Vitrified superfinishing grindstone

Abstract

To provide a vitrified superfinishing grindstone with sufficiently stabilized superfinishing accuracy, ensuring that the surface roughness (surface texture) of the grindstone surface does not vary over time during polishing, namely, that the stock removal amount (grinding amount) required for superfinishing the work does not vary depending on individual grindstones or the degree of progress of the polishing process.SOLUTION: A vitrified superfinishing grindstone comprises a grindstone structure in which abrasive grains are bonded with a vitrified bonding agent. The grindstone structure contains a large number of independent pores. The adjacent independent pores are arranged in close proximity to each other such that the grindstone structure forms a three-dimensional honeycomb structure. The pore diameter of the independent pores is 5 to 100 μm, and the porosity of the grindstone structure is 20 to 70 vol.%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a vitrified superfinishing grinding wheel capable of performing superfinishing on a workpiece.

Background Art

[0002] Generally, for superfinishing of precision machine parts and the like, a vitrified superfinishing grinding wheel in which abrasive grains are held by a vitrified bond (binder) made of ceramic is applied.

[0003] Superfinishing is a processing method that reduces the surface roughness of a workpiece and polishes it extremely smoothly. Generally, a grinding wheel is pressed against the surface of a workpiece rotating at a low speed under low pressure, and at this time, polishing is performed while applying small vibrations (oscillations: also referred to as oscillations) to the grinding wheel.

[0004] Examples of the material of the workpiece (workpiece) include metal materials, fine ceramics materials, and other non-metal materials. Using a vitrified superfinishing grinding wheel, it is possible to remove the altered layer, grinding burrs, undulations, etc. on the surface of these workpieces to improve the shape accuracy, and the smooth working surface of the grinding wheel is transferred. For example, when superfinishing is performed on the raceway surfaces of the inner and outer rings of a rolling bearing, the surface roughness (or also called surface roughness) becomes extremely small without breaking the cross-sectional shape of the raceway surface.

[0005] Generally, polishing with a surface roughness (arithmetic mean roughness Ra) below 0.2 μm is called mirror finishing, but using a vitrified superfinishing grinding wheel enables high-precision superfinishing with Ra of 0.1 μm or less.

[0006] To manufacture a vitrified grinding wheel, abrasive grains and a vitrified binder are added with an organic pore former and a molding aid and mixed. This mixture is molded into a green grinding wheel by casting or pressing, dried, and further fired to burn off or sublime the pore former dispersed in the green grinding wheel, thereby manufacturing a vitrified grinding wheel that encloses the hollowed-out portions as independent pores (open pores and closed pores).

[0007] Pores have the function of removing the grinding chips of the workpiece generated during grinding. If such a function is insufficient, the working surface of the grinding wheel is likely to wear, and the economic efficiency of the grinding wheel evaluated by the finishing ratio decreases.

[0008] The inventors of the present application disclosed a vitrified super-finishing grinding wheel that uses composite abrasive grains composed of hard abrasive grains and soft abrasive grains, improves the lubricity of the working surface of the grinding wheel, suppresses the wear of the working surface of the grinding wheel, thereby obtaining desirable cutting performance and surface roughness, and moreover excellent grinding wheel wear resistance and economic efficiency (finishing ratio) (Patent Document 1).

[0009] Also, the porosity of the independent pores obtained by firing the vitrified super-finishing grinding wheel can be adjusted by adjusting the amount of the organic pore former such as polystyrene particles compounded into the green grinding wheel, and a grinding wheel can be manufactured with a high porosity of about 75 to 95% by volume (Patent Document 2).

[0010] Further, as a ceramic molded body used for mechanical structures such as automobiles and filters for exhaust gas, a ceramic molded body having a three-dimensional network structure is known (Patent Document 3). This document (paragraph 0005 and paragraph 0010) discloses that coated particles are formed by coating the surface of gasifiable spherical synthetic resin particles with ceramic particles, and the coated particles are arranged in the mold in the closest packing form and the coated particles are thermally decomposed, thereby forming a plurality of cells and communication holes. As a result, a ceramic molded body in which cells of the same size are uniformly distributed and mechanical properties and physical properties are made uniform is obtained.

