There is a growing need for carbon-fiber-reinforced plastics (CFRP/CFRTP/GFRP) in the aircraft, aerospace, and automotive industries due to their high strength-to-weight ratio, high rigidity, and other features. Using these outstanding composites as machine components requires machining with the desired configuration, accuracy, and surface integrity. However, due to the composite structure of high-strength carbon fiber and the adhesive plastics, CFRP is difficult to machine without causing spalling or delamination, fluffing, fiber pullout, thermal degradation of the matrix resin, or other kinds of surface or subsurface damage. Rapid tool wear is also a serious problem that varies with the fiber orientation of the CFRP.

In order to avoid these problems, various innovative or careful approaches have been taken in drilling, trimming by milling, sawing, and grinding CFRP. Non-traditional machining techniques, including the use of abrasive waterjets, have been employed. In these techniques, the machining process, tool geometry, cooling system, and other machining parameters are optimized. In addition, the influence of surface integrity on the tensile and/or fatigue strength or on other mechanical properties of CFRP has also drawn interest.

In addition, regarded as a “machining process” in a broad sense, the press forming of continuous fiber reinforced thermoplastic (CFRTP) sheets is a promising technique used in the manufacture of structural components. In CFRTP forming, the effects that fiber layout naturally have on the deformation mechanisms must be understood, and temperature, pressure, speed, and stroke control should be optimized.

This special issue consists of twelve recent, high-quality research articles related to the machining of CFRP composite materials. These articles include one review and eleven technical papers on the topics of drilling, end milling, abrasive waterjet machining, and forming. The editors would like to express our deep appreciation to all the authors for their invaluable submissions and to the anonymous reviewers for their earnest efforts. Without these, this special issue could not have been published. We hope that further research on the machining of CFRP composites will make advances inspired by this special issue.

The characteristics of carbon-fiber-reinforced plastics (CFRP), which are being widely utilized in the aircraft industry as well as other fields, are reviewed, and challenges associated with their increasing application are discussed. The greatest feature of CFRP is that they can be tailor-made by arranging only the necessary amount of carbon fibers in the required directions. The material possesses unique characteristics, including heterogeneity, anisotropy, and a laminated structure, which must be taken into account in each stage of the design and manufacturing processes, including component design, molding, and machining. In particular, the machining stage requires a deeper understanding of the mechanisms involved, and it is hoped that further research and development will take place in this area.

This research was conducted to investigate the characteristics of electrodeposited diamond core drills when used to cut CFRP. An eccentric core drill was usedto improve cutting characteristics. First of all, the cutting characteristics of a normal core drill were investigated at a few different feed rates and compared with those of a diamond coated twist drill. The effect of air assistance on chip evacuation were also investigated. The cutting forces, surface roughness profile of the hole, and tool appearance were used for evaluation. At the same feed rate, more cutting force was necessary for the normal core drill than for the twist drill. When air was blown in, the cutting forces required by the core drill decreased drastically, but delamination was evident. When air was drawn out, the cutting forces of the normal core drill were almost the same as when there was no air assistance. On the other hand, when an eccentric core drill was used, the cutting force required was lower when air was drawn out than when it was blown in. Additionally, the surface quality of the hole when air was drawn out was greater than when it was blown in. When the eccentric core drill with slits was used while air was drawn out, the cutting forces, surface quality of the hole, and tool appearance were the same as when an eccentric core drill without slits was used. However, there was little core jamming. Therefore, the eccentric core drill with slits had the longest tool life.

A damage-free cutting method called the constant- load feeding method is proposed as a high-precision machining method in carbon-fiber-reinforced plastic (CFRP) cutting. The constant-load feeding method can minimize cutting defects such as burrs and scuffing during diamond-saw cutting. In addition, there was no apparent cutting damage to the cutting surface from drilling with a high-speed steel twist drill. The cutting resistance was kept at a constant value during the entire process. The constant-load feeding can self-regulate the optimal tool feed rate to realize damage-free machining of CFRP composites without any feedback control systems based on the cutting-force.

The drilling of carbon fiber-reinforced plastic (CFRP) has many important aspects, such as collecting the fine cutting chips. Serious problems relating to machining accuracy can arise when the fine cutting chips diffused into the air are deposited or mixed in the sliding surface and spindle unit of a machine tool. Moreover, the environmental aspects of fine cutting chips can seriously affect the health and safety of workers. Our group has developed a new hollow-type drilling device with a cyclone-type dust collection system, which aspirates and ejects fine cutting chips from a penetration hole in the central part of the drill shank to outside the cutting area. We produced the tools with drills both without a point angle for the counter-boring hole processing and with a point angle for general drilling. In this study the effect of the dust collection rate for cutting chip and the drilling hole shape accuracy are evaluated for the hollow-type drill with two different cutting edge shapes. The results demonstrate the possibility of suppressing the occurrence of fine cutting chips when the hollow-type drill edge is similar to the shape of general twist drill edges. The hole-finish surface properties were improved, and we obtained a higher dust collection rate.

