Exploring Advanced Manufacturing Technologies. Steve Krar
are revolutionizing the metal working industry. Conventional abrasives are ineffective, less productive, and uneconomical for use on these automatic computers controlled systems. Grinding wheels containing CBN abrasives last longer, provide more accurate parts, and require little or no reconditioning after initial truing and dressing. These wheels are more productive when grinding hardened steels, tool steels, and superalloys because of the properties of the CBN crystals. The CBN crystal is an extremely hard abrasive that is able to withstand the machining pressures and the heat of production grinding better than other abrasives. It has the toughness to match its hardness so that its cutting edges stay sharp longer and the crystal regenerates these edges to stay freer cutting over a longer period of time than conventional abrasives, Fig. 3-1-12.
Fig. 3-1-12 The advantages of grinding with CBN wheels. (GE Superabrasives)
POLYCRYSTALLINE TOOLS
Up until 1973, Diamond and CBN, in abrasive grain form, were the only available superabrasive products. With the development of polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) blanks and inserts, a new generation of superabrasive cutting tools became available. These tool blanks consist of a .025 in. (0.63mm) layer of diamond or CBN bonded to a cemented carbide substrate (base). Polycrystalline diamond tools are used to cut abrasive nonferrous and nonmetallic materials, while polycrystalline CBN tools are used to cut hard, ferrous metals, superalloys, and certain cast irons, Table 3-1-1. The cutting edges of polycrystalline cutting tool blanks and inserts are more wear resistant than the cutting edges of conventional tools such as cemented carbides, even when removing material at much higher rates. As a result, these tools need to be indexed or changed far less often than carbide or ceramic tools. This reduces the amount of downtime and as a result increases productivity. Properly designed polycrystalline tools have high impact resistance and therefore can be used to machine parts that require severe interrupted cuts or the removal of tough, abrasive forging scales.
MACHINE REQUIREMENTS
The performance of a superabrasive grinding wheel or polycrystalline cutting tool depends on the capabilities of the machine. Trying to use a superabrasive tool to make up for poor machine conditions will be doomed to failure right from the beginning. In order for superabrasive tools to work effectively, they should be used on machines that have the following characteristics:
▪Tight spindle bearings and snug machine slides to prevent vibration and chatter that could shorten the tool life, produce a poor surface finish, and inaccurate work, Fig. 3-1-13.
▪Consistent spindle speeds, to handle the torque required for high metal-removal rates, are necessary to keep superabrasive tools operating at best efficiency. Loss of spindle speed reduces the efficiency of the cutting action and shortens the life of the tool.
Fig. 3-1-13 A machine with a tight spindle and snug ways is necessary for machining and grinding with superabrasives. (GE Superabrasives)
GRINDING WHEELS
The first application of the superabrasives, diamond and cubic boron nitride, was in grinding wheels. Although the applications for diamond and cubic boron nitride grinding wheels are very different, these two superabrasives contain four main properties that cutting tools must have to cut extremely hard or abrasive materials at high metal removal rates.
Diamond grinding wheels are used to grind a variety of nonferrous and nonmetallic materials. In the metalworking industries, diamond wheels are widely used to regrind various types of tungsten carbide tools.
Cubic boron nitride (CBN) grinding wheels are recognized worldwide as superior cutting tools for grinding hard, abrasive ferrous-based metals. From their initial use in tool-room and cutter grinding applications, CBN wheels have made their presence felt in production grinding operations worldwide, where high technology CNC machines are revolutionizing the metalworking industry.
▪Grinding wheels containing CBN abrasives last longer, provide more accurate parts, and require little or no conditioning after initial truing and dressing.
▪These wheels are more productive when grinding hardened steels, tool steels, and superalloys, because of the properties of the CBN crystals.
▪Not only is the CBN crystal an extremely hard abrasive, it is able to withstand the machining pressures and the heat of production grinding better than other abrasives.
▪The CBN abrasive crystal has the toughness to match its hardness so that its cutting edges stay sharp longer, and the crystal resharpens itself to stay free cutting.
Superabrasive tools can be used effectively on conventional machines in good condition. Machine tool builders that have high speed and rigid spindles are now developing new machines specifically designed for superabrasives.
APPLICATIONS OF SUPERABRASIVES
Superabrasive tools are widely used in automotive, aerospace, and other manufacturing industries for turning and milling operations of hard, abrasive, and difficult-to-cut materials. Industry has found that these superabrasive cutting tools are among the most effective tools for production cost reduction and product improvement. In terms of the number of pieces per cutting edge, downtime and overall productivity, superabrasive tools have proven to be the most cost efficient tools available today.
General Applications Guidelines
To obtain the best tool performance and the most number of parts per cutting edge, the following guidelines should be closely followed.
▪Use PCD cutting tools for machining and grinding nonferrous and nonmetallic materials.
▪Select a rigid machine with good bearings and enough horsepower to maintain the cutting speed where PCD tools perform best.
▪Use a speed three times faster than for a cemented tungsten carbide tool.
▪Set speed and feed rates that give a good balance between productivity and long tool life.
▪Use rigid toolholders and keep the tool overhang as short as possible to avoid deflection, chatter, and vibration.
▪Use positive rake angles and the largest nose radius possible for better surface finishes and to spread the cutting force over a wider area.
▪Establish the life of each cutting edge or tool (usually after a certain number of pieces are cut) and change tools at the first sign of dullness.
▪Use coolant wherever possible to reduce heat, promote free cutting, and flush away the abrasive chips from the finished work surface.
POLYCRYSTALLINE SUPERABRASIVE TOOLS
With the development of polycrystalline diamond and polycrystalline cubic boron nitride (PCBN) tools in 1973, a new generation of cutting tools became available. Because of their excellent abrasion resistance and long wearing cutting edges, they greatly increase productivity while producing dimensionally accurate parts.
MANUFACTURE OF POLYCRYSTALLINE TOOLS
A layer of diamond or CBN crystals is deposited onto a substrate, usually tungsten carbide, Fig. 3-1-14. The assembly is placed into a high pressure, high temperature apparatus. It is subjected to a heat of 2200°F (1204°C) and pressure of nearly 1,000,000 pounds per square inch. The crystals form a very strong bond to each other and the substrate.
TYPES of PCD AND PCBN INSERTS
A variety of PCD and PCBN tool blanks and inserts are shown in Fig. 3-1-15.