Hard-carbon coated machine tool and cutting oil composition therefor

Abstract

A machine tool is used for machining a workpiece in the presence of a cutting oil composition. The machine tool includes a tool base and a hard carbon coating formed on the tool base. The hard carbon coating has 1 atomic % or less of hydrogen. The cutting oil composition contains a base oil and at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier.

Description

BACKGROUND OF THE INVENTION

[0001] The prevent invention relates to a hard-carbon coated machine tool and a cutting oil composition therefor.

[0002] It is desired that a machine tool (such as a drill or an end mill) be able to machine a workpiece with high precision, to reduce cutting resistance for improvement in machining efficiency and to maintain a high-precision high-efficiency machining capability over an extended time period. In order to respond to these desires, Japanese Laid-Open Patent Publication No. 2003-25117 and No. 2001-62605 propose forming a high-hardness, wear-resistant coating on the machine tool by chemical vapor deposition (CVD) or physical vapor deposition (PVD).

SUMMARY OF THE INVENTION

[0003] With the recent awareness of environmental issues, the use of a cutting oil in the machining process has become limited. In such a semi-dry machining process, however, the cutting point where the machine tool cuts a workpiece is not cooled sufficiently. As a result, there arises a problem that, in the case of the machine tool having a carbide base and a ceramic coating formed thereon, the machine tool is susceptible to adhesion with the workpiece. In addition, it is difficult to remove swarf from the cutting point. In the case of another diamond machine tool, there arises a problem that the machine tool is susceptible to chipping. The tool life of the machine tool is then shortened due to these problems.

[0004] It is therefore an object of the present invention to provide a hard-carbon coated machine tool and a cutting oil composition therefor, wherein the machine tool is, when lubricated with the cutting oil composition, capable of machining a workpiece with high accuracy and efficiency while attaining a long tool life even in a semi-dry machining process.

[0005] As a result of extensive researches, it was found by the present inventors that a hard-carbon coated machine tool shows excellent low-friction characteristics in the presence of a cutting oil composition that contains a specific ashless friction modifier or modifiers. The present invention has been accomplished based on the above finding.

[0006] According to a first aspect of the invention, there is provided a cutting oil composition for a hard-carbon coated machine tool, comprising: a base oil and at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier.

[0007] According to a second aspect of the invention, there is provided a machine tool for machining a workpiece in the presence of a cutting oil composition, the cutting oil composition containing at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier, the machine tool comprising: a tool base; and a hard carbon coating formed on the tool base, the hard carbon coating having 1 atomic % or less of hydrogen.

[0008] According to a third aspect of the invention, there is provided a machine tool unit, comprising: a machine tool having a tool base and a hard carbon coating formed on the tool base, the hard carbon coating having 1 atomic % or less of hydrogen; and a cutting oil composition to lubricate the machine tool, the cutting oil composition containing at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier.

[0009] The other objects and features of the invention will also become understood from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1A is an end view of a machine tool according to a first embodiment of the present invention.

[0011] FIG. 1B is a side view of the machine tool of FIG. 1A.

[0012] FIG. 2A is an end view of a machine tool according to a second embodiment of the present invention.

[0013] FIG. 2B is a side view of the machine tool of FIG. 2A.

[0014] FIG. 3A is an end view of a machine tool according to a third embodiment of the present invention.

[0015] FIG. 3B is a side view of the machine tool of FIG. 3A.

[0016] FIG. 4 is a schematic view showing a unit for friction test.

[0017] FIG. 5 is a graph showing a comparison of the tool life of a machine tool according to an exemplary embodiment of the present invention to that of a machine tool according to the earlier technology.

[0018] FIG. 6 is a graph showing a comparison of the machining accuracy and efficiency of a machine tool according to an exemplary embodiment of the present invention to those of a machine tool according to the earlier technology.

DESCRIPTION OF THE EMBODIMENTS

[0019] The present invention will be described below in detail. In the following description, all percentages (%) are by mass unless otherwise specified.

[0020] In the present invention, a hard-carbon coated machine tool is used for machining a workpiece under a condition that the machine tool and workpiece are lubricated with the cutting oil composition.

[0021] The hard-carbon coated machine tool of the present invention is not particularly restricted, and can be formed into a drill (such as a gun drill), a reamer or an end mill.

[0022] For example, the hard-carbon coated machine tool may be embodied as a drill 1, as shown in FIGS. 1A and 1B, and be used for creating a hole in the workpiece in the presence of the cutting oil composition. The drill 1 has a tool base made of a steel or carbide material and a hard carbon coating 3 formed on the tool base to cover the whole of the drill 1 including a cutting edge 2 (although it may seem as if the hard carbon coating 3 covers only the shaded areas in FIGS. 1A and 1B).

