NØRBYGAARD, Thomas (Cedervangen 70, Allerød, DK-3450, DK)
JENSEN, Alice, Zinger (Chr. Holmsvej 28, Klampenborg, DK-2930, DK)
NØRBYGAARD, Thomas (Cedervangen 70, Allerød, DK-3450, DK)
1. An abrasive disc for grinding a materialographic sample, the abrasive disc comprising a grinding layer having an exposed grinding surface to be brought into contact with a materialographic sample to be ground; and a base layer defining a mounting surface of the abrasive disc opposite the grinding surface for mounting the abrasive disc on a rotatable support member for rotating the disc around an axis projecting through the mounting surface and the grinding surface; the grinding layer comprising abrasive grains disposed in a bonding matrix; wherein the grinding surface includes a plurality of depressed portions having a depths between 5 and 60% of the thickness of the grinding layer.
2. An abrasive disc according to claim 1 , wherein the bonding matrix has a porosity of between about 30% and about 55%.
3. An abrasive disc according to claim 2, wherein the grinding layer has an open pore structure and an open pore porosity of between about 15% and about 55%.
4. An abrasive disc according to claim 2 or 3, wherein the porosity of the grinding layer varies along a direction parallel with the exposed grinding surface.
5. An abrasive disc according to claim 2 or 3, wherein the grinding layer has an apparent density that varies along a direction parallel with the exposed grinding surface.
6. An abrasive disc according to claim 4 or 5, wherein the porosity of the grinding layer defines a pattern of alternating areas of a first and a second type in a plane parallel with the exposed grinding surface, each area of the second type being surrounded by a number of areas of the first type, and having a higher porosity that each of the number of areas of the first type by which the area of the second type is surrounded.
7. An abrasive disc according to any one of claims 1 through 6, wherein the abrasive disc is a circular disc.
8. An abrasive disc according to any one of claims 1 through 7, wherein the bonding matrix comprises a ceramic material.
9. An abrasive disc according to any one of claims 1 through 7, wherein the bonding matrix is a phenolic resin.
10. An abrasive disc according to any one of claims 1 through 9, wherein the grinding layer has a larger diameter than the base layer.
11. An abrasive disc according to any one of the preceding claims, wherein the grinding layer comprises between about 25 vol. % and about 55 vol. % abrasive particles.
12. An abrasive disc according to any one of the preceding claims, wherein the depressed portions have a total cross sectional area measured in a plane parallel to the grinding surface, wherein the total cross sectional area varies as a function of the distance from the grinding surface.
13. An abrasive disc according to any one of the preceding claims, wherein at least two of the depressed portions have different depths.
14. An abrasive disc according to any one of the preceding claims, wherein one or more depressed portions have a dimension in a direction parallel with the grinding surface, which dimension decreases with increasing distance from the grinding surface.
15. Use of an abrasive disc according to any one of claims 1 through 14 for grinding materialographic samples of a soft and ductile material.
16. Use according to claim 15, wherein the soft and ductile material has a hardness smaller than 400 HV and is capable of suffering deformation of more than 10% strain without fracture.
17. Use according to claim 15 or 16, wherein the soft and/or ductile material is chosen from at least one of aluminium, an aluminium alloy, copper and a copper alloy, a PCB, lead, stainless steel.
18. Use according to any one of claims 15 through 17, comprising rotating the abrasive disc around an axis protruding through the grinding surface and bringing the materiallographic sample into contact with the grinding surface so as to grind at least one surface of the materiallographic sample.
19. Use according to any one of claims 15 through 18, further comprising polishing the ground surface of the materiallographic sample, and performing an analysis step to determine at least one property of the polished surface of the materiallographic sample.
This invention relates to an abrasive disc for matehalographic, in particular metal lographic, sample preparation and to a method for producing such a disc.
Materialographic sample preparation generally refers to methods for preparing a sample of a material prior to an analysis of the prepared sample. Materialographic sample preparation generally seeks to reveal properties of a materialographic sample, e.g. the structure of the examined material, without artefacts or contribution from the chosen preparation method. This is typically accomplished in several steps, from cutting up the specimen, followed by one or more grinding and polishing steps with increasingly fine abrasives. The grinding step(s) is/are a crucial part of the chain of preparation steps, where the aim is to remove all traces of the cutting procedure and to obtain a specimen surface having a high degree of planarity - prepared for the subsequent polishing - while at the same time causing minimal damage and deformation to the specimen surface.
Abrasive discs for materialographic sample preparation are widely used, and examples of abrasive discs for materialographic sample preparation are disclosed in WO 96/07508 and WO 99/08837. In use, such abrasive discs are placed on a rotatable support disc such that an abrasive top surface of the disc is exposed to be brought in contact with a sample to be prepared. The abrasive disc is rotated around an axis projecting out of the top surface while the sample is brought into contact with the exposed top surface of the abrasive disc. Even though such abrasive discs have proven suitable for a wide range of hard materials, and there exist suitable grinding solutions for hard steels and ceramics, there remains a range of materials which are notoriously difficult to plane grind in a continuous and reproducible manner. In particular, grinding of relative soft materials and/or materials with a relative high ductility has been found to be difficult when preparing metallographic samples. Examples of such materials include aluminium and aluminium alloys, stainless steels, copper and copper alloys, lead etc. Examples of samples of soft and/or ductile materials include printed circuit boards (PCBs) These samples/materials are easily deformed and damaged during grinding, and problems such as inclusions, smearing and clogging of the grinding tool are common.
