EP3590658B1 - Method for producing thin grinding bodies - Google Patents

Description

TECHNICAL AREA

The present invention relates to a method for producing vitrified bonded grinding wheels.

STATE OF THE ART

Bonded grinding tools or also grinding bodies are used in technology, in particular for surface treatment. To produce the bonded grinding tools, abrasive grains, for example those based on aluminum oxide or corundum, silicon carbide, diamond or CBN, are processed with a binder and other additives, such as fillers, active grinding substances, pore-forming agents or temporary adhesives, to form a mixture that is then pressed in a press mold to form the desired shape. The resulting green body is then dried, optionally freed from the added pore-forming agents at suitable temperatures and then ceramically fired.

Depending on the application, the grinding tools have different diameters and thicknesses, with the amount of work required to manufacture individual grinding wheels being almost independent of how thick a wheel is or what diameter it has. Thin grinding wheels can only guarantee a small amount of material removed during grinding. Thus, the cost of producing thin abrasives, calculated on the removal that can be achieved with these abrasives, is relatively high compared to thicker abrasives. Since the amount of work involved in the production of abrasive bodies makes up a large part of the production costs, the use of thin abrasive bodies is comparatively expensive. Although the manufacturing costs can be reduced through automation, this only pays off if large quantities of panes can be produced.

WO 2010/129813 A2 discloses a method of making a polycrystalline diamond abrasive article. Multiple layers of diamond grains are placed in a mold and each of these layers is surrounded by a substrate of cobalt containing tungsten carbide. A high temperature, high pressure sintering process is then carried out in the mold. During the sintering process, cobalt from the tungsten carbide substrate infiltrates the diamond layers where it catalyzes recrystallization of the diamond grains. This bonds the diamond grains together, creating a body of polycrystalline diamond (PCD) bonded to the tungsten carbide substrate. The resulting stack of tungsten carbide and PCD layers is removed from the mold and sliced. The tungsten carbide substrate is then removed and the catalyst is washed out of the remaining PCD material. Washing improves the thermal stability of the PCD material, resulting in a body of pure TSP.

U.S. 4,807,402 forms the basis for the preamble of claim 1 and discloses a method for the production of grinding wheels from diamond or CBN. Several layers of a layered structure are filled into a mold, each layer comprising a carrier layer and abrasive layers on both sides. Separating discs are provided between the individual layers. This layer structure is subjected to high pressure and high temperature at the same time in order to bond the carrier layer and the abrasive layers to one another.

PRESENTATION OF THE INVENTION

It is the object of the present invention to find a method for reducing the costs for the production of thin abrasive bodies without any loss of quality.

This object is achieved by a method according to claim 1.

When dealing with the stated problem, the inventors of the present application had the basic idea of first producing thick cylindrical shaped bodies in large press molds and then dividing them into individual thin grinding bodies. The first attempts, not claimed, consisted of cutting several thin discs from a cylinder to cut. For this purpose, a cylindrical base body for conventional vitrified bonded grinding wheels with corundum as the abrasive grain was manufactured. After sintering, the cylinder was cut into slices using a diamond saw, and the thin grinding bodies thus obtained were turned plane-parallel and then subjected to a grinding test. The grinding tests showed that the quality of the thin grinding wheels obtained in this way is comparable to that of conventionally manufactured individual grinding wheels. However, it soon turned out that cutting the cylinders into individual discs is relatively expensive and time-consuming, so that the original goal of producing thin grinding wheels at low cost could only be achieved to a limited extent in this way.

Building on the basic idea, the inventors of the present application then developed the idea further and produced several thin abrasive bodies in a single pressing process in a single mold by filling the mass of abrasive grain, binder and additives required for a thin abrasive body into the mold, the mass smoothed and covered with an intermediate disc, in order to then fill in the compound for the next grinding wheel. This method, which is not claimed, also delivered grinding bodies of impeccable quality, although the cost savings are limited, since the grinding bodies have to be processed further as individual grinding bodies after the pressing process and thus only the pressing of the grinding bodies was rationalized. In addition, inserting the intermediate panes requires additional handling, which further reduces the cost savings.

