JP3017697B2 - Method for manufacturing vitrified whetstone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a vitrified grindstone containing sintered alumina-based abrasive grains produced by a sol-gel method.

[0002]

2. Description of the Related Art Generally, a vitrified grinding wheel using sintered alumina manufactured by a sol-gel method as abrasive grains has a higher grinding wheel performance than a grinding wheel using conventional fused alumina abrasive grains. It is known to be excellent.

For example, US Pat. No. 4,543,107 discloses a vitrified grinding wheel using a sintered alumina-based abrasive grain produced by a sol-gel method and a vitrified binder.
No. 4,898,597.

In US Pat. No. 4,543,107, the viscosity and / or ripening temperature of the binder is properly controlled, ie less than 1100 ° C. for conventional binders and less than 1220 ° C. for high viscosity binders. It is disclosed that when the temperature is lowered, the excellent properties of the sintered alumina abrasive grains are exhibited.

Also, in US Pat. No. 4,898,597,
In order to substantially or drastically reduce the reduction in grinding performance when using a vitrified sintered alumina grinding wheel with an aqueous coolant, a frit (ie, pre-fired prior to use as a binder) is used as a binding medium. (A vitreous binder composition).

However, when a sintered alumina-based abrasive grain produced by the sol-gel method is used as the vitrified grindstone as described above, the crystal grain size is smaller than that of the conventional fused alumina and is active. It is susceptible to the influence of the vitrified binder of the agent, and it is conceivable that the reaction with the vitrified binder at the conventionally used sintering temperature will deteriorate the grinding wheel performance.

However, in addition to the above-mentioned US patents, the relationship between the degree of alteration of the sintered alumina-based abrasive grains produced by the sol-gel method and the reaction of the vitrified binder with the vitrified binder and the relationship between the performance of the wheel and the grinding wheel performance was investigated. Nothing was known and it was unknown.

In view of the above circumstances, the present invention has a completely different viewpoint from the prior art, that is, the degree of reactivity between a sintered alumina-based abrasive grain produced by a sol-gel method and a vitrified binder and the deterioration of grinding performance. The purpose is to control the thickness of the reaction layer between the sintered alumina-based abrasive grains and the vitrified binder so that the performance of the grindstone does not deteriorate. It is another object of the present invention to provide a production method, and a composition of a vitrified binder suitable for a sintered alumina-based abrasive grain produced by a sol-gel method, and a method for producing a vitrified grindstone using the binder.

[0009]

In view of the above, the present inventor has made extensive studies from the following points.

(1) In order to evaluate the reactivity between the sintered alumina-based abrasive grains produced by the sol-gel method and the vitrified binder, a modified oxide mainly acting as a flux in the glass component in the vitrified binder. Metal elements such as alkali metal oxides (Na ₂ O, Li ₂ O, K ₂ O, etc.) and alkaline earth metal oxides (CaO, MgO, BaO, etc.) erode from the surface of the sintered alumina abrasive grains, Measure the hardness of the surface layer of the part that has penetrated into the center of the abrasive grains (this is called the reaction layer between the abrasive grains and the vitrified binder). The decrease was measured.

(2) In addition, the modified oxide (Na ₂ O, Li ₂ O, K ₂ O, C
The depth of penetration of aO, MgO) into the abrasive grains was measured by an electron microanalyzer (E.P.M.A.).

(3) As a result of considering the relationship between the results of the measurements (1) and (2) described above and the grinding wheel strength and grinding wheel performance of the vitrified grinding wheel, the reaction layer of the vitrified binder and the bound alumina-based abrasive grains was found. By controlling the thickness to be equal to or less than a predetermined thickness, it has been found that a grinding wheel strength enough to be used as a vitrified grinding wheel can be obtained and that the grinding wheel performance does not deteriorate.

(4) Based on the above (3), a preferable production system including a vitrified binder composition and a calcination temperature was determined.

(5) The sintered alumina-based abrasive grains produced by the sol-gel method which are preferable for use in the present invention are the sintered alumina-based abrasive grains of US Pat. Nos. 4,744,802 and 4,881,951.

The present invention has been made based on the above (1) to (5).

