Mechanical Machining and Cutting Processes in Composites

Mechanical Processing Procedures for Composite Materials Composite materials offer users the beneficial integration of different material combinations and require less mechanical processing compared to standard materials. However, the widespread use of composite materials today necessitates mechanical processing procedures in composites. The mechanical processing of composite materials can be likened to the mechanical processing of metals, but this similarity should not be misleading. The mechanical processing of composite materials differs from that of ordinary materials due to the non-uniform distribution in composite material structures, high adhesion between layers, and their anisotropic nature. Therefore, when a composite material undergoes a mechanical processing procedure, the priority considerations are preventing damage to the composite workpiece and preventing machine tool wear. Because of the properties of composite materials, rapid wear can occur in the tools of the machine performing the mechanical processing, and due to the lattice structure of the composite material, there may be a reduction in processing quality. When appropriate processing conditions are provided, standard turning, milling, and drilling operations can be performed on composite materials.

Why Is Mechanical Processing Necessary?

1. In the production of composite parts, the manufacturing method of the composite material limits the creation of holes, channels, and similar features. 2. It is not possible to achieve surface tolerances specified in traditional mechanical processing operations through composite manufacturing methods. 3. It is difficult to achieve the surface smoothness and surface quality offered by mechanical processing through composite manufacturing methods. 4. In structural analysis tests, producing a prototype or sample through mechanical processing from a finished workpiece is more cost-effective than through another composite manufacturing process. Mechanical Processing Costs The percentage of composite material usage in different sectors continues to increase due to the advantages of composite materials in use. Looking at composite materials in the aerospace sector: in an aerospace application, assembly, subassembly, and labor costs constitute 50% of total cost. For example, a bomber aircraft has between 1,000,000 and 2,000,000 holes; therefore, mechanical processing procedures nearly compete with the main production cost [1]. Difficulties and Causes in Mechanical Processing of Composite Materials Deformation of cutting tools in turning, which is foremost among industrial mechanical processing operations, and surface roughness that may occur in the workpiece as a result of mechanical processing are problems that can be encountered in turning operations. Tool wear directly affects tool life and the surface quality of the processed part; surface roughness, in turn, affects wear resistance, fatigue behavior, lubrication properties, wear rate, and corrosion resistance of machine parts. Proper determination of processing parameters is a critical point for keeping the workpiece within the limits of desired tolerance and surface quality standards [2]. Based on this, the difficulties in mechanical processing of composite materials can be listed as follows [1]:

• If the mechanical processing procedure is not conducted under conditions suitable for the structure of composite parts, discontinuities may occur in the fiber structure.
• The mechanical processing procedure exposes the fiber structure to moisture and chemicals that have adverse effects.
• Due to the difference in thermal expansion coefficients between the matrix and fiber, achieving dimensional accuracy during the cutting operation of composites is difficult.
• Due to the abrasive properties of composite parts, tool life is generally shorter. To extend tool life, high-speed steel tools are coated with tungsten carbide or titanium nitride.
• Obtaining a smooth sharp edge in composites, and especially in aramid composites, is quite difficult because aramid fibers are hard and absorb cutting energy. For this reason, rough surfaces and fiber bending occur in aramid composite cutting.
• Delamination occurs at the cutting edges during the processing of composites; the critical points in the amount of delamination that occurs are the placement sequence and fiber orientations.

Cutting Tools and Processing Parameters

The fiber type and matrix content of the composite material are two critical points in determining cutting tools and processing parameters. In the processing of carbon fiber-reinforced plastic composites (CFRP) and glass fiber-reinforced polymer composites (GFRP), the use of carbide tools, coated carbide tools, and PCD (polycrystalline diamond) tools is advantageous in terms of preventing tool deformation and extending tool life. The material of the cutting tool in the mechanical processing of carbon and glass-reinforced composites is a parameter that affects tool selection [3].

Carbon Fiber-Reinforced Plastic Composites (CFRP)

When a hard carbide tool coated with TiAlN (titanium aluminum nitride) PVD (physical vapor deposition) is used as a cutting tool in the turning operation of carbon fiber-reinforced plastic composites (CFRP), research into the effects of cutting parameters such as workpiece rotation speed and feed rate on the surface roughness of the workpiece and wear of cutting tools yields the following results: an increase in the workpiece rotation speed resulted in a decrease in surface roughness values; an increase in feed rate, by contrast to an increase in rotation speed, caused deterioration of surface roughness. To improve surface quality in turning operations of CFRP composites, it is recommended to select a high workpiece rotation speed but a low feed rate [2].

Glass Fiber-Reinforced Polymer Composites (GFRP)

In researching the effects of cutting parameters on surface quality in the mechanical processing of glass fiber-reinforced polymer composites produced by the filament winding method, surface roughness was found to be at minimum levels at very low and very high speeds in the mechanical processing of GFRP pipes, while surface roughness increased at moderate cutting speeds [4]. Mechanical Processing Operations and Tips for Composites It is possible to divide the mechanical processing operations of composite materials into two categories: conventional and next-generation methods.

Conventional Methods

Conventional methods in the mechanical processing of composites can be examined in 4 sections: drilling, milling, grinding, and turning.