[0023] As shown in FIGS. 2A and 2B, the hard-carbon coated machine tool may be embodied as another type of drill (called a gun drill) 21 and be used for creating a deeper hole in the workpiece in the presence of the cutting oil composition. The gun drill 21 has a tool base made of a steel or carbide material and a hard carbon coating 23 formed on the tool base to cover a body portion 24 of the gun drill 21 including a cutting edge 22, a groove 26 and a core 27 (although it may seem as if the hard carbon coating 23 covers only the shaded areas in FIGS. 2A and 2B). An oil hole 25 is formed through the drill body portion 24 for supplying the cutting oil composition.

[0024] As shown in FIGS. 3A and 3B, the hard-carbon coated machine tool may be embodied as a reamer 31 and be used for enlarging, shaping, smoothing, or otherwise fining a hole in the workpiece in the presence of the cutting oil composition. The reamer 31 has a tool base made of a steel or carbide material and a hard carbon coating 33 formed on the tool base to cover a body portion 34 of the reamer 31 including a cutting edge 32 (although it may seem as if the hard carbon coating 33 covers only the shaded areas in FIGS. 3A and 3B).

[0025] The hard carbon coatings 3, 23 and 33 can be formed by various physical vapor deposition (PVD) processes, and each of the hard carbon coatings 3, 23 and 33 is preferably a diamond-like carbon (DLC) coating formed by arc ion plating. The DLC coating is a coating of amorphous carbon, such as hydrogen-free amorphous carbon (a-C), hydrogen-containing amorphous carbon (a-C:H) and metal-containing carbon or metal carbide (MeC) that contains metal elements of e.g. titanium (Ti) or molybdenum (Mo). In order to obtain a larger friction reducing effect, it is desirable to minimize the amount of hydrogen in the DLC coating. The amount of hydrogen in the DLC coating is preferably 1 atomic % or less, more preferably 0.5 atomic % or less, still more preferably substantially zero.

[0026] Further, the hard carbon coatings 3, 23 and 33 reflect the surface roughness of the tool bases, respectively. When the tool bases of the drill 1, the gun drill 2 and the reamer 3 have an arithmetic mean surface roughness Ra exceeding 0.3 &mgr;m, the hard carbon coatings 3, 23 and 33 become susceptible to cracking due to increased local contact of their surface roughness peaks with the workpieces. It is thus preferable to control the surface roughness Ra of the tool bases to be covered with the respective hard carbon coatings 3, 23 and 33 to 0.03 &mgr;m or lower.

[0027] The cutting oil composition of the present invention contains a base oil and at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier, and may be supplied in mist form to limit the amount of the cutting oil composition supplied effectively.

[0028] The base oil is not particularly limited, and can be selected from any base oil compounds commonly used for cutting oils, such as mineral oils, synthetic oils, and fats.

[0029] Specific examples of the mineral oils include normal paraffins and paraffin or naphthene oils each prepared by extracting cutting oil fractions from petroleum by atmospheric or reduced-pressure distillation, and then, purifying the obtained cutting oil fractions with at least one of the following treatments: solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, hydro-refining, wax isomerization, surfuric acid treatment and clay refining. Although a solvent-refined or hydro-refined mineral oil is often used as the base oil, it is more desirable to use a mineral oil prepared by Gas-To-Liquids (GTL) wax isomerization or by deep hydrocraking for reduction of an aromatics content in the oil.

[0030] Specific examples of the synthetic oils include: poly-&agr;-olefins, such as 1-octene oligomer, 1-decene oligomer and ethylene-propylene oligomer, and hydrides thereof; isobutene oligomer and a hydride thereof; isoparaffines; alkylbenzenes; alkylnaphthalenes; diesters, such as ditridecyl glutarate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate and dioctyl sebacate; polyol esters, such as trimethylolpropane esters (e.g. trimethylolpropane caprylate, trimetylolpropane pelargonate and trimethylolpropane isostearate) and pentaerythritol esters (e.g. pentaerythritol-2-ethyl hexanoate and pentaerythritol pelargonate); polyoxyalkylene glycols; dialkyl diphenyl ethers; and polyphenyl ethers. Among others, preferred are poly-&agr;-olefins, such as 1-octene oligomer and 1-decene oligomer, and hydrides thereof.

[0031] The above-mentioned base oil compounds may be used alone or in combination thereof. It the case of using as the base oil a mixture of two or more of the above base oil compounds, there is no particular limitation to the mixing ratio of the base oil compounds.

[0032] The sulfur content of the base oil is not particularly restricted, and is preferably 0.2% or less, more preferably 0.1% or less, still more preferably 0.05% or lower, based on the total mass of the base oil. It is desirable to use the hydro-refined mineral oil or the synthetic oil because the hydro-refined mineral oil and the synthetic oil have a sulfur content of not more than 0.005% or substantially zero (lower than a detection limit of e.g. 5 ppm).

[0033] The aromatics content of the base oil is also not particularly restricted. Herein, the aromatics content is defined as the amount of an aromatics fraction determined according to ASTM D2549. In order for the cutting oil composition to maintain its low-friction characteristics over time, the aromatics content of the base oil is preferably 15% or less, more preferably 10% or less, still more preferably 5% or less, based on the total mass of the base oil. The cutting oil composition undesirably deteriorates in oxidation stability when the aromatics content of the base oil exceeds 15%.

