Patent Grant
9168637
The present invention relates to a vitrified grinding stone formed by bonding superabrasive grains, using a vitrified bond and in particular, relates to a technology of suppressing occurrence of deformation, lowered hardness, and residual stress of a work material caused by a grinding heat.
A vitrified superabrasive grain grinding stone, because of bonding of superabrasive grains by melting down an inorganic vitrified bond at calcination temperature in the order of, for example, 500 to 1000° C., can have a high abrasive grain holding power, namely, a high adhesive power between the superabrasive grains and the vitrified bond, as compared with the case of using an organic resin bond. For example, in the case of CBN abrasive grain, it is considered that, since B element, and K or Na element, etc., within a catalyst, added during a synthesis process thereof, are present on a surface thereof, these elements react with the vitrified bond and their chemical bonding power heightens the abrasive grain holding power.
Conventionally, out of steel-made work materials, a shaft component such as a camshaft and a crankshaft as a main component of an automobile engine is subjected to a high-precision grinding process for enhancement of performance of the engine but there have been problems of processing deformation, lowered hardness, and residual stress caused to the shaft component as the work material by a grinding heat generated at the time of grinding. As to a general countermeasure to eliminate the occurrence of these problems, proposals are made such as (a) using a clean-cutting grinding stone, (b) reducing an amount of cutting at grinding time, using a porous grinding stone, (c) lessening processing conditions by using a soft grinding stone of a low binding degree, (d) cooling by a sufficient supply of a coolant to a grinding point, and (e) using a grind stone with the CBN abrasive grain and a diamond abrasive grain mixed at various ratios. Such grinding stones are those described, for example, in Patent Document 1, Patent Document 2, and Patent Document 3.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2009-072835
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-300165
Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-158347
Patent Document 4: Japanese Laid-Open Patent Publication No. 2008-200780
The grinding stones proposed in Patent Documents 1 to 3 are all designed to be less prone to cause the grinding heat at the time of grinding and therefore, are effective against grinding burn. All of these proposals, however, are qualitative and many man-hours are required for building-in of optimum conditions enabling high quality and high efficiency every time product specifications and production efficiency, i.e., grinding efficiency, change. For this reason, there has been a problem that, when the product specifications and the production efficiency, i.e., grinding efficiency, change, a structural limitation occurs and this has a great effect on quality of the work material, starting with processing accuracy and life of the grinding stone. The grinding stone proposed in Patent Document 4 has no finding at all with respect to the residual stress of the material to be processed.
In contrast, the present applicant made a proposal of suppressing generation of the grinding heat, the deformation of the material to be processed, and the wear of a wheel and lengthening the life of the wheel by using the CBN abrasive grain as a main abrasive grain as well as using the diamond abrasive grain of high thermal conductivity as an auxiliary grain. This proposal is Japanese Patent Application No. 2011-070354 as a prior application not known to the public. This has left the problems of an increase in processing resistance and lowering of dressing performance unsettled.
The present invention is conceived in light of the above circumstances and the object thereof is to provide a vitrified superabrasive grain grinding stone capable of not only suppressing generation of grinding heat, deformation of material to be processed, and wear of a wheel but also obtaining lowered processing resistance and enhanced dressing performance.
As a result of various studies on suppressing of the grinding heat by heightening the thermal conductivity of the vitrified superabrasive grain grinding stone in the context of the above circumstances, the present inventors, etc., when focusing on the high thermal conductivity of a diamond grain conventionally considered to be unsuitable for the grinding of the steel-made work material and mixing the diamond grain, at a predetermined ratio, into the vitrified superabrasive grain grinding stone with the CBN abrasive grain used as its main abrasive grain, have found out the fact that generation of the grinding heat is decreased and the residual stress becomes smaller than before while high-precision and high-efficiency grinding performance is maintained. At the same time, it has been found out that when the toughness value of the diamond abrasive grain used as the auxiliary grain is set at 0.4 to 1 when that of the CBN abrasive grain as the main abrasive grain is given as 1, the diamond grain becomes the auxiliary grain having optimum destructibility despite high Knoop hardness and can preferably suppress the increase of the processing resistance and the lowering of the dressing performance. The present invention is conceived based on this finding.