Prior Art Documents

Patent Documents

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-130635 [Patent Document 2] Japanese Patent No. 6737975 [Patent Document 3] Japanese Patent No. 4026835 [Summary of the Invention] [Problems to be Solved by the Invention]

[0012] In the super-finishing grindstones described in Patent Documents 1 and 2, since the independent pores are formed in random sizes and are randomly dispersed, when the wear of the grindstone progresses during the polishing of the workpiece, the ratio of pores per unit area (porosity), the uniformity of their distribution, and the size of the pores (particle size or average particle size) that appear on the polishing working surface of the grindstone change. Therefore, the cutting performance of the grindstone changes during the polishing process.

[0013] That is, in a grindstone in which the size and arrangement of the independent pores are randomly formed, since the surface roughness (surface roughness) of the grindstone surface fluctuates during the polishing process, the amount of workpiece replacement (thickness required for the same processing) required until the completion of the super-finishing process also fluctuates during the polishing process. The degree of such fluctuations also varies depending on the individual grindstones. Therefore, there is a problem that the amount of workpiece replacement and the surface roughness (surface roughness) of the workpiece are not stable, and the processing accuracy becomes unstable overall.

[0014] In addition, although the sizes and positions of the independent pores in the ceramic molded body for mechanical structures described in Patent Document 3 are aligned, such a ceramic molded body is not configured on the premise of wear and is in a technical field completely different from that of a grindstone. That is, a ceramic molded body for general mechanical structures does not contain abrasive grains, nor does it contain a vitrified binder that appropriately drops the abrasive grains. Therefore, it cannot improve the characteristics of a superfinishing grindstone, and there are no issues regarding the machining accuracy, surface accuracy of the grindstone, and stable cutting performance as a vitrified superfinishing grindstone, nor are there any disclosed technical means for addressing such issues.

[0015] Therefore, the problem of the present invention is to solve the problems related to the unstable machining accuracy caused by the pores of the above-described superfinishing grindstone, and to ensure that the surface roughness (surface roughness) of the grindstone surface does not change over time during the polishing process. That is, the replacement amount (thickness required for the same processing) required for superfinishing the workpiece does not vary depending on each individual grindstone or the progress of the polishing process, so that the accuracy of the superfinishing process is a vitrified superfinishing grindstone and its fired mixed material molded body with sufficiently stable performance.

Means for Solving the Problem

[0016] In order to solve the above problems, the present invention provides a vitrified superfinishing grindstone comprising a grindstone structure in which abrasive grains are bonded with a vitrified binder. This grindstone structure contains a large number of independent pores with uniform diameters, and the independent pores are arranged in proximity so that the grindstone structure forms a three-dimensional honeycomb structure. The pore diameter of the independent pores is 5 to 100 μm, and the porosity of the grindstone structure is 20 to 70% by volume, along with its fired mixed material molded body.

[0017] The vitrified superfinishing grindstone of the present invention configured as described above comprises a grindstone structure in which abrasive grains are bonded with a vitrified binder, contains independent pores, and the grindstone structure surrounding the entire circumference of its surface forms a three-dimensional honeycomb structure continuously in the three-dimensional direction, with a large number of independent pores arranged in proximity and evenly in the three-dimensional direction. And a large number of independent pores with uniform diameters have a pore diameter of 5 to 100 μm, and the porosity of the grindstone structure is 20 to 70% by volume for the vitrified superfinishing grindstone.

[0018] The vitrified super-finishing grinding wheel of the present invention configured as described above wears while gradually dropping abrasive grains by the vitrified binder being scraped off by cutting chips during the grinding process, and on the newly formed grinding wheel working surface (new surface), independent pores of a predetermined size appear at a predetermined ratio per predetermined area together with new abrasive grains.