That is, the method for producing a vitrified grindstone of the present invention includes a mixing molding step of mixing and molding an abrasive containing at least 5% by weight or more of a sintered alumina abrasive produced by a sol-gel method and a vitrified binder. A firing step of firing the molded body obtained in the mixed molding step at a temperature and a time of 1200 ° C. or less that regulate the penetration depth of the Na element in the vitrified binder in the surface layer of the abrasive grain after firing to less than 10 μm. Including, the thickness of the reaction layer at the interface between the abrasive grains and vitrified binder formed at the time of firing,
From the surface layer of the abrasive grain to the inside thereof, less than 10 μm (0
(excluding μm).

Further, the method for producing a vitrified grindstone of the present invention comprises a mixing and molding step of mixing and molding an abrasive containing at least 5% by weight or more of sintered alumina-based abrasive produced by a sol-gel method and a vitrified binder. The molded body obtained in the above-mentioned mixed molding step is fired at a temperature and a time of 1200 ° C. or less which regulate the rate of decrease in the hardness of the abrasive grains when the depth from the surface layer of the abrasive grains to the center part is 5 μm to less than 19%. Baking step, wherein the thickness of the reaction layer at the interface between the abrasive grains and the vitrified binder formed at the time of firing is less than 10 μm (excluding 0 μm) from the surface layer of the abrasive grains to the inside thereof. It is characterized by being manufactured.

The sintered alumina-based abrasive grains may be abrasive grains made of an aluminum oxide containing a rare earth metal, and the abrasive grains may be obtained by drying a sol containing α-alumina seed and / or a rare earth metal. It is preferable that the modified oxide is produced by heat treatment, and the total amount of the modified oxides contained in the vitrified binder is 20 wt% or less (preferably 15 wt% or less).

The reason why the firing temperature is set to 1200 ° C. or less is that if the temperature exceeds 1200 ° C., the abrasive grains will cause crystal growth, which may cause the abrasive grains to deteriorate.

The reason why the thickness of the reaction layer is set to less than 10 μm is that, when the thickness of the reaction layer becomes 10 μm or more, the degree of deterioration of the surface layer of the sintered alumina-based abrasive grains is reduced as shown in an experimental example described later. This is because the grinding wheel performance is deteriorated.

The above-mentioned reaction layer must be formed without fail in order to impart strength as a grindstone, and therefore 0 μm is excluded.

The reason why the content of the sintered alumina-based abrasive grains obtained by the sol-gel method is at least 5% by weight or more is that if less than 5% by weight, the effect of the sintered alumina-based abrasive grains obtained by the sol-gel method is hardly exhibited.

The vitrified grinding wheel manufactured by the method for manufacturing a vitrified grinding wheel of the present invention is obtained by firing an abrasive containing at least 5% by weight or more of a sintered alumina abrasive manufactured by a sol-gel method and a vitrified binder at a predetermined temperature. The thickness of a reaction layer at the interface between the abrasive grains and the vitrified binder formed during firing is less than 10 μm from the surface layer of the abrasive grains to the inside thereof. .

Further, the above-mentioned sintered alumina abrasive grains may be abrasive grains made of an aluminum oxide containing a rare earth metal in order to improve properties.

The alumina crystal grain size after calcination is preferably submicron, particularly 0.05 to 0.8 μm.
More preferably, it is within the range.

If the thickness is less than 0.05 μm, the degree of sintering of α-alumina is not advanced, so that an effective hardness as abrasive grains cannot be obtained. On the other hand, if it exceeds 0.8 μm, the bonding force between α-alumina crystals is weakened. This is because grain boundary destruction between crystals often occurs, and the mechanism for fine crushing of abrasive grains does not work.

Further, the total amount of the modified oxide contained in the vitrified binder is preferably not more than 20 wt%.

Here, the modified oxide is Na ₂ O, K
Alkali metal oxides such as ₂ O, Li ₂ O, CaO, Mg
Alkaline earth metal oxide such as O, BaO, etc., is a component mainly acting as a flux in the binder. The reason why the total amount of the modified oxide is set to 20 wt% or less is that if the content exceeds 20 wt%, even if the grindstone is fired at a low temperature of about 900 to 1100 ° C., the reaction layer at the interface between the abrasive grains and the vitrified binder at the time of firing. This is because the thickness increases to exceed 10 μm, and the deterioration of the grinding wheel characteristics becomes remarkable. In particular, among the above-mentioned modified oxides, Na ₂ O particularly easily penetrates into the inside of the abrasive grains and causes deterioration of characteristics. Therefore, the amount of Na ₂ O is desirably 10 wt% or less.