Namely, the present invention provides (a) a vitrified superabrasive grain grinding stone with superabrasive grains comprising a CBN abrasive grain as a main abrasive grain and a diamond abrasive grain as an auxiliary abrasive grain bonded together by use of a vitrified bond, wherein (b) the auxiliary abrasive grain has an average grain diameter equal to ½ to 1/10 of that of the main abrasive grain, and wherein (c) the auxiliary abrasive grain has a toughness value of 0.4 to 1 when that of the main abrasive grain is given as 1.
According to the vitrified superabrasive grain grinding stone of the present invention, since the superabrasive grains comprise a CBN abrasive grain as a main abrasive grain and a diamond abrasive grain as an auxiliary abrasive grain, and the auxiliary abrasive grain has an average grain diameter equal to ½ to 1/10 of that of the main abrasive grain, abrasive grain dispersibility of the CBN is heightened by the average grain diameter of the auxiliary abrasive grain and at the same time, by a presence of the diamond abrasive grain having the thermal conductivity in the order of 2 times of that of CBN abrasive grain and in the order of 20 times of that of the alumina abrasive grain used as the filler, the grinding heat is efficiently absorbed and the residual stress of the work material is made smaller. Since the auxiliary abrasive grain has the toughness value of 0.4 to 1 when that of the main abrasive grain is given as 1 and has optimum destructibility despite high Knoop hardness, an increased processing resistance and a lowered dressing performance are suppressed, lengthening a durability life of the grinding wheel.
Preferably, a contact angle of the auxiliary abrasive grain with the vitrified bond is 90 to 150°. Consequently, since the auxiliary abrasive grain is held by the vitrified bond without being embedded in the vitrified bond, a heat absorption effect by the auxiliary abrasive grain is maintained and at the same time, a dropout of the auxiliary abrasive grain is preferably prevented. If the contact angle of the vitrified bond relative to the auxiliary abrasive grain is less than 90°, then the auxiliary abrasive grain is embedded in the vitrified bond and the heat absorption effect by the auxiliary abrasive grain is lowered. On the contrary, if the contact angle of the vitrified bond relative to the auxiliary abrasive grain is more than 150°, then a holding power of the auxiliary abrasive grain is lowered, resulting in many dropouts.
Preferably, the auxiliary abrasive grain is contained at a volume ratio of 3 to 13 volume %. Consequently, this makes it possible to preferably obtain the heat absorption effect due to the high thermal conductivity of a diamond used as an auxiliary abrasive grain and the effect of suppressing the increased processing resistance and the lowered dressing performance due to the optimum destructibility despite the high Knoop hardness of the auxiliary abrasive grain. If the volume ratio of the auxiliary abrasive grain is less than 3 vol. %, then the heat absorption effect, and the effect of suppressing the processing resistance and the lowered dressing performance, coming from the diamond become hard to obtain and if the volume ratio of the auxiliary abrasive grain is more than 13 vol. %, then the clean-cutting quality, the grinding processing accuracy, and the dressing performance are lowered.
Preferably, since the vitrified bond is contained at a volume ratio of 15 to 30 volume %, the effect coming from the presence of the diamond abrasive grain can be obtained. If the volume ratio of the vitrified bond is less than 15 vol. %, then the ratio of the diamond abrasive grain exposing itself on the surface of the vitrified bond becomes high, a domination rate of the diamond abrasive grain contributing to the grinding relatively becomes high. As a result, the clean-cutting quality and a grinding accuracy are lowered. On the contrary, if the volume ratio of the vitrified bond is more than 30 vol. %, then the diamond abrasive grain is embedded in the vitrified bond, a function by the diamond abrasive grain is lowered, and an effect coming from the presence thereof cannot be sufficiently obtained.