Finally, the solution to the problem according to the invention provided for a thin intermediate layer to be formed by sprinkling inert particles onto the mass filled and smoothed for the desired individual abrasive body. The mass for the next individual abrasive body was then poured onto the intermediate layer, which was in turn covered with an intermediate layer of inert particles after smoothing. This process was repeated until the mold was completely filled, leaving the mass for the last single grinding wheel uncovered. During pressing at high pressure, the layered molded body is compressed in such a way that it can be removed from the mold as a coherent body and then dried and sintered or fired. The sintered shaped body obtained in this way is built up in layers, the cohesion of the thin intermediate layers being made of inert particles which do not have a binder included, is not very pronounced. It should be noted that the radial friction between the individual grinding bodies is very high, so that the grinding bodies can only be moved against one another with difficulty, but can be lifted off easily. The desired individual grinding bodies can now be removed from the sintered shaped body thus obtained at the level of the intermediate layers without great mechanical effort. The grinding wheels then only have to be rotated plane-parallel in order to be able to be used as thin grinding wheels.

Abrasives with a layered structure are described in numerous documents. That's how she describes it EP 1 189 731 B1 a method and an apparatus for producing thin grinding bodies by pressing at least one layer formed from abrasive grains and two layers containing no abrasive grains and enclosing the grinding wheel, the two layers without abrasive grains being provided as reinforcing layers.

In the DE 34 42 230 C2 describes a method for producing an abrasive body with a plurality of abrasive layers, between which there is a grinding-inactive, vibration-damping damping layer, which preferably consists of a polymer. In this case, it is a matter of providing a method for the production of sound-insulated grinding wheels.

The DE 100 62 473 A1 describes a honing ring and a method for producing a honing ring, the base body of the honing ring being formed by a stack of discs. The individual discs are firmly bonded to one another with layers of adhesive or binding agent to form the desired honing ring.

The DE 35 45 308 A1 describes multi-layer grinding bodies which are built up in layers with the interposition of at least one layer of vibration-damping materials for noise damping, the layer of vibration-damping materials being brought into the mold in the form of a fine-grained, free-flowing powder and/or granules.

All of the documents cited above are aimed at producing multi-layer abrasive bodies, with the multi-layer structure always having a technical background and is therefore an indispensable part of the finished grinding tool when used. A multi-layer grinding body, which serves as a base body for the production of individual grinding bodies in order to produce thin grinding bodies in this way at low cost, could not be found in the prior art.

The present invention thus specifies a method for producing vitrified bonded grinding bodies, in which, in a first step, a mass of at least one mixture of abrasive grains and binder, sufficient for several thin individual grinding bodies, is placed in a mold. The mixture can optionally additionally contain grinding-active and/or grinding-inactive fillers, additives and pore-forming agents. The mass is positively pressed in the mold using a press, and the green body thus obtained is then sintered to form a shaped body. After cooling, the sintered shaped body is divided into individual disks.

According to the method described above, an oversized abrasive body or cylindrical shaped body is thus produced in a press mold and then divided into thin individual discs.

The filling of the at least one mixture into the mold is done in portions, with a portion sufficient for a single abrasive element being initially filled into the mold, the mass then being smoothed and then covered with an intermediate layer, and this process being repeated until the mold is filled .

In an embodiment not claimed, the intermediate layer is formed by inserting an intermediate disk. The shim can be made of any material that will not react with the abrasive grain mixture. For example, thin sheet metal can be used, which can be produced relatively cheaply and reused without any problems.

According to the invention, however, the intermediate layer is formed by sprinkling inert particles onto the mass filled and smoothed for the desired individual abrasive body. Suitable inert particles are, for example, loose corundum grains, which are sprinkled onto the mass filled and smoothed for the desired individual grinding wheel. The The layer thickness of the intermediate layer of loose corundum grains is advantageously 0.5 mm to 5 mm, preferably about 1 mm. The layer thickness selected must be thick enough to prevent the adjacent grinding wheels from sticking together, and at the same time small enough so that the multi-layer molded body holds together after pressing and can be further processed as a whole. The grain size of the loose corundum grains is advantageously F60 to F1000 according to FEPA.

Of course, any other particles can also be used for the intermediate layer, as long as they do not react with the mixture of abrasive particles and binder. However, the use of corundum is advisable, since the method according to the invention is preferably intended for the production of vitrified bonded grinding bodies with abrasive grains based on aluminum oxide or corundum.

The process is not limited to the production of grinding wheels with the same strength and composition, but thin grinding wheels with different strengths (thickness) can be produced in one pressing process, which are then sintered as a coherent shaped body in a sintering cycle. It is even possible to produce grinding bodies with different compositions using the method according to the invention if corresponding different mixtures are poured into the compression mold in portions and separated with an intermediate layer. Provided that the green bodies obtained from the different mixtures can be fired with the same sintering cycle, a layer of loose corundum can again be used as an intermediate layer. If the respective green bodies require different sintering cycles, it makes sense, in contrast to the claimed invention, to insert metal sheets as intermediate layers, so that the green bodies can be further processed separately after the pressing process. It should be noted that in principle it is of course also possible to choose different intermediate layers for a pressing process if, for example, different grinding wheels are to be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

Figur 1

eine perspektivische Darstellung der einzelnen Komponenten einer Zusammenstellung für den Brennvorgang;

Figur 2

eine Explosionsdarstellung der Fig. 1;

Figur 3

eine perspektivische Ansicht von oben auf die Fig. 1;

Figur 4

einen schematischen Ablaufplan für das erfindungsgemässe Verfahren.