The thickness of the reaction layer between the vitrified binder and the crystalline alumina-based abrasive grains depends on the intrusion of the constituent components in the vitrified binder, particularly the respective metal elements of the modified oxide, from the surface layer of the abrasive grains into the interior of the abrasive grains. It can be represented by the largest value of the distance (depth).

[0030]

[Experimental Example 1] Generally, sintered abrasive grains produced by the sol-gel method are sintered at a lower temperature than conventional fused-type alumina abrasives fired at a maximum of 1300 to 1400 ° C. It is considered that the effect of heat is greater than that of alumina abrasive grains.

Therefore, as preparation before manufacturing a vitrified grindstone, an investigation was made on the alteration of the sintered alumina abrasive grains produced by the sol-gel method depending on the firing temperature.

(Test Method) First, two types of commercially available sintered alumina abrasive grains manufactured by the US 3M Company were prepared by the sol-gel method. These are two points based on the following US patents.

No. US Patent Specification Manufacturer Brand name 1 474 802 US 3M Cubitron 2 4881951 US 3M Cubitron-321

Next, the above-mentioned cubitron and cubitron-321 abrasive grains were placed in an alumina crucible and heated at 1000 ° C.
From 1300 ° C. for 6 hours, and then the changes in the density, hardness and crystal of the abrasive grains were investigated. Table 1 shows the results. The crystal size was measured from the results of SEM observation of the fracture surface of the abrasive grain shown in FIGS.

[0035]

[Table 1]

From the results shown in Table 1, it can be seen that the sintered alumina-based abrasive grains produced by the sol-gel method undergo crystal growth around 1200 ° C., and the abrasive grains are altered. Therefore, when manufacturing a vitrified grindstone using a sintered alumina-based abrasive grain manufactured by such a sol-gel method,
The firing temperature needs to be 1200 ° C. or less.

[0037]

[Experimental Example 2] Next, the crystalline alumina-based abrasive grains produced by the sol-gel method have a smaller crystal grain size than conventional fused-type alumina-based abrasive grains, and are more susceptible to the vitrified binder. Since a reaction was considered to occur, the presence or absence of alteration of the sintered alumina-based abrasive grains with respect to the vitrified binder was investigated.

(Test Method) A test piece for examining the degree of reaction was manufactured as follows.

(I) Sintered alumina-based abrasive grain No. 24 manufactured by 3M, USA 2 g (trade name: Cubitron-321) (ii) Vitrified binder No. 201 to No. 203 manufactured by Noritake Co., Ltd. …… 5g (iii) Tap water …… 5g

The used vitrified binders No. 201 to No. 203 manufactured by Noritake Co., Ltd. are made from a normal vitrified binder composed of feldspar, clay and frit glass. Are shown in Table 2.

[0041]

[Table 2]

The materials listed in (i), (ii), and (iii) are placed in a mortar in the order indicated, mixed thoroughly, and the
pellets with a height of 10 mm and a firing temperature of 100
The mixture was kept at a maximum temperature of 0 ° C. for 6 hours and allowed to cool to room temperature.

Next, in order to examine whether the surface layer of the abrasive grains has been altered and the surface layer of the abrasive grains has been altered, the pellets containing the sintered alumina-based abrasive grains after firing are polished, and a micro-hardness tester (manufactured by Akashi Seisakusho) is used. Micro Vickers), as shown in FIG.
The hardness of the boundary between the vitrified binder and the sintered alumina abrasive grains was measured at intervals of 25 μm from the position of 5 μm from the surface layer of the abrasive grains toward the center of the abrasive grains, and the degree of deterioration of the surface layer of the abrasive grains was measured.

The measurement conditions are as follows. (Measurement model: MVK-G1000 type (Akashi Seisakusho) 3
00g load, 10 seconds, Vickers indenter made of diamond

Table 3 shows the results of the depth from the surface layer of the abrasive grains to the center and the Vickers hardness. FIG. 4 shows a graph of the depth (μm) from the surface layer of the abrasive grains to the center and the Vickers hardness (GPa). Was.