Preferably, the vitrified superabrasive grain grinding stone comprises: a core having a cylindrical outer peripheral surface; and a plurality of segment grinding stones attached on the outer peripheral surface of the core, and the segment grinding stones have the superabrasive grains bonded together by use of the vitrified bond at least in an outer peripheral side layer thereof. Accordingly, expensive superabrasive grains are arranged solely in an area involved in the grinding out of the vitrified superabrasive grain grinding stone and the inorganic filler such as the general abrasive grain can be used for other portion and therefore, the vitrified superabrasive grain grinding stone becomes inexpensive.
An embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiment, the drawings are simplified or deformed appropriately and are not necessarily accurately drawn in the dimensional ratio, the shape, etc. of portions.
Embodiment
The surface layer 30 functions solely as a grinding stone to grind a work material 104 to be described later and includes the CBN abrasive grain 34 functioning as a main abrasive grain, the diamond abrasive grain 36 functioning as an auxiliary abrasive grain or a filler, and a pore 38. The CBN abrasive grain 34 is a cubic boron nitride particle and has, for example, the Knoop hardness in the order of 4700 kg/mm2 and the toughness value in the order of 55, and the CBN abrasive grain 34 of the size within a range of, for example, 60 mesh (average particle diameter of 250 μm) to 3200 mesh (average particle diameter of 5 μm) is preferably used.
The diamond abrasive grain 36 has a diameter smaller than that of the CBN abrasive grain 34 and has the Knoop hardness higher than that of the CBN abrasive grain 34, the Knoop hardness in the order of, for example, 6000 kg/mm2 and the toughness value equal to or smaller than that of the CBN abrasive grain 34, the toughness value in the order of, for example, 33. The diamond abrasive grain 36, while functioning as an abrasive grain to a certain extent, functions as a thermal conductor of grinding heat as well as having a function of exposing itself on the grinding surface 20 to suppress grinding stone wear. To have this function efficiently generated, the diamond abrasive grain 36 has an average grain diameter equal to, for example, ½ to 1/10 of that of the CBN abrasive grain 34 and is mixed to have the volume ratio of, for example, 3 to 13 vol. %. Namely, in the surface layer 30, for example, the volume ratio of the CBN abrasive grain 34 is 30 to 40 vol. %, the volume ratio of the diamond abrasive grain 36 is 3 to 13 vol. %, the volume ratio of the vitrified bond 32 is 20 to 30 vol. %, and the volume ratio of the remaining pore 38 is 17 to 47 vol. %.
The vitrified bond 32 is preferably configured by, for example, borosilicate glass or crystallized glass. As for the crystallized glass, there is such one as is precipitated, for example, from willemite. Sufficient abrasive grain holding power is considered to be, preferably, ±2×10−6(1/K) (room temperature to 500° C.) with respect to the CBN abrasive grain 34. Glass composition desirable as the vitrified bond 32 is, for example, as follows. SiO2: 40 to 70 wt. part, Al2O3: 10-20 wt. part, B2O3: 10 to 20 wt. part, RO (alkali earth metal): 20 to 10 wt. part, R2O: 2 to 10 wt. part
In
The diamond abrasive grain 36 is used that has the toughness value of 0.4 to 1 when that of the CBN abrasive grain 34 is given as 1. When, after 0.4 g of a sample sieved by a sieve net specified by grain size (sieve having the highest remaining rate in ISO6106:2005) and one steel ball of 2.040 g are put in a cylindrical metal tube of 12.5 mm diameter and 19 mm length and are crushed at 2400 rpm and 8 mm amplitude for a crush time specified depending on grain size as shown in
Then, in a molding process P2, a molded body of the shape shown in
