Читать книгу Exploring Advanced Manufacturing Technologies - Steve Krar - Страница 20
Оглавление(Michael Flaman – Portland Community Colllege)
Over the past three to four decades, industry in the United States has been greatly affected by intense global competition from offshore industries that are using the latest technologies in their manufacturing methods. Even though the United States continues to lead in the development of new technologies, other countries research, test, and implement them far sooner. Improved productivity and quality result in a larger share of the world market. Products that were previously produced in the United States are now being produced offshore; this has reduced employment opportunities in this country. U.S. industry must take advantage of the benefits of new technology as quickly as possible in order to maintain its leadership in manufacturing.
The superabrasives, Diamond and Cubic Boron Nitride, possess properties unmatched by conventional grinding wheels and cutting tools for grinders, lathes, and machining centers. The hardness, abrasion resistance, compressive strength, and thermal conductivity of superabrasives makes them the logical choice for many difficult grinding, sawing, lapping, machining, drilling, wheel dressing, and wire drawing applications. Superabrasives can cut and grind the hardest materials known, making difficult material-removal applications routine operations. Superabrasive cutting tools are designed to meet the challenge of today by increasing productivity, producing better quality products, and reducing manufacturing costs.
BACKGROUND
In 1954, The General Electric Company (GE), after years of research, produced Man-Made® Diamond in the laboratory. Carbon and a catalyst, such as iron, chromium, cobalt, and nickel, were subjected to tremendous heat and pressure to form diamond crystals, Fig. 3-1-1. Because the temperature, pressure, and catalyst solvent can be varied, it is possible to produce diamond abrasive of various sizes, shapes, and crystal structure to suit a range of grinding applications on nonferrous and nonmetallic materials.
In 1969, GE introduced an entirely new material, BORAZON® cubic boron nitride (CBN). Cubic boron nitride is synthesized in crystal form from hexagonal boron nitride and a catalyst using the same high pressure, high temperature technology perfected to produce diamond, Fig. 3-1-2. CBN, second only to diamond in hardness, is used for the grinding of hard alloy steels and other difficult to grind ferrous materials.
MANUFACTURED DIAMOND
Diamond is used for truing and dressing grinding wheels and for the manufacture of diamond wheels. The need for a reliable source of diamond during World War II was realized when natural diamond was not readily available.
Fig. 3-1-1 A combination of high pressure and high temperature plus carbon and a catalyst are necessary for diamond growth. (GE Superabrasives)
Fig. 3-1-2 The process for manufacturing CBN and the crystals produced. (GE Superabrasives)
To produce diamond by a manufacturing process, the conditions of pressure and temperature found far below the earth’s surface had to be duplicated. This required a high-pressure, high-temperature belt apparatus capable of reproducing the conditions necessary for diamond growth. Graphite (a form of carbon) and a catalyst (such as iron, chromium, cobalt, and nickel) were subjected to high temperatures (2550° to 4260°F, or 1400° to 2350°C) and high pressures (800,000 to 1,900,000 lbs./sq. in. of 55,000 to 130,000 atmospheres) to form diamond crystals, Fig. 3-1-3. Under these conditions, the graphite is transformed into diamond and remains that way when it is cooled and the pressure is removed.
Types of Manufactured Diamond
There are many different types of manufactured diamond to suit various grinding applications. Manufactured diamond is available for grinding cemented carbides, carbide/steel combinations, nonferrous and nonmetallic materials, and many products such as natural stone, concrete, and masonry. The four main manufactured diamonds are:
▪TYPE RVG DIAMOND is an elongated, friable crystal with rough edges, Fig. 3-1-4A, and consists of thousands of tightly bonded small crystals that make up each abrasive grain. Type RVG (resin and vitrified) wheels are used to grind ultra-hard materials, such as tungsten carbide, and tough, abrasive nonmetallic and nonferrous materials.
▪TYPE CSG 11 DIAMOND, Fig. 3-1-4B, is designed to grind cemented carbide brazed tools where it may be necessary to grind both the carbide and some of the steel shank supporting the carbide insert.
▪TYPE MBG-11 DIAMOND, Fig. 3-1-4C, is used for grinding glass, ceramics, and carbides. The wheels with MBG (metal-bond grinding) abrasive have a metal bond to hold the tough diamond crystals in the wheel.
▪TYPE MBS DIAMOND - The Type MBS (metal-bond sawing) diamond, Fig. 3-1-4D, is used in metal-bond saws to cut granite, concrete, marble, and a variety of masonry and refractory materials.