[0034] The kinematic viscosity of the base oil is not particularly restricted. The kinematic viscosity of the base oil is preferably 2 mm2/s or higher, more preferably 3 mm2/S, as measured at 100° C. At the same time, the kinematic viscosity of the base oil is preferably 20 mm2/s or lower, more preferably 10 mm2/s or lower, most preferably 8 mm2/s or lower, as measured at 100° C. When the kinematic viscosity of the base oil is less than 2 mm2/s at 100° C., there is a possibility that the cutting oil composition fails to provide sufficient wear resistance and causes a considerable evaporation loss. When the kinematic viscosity of the base oil exceeds 20 mm2/s at 100° C., there is a possibility that the cutting oil composition fails to exhibit low-friction characteristics and deteriorates in low-temperature performance. In the case of using two or more of the above-mentioned base oil compounds in combination, it is not necessary to limit the kinematic viscosity of each base oil compound to within such a specific range so long as the kinematic viscosity of the mixture of the base oil compounds at 100° C. is in the specified range.

[0035] The viscosity index of the base oil is not particularly restricted, and is preferably 80 or higher, more preferably 100 or higher, most preferably 120 or higher, in order for the cutting oil composition to attain improved oil-consumption performance and low-temperature viscosity characteristics.

[0036] As the fatty-ester friction modifier and the aliphatic-amine friction modifier, there may be used a fatty acid ester and an aliphatic amine having C6-C30 straight or branched hydrocarbon chains, preferably C8-C24 straight or branched hydrocarbon chains, more preferably C10-C20 straight or branched hydrocarbon chains, respectively. When the carbon number of the hydrocarbon chain of the friction modifier is not within the range of 6 to 30, there arises a possibility of failing to obtain a sufficient friction reducing effect. Specific examples of the C6-C30 straight or branched hydrocarbon chain include: alkyl groups, such as hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl and triacontyl; and alkenyl groups, such as hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl, icosenyl, heneicosenyl, docosenyl, tricosenyl, tetracosenyl, pentacosenyl, hexacosenyl, heptacosenyl, octacosenyl, nonacosenyl and triacontenyl. The above alkyl and alkenyl groups include all possible isomers.

[0037] The fatty acid ester is exemplified by esters of fatty acids having the above hydrocarbon groups and monofunctional aliphatic alcohols or aliphatic polyols. Specific examples of such fatty acid esters include glycerol monolate, glycerol diolate, sorbitan monolate and sorbitan diolate.

[0038] The aliphatic amine is exemplified by aliphatic monoamines and alkylene oxide adducts thereof, aliphatic polyamines, imidazolines and derivatives thereof each having the above hydrocarbon groups. Specific examples of such aliphatic amines include: aliphatic amine compounds, such as laurylamine, lauryldiethylamine, lauryldiethanolamine, dodecyldipropanolamine, palmitylamine, stearylamine, stearyltetraethylenepentamine, oleylamine, oleylpropylenediamine, oleyldiethanolamine and N-hydroxyethyloleylimidazolyne; alkylene oxide adducts of the above aliphatic amine compounds (C6-C28 alkyl or alkenyl amines), such as N,N-dipolyoxyalkylene-N-alkyl (or alkenyl) amines; and acid-modified compounds prepared by reacting the above aliphatic amine compounds with C2-C30 monocarboxylic acids (such as fatty acids) or C2-C30 polycarboxylic acids (such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid) so as to neutralize or amidate the whole or part of the remaining amino and/or imino groups. Above all, N,N-dipolyoxyethylene-N-oleylamine is preferably used.

[0039] The amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier contained in the cutting oil composition is not particularly restricted, and is preferably 0.05 to 3.0%, more preferably 0.1 to 2.0%, and most preferably 0.5 to 1.4%, based on the total mass of the cutting oil composition. When the amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier in the cutting oil composition is less than 0.05%, there arises a possibility of failing to obtain a sufficient friction reducing effect. When the amount of the fatty-ester friction modifier and/or the aliphatic-amine friction modifier in the cutting oil composition exceeds 3.0%, the solubility of the friction modifier or modifiers in the base oil becomes so low that the cutting oil composition deteriorates in storage stability to cause precipitations.

[0040] The cutting oil composition preferably includes polybutenyl succinimide and/or a derivative thereof as an ashless dispersant.

