Abstract: Metal-bonded diamond stone tools were introduced in the natural stone and civil engineering industries decades ago. Undoubtedly, their use has revolutionized material removal and increased machining productivity. However, despite the significant improvement in tool performance, the main factor limiting the widespread use of diamond tools is their high price. Therefore, it is particularly important for people to fully grasp the principle of turning and grinding performance. It is true that one can predict the best machining conditions, but knowledge of grinding during machining is the most fundamental condition for achieving optimal machining dimensions and improving diamond performance.
This article provides a brief overview of machining principles and associated tool and process design. Taking into account the stone and civil engineering industries, the development direction of some diamond tools has also been briefly described.
In the natural stone industry and related industries, diamond tools are widely used for cutting operations. One of the most widely used machining methods for diamond tools is called “sawing”, which is widely used not only in the field of stone but also in civil engineering. Therefore, rotary tools are used in the above industrial processing conditions. Diamond materials are also used in other processing in this field.
2. Cutting principle
The diamond cutting tip is shaped by machining the material with a diamond tool. This phenomenon occurs when cutting all materials. In cutting stone, there are some salient features, first: Although the term “sawing” is widely used, the beginning of the machining operation is the grinding process. The cutting edge is used for segmentation with a single geometric model to obtain two objects. The processing model is shown in Figure 1.
The factors that directly or indirectly affect the diamond cutting model are:
1. Physical material properties of stone, such as particle size and strength
2. The force between diamond and material
3. Tension distribution in stone
4. Tool temperature of workpiece contact surface
Deformation at the cutting edge due to elasticity of the plastic workpiece
Stone and diamond friction
Friction between stone and substrate
Friction between cutting chips and substrate
At the front of the diamond particles during machining, tension is caused by tangential forces. In this area, chips are produced by tension and pressure. This pattern is referred to as initial diamond cutting tip formation. Cutting debris escapes through the grooves in the front and the middle of the pellet.
During the cutting process, the stone exhibits stretchability to increase the maximum tension and, most importantly, to achieve the minimum cutting thickness. The material removed is deformed by the pressure induced by the lower part of the diamond. When the consolidated diamond particles are removed from the tool, elastic recovery occurs, resulting in critical stress, resulting in the fracture of the hard shape. The pattern that emerges from tensile stress is named second diamond chip tip formation. The result of the cutting process is a large amount of chips, which act as cooling for the machining process.
3. Stone cutting tools
In the processing of large blocks of natural stone, the processing of large blocks into modular profiles, such as ceramic tiles, is mainly done today with round diamond tools (see example 2). These diamond tools have two main parts: a steel core and fittings.
Tool design and appropriately related turned component dimensions are common sense and tooling, and the experience of the user and the end user needs to be carefully considered.
3.1 Steel core
The material of the steel core uses different steel grades, and it is necessary to consider brazing or laser welding the different parts to connect. The steel cores are relatively hard and have a fracture resistance of 43 to 45HRC (HRC: High-Rupturing Capacity, Rockwell hardness). Usually, the final forming of steel core parts requires laser cutting or milling. Holes need to be honed in the end, as a rounded appearance is obtained with minimal error. The most important point is that the fineness (minimum variation of plus or minus 0.01 mm) exceeds the overall geometry. The above technical requirements require the use of special grinding machines. In order to avoid tool chattering during cutting, the residual stress treatment of the tool is to heat or mechanically treat the center of the blade (tension adjustment). Parallel heads must be treated similarly to ensure that the blade will actually be used beyond the life of the tool.
3.2 Design of cutting part
The cutting part is welded on the steel core by brass welding or laser welding. The wear resistance of the matrix is to adjust the wear resistance of the diamond: if the grinding speed of the matrix is too fast, the grinding performance of the diamond cannot be fully exerted; if the grinding speed of the matrix is slower than that of the diamond, the space between the cutting edge and the matrix will continue to decrease. . If the above situation occurs, the chips will not be discharged in time, and the cutting part will continuously lose cutting performance. Therefore, the relatively soft substrate can be applied to the processing of stone with higher hardness, such as granite. In general, the soft matrix also holds good diamonds.