[0019] Therefore, during the grinding process, the surface roughness (surface finish) of the grinding wheel surface does not change over time, and the expected cutting performance is sufficiently obtained. That is, the replacement amount (grinding amount) required for super-finishing the workpiece does not vary depending on each individual grinding wheel or the progress of the grinding process, the processing accuracy of the super-finishing is sufficiently stable, and it becomes an excellent vitrified super-finishing grinding wheel.

[0020] The above-mentioned vitrified super-finishing grinding wheel is composed of a porous grinding wheel structure having communicating pores where the gaps between the abrasive grains communicate, and the cutting chips of the workpiece are easily discharged by cutting oil or the like flowing through the communicating pores. Also, such a lubricating function is stable without fluctuating even if the grinding wheel working surface wears over time.

[0021] Further, by being adjusted to a predetermined pore diameter of the independent pores being 5 to 100 μm and a predetermined porosity of the grinding wheel structure being 20 to 70% by volume, pores of a predetermined size appear at a predetermined density per predetermined area on the grinding wheel working surface newly generated by wear during the grinding process. Therefore, the surface roughness of the grinding wheel working surface and the super-finished workpiece is stable at a low value without fluctuating depending on each individual grinding wheel or the progress of the grinding process.

[0022] In order to further suppress the grinding wheel wear amount, it is preferable that the abrasive grains are composite abrasive grains composed of soft abrasive grains having chemical reactivity with respect to the material to be cut and having no cutting performance, and hard abrasive grains having cutting performance with respect to the material to be cut.

[0023] When using such composite abrasive grains, the soft abrasive grains without a cutting action cause a minute reaction layer on the polished surface through a chemical reaction. This layer is easily removed by the contact frictional force of the grinding wheel, and the soft abrasive grains act as a lubricant to reduce the damage of bond erosion caused by chips. Therefore, the grinding wheel wear amount is reduced compared to a grinding wheel without using composite abrasive grains, and it becomes a more excellent vitrified superfinishing grinding wheel in terms of each evaluation item of cutting performance, grinding wheel wear, economy (finishing ratio), and surface roughness.

[0024] To prepare such a vitrified superfinishing grinding wheel, the mixed material compact before firing of the vitrified superfinishing grinding wheel is composed of a mixture of particulate vitrified binder, abrasive grains, and particulate pore former, and it is preferable that the average particle size ratio of the particulate pore former to the average particle size of the vitrified binder is 1:4 to 13. As the abrasive grains, composite abrasive grains composed of soft abrasive grains having chemical reactivity with respect to the material to be cut and not having cutting performance, and hard abrasive grains having cutting performance with respect to the material to be cut can also be employed.

[0025] When firing such a compact for firing, a porous vitrified superfinishing grinding wheel in which independent pores encapsulated in the vitrified binder are evenly dispersed and arranged can be stably manufactured.

[0026] In particular, when the abrasive grains in the above mixed material compact are abrasive grains with an abrasive grain size (median diameter) of 0.5 to 22 μm, the particle size (median diameter) of the vitrified binder is 0.4 to 10 μm, and the particle size of the vitrified binder is smaller than the abrasive grain size, a large number of independent pores are likely to be arranged in a sufficiently close proximity with the intervals between adjacent pores being narrowed. The sphere composed of the pore former and the abrasive grains surrounding it approaches an ideal closest packing structure that occupies 74% of the volume of the compact. Therefore, the grinding wheel structure encapsulating the vitrified binder is likely to form an ideal three-dimensional honeycomb structure.

[0027] Taking such an ideal three-dimensional honeycomb structure as a standard, when a three-dimensional honeycomb structure is formed in a similar form, the adjacent independent pores are arranged sufficiently close to each other. Therefore, during the polishing process, the surface roughness (surface roughness) of the grinding wheel surface does not change over time, and the replacement amount (polishing amount) required for superfinishing the workpiece does not vary for each individual grinding wheel and with the progress of the polishing process. A vitrified superfinishing grinding wheel with sufficiently stable superfinishing accuracy is manufactured.