[0041] As the polybutenyl succinimide, there may be used compounds represented by the following general formulas (1) and (2). 1

[0042] In the formulas (1) and (2), PIB represents a polybutenyl group derived from polybutene having a number-average molecular weight of 900 to 3500, preferably 1000 to 2000, that can be prepared by polymerizing high-purity isobutene or a mixture of 1-butene and isobutene in the presence of a boron fluoride catalyst or aluminum chloride catalyst. When the number-average molecular weight of the polybutene is less than 900, there is a possibility of failing to obtain a sufficient detergent effect. When the number-average molecular weight of the polybutene exceeds 3500, the polybutenyl succinimide tends to deteriorate in low-temperature fluidity. The polybutene may be purified, before used for the production of the polybutenyl succinimide, by removing trace amounts of fluorine and chlorine residues resulting from the above polybutene production catalyst with any suitable treatment (such as an adsorption or washing process) in such a way as to control the amount of the fluorine and chlorine residues in the polybutene to 50 ppm or less, desirably 10 ppm or less, more desirably 1 ppm or less.

[0043] Further, n represents an integer of 1 to 5, preferably 2 to 4, in the formulas (1) and (2) in view of the detergent effect.

[0044] The production method of the polybutenyl succinimide is not particularly restricted. For example, the polybutenyl succinimide may be prepared by reacting a chlorinated product of the polybutene, or the polybutene from which fluorine and chlorine residues are sufficiently removed, with maleic anhydride at 100 to 200° C. to form polybutenyl succinate, and then, reacting the thus-formed polybutenyl succinate with polyamine (such as diethylene triamine, triethylene tetramine, tetraethylene pentamine or pentaethylene hexamine).

[0045] As the polybutenyl succinimide derivative, there may be used boron- or acid-modified compounds obtained by reacting the polybutenyl succinimides of the formula (1) or (2) with boron compounds or oxygen-containing organic compounds so as to neutralize or amidate the whole or part of the remaining amino and/or imide groups. Among others, boron-containing polybutenyl succinimides, especially boron-containing bis(polybutenyl)succinimide, are preferably used. The content ratio of nitrogen to boron (B/N) by mass in the boron-containing polybutenyl succinimide is usually 0.1 to 3, preferably 0.2 to 1.

[0046] The boron compound used for producing the polybutenyl succinimide derivative can be a boric acid, a borate or a boric acid ester. Specific examples of the boric acid include orthoboric acid, metaboric acid and tetraboric acid. Specific examples of the borate include: ammonium salts such as ammonium borates (e.g. ammonium metaborate, ammonium tetraborate, ammonium pentaborate and ammonium octaborate). Specific examples of the boric acid ester include: esters of boric acids and alkylalcohols (preferably C1-C6 alkylalcohols), such as monomethyl borate, dimethyl borate, trimethyl borate, monoethyl borate, diethyl borate, triethyl borate, monopropyl borate, dipropyl borate, tripropyl borate, monobutyl borate, dibutyl borate and tributyl borate.

[0047] The oxygen-containing organic compound used for producing the polybutenyl succinimide derivative can be any of C1-C30 monocarboxylic acids, such as formic acid, acetic acid, glycolic acid, propionic acid, lactic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, oleic acid, nonadecanoic acid and eicosanoic acid; C2-C30 polycarboxylic acids, such as oxalic acid, phthalic acid, trimellitic acid and pyromellitic acid, and anhydrides and esters thereof; C2-C6 alkylene oxides; and hydroxy(poly)oxyalkylene carbonates.

[0048] The amount of the polybutenyl succinimide and/or the derivative thereof added in the cutting oil composition is not particularly restricted, and is preferably 0.1 to 15%, more preferably 1.0 to 12%, based on the total mass of the cutting oil composition. When the amount of the polybutenyl succineimide and/or the derivative thereof in the cutting oil composition is less than 0.1%, there is a possibility of failing to attain a sufficient detergent effect. When the amount of the polybutenyl succineimide and/or the derivative thereof in the cutting oil composition exceeds 15%, the cutting oil composition may deteriorate in demulsification ability. In addition, there is a possibility of failing to obtain a detergent effect commensurate with the amount of the polybutenyl succineimide and/or the derivative thereof added.

[0049] Furthermore, the cutting oil composition preferably includes zinc dithiophosphate of the following general formula (3) as an antioxidant and as an anti-wear agent. 2