Fig. 3-1-3 The high-pressure, high-temperature belt apparatus used for manufacturing diamonds. (GE Superabrasives)
Metal Coatings
The RVG diamond abrasive can be coated to prevent the diamond crystals from being pulled out from the resin bond. Coatings, such as nickel and copper provide better retention (holding power) for the RVG crystal in the wheel bond.
▪TYPE RVG-W (Resin, Vitrified, Grinding—Wet) is an RVG diamond with a special nickel coating that covers all surfaces of the crystal, providing a better holding or bonding surface for the resin bond, and results in much longer grinding wheel life.
Fig. 3-1-4A Type RVG diamond is used to grind hard, abrasive nonferrous materials. (GE Superabrasives)
Fig. 3-1-4B Type CSG-11 diamond is used to grind carbide and steel combinations. (GE Superabrasives)
Fig. 3-1-4C Type MBG-11 diamond is used to grind carbides, glass, and ceramics. (GE Superabrasives)
Fig. 3-1-4D Type MBS diamond is used primarily for grinding stone, marble, concrete, and masonry products. (GE Superabrasives)
▪TYPE RVG-D (Resin, Vitrified, Grinding—Dry) is an RVG diamond with a special copper coating that improves the bonding strength of the diamond in the wheel and controls its fracturing (tiny particles breaking away) under the stresses of grinding.
Under the pressure and temperatures created when grinding ferrous metals, diamond will react chemically and result in excessive diamond wear.
WORK MATERIALS
Diamond is used to machine and grind hard, abrasive nonferrous, nonmetallic, and composite materials. It is not recommended for grinding and machining ferrous materials because of the chemical characteristic known as carbon solubility potential, where steels will react with any source of carbon to absorb carbon into their surface. The reaction occurs under the temperature and pressure created during the grinding or machining process, thus causing excessive wear of the diamond-cutting tool, Fig. 3-1-5.
CUBIC BORON NITRIDE
A major breakthrough in the precision high-production grinding of hard, difficult-to-grind ferrous metals, was the discovery and manufacture of cubic boron nitride. CBN is twice as hard as aluminum oxide, and its performance on hardened steels is far superior. CBN is cool cutting, chemically resistant to inorganic salts and organic compounds, and can withstand grinding temperatures up to 1832°F (1000°C) before breaking down. Because of the cool-cutting action of CBN wheels, there is little or no thermal (heat) damage to the surface of the part being ground. The main benefits of grinding wheels made of CBN abrasive are shown in Fig. 3-1-6.
Manufacture
CBN is manufactured in crystal form from hexagonal boron nitride, sometimes referred to as white graphite. Hexagonal boron nitride, which is composed of boron and nitrogen atoms along with a solvent catalyst, is converted into cubic boron nitride through the application of heat (3000°F or 1650°C) and pressure (up to 1,000,000 lbs./sq. in., or 68,500 atmospheres). The combination of high temperature and high pressure causes each nitrogen atom to donate an electron to a boron atom, which uses it to form another chemical bond to the nitrogen atom. This produces a strong, hard, blocky, crystalline structure similar to diamond.
Fig. 3-1-5 Diamond tools react chemically, under the proper temperature and pressure conditions, when cutting ferrous metals. (GE Superabrasives)
Fig. 3-1-6 The main benefits of CBN grinding wheels. (GE Superabrasives)
CBN Types
There are various types of CBN available to suit a variety of steel grinding applications; CBN does not perform well on nonferrous or nonmetallic materials. Two main classes of CBN abrasive are monocrystalline and microcrystalline.
▪MONOCRYSTALLINE CBN - Monocrystalline CBN abrasive contains a large number of cleavage (break) planes along which a fracture can occur. This macrofracture (large break) is necessary for the abrasive grains to resharpen themselves when they become dull, Fig. 3-1-7.
▪MICROCRYSTALLINE CBN - Microcrystalline CBN abrasive consists of thousands of micron-size crystalline regions tightly bonded to each other to form a 100% dense particle. When the grains dull and the grinding pressure increases, they resharpen themselves by microfracturing (creating very small breaks), Fig. 3-1-8.
Table 3-1-1 lists the various types of abrasives and the workpiece materials for which each is best suited.
CHARACTERISTICS OF SUPERABRASIVES
The main physical properties of superabrasives that make them superior to conventional abrasives are shown in Fig. 3-1-9
▪Hardness - The harder the abrasive with respect to the workpiece, the more easily it can cut and remove material. The basic principle in material removal is that the cutting tool must be harder than the material being removed. Hardness of the cutting tool with respect to the material allows higher cutting speeds and greater feeds to decrease the amount of time required to complete the work cycle. Due to the higher hardness, superabrasive tools last longer.