Carbon Fiber Reinforced Plastics (CFRP) is well known as a difficult-to-cut material that has very strong physical and mechanical characteristics. Drilling technique of CFRP that is one of the most important cutting operations is currently carried out in the aviation and automotive industries, among others. Parts manufactured from CFRP have many precision holes used as rivet holes and for various purposes. There are typicaly many problems involved in the precision drilling processes of CFRP plate such as burrs, chippings and delaminations of composite materials, and the rapid wear of the drilling tools. In this research study, various twist drill bits, square end mills and ball noses end mills made of materials including cemented-carbide, TiAlN PVD-coated cemented carbide, Diamond-Like Carbon (DLC) coated cemented carbide and high-speed steel, are tested. CFRP drilling tests without coolant are carried out on vertical machining centers. It is found that the ball nose end mill is the most suitable for drilling CFRP composite materials.

In recent years, many composite materials have been used in industry. Among such materials, the demand for carbon fiber-reinforced plastic (CFRP) is increasing. Although CRFP is used in various fields such as the aerospace industry, automotive industry, and sports equipment because of its light weight and high strength, it has poor production efficiency. Thus, carbon fiber-reinforced thermoplastic (CFRTP), with characteristics similar to CFRP but higher in production efficiency, has attracted attention in areas such as the automotive industry. Because CFRTP is used as a structural element, it is usually drilled to allow connection to other parts. However, it is a difficult-to-cut material requiring the development of a high-accuracy, high-efficiency processing method. This study proposes high-feed-speed processing as a method that achieves high-quality drilling at low cost. The cutting temperature was estimated to verify the effect of the high-feed-speed processing method, and processing conditions that prevented delamination and burring were identified.

This paper describes the temperature variations observed in the drilling of carbon fiber-reinforced plastic (CFRP)/Al, CFRP/Ti, Al/CFRP, and Ti/CFRP stacks. An infrared radiation pyrometer equipped with an optical fiber was used to measure the temperature. The optical fiber, inserted into the oil hole of an internal-coolant carbide drill, registered the infrared rays radiating from the bottom surfaces of the drilled holes. In drilling the CFRP/Ti stack, the temperature was ∼95°C in the CFRP layer. As the drill progressed into the Ti layer, the temperature increased to a very high value of 745°C at the bore exit. In the Ti/CFRP stack, meanwhile, the initial temperature was ∼170°C and increased to 695°C at the Ti/CFRP interface. Severe thermal damage, including fiber/resin pullout and matrix degradation, was caused by the heat at the bottom surface of the drilled hole in the Ti/CFRP stack. Along the edge of the entry point in the CFRP, the CFRP matrix was degraded by the heat. In drilling the CFRP/Al stack, the temperature in the Al layer was 200°C; little thermal damage was observed.

Because carbon fiber-reinforced plastics (CFRP) is used for various parts, it requires cutting. However, CFRP is well known to be difficult to cut. In this study, two types of tools were used to trim CFRP. At first, a conventional shaped tool was used. The cutting forces on the CFRP were measured and the influence of the fiber orientation relative to the cutting direction was investigated. Next, a herringbone-shaped tool was used and compared with the conventional end-mill. Specifically, cutting forces, burr formation, and surface roughness were measured to characterize the effect of this tool position. The characteristics of a diamond-coated tool were also investigated. The effectiveness of the diamond-coated herringbone tool was clarified. The main results obtained are as follows: 1) Cutting forces change by changing the tool axis position of the herringbone tool; the tool axis position is an important cutting condition. 2) By choosing the appropriate tool axis position, no uncut carbon fiber remains on the cut surface of the CFRP with the herringbone tool. 3) The burr formed under down-cut milling is smaller than that of up-cut milling at the cut end of CFRP by using the herringbone tool. 4) Small debonding of the diamond coating occurs on the cutting edge, but the wear value is very small, and the shape of the cutting edge is maintained despite small debonding of the diamond coating. 5) Diamond particles on the cutting edge do not work as those for grinding do. Sharp cutting edges with large rake angles and relief angles can cut the carbon fibers cleanly.

