Improving the efficiency of metal-bonded diamond abrasive end tools by improving manufacturing technology

OBRABOTKAMETALLOV MATERIAL SCIENCE Том 20 № 3 2018 EQUIPMEN . INSTRUM TS Vol. 3 No. 2 2021 of high-tension, hard-to-machine and nanostructured materials fully meet these criteria [1–5]. High quality indicators of such products are formed, as a rule, at finishing operations of shaping, among which abrasive processing is emphasized [6–13]. However, due to high physical and mechanical properties of the listed structural materials, not all abrasive materials deal with this task [14, 15]. Positive results are generally observed when using high-strength abrasive materials (such as diamond or cubic boron nitride). In addition, the performance of such a tool directly depends on the abrasive tool bonding [16]. Organic bonds, which are often used for machining high-strength materials, provide good cutting ability of the tool, but at the same time increase the consumption of expensive high-strength abrasives [17]. Wheels on metal bonds have a significantly lower consumption when processing high-strength materials, but, at the same time, they are more prone to loss of cutting ability as a result of the “glazing” phenomenon [15, 18, 19]. Diamond wheels on metal bonds are most effective when implementing combined processing methods [14, 19–26]. In this case, both the diamond-bearing layer and the tool body must meet the requirements of current conductivity, thermal conductivity, and strength. This is especially relevant when using diamond end tools (grinding heads) as a tool. The operational characteristics of diamond grinding heads on metal bonds, in addition to the physical and mechanical properties of the diamond layer, are also determined by the case (shank) strength properties. The choice of diamond tools case material also depends on the bonding material [16] and the way the diamond element is attached to the tool shank. For example, diamond tools with organic bonds are made by press-fitting a diamond layer onto a case or by simultaneously pressing and sintering (polymerizing) a dia - mond powder blend and a case in dies. In this case, the polymerization temperature doesn’t exceed 200 ° С, and carbon tool steel with a hardness of up to 63 HRC can be used as the material for the diamond grinding heads case on organic bonds, in accordance with the recommendations of GOST 17122–85. The common alloys based on Cu-Sn and Cu-Al-Zn [16] are used as materials for metal bonds of dia- mond tools for processing products made of high-strength materials. The temperature of diamond elements sintering on such metal bonds is 600…700 ° С. When using matrix-filled materials based on copper as a bond, the temperature of diamond part sintering can reach up to 750 ° С [5]. Such sintering temperatures require, accordingly, the use of heat-resistant steel for the cases (shanks) of grinding heads manufacture when using standard technology, when the tool case is connected to the diamond part in the process of cold pressing, sintering and subsequent hot pressing in a heat-resistant metal mold. Therefore, in order to maintain high hardness and bending strength of the tool case during sintering and hot pressing, tool cases are recommended to be made, in accordance with GOST 17122-85, from high-speed tool steel (GOST 19265–73). Such steels are capable of maintaining high hardness and strength when heated up to 650 ° С, and the bending strength of high-speed steel σ bend reaches 3000 MPa. Carbon tool steel, for example U8, has an ultimate bending strength of up to 2000 MPa. Steel 45 when quenched and low tempered (48 HRC) has an ultimate bending strength of 1200 MPa [27]. The required bending strength of the grinding head case material, which ensures safe and reliable work- ing conditions for the diamond tool, is determined by the pressing force P during grinding. In accordance with the recommendations of GOST 17122–85, when grinding by cylindrical diamond heads of the AW type, the pressing force is 0.4 N per 1 mm of the contact-forming head, which, with a diamond part of heads maximum height of 20 mm, is 8 H. Such a pressing force leads a maximum bending stress equal to 15 MPa in a shank with a diameter of 6 mm and a range of 40 mm in the dangerous case section, which is many times lower, for example, the permissible stress [ s bend ] = 270 MPa for steel 45 after quenching and low tempering, with a hardness of 48 HRC. However, the use of non-heat-resistant tool steel or carbon structural steel as the case of metal-bonded diamond grinding heads requires additional heat treatment, quenching and low tempering of the finished tool made using a standard technology. Such a final operation in the technology of diamond grinding head manufacture is not always acceptable, due to the possibility of softening the diamond layer metal bond. The most promising method of bonding a quenched steel case in the form of a cylindrical shank with a grinding head diamond part, as shown by preliminary studies [28, 29], is the method of butt condenser welding, carried out with arc ignition and melting of the connected surface [27]. This method is widely