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

OBRABOTKAMETALLOV Vol. 20 No. 3 2018 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 3 2 21 used in the industry for welding low-carbon steel studs with a diameter of 2 to 12 mm to thin-walled ones to case-shaped parts [16]. The use of an additional nozzle [28] to a standard condenser welding machine gun provides a butt welded joint of the tool cylindrical case (shank) with a diamond part with the required accuracy. In order to increase the joint efficiency between the case and the diamond part, the authors [29] propose an intermediate case made of steel St.3, inserted into the diamond part during its manufacture. The authors managed to show that the use of the condenser welding method, due to the high energy density and pulse shortness, does not lead to softening of the main case made of heat-strengthened tool steel U8. However, as practice shows, the use of quenched U8 carbon tool steel as a case does not always ensure the stability of joint efficiency properties between the shank and the diamond part, obtained by the condenser welding method. It is generally known [27] that the welding properties of steel deteriorate with increase in carbon content. This is usually associated with the appearance of brittle structural components in the form of martensite and residual stresses in the welded joint. Therefore, it is of practical interest to study the possibility of using intermediate-carbon structural steel as the case of diamond grinding heads, which, after quenching and low tempering, as was shown above, have a sufficient ultimate bending strength. The work objective is to increase the efficiency of the end diamond abrasive tool on a metal bond by using carbon steels as the case material, to increase the joint efficiency between the case and the diamond- bearing part, as well to simplify the manufacturing technology of the tool. Research methodology The proposed way to increase the efficiency of a diamond end tool manufacture, as already noted above, is the use of the butt condenser welding with arc ignition and melting of the connected surfaces to connect the case made of quenched carbon steel with the diamond-bearing working part [27]. The condenser welding method advantages are: minimum heat-affected zone due to high energy density and pulse shortness (1...3 ms); joint efficiency; high performance and technology simplicity, and the possibility of automation [30]. In this case, the surface layer to be melted does not exceed 0.2 mm [27]. Accordingly the quenched steel shank in the process of welding to the working diamond tool layer will not lose its strength characteristics in practice, especially in the area of maximum bending stresses impact during the tool operation. The process of condenser welding of a cylindrical case to the diamond part of the tool is recommended to be carried out with a pulsed arc using guns equipped with a spring mechanism for pressing the surfaces to be welded. Commercially available welding machines, for example, provide a bonding of such studs with a diameter from 2 to 12 mm. Amandatory requirement is the presence at the base of one of the welded elements of an axial contact tab in the form of a cylinder with a diameter of 0.6 to 0.75 mm and a height of 0.55 to 0.75 mm [31]. This is provided for two main purposes: – allows to determine the exact place of welding of the element on the surface of the case by applying a preliminary center marking; – provides ignition of the welding arc and steady-state combustion over the entire surface of the element at implementation of the discharge of the capacitor. To ensure a high joint efficiency between the diamond grinding head case and the working part, in this work, a transitional steel insert 3 (Fig.1) is used in the form of a screw stud with a contact end of the RT type (GOST R 55738-2013 (ISO 13918:2008) and a height of no more than 2/3 of the head working part height. When manufacturing the working diamond part, such a stud is installed in the mold along its axis and fixed in this position with its contact end in the lower punch end hole of the press tool for cold pressing. Then the ready-made diamond powder blend is filled in a mold, and the blend is subjected to cold pressing at a pressure of 300...400 MPa. A finished diamond briquette with an insert in the form of a screw stud is removed from the mold and sintered in a vacuum or in a shielding atmosphere at a temperature of 700 ° С. The process of manufacturing a diamond-bearing working layer with an insert is completed by hot pressing (compaction) in a heat-resistant molding tool at a pressure of no more than 300 MPa and a temperature of no more than 500 ° С. Then, the finished tool is ejected from the mold. For the convenience of implement -