The development direction of the cutting part of diamond tools, one focuses on its role in the machining process, and the other focuses on its materials (diamond and metal powder) and their properties. The cutting section technology, including diamond, mainly uses hot isobaric treatment, and finally the so-called green prefabricated cast parts are pressed into shape. In recent years, a new technology has come out: the use of non-forging hot sintering technology to machine tools, this excellent new technology is called “cold pressing sintering”. The new technology has some outstanding advantages over traditional hot isobaric processing, including even an intermediate frequency hot pressing sintering process. Cold-pressed sintering can maximize the closed-cell porosity properties of metal powders. Another unique aspect of cold press sintering is the reduction in diamond capacity after cooling or (shrinking) during cutting, the diamond particles will penetrate deep into the cut section. As the density of the sintered part increases, the shrinkage of the diamond capacity will gradually decrease, so the grinding particles will be partially exposed, and the normal grinding action can be achieved without wearing the tool itself.
Looking at the latest technological developments in diamond particles and matrix materials, diamond tools will be coated with high-quality diamond particles using titanium or chromium, joined using low-cobalt wear-resistant bonding materials. Today’s diamond coating technology successfully achieves the grasp of the diamond material on the substrate.
In order to meet the ever-changing environmental and economic requirements, new metal powders in diamond bonds that are newly introduced to the market can partially replace cobalt. The properties of these new metal powders are close to pure cobalt. They are ultra-fine alloy powders and each grain consists of three base metals (copper, iron and cobalt). After sintering, the metal powder exhibits a single ultrafine microstructure. The most special aspect is that these metal powders are compatible, and when combined with diamond and coated diamond, the performance of diamond tools will be greatly improved.
4. Wear resistance of diamond tools
Influenced by the mineral composition in the stone, the wear resistance index of the diamond bond is determined by the composition of the cutting part and the processing parameters. For the technical applications discussed above, the wear resistance of the joint is on the one hand the wear resistance of the matrix and on the other hand the wear resistance of the diamond.
One kind of grinding processing situation: due to the high-level machinery and processing of stone processing, especially the stone grinding treatment under thermal stress load grinding conditions, the diamond particles will be affected. On the other hand, debris builds up in the abrasive and causes matrix bond wear to occur. Both machining methods have an impact on tool wear resistance and machining results.
Tool wear caused by chips depends on the composition of the stone. Unlike cutting tools, detrital particles consist of deformed and smaller grains of deep rock. During the cutting process, the fracture of the sedimentary rock is close to the bottom of the diamond particles, mainly because of the low strength of the tool bond material. Small pieces and hard stone chips appear after the cutting process. In this example, the wear-resisting effect of diamond particles is not significant compared with the high-grinding stone chips, but the wear rate of the diamond tool base body is significantly improved.
4.1 Diamond Grinding
The grinding mode affects the diamond particles in the following four forms:
1. Adhesive grinding: diamond contacts the stone surface and the particles are crushed;
2. Friction grinding: very hard stone particles scrape the diamond surface;
3. Diffusion grinding: a chemical reaction between the workpiece and the diamond surface will reduce the strength or hardness of the diamond;
4. The fracture of diamond particles and the fracture of diamond is due to mechanical, thermal overload and fatigue reasons.
The above investigation studies bonding and diffusion grinding are not the most significant in stone processing. The fracture and friction of diamond are mainly caused by grinding, which leads to the reduction of diamond particles. Under the condition of low speed upward cutting of homogeneous stone, the diamond particles will be ground into a flat shape. In the high-speed downward cutting method, a hard stone is processed, which will break the diamond due to the impact load.
The characteristics of diamond grinding under normal conditions are described by Schultz, demonstrating two different grinding states: smooth and grained.
Grind out the plane
cracked abrasive particles
Grinding diamond grains is due to the high thermal load on the tip of the grains, and the mechanical shock does not cause them to break, for example: the surface of the diamond grains is too small or the cutting speed is too low. Diamond particle fracture requires a certain diamond particle area or cutting speed to generate mechanical impact.
4.2 Matrix grinding
In addition to diamond particle wear, cooled grinding chips and stone particles can erode the bond. Considering the cut of the stone, the bonding material needs to be carefully selected. In addition, base grinding is divided into the following two cases:
If the matrix is ground too fast, its processing ability cannot be fully exerted before the diamond particles are exposed.