Preferred embodiments of the invention are described below with reference to Drawings described, which are intended to be illustrative only and not to be construed as limiting. In the drawings show:

figure 1

a perspective view of the individual components of an assembly for the burning process;

figure 2

an exploded view of the 1 ;

figure 3

a perspective view from above of the 1 ;

figure 4

a schematic flow chart for the inventive method.

DESCRIPTION OF PREFERRED EMBODIMENTS

The figure 1 shows the individual components of a compilation for the burning process. This combination comprises a shaped body 1 which is made up of a plurality of grinding bodies 2 and intermediate layers 3 . The assembly also includes a trivet 4 on which the shaped body 1 is supported during the firing process and during transport to and from the kiln. For a better overview, the intermediate layer 3 and the abrasive body 2 in the figure 1 also shown separately. The intermediate layer 3 can be formed from loose corundum grains. The coaster 4 is a refractory ceramic carrier that can be used again after the firing process for the next firing cycle.

The figure 2 shows, from a side perspective, an assembly for a firing process of a multi-layer sintered shaped body 1, which consists of a total of four grinding bodies 2, which are each connected to one another via an intermediate layer 3. In the present case, the intermediate layer 3 can be formed from loose corundum grains, for example, which are compressed to such an extent during pressing that the intermediate layer 3 holds the shaped body 1 together without an additional binder. In comparison to the grinding bodies 2, the intermediate layer 3 is relatively thin, so that it is ensured that the shaped body 1 holds together as a whole and can be processed accordingly. However, the stability of the intermediate layer 3 is low enough that the individual grinding bodies 2 can be lifted off the stack and separated from one another after sintering with a low mechanical load along the intermediate layer 3 . The intermediate layer 3 thus acts like a perforation. The sintered one Shaped body 1 is mounted on a ceramic coaster 4, on which it is placed after pressing in order to then be driven into the sintering furnace.

The figure 3 shows the sintered body 1 from figure 2 from a slightly different perspective, with the viewer looking inside the stack of abrasives in the top view. The multi-layer molded body 1 made up of abrasive body layers 2 and intermediate layers 3 is deposited on a ceramic base 4, which is provided as a carrier for transporting the green body from the press to the sintering furnace.

In figure 4 Illustrates a simplified flow chart for the cost-effective manufacture of thin vitrified bonded grinding tools. First of all, abrasive grains, a ceramic binder, a pore-forming agent and, if necessary, adhesive and additives are mixed (step 21). The resulting mass is then removed from the mixer, sieved and placed in a mold (step 22). The filling into the mold takes place in portions, with a portion sufficient for a single abrasive element being initially filled into the mold, the mass then being smoothed and then covered with an intermediate layer, with this process being repeated until the mold is filled. Then the filled mass together with the intermediate layers is pressed with a press (step 23). The green body thus obtained is optionally dried and fired in an oven (step 24). After firing and cooling, the grinding bodies are separated from the sintered shaped body, for example by low mechanical stress along the intermediate layer, and separated (step 25).

Manufacture of abrasives

In a grinding test, it should be checked whether the changed manufacturing process has an influence on the grinding performance. For this purpose, the same raw material components shown in Table 1 were used for all tested abrasives, so that a direct comparison of the abrasives produced under different conditions is possible through the grinding test. The quantities of the individual components are each based on the abrasive grain (100%). <u>Table 1</u> raw material characterization Amount (% by weight) abrasive grain corundum 100 ceramic binder oxide mixture 11 Glue polymer solution 2 additives Various additives 3 pore builder charcoal 6

The components were placed in a drum mixer and mixed in 23 mixing steps for about 60 minutes until the composition was visually homogeneous and free-flowing. The mass was then removed from the mixer and screened out. The screened mass was placed in a mold to produce a comparison pane and pressed with a press at a pressure of 90 bar to form a positive fit. The green body obtained in this way had the dimensions (diameter×height×bore) 280×35×157 mm and was fired in a furnace to a maximum temperature of 1200° C. using a predetermined firing program.