[0050] In the general formula (3), R4, R5, R6 and R7 each represent C1-C24 hydrocarbon groups. The C1-C24 hydrocarbon group is preferably a C1-C24 straight-chain or branched-chain alkyl group, a C3-C24 straight-chain or branched-chain alkenyl group, a C5-C13 cycloalkyl or straight- or branched-chain alkylcycloalkyl group, a C6-C18 aryl or straight- or branched-chain alkylaryl group, or a C7-C19 arylalkyl group. The above alkyl group or alkenyl group can be primary, secondary or tertiary. Specific examples of R4, R5, R6 and R7 include: alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl and tetracosyl; alkenyl groups, such as propenyl, isopropenyl, butenyl, butadienyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl (oleyl), nonadecenyl, icosenyl, heneicosenyl, docosenyl, tricosenyl and tetracosenyl; cycloalkyl groups, such as cyclopentyl, cyclohexyl and cycloheptyl; alkylcycloalkyl groups, such as methylcyclopentyl, dimethylcyclopentyl, ethylcyclopentyl, propylcyclopentyl, ethylmethylcyclopentyl, trimethylcyclopentyl, diethylcyclopentyl, ethyldimethylcyclopentyl, propylmethylcyclopentyl, propylethylcyclopentyl, di-propylcyclopentyl, propylethylmethylcyclopentyl, methylcyclohexyl, dimethylcyclohexyl, ethylcyclohexyl, propylcyclohexyl, ethylmethylcyclohexyl, trimethylcyclohexyl, diethylcyclohexyl, ethyldimethylcyclohexyl, propylmethylcyclohexyl, propylethylcyclohexyl, di-propylcyclohexyl, propylethylmethylcyclohexyl, methylcycloheptyl, dimethylcycloheptyl, ethylcycloheptyl, propylcycloheptyl, ethylmethylcycloheptyl, trimethylcycloheptyl, diethylcycloheptyl, ethyldimethylcycloheptyl, propylmethylcycloheptyl, propylethylcycloheptyl, di-propylcycloheptyl and propylethylmethylcycloheptyl; aryl groups, such as phenyl and naphthyl; alkylaryl groups, such as tolyl, xylyl, ethylphenyl, propylphenyl, ethylmethylphenyl, trimethylphenyl, butylphenyl, propylmethylphenyl, diethylphenyl, ethyldimethylphenyl, tetramethylphenyl, pentylphenyl, hexylphenyl, heptylphenyl, octylphenyl, nonylphenyl, decylphenyl, undecylphenyl and dodecylphenyl; and arylalkyl groups, such as benzyl, methylbenzyl, dimethylbenzyl, phenethyl, methylphenethyl and dimethylphenethyl. The above hydrocarbon groups include all possible isomers. Above all, preferred are C1-C18 straight- or branched-chain alkyl groups and C6-C18 aryl or straight- or branched-chain alkylaryl groups.

[0051] The zinc dithiophosphate is exemplified by zinc diisopropyldithiophosphate, zinc diisobutyldithiophosphate, zinc di-sec-butyldithiophosphate, zinc di-sec-pentyldithiophosphate, zinc di-n-hexyldithiophosphate, zinc di-sec-hexyldithiophosphate, zinc di-octyldithiophosphate, zinc di-2-ethylhexyldithiophosphate, zinc di-n-decyldithiophosphate, zinc di-n-dodecyldithiophosphate and zinc diisotridecyldithiophosphate.

[0052] The amount of the zinc dithiophosphate contained in the cutting oil composition is not particularly restricted. In order to obtain a larger friction reducing effect, the amount of the zinc dithiophosphate in the cutting oil composition is preferably 0.1% or less, more preferably 0.06% or less, still more preferably a minimum effective amount, in terms of the phosphorus element, based on the total mass of the cutting oil composition. When the amount of the zinc dithiophosphate in the cutting oil composition exceeds 0.1%, there is a possibility of inhibiting the effect of the ashless fatty-ester friction modifier and/or the ashless aliphatic-amine friction modifier.

[0053] The production method of the zinc dithiophosphate is not particularly restricted, and the zinc dithiophosphate can be prepared by any known method. For example, the zinc dithiophosphate may be prepared by reacting alcohols or phenols having the above R4, R5, R6 and R7 hydrocarbon groups with phosphorous pentasulfide to form dithiophosphoric acid, and then, neutralizing the thus-formed dithiophosphoric acid with zinc oxide. It is noted that the molecular structure of zinc dithiophosphate differs according to the alcohols or phenols used as raw materials for the zinc dithiophosphate production.

[0054] The above-mentioned zinc dithiophosphate compounds may be used alone or in the form of a mixture of two or more thereof. In the case of using two or more of the above zinc dithiophosphate compounds in combination, there is no particular limitation to the mixing ratio of the zinc dithiophosphate compounds.

[0055] The cutting oil composition may further include any other additive or additives, such as a metallic detergent, an antioxidant, a viscosity index improver, a friction modifier other than the above-mentioned fatty-ester friction modifier and the aliphatic-amine friction modifier, an ashless dispersant other than the above-mentioned polybutenyl succinimide and the derivative thereof, an anti-wear agent or extreme-pressure agent, a rust inhibitor, a nonionic surfactant, a demulsifier, a metal deactivator and/or an anti-foaming agent, so as to meet the performance required of the cutting oil composition.

[0056] The metallic detergent can be selected from any metallic detergent compounds commonly used for cutting oils. Specific examples of the metallic detergent include sulfonates, phenates and salicylates of alkali metals, such as sodium (Na) and potassium (K), or alkali-earth metals, such as calcium (Ca) and magnesium (Mg); and a mixture of two or more thereof. Among others, sodium and calcium sulfonates, sodium and calcium phenates, and sodium and calcium salicylates are suitably used. The total base number and amount of the metallic detergent can be determined in accordance with the performance required of the cutting oil composition. The total base number of the metallic detergent is usually 0 to 500 mgKOH/g, preferably 150 to 400 mgKOH/g, as measured by perchloric acid method according to ISO 3771. The amount of the metallic detergent is usually 0.1 to 10% based on the total mass of the cutting oil composition.