Then, in a pasting process P4, the vitrified grinding stone strip 26 is attached tightly on the cylindrical outer peripheral surface 24 of the pre-manufactured base metal 18, using, for example, epoxy resin adhesive agent, etc. Then, in a finishing process P5, a surface of the base metal 18 with the vitrified grinding stone strip 26 attached thereto, namely, the superabrasive grain grinding wheel 10, is adjusted in respect of the outer diameter dimension D, the roundness of the outer diameter dimension D, the thickness dimension, etc., of the superabrasive grain grinding wheel 10, using a dressing tool and a cutting tool. The vitrified grinding stone strip 26 is manufactured to have predetermined dimensions that are larger by the above grinding tolerances at the time of finishing the calcination process P3. By undergoing the above processes, the superabrasive grain grinding wheel 10 is manufactured in which the vitrified grinding stone strip 26 having the superabrasive grains bonded by the inorganic bonding agent is attached on the outer peripheral surface 24 of the base metal 18, as shown in
The vitrified grinding stone strip (vitrified superabrasive grain grinding stone) 26 contains the CBN abrasive grain 34 as a main abrasive grain and the diamond abrasive grain 36 as an auxiliary abrasive grain, and the diamond abrasive grain 36 has the toughness value of 0.4 to 1 when that of the CBN abrasive grain 34 is given as 1, has an average grain diameter equal to ½ to 1/10 of that of the CBN abrasive grain 34, and is included at the volume ratio of 3 to 13 vol. %. From this, since the diamond abrasive grain 36 as an auxiliary abrasive grain has the average grain diameter equal to ½ to 1/10 of that of the CBN abrasive grain 34 as a main abrasive grain, dispersibility of the CBN abrasive grain 34 is heightened by the average grain diameter of the diamond abrasive grain 36 and at the same time, by a presence of the diamond abrasive grain 36 having the thermal conductivity in the order of 2 times of that of CBN abrasive grain 34 and in the order of 20 times of that of the alumina abrasive grain used as the filler, the grinding heat is efficiently absorbed by the vitrified grinding stone strip 26. Since the diamond abrasive grain 36 has the toughness value of 0.4 to 1 when that of the CBN abrasive grain 34 is given as 1 and has optimum destructibility despite high Knoop hardness, an increased processing resistance and a lowered dressing performance are suppressed of the superabrasive grain grinding wheel 10, lengthening a durability life of the superabrasive grain grinding wheel 10.
According to the vitrified grinding stone strip 26 of this embodiment, since the diamond abrasive grain 36 as an auxiliary abrasive grain, because of a contact angle of 90 to 150° with vitrified bond 32, is held by the vitrified bond 32, without being embedded in the vitrified bond 32, a heat absorption effect by the diamond abrasive grain 36 is maintained and at the same time, a dropout of the diamond abrasive grain 36 is preferably prevented. If the melting-time contact angle of the vitrified bond 32 relative to the diamond abrasive grain 36 is less than 90°, then the diamond abrasive grain 36 is embedded in the vitrified bond 32 and the heat absorption effect by the diamond abrasive grain 36 is lowered. On the contrary, if the melting-time contact angle of the vitrified bond 32 relative to the diamond abrasive grain 36 is more than 150°, then a holding power of the diamond abrasive grain 36 is lowered, resulting in many dropouts and absorption of the grinding heat by the diamond abrasive grain 36 becomes insufficient. In either case, the heat absorption effect of the grinding heat by the diamond abrasive grain 36 is lowered and therefore, the effect of suppressing the processing resistance and the lowered dressing performance becomes hard to obtain and clean-cutting quality, a grinding processing accuracy, and the dressing performance are lowered.
According to the vitrified grinding stone strip 26 of this embodiment, the diamond abrasive grain 36 as an auxiliary abrasive grain is contained at the volume ratio of 3 to 13 vol. %. This makes it possible to preferably obtain the heat absorption effect due to the high thermal conductivity of the diamond abrasive grain 36 and the effect of suppressing the increased processing resistance and the lowered dressing performance due to the optimum destructibility despite the high Knoop hardness of the diamond abrasive grain 36. If the volume ratio of the diamond abrasive grain 36 is less than 3 vol. %, then the heat absorption effect, and the effect of suppressing the processing resistance and the lowered dressing performance, coming from the diamond become hard to obtain and if the volume ratio of the diamond abrasive grain 36 is more than 13 vol. %, then the clean-cutting quality, the grinding processing accuracy, and the dressing performance are lowered.