Advantages of the Invention

[0028] This invention has the following advantages: the grinding wheel structure contains a large number of independent pores with uniform diameters, and the adjacent independent pores are arranged closely to form a three-dimensional honeycomb structure. Since the large number of independent pores with uniform diameters are evenly arranged close to each other in the three-dimensional direction, the surface roughness (surface roughness) of the grinding wheel surface does not change over time during the polishing process. That is, the replacement amount (polishing amount) required for superfinishing the workpiece does not vary depending on each individual grinding wheel or the progress of the polishing process, and it becomes a vitrified superfinishing grinding wheel with sufficiently stable superfinishing accuracy.

[0029] In addition, for the fired mixed material compact which is an intermediate product of the above-mentioned vitrified superfinishing grinding wheel, since the average particle size of the pore-forming agent is in a predetermined average particle size ratio of 1:4 to 13 with respect to the average particle size of the vitrified binder, there is an advantage that a high-quality vitrified superfinishing grinding wheel with excellent cutting performance as described above can be stably obtained by firing.

Brief Description of the Drawings

[0030]

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Mode for Carrying Out the Invention

[0031] The vitrified super-finishing grinding wheel as an embodiment of the present invention is composed of a grinding wheel structure in which abrasive grains are bonded with a vitrified binder. This grinding wheel structure contains a large number of independent pores, and the adjacent independent pores are arranged closely so that the grinding wheel structure forms a three-dimensional honeycomb structure. The pore diameter of the independent pores is 5 to 100 μm, and the porosity of the grinding wheel structure is 20 to 70% by volume.

[0032] As for the abrasive grains of such a vitrified super-finishing grinding wheel, well-known abrasive grains can be selected so that super-finishing processing using the grinding wheel is possible for a workpiece made of a steel material. For example, as hard abrasive grains, chromium oxide (CrO2), alumina (Al2O3), silicon carbide (SiC), or super abrasive grains such as diamond (SD) and cubic boron nitride (CBN) can be mentioned. Among these, SiC, SD, and CBN, which are non-oxides, are easily affected by heat, but the grinding wheel can be fired at 800°C or higher.

[0033] In addition, composite abrasive grains in which the above hard abrasive grains (including super abrasive grains are referred to as hard abrasive grains) and soft abrasive grains are used in combination can also be adopted. As an example of a particularly preferred composite abrasive grain, there is a composite abrasive grain composed of a soft abrasive grain having chemical reactivity with the material to be cut and no cutting ability, and a hard abrasive grain having cutting ability with respect to the material to be cut.

[0034] Examples of the soft abrasive grains include one or more soft abrasive grains selected from cerium oxide, barium sulfate, calcium carbonate, silicon oxide, and zirconium oxide, which are soft abrasive grains having reactivity to chemically weaken, such as oxidizing the surface layer of the contact point with respect to the steel material, and having no cutting ability.

[0035] Such soft abrasive grains are abrasive grains used for mechanochemical polishing (MCP), and are abrasive grains having chemical weakening reactivity such as an oxidation reaction with respect to the iron-based steel material as the workpiece and being sufficiently soft and non-cutting. The material thereof is cerium oxide (CeO2, new Mohs hardness 5), barium sulfate (BaSO4, 3 - 4), calcium carbonate (CaCO3, 3 - 4), etc., or silicon oxide (SiO2, 6.5 - 7), zirconium oxide (ZrO2, 8 - 9), etc., and one or more of these may be included.

[0036] When the mixing ratio of the hard abrasive grains and the soft abrasive grains is 20 - 95% for the hard abrasive grains and 5 - 80% for the soft abrasive grains by volume ratio, the layered fine reactants generated by the chemical reaction between the workpiece to be cut and the soft abrasive grains are easily removed, so-called "clogging" and "chipping" are less likely to occur during cutting, and the grinding wheel wear amount is small, enabling super-finishing with a high finishing ratio.