[0057] The antioxidant can be selected from any antioxidant compounds commonly used for cutting oils. Specific examples of the antioxidant include: phenolic antioxidants, such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; amino antioxidants, such as phenyl-&agr;-naphthylamine, alkylphenyl-&agr;-naphthylamine and alkyldiphenylamine; and a mixture of two or more thereof. The amount of the antioxidant is usually 0.01 to 5% based on the total mass of the cutting oil composition.

[0058] As the viscosity index improver, there may be used: non-dispersion type polymethacrylate viscosity index improvers, such as copolymers of one or more kinds of methacrylates and hydrogenated products thereof; dispersion type polymethacrylate viscosity index improvers, such as copolymers of metacrylates further containing nitrogen compounds; and other viscosity index improvers, such as copolymers of ethylene and &agr;-olefin (e.g. propylene, 1-butene or 1-pentene) and hydrogenated products thereof, polyisobutylenes and hydrogenated products thereof, styrene-diene hydrogenated copolymers, styrene-maleate anhydride copolymers, and polyalkylstyrenes. The molecular weight of the viscosity index improver needs to be determined in view of the shear stability. For example, the number-average molecular weight of the viscosity index improver is desirably in the range of 5000 to 1000000, more desirably 100000 to 800000, for the dispersion or non-dispersion type polymethacrylate; in the range of 800 to 5000 for the polyisobutylene or hydrogenated product thereof; and in the range of 800 to 300,000, more desirably 10,000 to 200,000 for the ethylene/&agr;-olefin copolymer or hydrogenated product thereof. The above viscosity index improving compounds can be used alone or in the form of a mixture of two or more thereof. The amount of the viscosity index improver is preferably 0.1 to 40.0% based on the total mass of the cutting oil composition.

[0059] The friction modifier other than the above-mentioned fatty-ester and aliphatic-amine friction modifiers can be any of ashless friction modifiers, such as boric acid esters, higher alcohols or aliphatic ethers, and metallic friction modifiers, such as molybdenum dithiophosphate, molybdenum dithiocarbamate and molybdenum disulfide.

[0060] The ashless dispersant other than the above-mentioned polybutenyl succinimide and derivative thereof can be any of polybutenylbenzylamines and polybutenylamines each having polybutenyl groups of which the number-average molecular weight is 900 to 3500, polybutenyl succinimides having polybutenyl groups of which the number-average molecular weight is less than 900, and derivatives thereof.

[0061] As the anti-friction agent or extreme-pressure agent, there may be used: disulfides, sulfurized fats, olefin sulfides, phosphate esters having one to three C2-C20 hydrocarbon groups, thiophosphate esters, phosphite esters, thiophosphite esters and amine salts of these esters.

[0062] As the rust inhibitor, there may be used: alkylbenzene sulfonates, dinonylnaphthalene sulfonates, esters of alkenylsuccinic acids and esters of polyalcohols.

[0063] As the nonionic surfactant and demulsifier, there may be used: noionic polyalkylene glycol surfactants, such as polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers and polyoxyethylene alkylnaphthylethers.

[0064] The metal deactivator can be any of imidazolines, pyrimidine derivatives, thiazole and benzotriazole.

[0065] The anti-foaming agent can be any of silicones, fluorosilicones and fluoroalkylethers.

[0066] Each of the friction modifier other than the fatty-ester and aliphatic-amine friction modifiers, the ashless dispersant other than the polybutenyl succinimide and derivative thereof, the anti-wear agent or extreme-pressure agent, the rust inhibitor and the demulsifier is usually contained in an amount of 0.01 to 5% based on the total mass of the cutting oil composition, the metal deactivator is usually contained in an amount of 0.005 to 1% based on the total mass of the cutting oil composition, and the anti-foaming agent is usually contained in an amount of 0.0005 to 1% based on the total mass of the cutting oil composition.

[0067] As described above, the hard-carbon coated machine tool shows a considerably low friction coefficient in combination with the cutting oil composition containing the ashless fatty-ester friction modifier and/or the ashless aliphatic-amine friction modifier so that the adhesion of the workpiece or swarf to the machine tool does not occur. This makes it possible to increase machining precision and efficiency and to avoid tool breakage. The machine tool is thus able to machine a workpiece with high precision and efficiency while attaining a longer tool life.

[0068] The present invention will be described in more detail by reference to the following examples. It should be however noted that the following examples are only illustrative and not intended to limit the invention thereto.

