The present invention relates to the field of machining of materials by cutting (e.g., shaping parts by removing excess material in the form of chips), and more particularly machining of materials by cutting with cryogenically cooled oxide-containing ceramic cutting tools.
As used herein, the term “cutting” includes but is not limited to the following operations: turning, boring, parting, grooving, facing, planing, milling, drilling and other operations which generate continuous chips or fragmented or segmented chips. The term cutting does not include: grinding, electro-discharge machining, ultrasonic cutting, or high-pressure jet erosion cutting, i.e., operations generating very fine chips that are not well defined in shape, e.g., dust or powder.
The term “oxide-containing ceramic cutting tool,” as used herein, includes cutting tools (or cutting tips or cutting bits) made of oxide-containing ceramic materials and/or any other advanced tool materials containing at least 5% by weight of an oxide ceramic phase.
The “material removal rate,” a measure of machining productivity, is the volume of material removed by a tool per unit time and is defined by the machining parameters selected for the operation. In the case of turning, the most generic cutting operation, the material removal rate is the product of cutting speed, tool feed-rate, and depth of cut. The objective is to enable machining at a higher cutting speed, a higher feed-rate, a greater depth of cut, or at any combination of these parameters leading to an overall increase in material removal rate. Alternatively, the objective is to enhance the life of cutting tools in order to minimize the down-time spent for tool change-over and/or to reduce worn tooling costs. In certain machining operations, it is sometimes desired to increase cutting speed only while keeping material removal rate constant, or even reducing it, in order to produce an improved surface finish of a machined part or to reduce cutting force and/or part fixturing requirements. This can be accomplished by a corresponding reduction in feed-rate or depth of cut, or both. The undesired effect of such a manipulation with machining parameters is a significant increase in tool temperature leading to its premature wear and failure. The objective is to minimize this undesired effect.
Driven by economic factors, the machining industry is interested in achieving cost-reductions by:                increasing material removal rates without increases in worn tool and tool change-over costs, thereby increasing productivity;        increasing cutting speeds without increases in worn tool and tool change-over costs;        turning or milling hard parts which, in the past, could have been produced only via expensive grinding operations; and        using cleaner, safer, and more health-acceptable machining methods to eliminate numerous costs associated with conventional cutting fluids (e.g., emulsions) and clean-up operations.        
New, advanced cutting tool materials recently have been developed and commercialized to address these needs and improve the cutting performance of conventional tools made of high-speed steel (HSS) or tungsten carbide-cobalt (WC/Co). Compared to tools made of HSS and WC/Co, these new tools are significantly harder but also are much more brittle and sensitive to load stress and/or thermal stress shocking. Some of these advanced cutting materials, such as oxide ceramics and cermets, are capable of operating at relatively high temperatures. (Cermets are dense composite materials comprising both ceramic and metallic phases. As applied in the field of machining technologies, the term cermet includes carbide, nitride, boride, oxide and/or other more complex ceramic particles bonded or infiltrated with alloyed metals, but excludes the conventional WC/Co “hard metals.”) However, the wear behavior of oxide ceramics is less predictable than that of HSS, WC/Co or other advanced tool materials. After an initial, usually negligible, cratering, flank wear, and/or notching, the oxide ceramic tools usually fracture catastrophically within the cutting edge area or nose, resulting in machining down-time and, frequently, in a damaged work-piece surface.
Table 1 below compiles typical values of thermo-mechanical properties of some of the most popular cutting tool materials. Compared to carbide, nitride, and diamond-based cutting tools, the oxide ceramic-based tools show significantly lower values of a combined traverse rupture strength, fracture toughness, and thermal conductivity, while revealing a dangerously high thermal expansion coefficient. This makes the oxide ceramic tools prone to brittle fracture under mechanical load as well as cracking due to a localized thermal expansion in thermal gradient.
TABLE 1Thermo-mechanical Properties of Popular Cutting Tool MaterialsTraverseFractureThermalThermalrupturetoughnessexpansionconductivitystrength(K1C)coefficientat 20° C.Tool material(M Pa)M Pa m−1/2(ppm/° C.)(W/m° C.)Al2O3550489Al2O3—TiC8004.5816–21 Al2O3—1% ZrO27005.58.510SiAlON8006.532–20Si3N4100–8001.5–5.53.57–54SiC550–8604.64.557–77 Polycryst. CBN 800–11004.55100(PCBN)Polycryst. 390–15506–84560Diamond (PCD)WC/Co2000–340094–680–121(TiC—TaCaddit.)Data compiled from: “Ceramics and Glasses, Engineered Materials Handbook”, Vol.4, ASM Int., The Materials Information Soc., 1991, “Microstructural Effects in Precision Hard Turning”, Y. K. Chou and C. J, Evans, MED-Vol. 4, Mfg. Sci. and Engr., ASME 1996., and “Temperature and Wear of Cuffing Tools in High-speed Machining of Inconel 718 and Ti-6Al-6V-2Sn”, T. Kitagawa et al., Wear 202 (1997), Elsevier, pp. 142–148.
It is recognized that all conventional coolants and cutting fluids, including room-temperature water and an emulsified oil, as well as evaporative-cooling fogs or oil mists, can thermally shock and fracture oxide ceramics. The machining community is well aware of the need to avoid the use of these cutting fluids and coolants when machining with oxide ceramic cutting tools. Numerous publications, research papers, and tool manufacturers' recommendations warn machining operators about a drastic reduction of ceramic tool life on contact with conventional cutting fluids or even with a small residue of such fluids on workpiece surfaces. Despite numerous inherent deficiencies, e.g., overheated workpiece, reduced dimensional accuracy, and risk of chip fires, dry machining is recommended when ceramic cutting tools are used.