[0046]

[Table 3]

From Table 3 and FIG. 4, it can be seen that the vitrified binder
_{No.203 (Na 2 O6.0wt%, Na} 2 O, K 2 O, C
In the case of using the total amount of the four types of aO and MgO (hereinafter, referred to as the total amount of modified oxides: 10 wt%) and the unfired (raw) type, there was almost no decrease in hardness at a position 5 μm from the surface layer of the abrasive grains. However, binder No. 201 (Na ₂ O 13.5w
(t%, total amount of modified oxides: 16 wt%), the hardness was significantly lower at a position 5 μm from the surface layer than at the center. Also, binders No.202 (Na ₂ O9.
(0 wt%, the total amount of the modified oxides was 12.5 wt%), it was slightly deteriorated at a position 5 μm from the surface layer.

Therefore, the binder No. 201 (Na ₂ O 13.5)
wt%, total amount of modified oxides 16 wt%) and 1000
It was found that the material fired at ℃ was deteriorated by the vitrified binder up to the position of the surface layer 5μ.

[0049]

Example 1 Next, in order to examine how much the degree of reactivity between the vitrified binder and the sintered alumina abrasive grains affects the grinding performance, a grinding test wheel was manufactured and a grinding test was performed. .

(Test method) Vitrified binder (No.
201, 202, 203), sintered alumina-based abrasive cubitron-321 and dextrin were mixed at the ratios shown in Table 4 and molded, and then calcined at 1000 ° C. for 6 hours to produce grindstones A, B, and C.

The dimensions of these whetstones are 180 m in outer diameter.
m, thickness 19 mm, inner diameter 76.2 mm. The reason for the difference in the amount of the vitrified binder between the grindstones A to C is to make the working hardness of the grindstones at the time of grinding substantially equal.

[0052]

[Table 4]

Next, the abrasive grain ratio, binder ratio,
The specific gravity of the grindstone, the degree of Ogoshi bond, and the bending strength were measured. Table 5 shows the results.

[0054]

[Table 5]

Further, a grinding test was performed on these grindstones A, B and C. For comparison, a grindstone D made of fused alumina single crystal abrasive grains (No. 32A, No. 60, manufactured by Taiheiyo Random Co., Ltd.) having the same working hardness as the grindstones A, B and C was manufactured.

The grindstone D of the comparative example had a firing temperature of 1200 ° C.
Whetstone specific gravity 1.95, abrasive grain ratio 46.1% by volume, binder ratio 7.2% by volume, Ogoshi bonding degree 0.63mm, flexural strength 306kg / cm ²
It is a normal whetstone.

(Grinding Test Conditions) Machine: Hiraoka Okamoto CFG-52AN Grinding wheel peripheral speed: 2000 m / min Depth of cut: Wet plunge Do of ΔR 20 μm / pass
wn Cut Work material: SUJ-2 (HRC60) Dimensions: length 100 x height 50 x width 10 (mm) Work width: 10 mm Grinding oil: water-soluble grinding oil Noritake Co., Ltd. Noritake Cool K82B manufactured by Limited 80 times liquid dresser: single stone

Table 6 shows the results of the grinding test.

[0059]

[Table 6]

As is clear from Table 6, the grindstones of Examples B and C have a grinding ratio of 2.2 to 2.8 times, a better surface roughness, lower power consumption and lower noise than the grindstones of Comparative Example D. Showed the grinding performance.

However, Example A (vitrified binder)
The grindstone No. 201) had the same grinding ratio, surface roughness, and noise as the grindstone of Comparative Example D, but the power consumption was higher.

Therefore, it was found that the difference in performance between the grindstone A and the grindstones B and C was caused by the fact that the surface layer of the sintered alumina-based abrasive grains of the grindstone A was more deteriorated by the vitrified binder.

Then, next, grinding wheel A (fired at 1000 ° C.)
The thickness of the reaction layer between the surface layer of the sintered alumina abrasive grains of the grindstone C (fired at 1000 ° C.) and the vitrified binder is measured by an electronic microanalyzer (E.P.M.A.) manufactured by Shimadzu Corporation (model: wavelength dispersion type). , ASM-SX).

The test piece used was one in which fragments of the grindstones A to C were embedded in an epoxy resin and wrapped so that the grindstone surface became a mirror surface after the epoxy resin was cured.