EXAMPLE 1

[0069] A test unit was set up with a sliding member 10 (as a test sample) and a counterpart member 11 as shown in FIG. 4. The sliding member 10 was prepared by cutting a semi-cylindrical base piece from S45C steel (compliant with JIS G4051), and then, forming a DLC coating on the base piece by PVD arc ion plating to cover a curved portion 10a of the sliding member 10. The sliding member 10 had a size of 8×10×40 mm, and the DLC coating had a hydrogen content of 0.5 atomic % or less, a Knoop hardness Hk of 2170 kg/mm2, a surface roughness Ry of 0.03 &mgr;m and a thickness of 0.5 &mgr;m. On the other hand, the counterpart member 11 was formed into a plate of ADC12 alloy (compliant with JIS H5302) having a size of 40×60×7 mm. The sliding member 10 and the counterpart member 11 were lubricated with a cutting oil composition A. The chemical makeup of the cutting oil composition A is shown in TABLE 1. In TABLE, the amount of each component in the cutting oil composition A is indicated with respect to the total mass of the cutting oil composition A.

[0070] The coefficient of friction of the sliding member 10 was measured by sliding the curved portion 10a of the sliding member 10 over the counterpart member 11 in such a manner as to cause a reciprocating motion of the sliding surface 10 in the direction of arrows Q and R within a range A of the counterpart member 11 while pressing the sliding member 10 against the counterpart member 11 under a load P. The test was conducted under the following test conditions. The test result is shown in TABLE 2.

[0071] [Test Conditions]

[0072] Test unit: Reciprocating type friction/wear tester

[0073] Sliding member: 8×10×40 mm (JIS S45C base with DLC coating)

[0074] Counterpart member: 40×60×7 mm (JIS ADC12 plate)

[0075] Reciprocating speed: 600 cpm (counts per minute)

[0076] Test temperature: 25° C.

[0077] Load (P) applied: 10 kgf

[0078] Measuring time: 60 min. after the test start.

COMPARATIVE EXAMPLE 1

[0079] The same test unit as used in Example 1 was set up, except that the sliding member 10 and the counterpart member 11 were lubricated with a cutting oil composition B. The chemical makeup of the cutting oil composition B is also indicated in TABLE 1. In TABLE 1, the amount of each component in the cutting oil composition B is indicated with respect to the total mass of the cutting oil composition B. The coefficient of friction of the sliding member 10 was measured under the same conditions as used in Example 1. The test result is shown in TABLE 2.

COMPARATIVE EXAMPLE 2

[0080] The same test unit as used in Comparative Example 1 was set up, except that the sliding member 10 was made of K10 carbide (compliant with ISO 513) with no DLC coating. The coefficient of friction of the sliding member 10 was measured under the same conditions as used in Example 1 and Comparative Example 1. The test result is shown in TABLE 2. 1 TABLE 1 Oil composition (mass %) A B Base oil 87 100 (mineral oil) Fatty-ester friction modifier 1.0 — (glycerol monolate) Aliphatic-amine friction modifier — — Ashless dispersant 5.0 — (polybutenyl succinimide) Other additives (including an 7.0 — antioxidant and a rust inhibitor)

[0081] 2 TABLE 2 Friction coefficient Example 1 0.05 Comparative Example 1 0.08 Comparative Example 2 0.11

[0082] As is apparent from TABLE 2, the sliding member 10 of Example 1 had a much lower friction coefficient than those of Comparative Examples 1 and 2.

EXAMPLE 2

[0083] The same type of drill as shown in FIG. 1 was produced by preparing a tool base of K10 carbide (compliant with ISO 513) and forming a DLC coating on the tool base. The DLC coating had a hydrogen content of 0.5 atomic % or less, a Knoop hardness Hk of 2170 kg/mm2, a surface roughness Ry of 0.03 &mgr;m and a thickness of 0.5 &mgr;m. The thus-produced drill was set to a main shaft of a machining center, thereby machining a workpiece while supplying the above cutting oil composition A in mist form. The machining conditions are indicated below. In the process of machining, the drill was tested for cutting resistance (i.e., a cutting force applied to the main shaft). The test result is shown in FIG. 5.

[0084] [Machining Conditions]

[0085] Workpiece: ADC 12/AC2A alloy (JIS H53 02/H5202)

[0086] Cutting speed: 213.52 m/min.

[0087] Shaft rotation speed: 10000 rpm

[0088] Feed rate: 0.2 mm/rev.

[0089] Oil mist discharge rate: 5 cc/hr.

COMPARATIVE EXAMPLE 3

[0090] The same drill as used in Example 2 was produced, except that no DLC coating was formed on the drill. The produced drill was set to a machining center, thereby machining a workpiece while supplying the above cutting oil composition B in mist form. The machining conditions were the same as in Example 2. In the process of machining, the drill was tested for cutting resistance. The test result is shown in FIG. 5.

[0091] As is apparent from FIG. 5, the drill of Example 2 had much lower cutting resistance than that of Comparative Example 3.

EXAMPLE 3

[0092] The same type of reamer as shown in FIG. 3 was produced by preparing a tool base of K10 carbide (compliant with ISO 513) and forming a DLC coating on the tool base. The DLC coating had a hydrogen content of 0.5 atomic % or less, a Knoop hardness Hk of 2170 kg/mm2, a surface roughness Ry of 0.03 &mgr;m and a thickness of 0.5 &mgr;m. The thus-produced reamer was set to a machining center, thereby finishing holes in a workpiece while supplying the cutting oil composition A in mist form. The machining conditions are indicated below. The finished holes were tested for surface roughness Ra. The test result is shown in FIG. 6. In FIG. 6, the degree of machining represents the number of holes finished by the reamer.