P. K. Mehrotra of Kennametal teaches in the “Applications of Ceramic Cutting Tools”, Key Engineering Materials, Vol. 138–140 (1998), Chapter 1, pp. 1–24 that: “the use of coolants is not recommended when these [ceramic] tools are used to machine steels due to their low thermal shock resistance”. R. Edwards states: “this ceramic [Al2O3-ZrO2 white ceramic] has a low thermal conductivity which makes it susceptible to thermal shock and so the use of coolant should be avoided”, “Cutting Tools”, The Institute of Materials, 1993, p. 20. According to D'errico, et al, “when a coolant is used, alumina, alumina/zirconia, and alumina/TiC tools, with one exception, tend to have poor performance probably due to limited thermal shock capability resulting from high thermal expansion coefficients”, “Performance of Ceramic Cutting Tools in Turning Operations”, Industrial Ceramics, Vol 17, #2, 1997. A 1995 ASM handbook adds: “water or oil coolants are not recommended for use with cold-pressed Al2O3-base ceramics because they may cause the insert to crack. If carbide tooling is used to machine a part run with coolant and a subsequent operation is planned using a cold-pressed oxide-base ceramic, the residual coolant should be blown away from the part”, ASM Specialty Handbook, “Tool Materials”, 1995, p. 73. R. C. Dewes and D. K. Aspinwall (“The Use of High Speed Machining for the Manufacture of Hardened Steel Dies”, Trans. of NAMRI/SME, Vol. XXIV, 1996, pp.21–26) tested a range of oxide and nitride tools including: 71% Al2O3-TiC (mixed alumina), 75% Al2O3-SiC (whisker reinforced alumina), 50% CBN-AlB2-AlN, 50%-TiC-WC-AlN-AlB2, 80% CBN-TiC-WC, as well as 95% CBN-Ni/Co. They found that the use of conventional cooling fluid applied by flooding or spraying resulted in the reduction of tool life by more than 95% except for the whisker reinforced alumina for which the life was shortened by about 88%.
The oxide ceramics have one thing in common with all of the other cutting tool materials—as their temperature increases, they soften, weaken, and build-up localized, internal stresses (due to thermal expansion frequently compounded with a limited conductivity) which ultimately leads to a limit in the cutting speed, material removal rate, and/or the hardness of workpieces machined. This common characteristic of tool materials is well described by E. M. Trent and P. K. Wright in “Metal Cutting”, 4th Ed., Butterworth, Boston, Oxford, 2000, and in the ASM Handbook on “Machining, Ceramic Materials”. 
Thus, a problem facing the machining industry is the inability to use conventional cooling methods with oxide ceramic cutting tools, i.e., the thermo-mechanical limitation on further increases in cutting speed, material removal rate, and/or the hardness of workpieces being machined.
Other problems facing the machining industry include significant environmental and health related problems associated with the conventional cutting fluids and coolants presently used in the industry. For example, carbon dioxide (CO2), a commonly used coolant, is a greenhouse generator. Also, since CO2 is denser than air it presents a potential asphyxiation concern. In addition, CO2 also has the potential to cause acid corrosion, since it is soluble in water. Freons and freon substitutes, some other commonly used coolants, also are greenhouse generators and ozone depleters. These substances also are explosive and/or toxic when heated on contact with red-hot solids. Other coolants which can be explosive include hydrocarbon gases and liquefied ammonia. Coolants such as cryogenic/liquefied air with oxygen in it can result in chip fires.
There exists a relatively large body of prior art patents pertaining to cryogenic cooling of cutting tools, including: U.S. Pat. No. 5,761,974 (Wang, et al.), U.S. Pat. No. 5,901,623 (Hong), U.S. Pat. No. 3,971,114 (Dudley), U.S. Pat. No. 5,103,701 (Lundin, et al.), U.S. Pat. No. 5,509,335 (Emerson), U.S. Pat. No. 4,829,859 (Yankoff), U.S. Pat. No. 5,592,863 (Joskowiak, et al.) and WO 99/60079 (Hong). However, neither these patents nor the other prior art references discussed herein solve the problems discussed above or satisfy the needs set forth below.
It is desired to have an apparatus and a method that enables machining operators to increase machining speeds and/or material removal rates without shortening the useful life of tools made of oxide-containing ceramic materials and/or any other advanced tool materials containing a significant fraction of oxide ceramic phase.
It is further desired to have an improved apparatus and a method for cooling and strengthening cutting tools made of materials revealing a tendency to wear and fail by brittle cracking so as to enable cutting at increased speed without reducing the useful life of cutting tools.
It is still further desired to have an apparatus and a method that increase material cutting speeds and/or productivity, which are limited by the lifetime (and cost) of cutting tools.
It is still further desired to have an apparatus and a method for machining materials and/or parts that cannot tolerate elevated temperatures generated on contact with the hot edge(s) of cutting tools.
It is still further desired to have an apparatus and a method for machining a workpiece which improves safety and environmental conditions at work places by minimizing the risks of chip fires, burns and/or chip vapor emissions while using an environmentally acceptable, safe, non-toxic and clean method of cooling cutting tools.
It also is desired to have an apparatus and a method for machining a workpiece which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.
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