[0037] As a practical guide for adjusting the mixing ratio, it is adjusted so that the relative reduction of the hard abrasive grains due to the blending of the soft abrasive grains does not reduce the cutting performance of the grinding wheel. For grinding wheels of super abrasive grains (CBN, SD), on the premise of maintaining a grain ratio of 12.5 - 17.5% (concentration 50 - 70), a suitable mixing ratio is to blend 40 - 80% of hard abrasive grains and 20 - 60% of soft abrasive grains.

[0038] In the case of ordinary abrasive grains (WA, GC) grinding wheels, taking the abrasive grain ratio of 35 - 40% as a guide, if 80 - 90% of hard abrasive grains and 10 - 20% of soft abrasive grains are blended, the desired cutting performance can be easily obtained.

[0039] Also, in the case of fine-grained grinding wheels mainly for mirror finishing, since the tribological (friction, wear, lubrication) effects are strongly required, it is preferable to mix hard abrasive grains and soft abrasive grains at a ratio of 20 - 40% and 60 - 80% respectively.

[0040] Moreover, the particle size (median diameter) of the above-mentioned abrasive grains used in this invention is preferably 0.5 - 22 μm. When the particle size is less than 0.5 μm, it becomes difficult to make the particle size (median diameter) of the vitrified binder smaller than that of the abrasive grains, and it becomes difficult to sufficiently form voids between the abrasive grains. Also, for abrasive grains with a particle size exceeding 22 μm, it is not easy to evenly form spherical independent pores of a certain size within the grinding wheel structure.

[0041] As the vitrified binder used in this invention, an inorganic vitrified binder that is usually widely used can be used, and in addition, an appropriate one can be used as the bond for composite abrasive grains.

[0042] The vitrified binder applicable to grinding wheels using composite abrasive grains is adjusted to a predetermined softening temperature so as not to chemically or thermally modify the soft abrasive grains having chemical reactivity, and it is preferable to use a combination of 80 - 95% by mass of low-melting inorganic glass and 5 - 20% by mass of high-melting inorganic minerals.

[0043] Soft abrasive grains such as CeO2, SiO2, ZrO2, BaSO4, etc. show alterations such as sintering, granulation, and weight loss when fired at a high temperature exceeding 800°C. Therefore, the firing temperature of the grinding wheel containing soft abrasive grains needs to be set below 800°C. Further, similar to the case of hard abrasive grains such as WA, GC, and super abrasive grains SD, CBN, etc., it is preferable to set the grinding wheel firing temperature at 650 - 800°C so that the heating change of the abrasive grains is small and an appropriate abrasive grain bonding force can be easily obtained.

[0044] When the grinding wheel firing temperature is 650 to 750 °C, in order to obtain a stable bonding force, it is necessary to adopt a binder that melts at a lower temperature, and it is preferable to adopt a borosilicate (SiO2 - B2O3 - R2O - RO) glass formulated with several frit raw materials as a low - melting inorganic glass.

[0045] High - melting binders are mainly made from natural minerals such as feldspar, pottery stone, and clay, and an appropriate amount of such high - melting inorganic minerals is added to the low - melting inorganic glass. Specifically, 5 to 20% by mass of a high - melting inorganic mineral mainly composed of silica (SiO2) and alumina (Al2O3) is blended with 80 to 95% by mass of the low - melting inorganic glass to adjust the softening temperature of the vitrified binder.

[0046] The vitrified binder formulated in this way contains an appropriate amount of SiO2 and Al2O3 components, thereby suppressing the reactivity between the soft abrasive grains and the alkali, and reducing the fluidity and thermal expansion at high temperatures. As a result, the thermal expansion difference from the abrasive grains is reduced, and the mechanical strength of the grinding wheel also increases. Also, when the proportion of the high - melting inorganic mineral is less than 5% by mass, the desired effect is not achieved, and when it exceeds 20% by mass in a large amount, a sufficient molten state cannot be obtained and the abrasive grain bonding force becomes weak.