Portions of the same mass were placed in a higher mold with the same diameter, with a portion sufficient for a single grinding wheel being filled into the mold, the mass then smoothed and then covered with an intermediate layer and this process repeated until the mold with was filled with a compound for a total of four grinding wheels. A layer of fine corundum white with a grain size of F80 and a layer thickness of approx. 1 mm was chosen as the intermediate layer.

grinding tests

The finished abrasives had the properties described in Table 2 below. Sample A was conventionally manufactured as a single disk in a small mold. Sample B was an edge piece of the sintered multi-layer molded body. Sample C was taken out from the center of the sintered multi-layer molded body. The first step in testing the discs was Limit chip volume or the equivalent limit chip thickness h _{eq_th} until grinding burn occurs and in a second step the limit chip volume or the equivalent limit chip thickness h _{eq_v} with regard to exceeding the permissible wear limit. Both values are also recorded in Table 2. <u>Table 2</u> specimen A (comparison) B (Border) C (middle) Dimensions (mm) 275×30×160 275×30×160 275×30×160 weight (g) 3907 3921 3911 Density (g/cm ³ ) 1,881 1,874 1,897 Grinding burn limit (%)* 103 105 102 Wear limit (%)** 105 110 104 * Grinding burn limit = maximum achievable equivalent chip thickness h _{eq_th} that can be used without grinding burn, ie without thermo-mechanical surface layer damage.
** Wear limit = maximum achievable equivalent chip thickness h _{eq_v} that can be used while maintaining a specified wear criterion.

The discs were tested on a Reishauer RZ 260 machine using cooling oil and a diamond dressing tool. A test wheel made of the material 16MnCr5 was selected as the workpiece.

In the grinding burn test, work was carried out in three stages with three leveling strokes, one roughing stroke and one finishing stroke by systematically increasing the axial feed (Z feed) with otherwise the same cutting values and cutting conditions. In this way, a uniform infeed could be ensured in the 2nd stage for the roughing stroke. The evidence of overheating was carried out after the finishing stroke (3rd stage) by means of nital etching.

The wear test was carried out with a comparable technology, whereby the 2nd stage worked with a variable Z feed during the roughing stroke and after the finishing stroke (3rd stage) the wear in the area of use of the grinding worm was determined during the roughing stroke. If a form deviation of the profile f _ff > 6 µm according to DIN 3960 is exceeded at a specified grinding speed, the performance limit is reached.

It can be said that several thin vitrified bonded abrasives can be easily manufactured in one pressing process without any loss of quality. The production costs per grinding wheel can be significantly reduced, in particular due to the rationalization of the pressing process and the sintering step. When using loose corundum as inert particles for the intermediate layer, cost savings of up to 30% can be achieved.

REFERENCE LIST

1

molding

2

grinding wheel

3

intermediate layer

4

coasters

21

process step (mixing)

22

Process step (filling)

23

process step (pressing)

24

process step (sintering)

25

Process step (separation)

Claims (7)

1. A method for the production of vitrified bonded grinding bodies (2), wherein

   - a mass of at least one mixture of abrasive grains and binder, sufficient for several individual abrasive bodies (2), is introduced into a mold, the mixture optionally additionally containing abrasive and/or non-abrasive fillers, additives and pore-forming agents,

   - the filling of the at least one mixture into the mould is carried out in portions, wherein first a portion sufficient for a single grinding body (2) is filled into the mould, the mass is subsequently smoothed and then covered with an intermediate layer (3) and this process is repeated until the mould is filled, and

   - the mass is positively pressed in the mold by means of a hydraulic press,

   characterized in that

   - the intermediate layer (3) is formed by sprinkling inert particles, which do not show any interaction with the abrasive grain-binder mixture, onto the mass filled and smoothed for the desired individual abrasive body (2),

   - the green body thus obtained is subsequently sintered to form a shaped body (1), and

   - the sintered shaped body (1) is divided into individual grinding bodies (2) after cooling.
2. The method according to claim 1,
   characterized in that
   the intermediate layer (3) of inert particles has a layer thickness of 0.5 to 5 mm.
3. The method according to claim 1,
   characterized in that
   the intermediate layer (3) of inert particles has a layer thickness of approx. 1 mm.
4. The method according to any one of claims 1 to 3,
   characterized in that
   the intermediate layer (3) is formed by sprinkling loose corundum grains onto the mass filled and smoothed for the desired individual grinding body (2).
5. The method according to claim 4,
   characterized in that
   the grain size of the loose corundum grains is F60 to F 1000 according to FEPA.
6. The method according to any of the preceding claims,
   characterized in that
   all grinding bodies (2) have the same thickness and the same composition.
7. The method according to any one of claims 1 to 5,
   characterized in that
   the grinding bodies (2) do not all have the same thickness and the same composition.

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