Fig. 3-1-7 Monocrystalline CBN crystals macrofracture (large break) under high grinding forces and expose new sharp cutting edges. (GE Superabrasives)
Fig. 3-1-8 Microcrystalline CBN crystals microfracture (very small breaks) to resharpen a wheel and promote long wheel life. (GE Superabrasives)
Table 3-1-1 Abrasive-Workpiece profiles. (GE Superabrasives)
▪Diamond is four times harder than silicon carbide and is used for machining and grinding nonferrous and nonmetallic materials.
•Cubic boron nitride (CBN) is two and one half times harder than aluminum oxide and is used for machining and grinding ferrous materials.
▪Abrasion Resistance - Resistance to abrasive wear is a desirable property in a cutting tool, it increases the productivity by maintaining a sharp cutting edge longer. It also allows increased cutting speeds and feeds, decreasing the time required to complete the work cycle and lessen the time required to maintain the cutting tool.
•Diamond has three times the abrasive resistance of silicon carbide.
•CBN has about four times the abrasive resistance of aluminum oxide.
▪Compressive Strength - The physics of metal removal consists of high pressures created in the shear zone as a result of the materials resistance to rupture. Resistance to compressive pressure allows the material to fracture, thus producing a chip of material removed. Compressive strength of a material is a linear relationship to its density; the higher the density, the higher the compressive strength.
•Diamond is eighteen times greater than silicon carbide.
•CBN is about two and one half times greater than aluminum oxide.
Superabrasives can withstand forces of interrupted cuts and high material-removal rates.
▪Thermal Conductivity - The majority of the heat produced in a material-removal process takes place in the shear zone because of the plastic deformation of the material being cut. As the shear angle decreases, due to heat, the length of the shear plane increases, producing more heat. The remainder of the heat produced is a result of friction as the chip slides over the tool. High thermal conductivity of the cutting tool allows the heat to be dissipated quicker. This decreases the friction at the chip/tool interface increasing the shear angle and decreasing the length of the shear plane.
Fig. 3-1-9 The properties that make superabrasives super-hard, super wear-resistant cutting tools. (GE Superabrasives)
•Diamond has 27 times the thermal conductivity of silicon carbide.
•CBN has 55 times the thermal conductivity of aluminum oxide.
The superior qualities of diamond and CBN allow cutting tools to stay sharp longer and allow free cutting at high temperatures and cutting speeds. Diamond and CBN tools increase productivity while producing dimensionally accurate parts.
MACHINABILITY AND GRINDABILITY
The type of material and its physical, mechanical, and chemical properties have a major effect on its machinability. The mechanical properties of hardness, strength, toughness, and abrasion resistance are the main properties that determine the machinability and grindability of any given material. Naturally softer materials will have a higher machinability than would hard abrasive materials, making harder materials more expensive to machine, Table 3-1-2.
▪Hardness - The depth and distribution of hardness in a work material is generally the result of a heat treating operation, the alloy content, or a combination or both
▪Strength - Refers to the tensile strength; the higher the tensile strength, the more difficult the material is to cut
▪Toughness - Refers to the property of a material to absorb considerable energy before fracture and involves both ductility and strength
▪Abrasion resistance - Refers to the material content of alloys or particles, which can cause rapid tool, wear
Low machinability and grindability materials
The machining and grinding characteristics have an effect on tool life, the frequency of tool changes, and the cost of tools.
The types of metals generally having a low machinability rating are tool steels, hardened alloy steels, high-temperature alloys, shape memory alloys, and highly abrasive materials.
MATERIAL-REMOVAL RATES
In grinding operations, the material-removal rates are expressed as a grinding ratio (G ratio). It is calculated by dividing the volume of work material removed by the volume of wheel material used. The higher the grinding ratio number, the more efficient the grinding operation.
Table 3-1-2 The machining cost for material increases as the hardness increases. (TechSolve, Inc.)
The following examples in Fig. 3-1-10 show the differences in G ratio for various grinding wheels and work materials:
▪The grinding ratio for a CBN wheel is 229 times greater than aluminum oxide when grinding M-2 tool steel.
▪The grinding ratio for a CBN wheel is 150 times greater than aluminum oxide when grinding T-15 tool steel.
▪The grinding ratio for a CBN wheel is 217 times greater than aluminum oxide when grinding D-2 tool steel.
It is wise to try to match the cutting tool to the work material to get the best balance between productivity and tool life. Low material-removal rates extend the cutting tool life at the expense of productivity, while high material-removal rates increase productivity at the expense of tool life.
GRINDING WHEELS
The first application of superabrasives was in grinding wheels for sharpening milling cutters, Fig. 3-1-11. Although the applications for diamond and CBN grinding wheels are very different, these two superabrasives contain the four main properties that cutting tools must have to cut extremely hard or abrasive materials at high-material removal rates.
