Exploring Advanced Manufacturing Technologies. Steve Krar
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