[0047] In this way, by adding the high - melting inorganic mineral, the over - reaction between the unnecessary abrasive grains and the low - melting inorganic glass is improved, and while maintaining a preferable bonding state, the variation due to the shortage of the alkali component in the vitrified binder is eliminated. Furthermore, in order to achieve more stable high quality, it is preferable to add zinc chloride (ZnCl2) as a solvent to the vitrified binder.

[0048] The particle size (median diameter) of the vitrified binder is 0.4 to 10 μm, and it is preferably smaller than the abrasive grain size. This is because when the particle size of the vitrified binder is less than 0.4 μm, the vitrified binder becomes relatively too small compared to the abrasive grains of the predetermined particle size, the grinding wheel structure becomes densified, and almost no voids are formed between the abrasive grains, resulting in a decrease in the lubricating effect of cutting oil and the like and the discharge effect of cutting chips during superfinishing. Also, when the particle size of the vitrified binder is larger than 10 μm, the vitrified binder becomes relatively too large compared to the abrasive grains of the predetermined particle size, and it becomes impossible to stably form spherical independent pores of a predetermined size in the grinding wheel structure.

[0049] The grinding wheel structure of the vitrified superfinishing grinding wheel of this invention preferably has a porous grinding wheel structure having communicating pores in which the voids between the abrasive grains communicate, and further contains a large number of independent pores of uniform diameter, and the pore diameter of such independent pores is 5 to 100 μm.

[0050] When the pore diameter of the independent pores or the particle size (median diameter) of the particulate pore former is less than 5 μm, it becomes difficult to sufficiently form the voids between the abrasive grains as described above, which is not preferable. Also, when the pore diameter of the independent pores or the particle size of the particulate pore former exceeds 100 μm, the working efficiency of superfinishing is reduced and the practicality is impaired, which is not preferable.

[0051] The pore diameter of such independent pores substantially coincides with the particle size of the pore former. This is predictable because the process of forming independent pores is that the pore former vaporizes and disappears during the firing of the mixed material compact for firing the vitrified superfinishing grinding wheel.

[0052] The mixed material compact for firing the vitrified superfinishing grinding wheel is composed of a mixture of particulate vitrified binder, abrasive grains, and particulate pore former, and the average particle size of the particulate pore former is in an average particle size ratio of 1:4 to 13 with respect to the average particle size of the vitrified binder.

[0053] If the average particle size of the particulate pore former is less than 8 with respect to the average particle size 1 of the vitrified binder, the average particle size of the vitrified binder becomes relatively too large with respect to the average particle size of the pore former, so that a large number of independent pores with uniform diameters are not formed.

[0054] Also, when it exceeds 13 with respect to the average particle size ratio 1 of the vitrified binder, the average particle size of the vitrified binder becomes relatively too small with respect to the average particle size of the pore former, so that the blending efficiency of the binder deteriorates to provide the required binding force for the abrasive grains, which is not preferable. For such reasons, a more preferable average particle size ratio of the pore former with respect to the average particle size of the vitrified binder is 1:8 to 12, more preferably 1:9 to 11, and particularly preferably 1:9.5 to 10.4 with 1:10 as a reference.

[0055] The porosity of the grinding wheel structure is preferably 20 to 70% by volume. If the porosity is less than 20% by volume, the grinding wheel structure becomes dense and the chip discharge effect during superfinishing is reduced, which is not preferable. If the porosity exceeds 70% by volume, the working efficiency of superfinishing is reduced and the practicality is impaired, which is not preferable. From such a viewpoint, the porosity of the grinding wheel structure is more preferably 20 to 50% by volume.

[0056] In the grinding wheel structure, preferably, pores having the same shape and uniform diameters are formed in a large number so that the porosity is 20 to 70% by volume. As a result, adjacent independent pores are arranged in close proximity at equal intervals, and the grinding wheel structure forms a three-dimensional honeycomb structure.