Conventional Methods: Drilling

Drilling, one of the conventional and common methods, is important for the joining of different parts and subassemblies [5]. The parameters on which thrust and torque during drilling depend are feed rate, feed speed, tool geometry, and tool coating [6]. To provide an example: in glass fiber-reinforced plastic composites, the effect of feed rate and drill point angle, the two most important criteria in drilling operations, on delamination, which is one of the major problems in the general mechanical processing of composite materials, has been determined as follows: high feed rates significantly trigger delamination of the composite material, and keeping the drill point angle at minimal levels yields better results [7]. Regarding another parameter, feed speed: when the drill enters the composite material during drilling, it can damage the structure of the composite material; the reason for this is that the feed speed is not well adjusted. To keep defects in the composite workpiece at minimum levels, it is necessary to adjust the optimum feed speed [8].

Conventional Methods: Milling

After drilling operations are performed on composite workpieces, a milling operation is needed to bring the holes to their final dimensions [9]. To obtain the necessary benefit from the milling operation, the selection of milling cutters is important. Next-generation diamond-coated milling tools can significantly extend tool life and, by minimizing heat generation during operation, ensure that the composite workpiece has a high-quality machined surface [10].

Conventional Methods: Grinding

Grinding, one of the final processing steps, is an operation performed after the initial mechanical processing of the composite workpiece is completed. Solutions exist to obtain better efficiency from the grinding operation and minimize thermal damage, and the basis of these solutions is increasing the thermal conductivity of the composite material. One solution is the placement of very thin metal wires alongside the reinforcement fibers of the composite material, which rapidly and efficiently transfers the heat generated during the grinding process away from the ground area. Another solution is strengthening the epoxy matrix of the composite material with nickel-coated graphite fibers to increase the material's thermal conductivity [11].

Conventional Methods: Turning

The turning operation, which has a very wide range in industrial applications, can be defined as one of the cutting processes. In turning composites, conventional mechanical processing is preferred because the reinforcement materials of composites have a brittle and fragile structure. In turning operations, various precautions are taken to prevent the composite workpiece from wearing down the tool; for example, turning of metal-matrix composites becomes more difficult due to the presence of reinforcement materials and causes rapid tool wear with their abrasive properties. As a solution to this, it is possible to make the turning operation easier by adding graphite particles with self-lubricating properties to the reinforcement materials of metal-matrix composites [12].

Next-Generation Methods

Next-generation methods used when conventional methods are unsuitable or insufficient can be examined in 4 sections: laser cutting, waterjet cutting, electroerosion machining, and ultrasonic machining. Next-Generation Methods: Laser Cutting With the increased use of composites, the varied applications of composites have led to the insufficiency of traditional cutting methods. Additionally, to provide examples from composite materials: the different density of carbon fibers and matrix material, the abrasive properties of reinforcement materials, and the difficulty of achieving high-quality cutting edges with conventional milling, the cost increase from wear of milling cutters, and damage and delamination on thin and fragile parts [13] have increased the need for laser cutting, a contactless and force-free system that does not require reprocessing after cutting and prevents tool wear. The high precision during laser cutting and the burr-free straight cutting edges following the cutting operation help achieve the desired surface quality. There are even newer-generation laser cutting machine models capable of providing vacuum fixtures. This also prevents the time loss required for fixing the workpiece [14]. Next-Generation Methods: Waterjet Cutting Waterjet cutting is one of the next-generation cutting methods for composite materials; with advanced machine technologies, the cutting of glass fiber, composite foam, and other composite materials becomes easier with each passing day. The supersonic erosion created by the water jet on the composite workpiece eliminates material-tool interaction, minimizing problems such as cracking, breaking, or delamination [15]. The supersonic erosion created by the water jet eliminates friction forces, preventing wear and delamination of the cut surface without requiring a secondary processing operation. Next-Generation Methods: Electroerosion Machining (EDM) Electroerosion (EDM - Electrical Discharge Machining) is one of the next-generation processing operations. In an electroerosion machine, chip removal is performed using controlled electric arcs between two electrodes, one of which is the cutting tool and the other is the workpiece to be machined [16]. Burr-free parts processing and lower surface roughness compared to other manufacturing methods are notable advantages due to the elimination of cutting force by electric arc chip removal. Next-Generation Methods: Ultrasonic Machining (USM) In Ultrasonic Machining (USM), a high-frequency (20-40 KHz) electrical signal is converted into vertical mechanical motion (20,000 times per second) to create vibration between the workpiece and tool. By passing a multiphase abrasive slurry between the part and processing tool, the particles in the abrasive liquid mud gain very high speed due to vibration and strike the workpiece, completing the chip removal operation with rotational motion [17]. With this method, negative effects such as stress, distortion, or heating are eliminated in the processing of composite parts. Acknowledgments I express my gratitude to my valued advisor Dr. Aytaç Gören, Associate Professor, and the Dokuz Eylül Üniversitesi Automatic Control and Robotics Laboratory for providing all kinds of resources and support during the writing and publication process.   Zeynep Pasinli Dokuz Eylül Üniversitesi Faculty of Engineering Department of Mechanical Engineering

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