If the substrate is ground at a slower rate than diamond, the gap between the cutting edge and the substrate will be reduced. The fragments cannot be expelled better, and the composite subsequently loses the ability to cut.
Matrix grinding is typically characterized by a single crystal grain with an annular front. With the exception of the diamond particles, the matrix wears very little, so the bond is formed at the end, so the diamond is retained in the bond during processing.
4.3 Influence of machining parameters and motion
Material mobility is generally limited by the maximum thermal and mechanical loads that diamond can handle. The load that the diamond can withstand is affected by the machining parameters. As a machining workpiece material removal rate, the following machining environmental conditions need to be considered:
ae: a is the huge depth of cut, e is the area of the positive contact surface of the abrasive grains. Because of the great depth of contact, the thermal load on the diamond tip increases.
vft: v is the feed rate, and ft is the frontal area of the abrasive particles. This symbol indicates the diamond thermal load.
vc: v is the grinding speed, and c is the thermal load of the diamond particle tip. The diamond particle front contact surface becomes smaller and smaller, but the mechanical shock pulse becomes shorter and more pronounced when the abrasive tip is initially removed.
In addition to the machining parameters, there are also influences in the machining motion. The initial contact surface of the single crystal grains can reach the maximum grain tip thickness if the machining process adopts the up and down cutting mode (see example Fig. 3). Conversely, when the up-down mode is maintained, the diamond grain thickness increases to a maximum value during processing. In this condition, the tensile deformation of diamond particles mainly occurs in the initial stage, and the optimal thickness of the particle tip is reached and formed. The up-cut and down-cut modes result in very different grinding characteristics. During down-cutting, high mechanical shock pulses occur, taking into account the type of abrasive and bond as well as diamond breakage or exposure. The diamond tip thickness is increased from zero to maximum in the up-cut mode, initially only the diamond tip contacts the stone, and finally the stone is contacted with ground and flat diamond particles.
5. Machining design and optimization technology
In a series of different experiments, the diamond at the end of the cutting process could not achieve the best performance, or could not get the most satisfactory results. Mismatched tooling, turning operations, and process design in these experiments are factors that require statistical considerations, as well as systematic machining methods to consider, as well as all survey data related to the machining process. The measurement and analysis of material properties, machining forces, temperature, and vibration are first-hand information for improving tools and machine tools (see example Figure 4). In particular, the mechanical and thermal loads on the workpiece-to-workpiece interface require detailed consideration. These aspects not only depend on the parameters of the process but also take into account the occurrence of cutting and vibration of the same level of material during the process.
The performance of the tool can only be improved by conducting basic research on the weak points of successful implementation. These studies are most necessary to improve tools and machine tools. In order to gain knowledge of the grinding of diamond tools, one needs to investigate the changes in the matrix and diamond grinding. In order to obtain optimum machining parameters and tool component performance, machining personnel must have knowledge of the grinding process, including the many effects on diamond and bond during cutting. For these research studies, of course, also consider reducing the cost of purchasing expensive tools. In general, a satisfactory tool requires high vibration stability of the insert and optimum grinding resistance.
To achieve these goals, on the one hand, we need to improve the vibration characteristics of machine tools and tools with the help of modal analysis methods. For material results, these results can avoid increasing the vibration intensity at a strict cutting speed. On the other hand, the feed rate should be as high as possible to increase productivity. The grinding rate can be exploited to increase the dynamic stability of the insert and to change the thermal and mechanical loading of the diamond particles, but reduce the diamond particle frontal area and tangential force. To achieve such a suitable balance: machining force and grinding volume can only be achieved under different turning conditions.
In addition, lathe components such as lathe structure, spindle drive mechanism, beams and bearings, etc. all need to be checked and improved or meet the necessary requirements.
Summary and Outlook
For the cutting summary and outlook for natural stone diamond tool applications in this article, a complex system is described through a set of different influencing factors. Furthermore, if the above sections are to be explained, one needs to consider the basic machining process (with an emphasis on the grinding process) as well as all the associated features and associated methods needed to solve the problem in the cutting application. In most cases, just this systematic approach tries to find weak points as much as possible, rather than just looking at one aspect of the problem.