According to the vitrified grinding stone strip 26 of this embodiment, since the vitrified bond 32 is contained at the volume ratio of 15 to 30 vol. %, the effect coming from the presence of the diamond abrasive grain 36 can be obtained. If the volume ratio of the vitrified bond 32 is less than 15 vol. %, then the ratio of the diamond abrasive grain 36 exposing itself on the surface becomes high, the holding of the diamond abrasive grain 36 becomes unsteady, and the clean-cutting quality and grinding efficiency are lowered. On the contrary, if the volume ratio of the vitrified bond 32 is more than 30 vol. %, then the diamond abrasive grain 36 is embedded in the vitrified bond 32, a heat absorption function by the diamond abrasive grain 36 is lowered, and the effect coming from the presence thereof cannot be sufficiently obtained.
According to the superabrasive grain grinding wheel 10 of this embodiment, since the superabrasive grain grinding wheel 10 has the core, i.e., base metal 18, having the cylindrical outer peripheral surface 24 and plural pieces of the vitrified grinding stone strip 26 attached on the outer peripheral surface of the base metal 18 and at least the surface layer 30 out of the vitrified grinding stone strip 26 has the CBN abrasive grain 34 and the diamond abrasive grain 36 bonded together by use of the vitrified bond 32, expensive superabrasive grains are arranged solely in an area involved in the grinding out of the vitrified grinding stone strip 26 and the inorganic filler such as the general abrasive grain can be used for other portion and therefore, the superabrasive grain grinding wheel 10 becomes inexpensive.
Evaluation tests 1 to 6 will now be described that were performed by the present inventors for evaluation of grinding performance of the superabrasive grain grinding wheel 10.
[Grinding Performance Evaluation Test 1]
In this evaluation test 1, a vitrified grinding stone composed of a control product described below and a vitrified grinding stone composed of a product of the present invention were prepared, basically from the following materials and at the following ratio, using the process shown in
<Control Product>
<Product of Present Invention>
<Grinding Test Condition>
<Measurement Item>
Measuring device: X-ray stress measuring device (made by Rigaku Co., Ltd.)
Measuring location: cam lift portion
The residual stress (MPa) of a cam lift portion out of the cam surface of the work material was measured, using the X-ray stress measuring device AutoMATE made by Rigaku Co., Ltd., at a predetermined interval corresponding to an increase in the number of pieces processed.
Measuring device: surface roughness meter (Taylor Hobson-made)
Measuring location: carbon pattern-taking, cross-sectional step measurement
A step (μm) in the direction of the rotating shaft center corresponding to the depth of a concave formed by being in slide contact with the camshaft at the grinding surface of the grinding stone used for the grinding test was measured, using the surface shape roughness measuring device PGI1250A made by Taylor Hobson, at a predetermined interval corresponding to an increase in the number of pieces processed.
Measuring device: a power meter (made by Hioki E. E. Corporation)
Measuring location: grinding stone shaft motor
Power consumption (kW) of the grinding stone shaft drive motor of the grinding machine during grinding was measured, using the power meter made by Hioki E. E. Corporation, at a predetermined interval corresponding to an increase in the number of pieces processed.
Measuring device: contour shape measuring device (made by Mitutoyo Corporation)
Measuring location: dressing surface of a rotary dresser
The outer diameter of the rotary dresser before and after the dressing of the outer peripheral surface of the vitrified grinding stone was measured, using the contour shape measuring device CV-2000 made by Mitutoyo Corporation, to obtain the wear amount by the dressing and at the same time, the ratio of the wheel radius wear amount (step μm) to the wear amount by the dressing, namely, the dressing rate (%), was calculated for each grinding.
[Grinding Performance Evaluation Test 2]
In grinding performance evaluation test 2, under the conditions of same composition and vol. % as those of the vitrified grinding stone of the product of the present invention used in grinding performance evaluation test 1, 9 kinds of samples 1 to 9 were prepared by varying the ratio of the average grain diameter of the diamond abrasive grain to that of the CBN abrasive grain and the same grinding test as described above was performed, using these samples 1 to 9.