This study presents a method for extending the life of tools in cutting of Carbon-fiber-reinforced plastics (CFRP). In the previous study, our research group found that the use of two layer tool, which has a wear resistance distribution due to the use of a combination of two different materials with different hardnesses, is effective for decreasing cutting force during machining of CFRP composites. In the two layer tool, a narrow region of the flank face close to the tool edge and the whole of the rake face were made of a material with a relatively high wear resistance, e.g., diamond or polycrystalline diamond (PCD). In contrast, the other region of the flank face was made of a material with a relatively low wear resistance, e.g., tungsten carbide (WC-Co). In this study, based on some experiments, the effect of the thickness of high wear resistance region on the reduction effects of cutting forces was investigated.

Side milling tests of CFRP (carbon fiber reinforced plastics) containing thermosetting resin are carried out by TiAlN/AlCrN-coated, H₂-free DLC (diamond-like carbon)-coated, and CVD diamond-coated carbide end mills without coolant. Two types of end mills having different helix angles of 30° and 60° are used. The film thickness and surface smoothness are varied for the DLC-coated end mills. The cutting characteristics are evaluated by tool wear and surface integrity (i.e., 3D profiles of the machined surface, generation of fluffing, delamination, and pull-out of the carbon fibers). The cutting force and tool flank temperature are also examined for the two types of CFRP composites and the helix angle of the end mill. “Inclination milling,” in which the end mill is tilted so that the resultant cutting force acts parallel to the work surface, is proposed as a novel technique to be used with a high-helix angle end mill. This unique approach enables the reduction of tool wear and improves the surface integrity of machined CFRP surfaces.

Carbon-fiber-reinforced thermoplastic (CFRTP) is viewed as a prospective material for high-cycle production of CFRP parts. This paper deals with a process whereby a preheated thermoplastic plain-woven carbon fiber fabric sheet is formed into a circular cup by a mechanical servo-press. The effects of press parameters, specifically the bottom dead center and slide speed in the forming of CFRTP cup, on the press load, pressure, internal temperature, shape accuracy, and internal structure have been investigated. A plain-woven carbon-fiber-reinforced PA6 thermoplastic sheet was used. The sheet consisted of four layers of woven 3K carbon and had a thickness of 1 mm. The sheet was heated to 320°C under a halogen heater so that it would be around the recommended temperature for forming 260°C after transfer to the mold. The sheet was pressed into a circular cup shape by a cold mold while the periphery was cramped by a heated holder so as not to cool the sheet before it was pulled into the mold cave. Die clearance was designed considering the thickness increase due to the fiber concentration during the forming. By increasing the slide stroke to the bottom dead center, the applied press load was increased and the internal structure was improved, showing no voids. By increasing the slide speed, the final press load was reduced and shape accuracy was improved through a good pressure distribution on the mold. Measurement of the surface temperature of the sheet during the forming revealed that it remained in the melting region of the resin in the case of fast slide speed, but dropped below the melting temperature in the case of low slide speed. This difference apparently led to spring-in or spring-back after the forming. The experimental results indicate that appropriate balance among press speed, bottom dead center, and sheet temperature is important in the high-cycle forming of CFRTP.

High temperature Fiber Metal Laminate – Titanium/Graphite (Ti/Gr) is an advanced material system, developed to meet the high temperature requirements in aerospace applications. High specific strength and stiffness of composite core along with its protection from aggressive environment by tough titanium alloy sheets qualify FMLs for a promising alternative material where metallic and composites overcome each other’s limitations. However, industrial employability of this three phase system is often limited by the machining challenges posed by the difference in material removal mechanisms of Titanium alloy, PIXA thermoplastic polyimide resin and graphite fibers. An experimental investigation was conducted to evaluate the machinability of 1 mm thick Ti/Gr laminate sheets through Abrasive Waterjet (AWJ) machining process in terms of kerf characteristics and material removal rate. The parametric influence of AWJ operating variables on machining performance was studied by systematically measuring operating variables (traverse speed and Abrasive flow rate) using fully crossed Design of experiment (DOE) scheme, and statistically analyzing using ANOVA (Analysis of variance) technique. Empirical models were developed to quantify these effects and predict the influence of process parameters on material removal rate, kerf taper, entry damage width and overcut in straight cutting of Ti/Gr sheets.