E. P. M. 5 and 6 show the results obtained by observing the reaction layer between the abrasive grains of the grindstone A and the vitrified binder at A. FIG. (A) in FIG. 5 is an SEM photograph of an interface between the abrasive grains and the vitrified binder, and FIGS. 5 (b) and (c) are Na and K elements at the same location as the SEM observation location in (a), respectively. E. P. M. A is the result of the surface analysis. FIG.
(A), (b), and (c) show the E.E. of each element of Al, Mg, and Ca at the same place as the SEM observation place in FIG. P.
M. A is the result of the surface analysis.

Here, the E.E. of the Na element shown in FIG. P.
M. When the result of the surface analysis of A was carefully observed, it was observed that the Na element slightly penetrated into the sintered alumina-based abrasive grains.

Therefore, the same Na surface analysis of the same grindstone A is performed by the line analysis of the Na element, and depending on the difference in the Na element concentration, the Na element in the vitrified binder is transferred into the sintered alumina-based abrasive grains. The degree of penetration was measured.

FIG. 7 shows the result of actual line analysis of the Na element. As shown in FIG. 8, the penetration depth of the Na element was defined as the distance ^* A between the portion where the concentration of the Na element was high and the portion where the concentration was low.

Table 7 shows the penetration depth of the Na element into the grindstones A to C.

[0070]

[Table 7]

From the results shown in Table 7, when the penetration depth of the Na element is the thickness of the reaction layer with the vitrified binder, the grinding stone A
Is about 2.5 times as thick as the grindstone C. Therefore, it is considered that the sintered alumina-based abrasive grains of the grindstone A were considerably deteriorated by the vitrified binder, so that the performance of the original abrasive grains could not be exhibited.

[0072]

Example 2 Using vitrified binder No. 112 manufactured by Noritake Co., Ltd., the materials listed in Table 8 were mixed at a predetermined ratio, molded, and fired at 1000 ° C. to produce grinding wheels E and F. .

The chemical components of vitrified binder No. 112 are 65% silica, 11% alumina, 13% boron oxide, 7% sodium oxide, 2.5% potassium oxide,
% Magnesium oxide and 2.5% calcium oxide (the total amount of the modified oxides is 13.0 wt%).

[0074]

[Table 8]

Next, the abrasive grain ratio, binder ratio,
Table 9 shows the results of measuring the specific gravity of the grindstone, the degree of Ogoshi bond, and the bending strength.
Shown in

[0076]

[Table 9]

On the other hand, pellets of abrasive grains and a vitrified binder were produced in the same manner as in Example 1, fired at 1000 ° C., the pellets were polished, and the hardness at a position of 5 μm from the surface layer of the abrasive grains was measured. there were.

Further, grinding tests were performed on the grinding wheels E and F. The grinding test conditions are the same as in Example 1. In Comparative Example D, the same 32A grindstone as that of Example 1 was used. Table 10 shows the results.

[0079]

[Table 10]

As is clear from Table 10, the grindstones of Examples E and F showed a remarkable improvement in grinding performance as compared with the grindstone of Comparative Example D.

Next, Na, K, Mg and Ca elements in the vitrified binder from the surface layer of the abrasive grains of the grindstone E were used.
P. M. A-line analysis was performed (see FIGS. 9 and 10), and Na,
The penetration depth of each element of K, Mg and Ca was measured in the same manner as in Example 1.

Table 11 shows the results.

[0083]

[Table 11]

From the results shown in Table 11, the penetration depth of each element is 10
μ, the reaction layer of the grinding wheel E was 10 μm
Was found to be less than.

[0085]

As described above, according to the method for manufacturing a vitrified grindstone according to claims 1 to 4, the performance of the sintered alumina-based abrasive grain manufactured by the sol-gel method is not deteriorated.
Vitrified whetstones that exhibit the excellent performance of sintered abrasives can be easily manufactured and provided. The performance of the vitrified grindstone is much superior in terms of grinding ratio and grinding amount as compared with conventional fused alumina abrasive grains.

[Brief description of the drawings]

FIG. 1 is an SEM photograph showing the particle structure of a broken abrasive surface of a sol-gel method sintered alumina (No. 1) used in Experimental Example 1, (a) is a raw abrasive material, (b) is at 1200 ° C. After firing for 6 hours, (c) after firing at 1250 ° C. for 6 hours.

