the requirements on high quality and efficient holes creation. Hence a so-called tilt helical milling (THM) method has been proposed. This new method is performed by replacing the revolving motion of the tool in conventional helical milling (CHM) with a conical pendulum motion, in which the tool axis is tilted towards the hole axis at a certain angle. As a step toward the establishment of the new method, in this work, the fundamental drilling characteristics of CFRP by the THM is elucidated by experimentally investigating the effects of tilt angle on thrust force and delamination factor. The obtained experimental results demonstrated that thrust force and delamination factor can be reduced with THM technique. In addition, THM can achieve better hole surface finish than CHM.

Proposal of a tilted helical milling technique for high-quality hole drilling of CFRP: analysis of hole surface finish

An innovative machining process, i.e., tilted helical milling (THM), was proposed previously by the authors for high-quality hole drilling of CFRP as an alternative technique to conventional helical milling (CHM). To further develop the THM technique, hole surface finish with THM is analyzed and compared to that finished with CHM in this article. Theoretical analysis results show that a large amount of fiber bending and fiber-matrix debinding in CHM will occur in the hole surface when instantaneous fiber cutting angle α lies in the range of 90° to 180° due to the tool pushing. This phenomenon can be mitigated in THM owing to a downward cutting force component along the hole axis, which makes fibers fracture easily. The theoretical analysis results are confirmed by CT scans and SEM results of surface morphology of holes, indicating THM is useful for obtaining high-quality hole surface finish. In addition, compared with CHM, damage generation, such as entrance split and exit delamination of the hole, can be restrained with THM technique.

Mechanism for material removal in ultrasonic vibration helical milling of Ti 6Al 4V alloy

Abstract High quality hole-making technology in the aviation industry is urgently needed due to the application of difficult-to-cut materials, such as titanium alloy, composite materials and the stacks in aircraft fuselage skins. To improve the hole-making quality, an ultrasonic vibration helical milling (UVHM) technology was developed for machining of Ti 6Al 4V alloy, meanwhile, comparison experiments were conducted between UVHM and conventional helical milling (HM) processes. Material removal mechanism of UVHM was investigated by modeling of cutting trajectories and the analysis of tool-workpiece contact behavior for bottom and peripheral cutting edges. The actual vibration frequency in UVHM was also determined by a theoretical-experimental combined method. Due to the vibration in UVHM, the bottom cutting edges generate discontinuous contact with workpiece. Unit forces considering material removal were modeled and applied to analyze the axial force reduction. The axial cutting forces of UVHM were reduced by 38–64% compared with HM at different cutting speeds. The cutting speed of peripheral cutting edge changes periodically. The cutting edges can separate with chips due to axial vibration, which will contribute to reducing the cutting forces and improving heat dissipation. Meanwhile, a friction effect was generated by the peripheral cutting edge which can improve the micro-scale surface roughness. Due to the effects of periodical friction and compression by ultrasonic vibration, UVHM increases the surface compressive stresses by 85% and 99% at the hole surface for axial and circumferential directions, respectively.

Proposal of a tilted helical milling technique for high quality hole drilling of CFRP: kinetic analysis of hole formation and material removal

Carbon fiber reinforced plastic (CFRP) is used for various mechanical structures because of its superior mechanical and physical properties. Industrial applications of CFRP require high quality, efficiently drilled holes. However, it is difficult for conventional methods, such as drilling, helical milling, and so on to meet industry requirements. Therefore, this study proposed a novel method for drilling holes in CFRP products. This new method is performed by replacing the revolving motion of the tool in conventional helical milling (CHM) with conical pendulum motion, in which the tool axis is tilted toward the hole axis at a certain angle, consequently called tilted helical milling (THM). As a step toward the establishment of the new method, in this work, the hole formation process, the cross-sectional area of work materials removed per tool revolution, and the zero cutting speed point problem in THM were theoretically compared with those in CHM. Then, experiments were performed to confirm the theoretical analysis in terms of the variations in the cross section profile of the drilled hole, the drilling forces, the hole quality at entrance/exit, and the chip removal. The obtained results revealed that during hole drilling by THM, a circumferential V-groove exists between the end face of the tool and the bottom of the hole, whereas in CHM, this phenomenon does not occur. This fact is beneficial to timely chip removal, drilling force reduction, and zero cutting speed point problem avoidance. Subsequently, high quality holes can be successfully obtained with high efficiency by THM.

Proposal of Tilt Helical Milling Method for Hole Creation of Carbon Fiber Reinforced Plastic (CFRP)

Carbon fiber reinforced plastic (CFRP) are used for various aircraft structural components because of their superior mechanical and physical properties such as high specific strength, high specific stiffness, etc. In order for CFRP materials to be used in aircraft structures or machine elements, high quality holes must be created on them efficiently. However, it is difficult for conventional methods such as drilling, helical milling and so on to meet the requirements. Therefore, this study proposes a novel method for creating holes on CFRP products. This method is just performed by tilting the workpiece at a certain angle on the basis of conventional helical milling (CHM) and hence called tilt helical milling (THM). In this paper, the processing principle of the THM is described in detail at first. An experimental apparatus is then constructed by installing a work holding unit produced in house onto a CNC milling machine. Finally, experiments are carried out on the apparatus. The obtained experimental results demonstrated that the THM is capable of creating holes on CFRP with lower burrs and chippings formation compared with those by CHM. By this new method, high quality holes could be obtained with high efficiency compared with conventional methods.

An Analytic Mechanistic Cutting Force Model for Milling Operations: A Case Study of Helical Milling Operation

The milling force theory developed (Bayoumi et al., 1994) is applied to conventional helical milling operations. Force relationships for this case were obtained. The process parameters were treated as variables, and a methodology to compute them as a function of rotation angle from the sampled force data was developed. Experiments were carried out to test and/or refine the accuracy of the developed force model. The effect of cutting conditions such as radial and axial engagements and cutting speed on the process parameters was investigated. Some cutting tests were carried out using multiedges cutters at different cutting conditions and different tool and workpiece materials to verify the mechanistic model. The predicted results compared well with the experimental measurements.