The distribution of dynamic stress in sheet glass, stress which is caused by a continuous step input from a cylindrical loader, was estimated by considering elastic wave propagation. In modeling the dynamic stress behavior, we used a two-dimensional dynamic stress model combining a plane stress model and the equations of motion. A finite-difference method was used in the numerical calculation. Under damped vibration mode conditions, the dynamic stress behavior in the sheet glass was investigated in both the depth (Z) and horizontal (X) directions. The stress component in the Z direction changed from tensile to compressive near the outside glass surface of the contact stress distribution. The stress component in the X direction changed from compressive to tensile in the Z direction under the glass surface at the center of the contact stress distribution. The overshoot of the dynamic stress in the Z direction was 1.8 times that of the steady stress during an elapsed time of less than 1 ns from the beginning of loading.

In this study, the tool performance of two types of binderless diamond tools – single-crystal diamond (SCD) and nano-polycrystalline diamond (NPD) – is investigated in the high-speed cutting of titanium alloy (Ti-6Al-4V) with a water-soluble coolant. The NPD tool allows for a larger cutting force than the SCD tool by dulling of the cutting edge, despite NPD being harder than SCD. This large cutting force and the very low thermal conductivity of NPD yield a high cutting temperature above 500°C, which promotes the adhesion of the workpiece to the tool face, thereby increasing tool wear. Based on the morphology of the tool edge without scratch marks and the elemental analysis by energy-dispersive X-ray spectroscopy (EDX) of both the flank face and the cutting chips, diffusion-dissolution wear is determined to be the dominant mechanism in the diamond tool. A thin TiC layer seems to be formed in the boundary between the diamond tool and the titanium alloy at high temperatures; this is removed by the cutting chips.

A non-sinusoidal periodic waveform, such as a triangle wave, is often used as the testing input signal in force excitation control systems. Limited by the bandwidth of the system, the output waveform is often distorted, and the dynamic tracking accuracy is reduced. As an alternative to a hardware upgrade, a novel method of improving the tracking precision of the non-sinusoidal periodic waveform in the force excitation control system is presented. A key technique of the method is the adjustment of the spectrum of the non-sinusoidal periodic waveform in advance according to the frequency response characteristic of the control system. The working principle of the frequency-box regulator, the formula derivation, and the operation steps are explained. Simulations and tests are conducted to confirm the effectiveness of the method. The results show that the novel method (frequency-box regulation) works well in the force excitation control system.

The accurate estimation of cutting time before beginning a cutting process is necessary to improve the productivity of machining. Commercial computer-aided machining (CAM) systems estimate the cutting time by dividing the tool path length by the designated feed rate in a numerical control (NC) program. However, the actual cutting time generally exceeds the estimated cutting time for curved surfaces because of the acceleration and deceleration of the NC machine tool. There are systems that estimate cutting time while considering acceleration and deceleration along the controlled axes, but these are applicable only to particular machine tools. In this study, a flexible system for the accurate estimation of cutting time, based on the control principle of a machine tool, is developed. Experiments to estimate cutting time are conducted for the machining of complex shapes using two different NC machine tools. The actual cutting time is compared with the cutting time estimated by the developed system and that by a commercial CAM system. The estimation error of the proposed system is only 7%, while that of the commercial CAM system is 51%.

Augmented-reality (AR) technology was implemented for vision-based microassembly operations. A computer aided design (CAD) model for a virtual microassembly system was generated and calibrated using data from a real system to simulate the same image features in a virtual environment. By employing static AR, a hidden feature of a mating hole in a rod was reconstructed in an image. In real-time operation, a dynamic AR system was implemented to handle the issue of difference in velocities of a moving object between the virtual environment and real system. By utilizing the image from the AR system, the performance in manual and automatic assembly was experimentally tested. The assembly time and failure rate in automation were compared with those obtained in a visual servo without utilizing AR. The advantages of employing AR for the peg-in-hole microassembly were identified.

The purpose of this study is to develop a new machine bed support mechanism for reducing the vibration generated during the high-speed tracking motion of numerical control machine tools. In order to achieve this, the frequency response and motion trajectory of a machine tool with the proposed machine bed, which has a sliding surface, are measured and compared with that of the conventional support. Based on the modal analysis of the machine tool structure, a mathematical model representing the influence of the machine bed characteristics on the vibration is also developed. The model consists of a bed, saddle, table, column, and spindle head. Every component has three degrees of freedom for each of the translational and rotational axes. In order to evaluate the characteristics of the machine bed, the mathematical model determines the stiffness and damping along the X-, Y-, and Z-axis between the bed and the ground. The frequency response curves simulated by using the mathematical model are compared with that of the measured ones. From the results of the experiments and simulations, it is confirmed that the vibration generated during high-speed tracking motions can be reduced by using the proposed machine bed with a sliding surface.