[0093] [Test Conditions]

[0094] Workpiece: ADC 12/AC2A alloy (JIS H5302/H5202)

[0095] Cutting speed: 339.12 m/min.

[0096] Rotation speed: 6000 rpm

[0097] Feed rate: 0.24 mm/rev.

[0098] Oil mist discharge rate: 5 cc/hr.

COMPARATIVE EXAMPLE 4

[0099] The same reamer as used in Example 3 was produced, except that no DLC coating was formed on the reamer. The produced reamer was set to a machining center, thereby finishing holes in a workpiece while supplying the above cutting oil composition B in mist form. The machining conditions were the same as in Example 3. The finished holes were tested for surface roughness Ra. The test result is shown in FIG. 6.

[0100] As is apparent from FIG. 6, the surface roughness Ra of the holes finished by the reamer of Example 3 was much lower than that of Comparative Example 4.

[0101] It is thus proved by the test results of TABLE 2 and FIGS. 5 and 6 that the machine tool of the present invention has the advantages of not only a longer tool life but also higher machining precision and efficiency over the machine tool of the earlier technology.

[0102] The entire contents of Japanese Patent Application Nos. 2003-151855 (filed on May 29, 2003) and 2003-409856 (filed on Dec. 9, 2003) are herein incorporated by reference.

[0103] Although the present invention has been described with reference to a specific embodiment of the invention, the invention is not limited to the above-described embodiment. Various modifications and variations of the embodiment described above will occur to those skilled in the art in light of the above teaching. The scope of the invention is defined with reference to the following claims.

Claims

1. A cutting oil composition for a hard-carbon coated machine tool, comprising: a base oil and at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier.

2. A cutting oil composition according to claim 1, wherein said at lease one of the ashless fatty-ester friction modifier and the ashless aliphatic-amine friction modifier has a C6-C30 hydrocarbon group, and is contained in an amount of 0.05 to 3.0% by mass based on the total mass of the cutting oil composition.

3. A cutting oil composition according to claim 1, further comprising a polybutenyl succinimide and/or a derivative thereof.

4. A cutting oil composition according to claim 3, wherein the polybutenyl succinimide and/or the derivative thereof is contained in an amount of 0.1 to 15% by mass based on the total mass of the cutting oil composition.

5. A cutting oil composition according to claim 1, further comprising 0.1% or less by mass of zinc dithiophosphate in terms of phosphorus based on the total mass of the cutting oil composition.

6. A cutting oil composition according to claim 1, wherein the oil is supplied in mist form.

7. A machine tool for machining a workpiece in the presence of a cutting oil composition, the cutting oil composition containing at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier, the machine tool comprising:

a tool base; and

a hard carbon coating formed on the tool base, the hard carbon coating having 1 atomic % or less of hydrogen.

8. A machine tool according to claim 7, wherein the hard carbon coating has 0.5 atomic % or less of hydrogen.

9. A machine tool according to claim 7, wherein the hard carbon coating is formed by physical vapor deposition.

10. A machine tool according to claim 7, wherein the tool base has a surface roughness Ra of 0.03 &mgr;m or lower.

11. A machine tool unit, comprising:

a machine tool including a tool base and a hard carbon coating formed on the tool base, the hard carbon coating having 1 atomic % or less of hydrogen; and

a cutting oil composition to lubricate the machine tool, the cutting oil composition containing at least one of an ashless fatty-ester friction modifier and an ashless aliphatic-amine friction modifier.

12. A machine tool unit according to claim 11, wherein the hard carbon coating has 0.5 atomic % or less of hydrogen.

13. A machine tool unit according to claim 11, wherein the hard carbon coating is formed by physical vapor deposition.

14. A machine tool unit according to claim 11, wherein the tool base has a surface roughness Ra of 0.03 &mgr;m or lower.

15. A machine tool unit according to claim 11, wherein said at lease one of the ashless fatty-ester friction modifier and the ashless aliphatic-amine friction modifier has a C6-C30 hydrocarbon group, and is contained in an amount of 0.05 to 3.0% by mass based on the total mass of the cutting oil composition.

16. A machine tool unit according to claim 11, wherein the cutting oil composition further contains a polybutenyl succinimide and/or a derivative thereof.

17. A machine tool unit according to claim 16, wherein the polybutenyl succinimide and/or the derivative thereof is contained in an amount of 0.1 to 15% by mass based on the total mass of the cutting oil composition.

18. A machine tool unit according to claim 11, wherein the cutting oil composition further contains 0.1% or less by mass of zinc dithiophosphate in terms of phosphorus based on the total mass of the cutting oil composition.

19. A machine tool unit according to claim 11, wherein the cutting oil composition is supplied in mist form.