FIGS. 2A and 2B are SEM photographs showing a grain structure of a broken abrasive surface of a sol-gel method sintered alumina (No. 2) used in Experimental Example 1, wherein FIG. 2A is a raw abrasive, and FIG. After firing for 6 hours, (c) after firing at 1250 ° C. for 6 hours.

FIG. 3 is an SEM photograph of a structure of a ceramic material showing a press-fit position of a Vickers indenter at a boundary portion between a vitrified binder and abrasive grains.

FIG. 4 is a graph showing the relationship between the depth (μm) from the surface layer of the abrasive grains to the center and Vickers hardness (GPa).

FIG. 5 is an SEM observation photograph showing the structure of the ceramic material of the reaction layer of the grindstone A in Example 1, and an X-ray photograph showing the results of the EPMA surface analysis. S showing the texture of the ceramic material at the interface of the binder
EM photographs, (b) and (c) show SEMs of (a), respectively.
E. of each element of Na and K at the same place as the observation place. P. M. A
9 is an X-ray photograph showing the result of surface analysis.

FIG. 6 shows an E.C. in the same place as the SEM observation place in FIG. 5 (a).
It is an X-ray photograph which shows a PMA surface analysis result, (a),
(B) and (c) show E.E. of each element of Al, Mg and Ca, respectively. P. M. 3 is an X-ray photograph showing the result of the area analysis of A.

FIG. 7 is an SEM photograph showing the structure of the ceramic material in which an X-ray photograph of the result of Na elemental ray analysis of the reaction layer of the grindstone in Example 1 is also shown, and (a) shows the line analysis result of the grindstone C; X-ray photograph and SEM showing structure of ceramic material
(B) X showing the line analysis result of grinding wheel A
2 shows a superposition of a line photograph and an SEM photograph showing the structure of a ceramic material.

FIG. 8 is a schematic diagram for explaining a penetration depth of a Na element.

9 is a graph showing the E.E. of a reaction layer of a grindstone E in Example 2. FIG. P.
M. It is a SEM photograph which shows the structure of the ceramic material which also displayed the X-ray photograph of the A-ray analysis result, (a), (b)
Is a superposition of an X-ray photograph showing the result of each element of Na and K and an SEM photograph showing the structure of the ceramic material.

10 is a graph showing the E.E. of a reaction layer of a grindstone E in Example 2. FIG. P.
M. It is a SEM photograph which shows the structure of the ceramic material which also displayed the X-ray photograph of the A-ray analysis result, (a), (b)
Is a superposition of an X-ray photograph showing the result of each element of Mg and Ca and an SEM photograph showing the structure of the ceramic material.

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Claims (4)

(57) [Claims] A mixed molding step of mixing and molding an abrasive containing at least 5% by weight of sintered alumina-based abrasive produced by a sol-gel method with a vitrified binder, and a vitrified binder in a surface layer of the abrasive after firing. N in
1200 ° C that limits the penetration depth of element a to less than 10 μm
A firing step of firing the molded body obtained in the mixed molding step at the following temperature and time, wherein the thickness of the reaction layer at the interface between the abrasive grains and the vitrified binder formed during firing is reduced by the abrasive A method for producing a vitrified grindstone, comprising producing a vitrified grindstone having a particle size of less than 10 μm (excluding 0 μm) from the surface layer of the grain to the inside thereof. 2. A mixing and molding step of mixing and molding an abrasive containing at least 5% by weight or more of sintered alumina-based abrasives produced by a sol-gel method and a vitrified binder; Baking the molded body obtained in the mixing molding step at a temperature and a time of 1200 ° C. or less that regulates the hardness reduction rate of the abrasive grains at a depth of 5 μm to less than 19%. A vitrified grindstone having a thickness of a reaction layer at an interface between the abrasive grains and the vitrified binder, which is less than 10 μm (excluding 0 μm) from the surface layer of the abrasive grains to the inside thereof, Production method. 3. The abrasive grains according to claim 1, wherein said sintered alumina-based abrasive grains are made of aluminum oxide containing a rare earth metal.
The method for producing a vitrified whetstone according to claim 1 or 2, wherein the sol containing alumina seed and / or rare earth metal is dried and then heat-treated. 4. The method for producing a vitrified grinding wheel according to claim 1, wherein a total amount of the modified oxide contained in the vitrified binder is 20 wt% or less.