[Grinding Performance Evaluation Test 3]
In grinding performance evaluation test 3, samples 10 to 18 were prepared, with the same composition as that of the vitrified grinding stone of the product of the present invention used in grinding performance evaluation test 1, but with only the volume % of the diamond abrasive grain varied, and the same grinding test as described above was performed.
[Grinding Performance Evaluation Test 4]
In grinding performance evaluation test 4, samples 19 to 28 were prepared, with the same composition as that of the vitrified grinding stone of the product of the present invention used in grinding performance evaluation test 1, but with only the volume % of the vitrified bond varied, and the same grinding test as described above was performed.
[Grinding Performance Evaluation Test 5]
In grinding performance evaluation test 5, samples 29 to 36 were prepared, with the same composition as that of the vitrified grinding stone of the product of the present invention used in grinding performance evaluation test 1, but with only the toughness value of the diamond abrasive grain varied, and the same grinding test as described above was performed.
[Grinding Performance Evaluation Test 6]
In grinding performance evaluation test 6, samples 37 to 44 were prepared, with the same composition as that of the vitrified grinding stone of the product of the present invention used in grinding performance evaluation test 1, but with only the contact angle of the vitrified bond with respect to the diamond abrasive grain varied by the composition or the calcination temperature of the vitrified bond, and the same grinding test as described above was performed.
When the melting vitrified bond is considered as a liquid, the contact angle of the vitrified bond is an angle formed by a surface of the liquid and a wall surface of a solid in contact therewith. The contact angle of the vitrified bond is formed not only with respect to the diamond abrasive grain but it is similarly formed with respect to the CBN abrasive grain and the general abrasive grain used as the filler. This can be measured from a cross-section of an adhesion surface (sample) of the vitrified bond and the diamond, using a scanning electron microscope (SEM).
As shown in
While the above has described in detail one embodiment of the present invention with reference to the drawings, the present invention is not limited to this embodiment but can also be carried out in other modes.
For example, in the embodiment described above, the vitrified superabrasive grain grinding stone of the present invention was applied to the surface layer 30 of the vitrified grinding stone strip 26 but may be applied to a whole of the vitrified grinding stone strip 26 not having the base layer 28 or may be applied to a whole or a surface layer of a disc-shaped grinding stone, a cup-shaped grinding stone, a honing grinding stone, and a block-shaped grinding stone.
While, in the surface layer 30 of the vitrified grinding stone strip 26 of the embodiment described above, only the diamond abrasive grain 36 was used as an auxiliary abrasive grain, other abrasive grains or fillers may be added.
What was described above is merely one embodiment and, though this is not illustrated specifically, the present invention can be carried out in the modes in which various changes and improvements are added, based on the knowledge of those skilled in the art, without departing from the scope of the present invention.
10: superabrasive grain grinding wheel 18: base metal (core) 24: outer peripheral surface 26: vitrified grinding stone strip (segment grinding stones, vitrified superabrasive grain grinding stone) 30: surface layer 32: vitrified bond 34: CBN abrasive grain (superabrasive grain) 36: diamond abrasive grain (superabrasive grain) 38: pore
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-008512 | Jan 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/050995 | 1/18/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/108898 | 7/25/2013 | WO | A |
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| Number | Date | Country |
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| A-49-059387 | Jun 1974 | JP |
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| A-06-262527 | Sep 1994 | JP |
| A-2000-158347 | Jun 2000 | JP |
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| A-2002-178265 | Jun 2002 | JP |
| A-2003-025232 | Jan 2003 | JP |
| A-2003-300165 | Oct 2003 | JP |
| A-2008-200780 | Sep 2008 | JP |
| A-2009-072835 | Apr 2009 | JP |
| A-2012-200847 | Oct 2012 | JP |
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| Entry |
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| Feb. 19, 2013 International Search Report issued in International Patent Application No. PCT/JP2013/050995. |
| Number | Date | Country | |
|---|---|---|---|
| 20140349557 A1 | Nov 2014 | US |
