processes Laser
processes Laser technology: Opportunities for cutting tool manufacturers written by: Philipp Esch, Thomas Götz and Andreas Gebhardt, authors from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA The manufacturing industry makes a significant contribution to the overall economic value added and is the key to the international competitiveness of industrialized countries. Of particular importance for the manufacturing industry are machining production processes that are characterized by precision, geometric variety and a high degree of automation. Reliable and efficient cutting tools can therefore be found in almost every manufacturing company. However, the sector of cutting tool manufacturers which is dominated by small and medium-sized enterprises (SMEs) is currently facing a multitude of challenges. On the customer side, the requirements regarding quality, productivity and tool performance are increasing. In addition, the industry is facing a profound change in response to digitalization as well as fundamental upheavals in the automotive industry due to the increasing hybridization and electrification of powertrains. A further challenge on the market side is the growing competition due to low-cost cutting tools from low-wage countries. Hence, innovation and a reduction of manufacturing costs, access to new technologies and manufacturing processes are required to maintain the technological leadership of cutting tool manufacturers, especially SMEs on the world market in the medium and long term. Conventional production chain of cutting tool manufacture The materials, procedures and process steps used in the cutting tool manufacturing process influence the quality of the cutting tools and their performance in machining [1] . In the following, the individual process steps in the production of cutting tools are illustrated by way of example, as shown in figure 1. Depending on the cutting tool design, individual process steps can be omitted or their sequence can be reversed. In a first process step, the blank is typically manufactured from powder metallurgical materials in a pressing and sintering process at high temperatures. To produce its final contour, the blank must then pass through vari ous process steps for post-processing and finishing. Usually, the tool geometry is created by machining processes with a geometrically undefined cutting edge, such as grinding using ultra-hard abrasives like diamond or boron carbides. In addition, ablative processes such as electrical discharge machining are used especially for ultra-hard materials. For the later use of a cutting tool, its cutting edge quality is of decisive importance. In order to achieve low process forces and high surface qualities, sharp cutting edges are increasingly required, which are mechanically rounded to increase tool life. Various edge preparation techniques such as grinding, blasting or brushing are available to Edge-preparation Grinding Tool blank Sintering Preparation Coating Post-treatment Cutting tool figure 1 Steps of the cutting tool manufacturing [Fraunhofer IPA] 28 no. 2, June 2020
processes produce a defined and reproducible cutting edge geometry and high cutting edge qualities without damaging the cutting material [1] . In the case of a subsequent coating, surface and edge zone preparation of the substrate material are carried out by means of mechanical process steps such as microblasting or brushing to achieve good coating adhesion properties [1] . The coating usually consists of a composite of a tough substrate and a hard material layer, which is applied by means of chemical (Chemical Vapour Deposition, CVD) or physical (Physical Vapour Deposition, PVD) coating processes [2] . The coating systems serve as protection against wear, oxidation, corrosion and heat and lead to an increase in productivity and tool life [1] . Laser technology as a complementary procedure to conventional grinding Cutting materials and their wear mechanisms affect both production and tool lifetime and hence the overall production costs. Therefore, wear-resistant, hard and tough cutting materials, preferably hard metals based on tungsten carbide, cermets based on titanium carbide or titanium nitride as well as diamond based cutting materials are increasingly used for the production of cutting tools to meet the requirements for cutting new high-performance materials [2] . While the machining of tool steels like cold and highspeed steel (HSS) proves to be unproblematic, the processing of extremely hard cutting materials is consid erably more complex. For sintered carbides, cutting processes with geometrically undefined cutting edges are essen tially used, such as grinding with diamond- and CBN grinding wheels [2] . On the other hand, ultra-hard high-performance cutting materials such as PCD, MCD and CBN are very difficult to grind due to their hardness and are subjected to high wheel wear. Hence, such cutting materials are generally machined using ablative techniques such as electro chemical machining and are subsequently finish-ground, if necessary. This makes the process very time-consuming and cost-intensive [3] . In addition to conventional grinding, laser technology is increasingly applied as a technical and economical complement for the production of cutting tools. The laser machining, as depicted in figure 2, allows cutting materials such as iron-based tool steel but also carbides and dia - mond-based ultra-hard cutting materials to be processed gently and almost athermal providing the highest quality in a short time. The laser beam generates tool cutting edges and geometries by vaporization of the material [4, 5] . This minimizes material heating in the processing area, even with materials that conduct heat well, such as carbide, and reduces the risk of damaging the edges of the substrate [6] . Cutting ceramics, which are brittle due to their hardness and therefore difficult to process, are also suitable for machining by laser technology. Here, short-pulse laser technology is used, which enables selective sublimation of the material without micro-crack formation and damage [7] . The laser also enables chip grooves or chip breakers to be inserted into the cutting tool and thus increase the tool life. The implementation of such fine three-dimensional structures and the high-precision positioning on the cutting tool itself cannot be achieved mechanically, e.g. by milling or grinding [8] . Laser beam Cutting nozzle Workpiece Slag Shield gas Direction of machining Material vapor, smoke figure 2 Schematic of laser machining [Trumpf] Cutting edge preparation Tool life and working behaviour are decisively determined by the micro geometry of the cutting edge. The impact of thermal and mechanical loads during cutting processes induce wear. Typical wear mechanisms in the cutting of metals are abrasive, adhesive, diffusive and tribochemical wear. The cutting of fibre-reinforced plastics is mainly dominated by abrasion. Especially in terms of abrasion the resulting form of wear is cutting edge rounding, flank or crater wear. Due to high mechanical load outbreaks or bursts are also likely. As a result of the production process by grinding technologies, the cutting edge topography is characterized by microscopic chipping. Although the cutting edge is macroscopically sharp, the microscale geometry is reigned by undefined roughness. This roughness induces force peaks and stress localization in the tooling material which can result in premature failure [6, 9] . The accumulation of this failure namely the successive erosion of hard particles in case of cemented carbides leads to strong and uncontrollable tool wear, often resulting in abrupt total failure of the entire cutting wedge [10] . Also, against earlier assumptions a cutting edge of infinite sharpness without a measurable edge radius is not suitable for most applications, especially for cutting metals, due to high mechanical load and cutting forces. The unstable edges are feasible for bursts and account for high initial wear [11, 12] . In order to control both wear behavior and edge stability, edge preparation techniques are applied. Common methods no. 2, June 2020 29