[0057] The honeycomb structure is generally recognized as a structure in which regular hexagons, regular hexagonal prisms, etc. are arranged without gaps. In a broad sense, it is not limited to regular hexagonal prisms, but includes polyhedral solids such as dodecahedrons and tetrakaidecahedrons or spheres arranged without gaps, or those in which the gaps are filled with a material of the same quality as the partition walls (three-dimensional space filling). In this invention, such a honeycomb structure in a broad sense is also formed.

[0058] [Example 1] Super abrasive grains (CBN, median diameter 4 μm), vitrified bond (glass frit in classification (ii) of Table 1, median diameter 3 μm), and pore former (spherical acrylic resin, median diameter 20 μm) were blended at the ratios (volume %) shown in Table 2, and these were uniformly mixed. The resulting mixed material was filled into a grinding wheel mold and compression molded. The green compact obtained was heated and fired at 650 to 800 °C, and after cooling, it was demolded to produce the super finishing grinding wheel of Example 1.

[0059] [Comparative Example 1-2] Super finishing grinding wheels of Comparative Examples 1 and 2 were produced in exactly the same manner as in Example 1, except that the median diameter of the vitrified bond (glass frit) was set to 7 μm or 11 μm and it was blended at the ratios (volume %) shown in Table 2.

[0060] [Example 2] The grinding wheel components were blended such that CBN was used as the hard abrasive grains (super abrasive grains) and CeO2 (primary particle diameter 0.5 μm, abrasive grain diameter 4 μm, purity 65.4%) was used as the soft abrasive grains, with abrasive grain ratios of 28.42 volume % and 6.85 volume %, respectively. A pore former (spherical acrylic resin, median diameter 20 μm) was blended at a pore material ratio of 35.86 volume %, and a vitrified bond (glass frit in classification (i) of Table 1, median diameter 3 μm) was blended at a binder ratio of 28.87 volume %. Otherwise, the super finishing grinding wheel of Example 2 was produced in exactly the same manner as in Example 1.

[0061]

Table 1

[0062]

Table 2

[0063] What was cut out from Example 1 or Comparative Example 2 into a prismatic shape with a width of 5.5 mm and a thickness of 5.0 mm and finished to the dimensional accuracy for super finishing the inner ring raceway surface of a ball bearing was used as a test grinding wheel, and the following super finishing test was conducted.

[0064] <Superfinishing Process Test> The actual grinding test of the raceway surface of the ball bearing inner ring was used as the superfinishing process test. For the raceway surface of the ball bearing inner ring, which is the workpiece for the test, first under rough conditions and then under finishing conditions, the workpiece peripheral speed (m / min), the grinding wheel oscillation frequency of 600 cycles / min, the grinding wheel surface pressure (MPa), and the processing time (seconds) shown in Table 3 were applied, and a sulfurized fatty oil-based water-insoluble oil was used as the processing oil.

[0065]

Table 3

[0066] Regarding the test grinding wheel of Example 1 or Comparative Example 2, the above-mentioned processing test was performed on 100 workpieces for the test, the replacement amount (μm) of the workpiece for the test with respect to the number of processed workpieces was measured, and the results are shown in Fig. 9. Furthermore, the surface roughness Ra (μm) of the test grinding wheel with respect to the number of processed workpieces was measured, and the results are shown in Fig. 10.

[0067] From the results of the above examples and comparative examples, the grinding wheel structure of Example 1 (median diameter of the binder is 3 μm) shown in Figs. 1 and 2 formed a three-dimensional honeycomb structure, in which spherical independent pores with substantially the same pore diameter were uniformly dispersed and encapsulated in the matrix, and the adjacent independent pores were arranged close to each other.