Fig. 3-1-10 The grinding ratio examples between aluminum oxide and CBN grinding wheels. (GE Superabrasives)
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 tungsten carbide tools. Diamond wheels are also important in the production of electronic components from materials such as silicon, germanium, and ferrites. Automotive, optical, and decorative glass finishing operations, and many non-metal space age materials such as silicon nitride, aluminum oxide, metal-matrix composites, PEAK, and silicon carbide are ground with diamond wheels.
Fig. 3-1-11 One of the first applications for superabrasive grinding wheels was in resharpening milling cutters. (GE Superabrasives)
Cubic Boron Nitride (CBN) grinding is often recognized worldwide as a superior cutting tool for grinding difficult to cut ferrous-based metals. From their initial use in toolroom grinding applications, CBN wheels have made their presence felt in production grinding operations worldwide, where high technology CNC machines 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.
Fig. 3-1-14 Polycrystalline tool blanks and inserts consist of a thin diamond or CBN layer bonded to a cemented carbide substrate. (GE Superabrasives)
Fig. 3-1-15 Polycrystalline tool blanks and inserts are available in a wide range of shapes and sizes. (GE Superabrasives)
▪Triangular, the most versatile tool shape, can be used to produce a wide range of shapes or forms on a workpiece
▪Square, a strong insert that provides good support for the cutting edge, but cannot produce sharp corners on the workpiece
▪Round, the strongest insert shape, provides more cutting edges than other shapes, but cannot produce corners on the workpiece
ADVANTAGES OF POLYCRYSTALLINE TOOLS
The advantages that PCBN tools offer industry more than offset their initial higher cost, Fig. 3-1-16.
▪Long Tool Life that consistently outperforms conventional tools from 10 to 700% and reduces tool wear, resulting in less machine downtime and dimensionally accurate workpieces.
Fig. 3-1-16 The main advantages of PCBN cutting tools. (Kelmar Associates)
▪High Material Removal Rates that allow higher cutting speeds to be used because tools can withstand the heat and excessive wear encountered.
▪Cuts Hard and Tough Materials including materials of Rc35 hardness and higher and, in some cases can replace grinding, which is a relatively slow material-removal process.
▪High Quality Products that are produced faster and at reduced costs, that reduces the need for frequent inspections.
▪Uniform Surface Finish, often in the single digit microinch range, because of reduced tool wear.
▪Lower Tool Cost per Piece because tools stay sharp longer and cut efficiently, producing longer production runs.
▪Reduced Machine Downtime that results in more machine time spent producing parts and less time spent changing and resetting cutting tools.
TYPES OF MATERIALS MACHINED WITH PCD (COMPAX®) TOOLS
Polycrystalline diamond (PCD) tools are used for turning and milling nonferrous or nonmetallic materials, especially where the workpiece is hard and abrasive. The largest group of nonferrous metals is generally soft, but can have hard particles in them, such as silicon suspended in aluminum or glass fibers in plastic. These hard abrasive particles destroy the cutting edge of conventional tools. PCD tools often have a wear life of 100 times more than cemented carbide tools in such an abrasive machining application.
The materials most successfully machined with PCD tools fall into three general categories: nonferrous metals, nonmetallic materials, and composites, Table 3-1-3.
TYPES OF MATERIALS MACHINED WITH PCBN (BORAZON®) TOOLS
Polycrystalline cubic boron nitride (PCBN) tools are used for turning and milling operations on abrasive, and difficult-to-cut (DTC) materials. PCBN tools can remove material at much higher rates than conventional cutting tools, with far longer tool life. Wherever PCBN cutting tools were used to replace a grinding operation, machining time was greatly reduced because of the higher metal-removal rate.
Table 3-1-3 The materials best suited for machining with PCD cutting tools. (GE Superabrasives)
The best applications for PCBN cutting tools are on materials where conventional cutting-tool edges of cemented carbides and ceramics are breaking down too quickly. Their long-lasting cutting edges are capable of transferring the accuracy of computer controlled machine tools and flexible manufacturing systems, thereby producing accurate parts, increasing productivity, and reducing expensive machine downtime. Table 3-1-4 lists some of the common metals that are machined efficiently with PCBN cutting tools.
Table 3-1-4 PCBN cutting tools are best for machining ferrous metals. (GE Superabrasives)
Compax® is a registered trademark of GE Superabrasives of Worthington, Ohio.
Borazonz® is a registered trademark of GE Superabrasives of Worthington, Ohio.
For more information on SUPERABRASIVE TECHNOLOGY see the Website: www.geplastics.com/superabrasives