[0068] Also, as shown in Fig. 7, from the results of the superfinishing process test of Example 1, the replacement amount of the 100 workpieces for the test after processing had little variation and was stable compared to the initial stage of processing. Moreover, regarding the surface roughness shown in Fig. 8, the surface roughness Ra of the working surface of the grinding wheel remained stable within a narrow range of around 0.04 μm even after processing 100 workpieces, similar to the initial stage of processing. Furthermore, no obvious unevenness was observed in the microscope photograph of the working surface of the grinding wheel (after processing 100 workpieces) shown in Fig. 9.

[0069] On the other hand, the grinding wheel structure of Comparative Example 1 (median diameter of the binder is 7 μm) shown in Figs. 3 and 4 had independent pores observed, but there were differences in pore diameter size and they were random. Also, the independent pores were randomly dispersed in the matrix, and a three-dimensional honeycomb structure was not formed.

[0070] In addition, although independent pores are observed in the grinding wheel structure of Comparative Example 2 (median diameter of the binder: 11 μm) shown in FIGS. 5 and 6, they are fewer than those in Comparative Example 1, the difference in pore size is large, the distribution of independent pores is random and patchy in the matrix, and a three-dimensional honeycomb structure is not formed.

[0071] Also, as shown in FIG. 7, from the results of the superfinishing test of Comparative Example 2, the replacement amount of 100 machined test workpieces was stable with little variation from the beginning of machining. However, regarding the surface roughness shown in FIG. 8, during the machining of 100 workpieces, the surface roughness Ra of the grinding wheel working surface varied in the range of 0.04 to 0.055 μm and was not stable. Furthermore, unevenness was observed on the surface in the microscope photograph of the grinding wheel working surface (after machining 100 workpieces) shown in FIG. 9.

Claims

1. A vitrified superfinishing grinding wheel comprising a grinding wheel structure in which abrasive grains are bonded with a vitrified binder, the grinding wheel structure enclosing a large number of independent pores having a uniform diameter, and the adjacent independent pores being arranged in proximity so that the grinding wheel structure forms a three-dimensional honeycomb structure, the pore diameter of the independent pores being 5 to 100 μm, and the porosity of the grinding wheel structure being 20 to 70% by volume.

2. The vitrified superfinishing grinding wheel according to Claim 1, wherein the grinding wheel structure is a porous grinding wheel structure having communicating pores in which the voids between the abrasive grains communicate with each other.

3. The vitrified superfinishing grinding wheel according to Claim 1 or 2, wherein the abrasive grains are composite abrasive grains composed of soft abrasive grains having chemical reactivity with a workpiece to be cut and no cutting ability, and hard abrasive grains having cutting ability with respect to the workpiece to be cut.

4. A molded body of a mixed material for firing a vitrified superfinishing grinding wheel, comprising a grinding wheel structure in which abrasive grains are bonded with a vitrified binder, the grinding wheel structure enclosing a large number of independent pores having a uniform diameter, and the adjacent independent pores being arranged in proximity so that the grinding wheel structure forms a three-dimensional honeycomb structure, the pore diameter of the independent pores being 5 to 100 μm, and the porosity of the grinding wheel structure being 20 to 70% by volume, and comprising a mixture of abrasive grains, particulate vitrified binder, and particulate pore former, the average particle size ratio of the particulate pore former to the average particle size of the vitrified binder being 1:4 to 13.

5. The molded body of a mixed material for firing a vitrified superfinishing grinding wheel according to Claim 4, wherein the abrasive grains are composite abrasive grains composed of soft abrasive grains having chemical reactivity with a workpiece to be cut and no cutting ability, and hard abrasive grains having cutting ability with respect to the workpiece to be cut.

6. The molded body of a mixed material for firing a vitrified superfinishing grinding wheel according to Claim 4 or 5, wherein the abrasive grains have an abrasive grain size (median diameter) of 0.5 to 22 μm, the particle size (median diameter) of the vitrified binder is 0.4 to 10 μm, and the particle size of the vitrified binder is smaller than the abrasive grain size.

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