The term "inclusions" refers to hard particles such as broken off abrasive grains that may be buried into the softer sample material obstructing subsequent polishing, or giving rise to erroneous structural conclusions.
The term "clogging" refers to grinding chips of the soft sample material becoming irreversibly lodged in crevasses of the grinding surface, thus building up and closing the pore structure of the grinding tool material. This will usually result in a (rapid) decrease in performance of the grinding tool, with reduced removal rate and poor grinding quality (scratches or smearing).
The term "smearing" refers to the built up of a thin layer of non-crystalline material covering the true structure of the material. Instead of being removed (as chips) during the grinding process, such a thin layer may be created by the abrasive tool when the sample material is translated parallel to the surface. Such smearing may be caused by unsuitable combinations of sample material and grinding tool or by a dysfunctional grinding tool, e.g. as a result of clogging. Despite continued efforts in this area it has so far been found difficult to find a solution to the above problems. It is thus generally desirable to develop an abrasive disc that solves at least one of the above problems. Even though attempts have been made to suggest alternative tools, e.g. in GB 225,436 published in 1924, a widely used procedure for plane grinding soft and/or ductile materials today still involves the use of single-use grinding paper such as SiC paper or similar materials. Thus, there remains a need for a multi-use grinding tool which offers continuous, uniform grinding without frequent cleaning, truing or dressing. Such a product would allow increased automation, and offer improved reproducibility and flexibility.
Disclosed herein are embodiments of an abrasive disc for grinding of materialographic samples. Embodiments of an abrasive disc for grinding a materialographic sample comprises a grinding layer having an exposed grinding surface to be brought into contact with a materialographic sample to be ground; and a base layer defining a mounting surface of the abrasive disc opposite the grinding surface for mounting the abrasive disc on a rotatable support member for rotating the disc around an axis projecting through the mounting surface and the grinding surface; the grinding layer comprising abrasive grains disposed in a bonding matrix. In embodiments of the grinding disc described herein the grinding layer provides a grinding surface that includes a plurality of depressed portions having a depth between 5% and 60%, e.g. between 10% and 50%, e.g. between 10% and 60%, e.g. between 20% and 50%, such as between 25% and 40%, e.g. about 30% of the thickness of the grinding layer. Hence, the depressions only extend partially into the grinding layer, thus a part of the grinding layer remains intact between the bottom of the depression and the base layer without exposing any non-grinding layers at the bottom of the depressions. In some embodiments, the depressed portions have a depth of between about 0.1 mm and about I mm, such as between about 0.4mnn and about O.δmnn, such as between about 0.5mnn and about 0.7 mm, e.g. about 0.6 mm.
For the purpose of the present description, the term grinding layer is intended to refer to the layer of the abrasive disc that comprises abrasive material and provides the grinding function to a specimen brought into contact with the abrasive disc. The term base layer is intended to refer to any non-grinding layer(s) that is/are provided for mounting and/or for providing the desired stiffness and/or stability to the abrasive disc, but which does/do not include abrasive material and which during normal operation are not brought into direct contact with the specimen to be ground.
A production process for manufacturing embodiments of the abrasive disc described herein may thus include a pressing step and a thermal treatment step, e.g. a baking step. Due to the pressing and thermal treatment/baking processes during production of embodiments of the abrasive disc described herein, the grinding surface of an unused abrasive disc may contain relatively fewer exposed abrasives as the remainder (deeper layers) of the grinding layer. Therefore, the initial grinding will be significantly slower (i.e. with a lower removal rate) than the average grinding rate for the abrasive disc, as initially a surplus of binding matrix needs to be removed. This process is sometimes referred to as a need for "opening" the disc. Once the outer layer has been ground away, the grinding process enters a steady state - where new abrasive grains are exposed at the same rate as old abrasive grains are worn down or pulled out of the disc.
The depth of each depressed portion may be defined as the largest depth of the depressed portion relative to the exposed grinding surface of the grinding layer that during operation is brought in contact with a specimen to be ground. The surface area of the exposed grinding surface will also be referred to as the specimen contact area. By applying a pattern of depressed portions, e.g. in the form of a strong pressure pattern, covering most of the upper surface of the disc, the specimen contact area is smaller during the initial grinding by a new disc compared to the same disc after a certain period of use, thus providing an increased actual grinding force (given a constant nominal force) during initial use. During continued use of the grinding disc, the deeper layers of the grinding layer are exposed. When the upper layers of the grinding layer up to the depth of the depressions have been removed due to the grinding action, the entire surface area of the remaining grinding layer is exposed and the specimen contact area is increased accordingly. In some embodiments, the pressure pattern covers more than 50% of the surface area, e.g. between about 60% and about 90% of the surface area, e.g. between about 75% and about 85% of the surface area. At the same time multiple edges are introduced to the surface, making it more aggressive. These two factors counteract the inherent slowness of the closed surface, resulting in a surface that provides improved grinding during the initial grinding period of a new disc. As the pattern is removed by wear the disc functions on a full surface after the initial start-up period.
Embodiments of the abrasive disc described herein thus reduce the need for dressing.
In some embodiments the total cross sectional area of the depressions (in a plane parallel to the grinding surface) may vary as a function of the distance from the exposed grinding surface of the unused abrasive disc. The total cross sectional area may be defined as the sum of the cross sectional areas of all depressed portions at a given distance from the exposed grinding surface. In particular when the total cross sectional area of the depressions decreases with increasing distance from the exposed grinding surface of the unused abrasive disc, the specimen contact area is gradually increased during use until the full surface area of the grinding layer is exposed. Hence, during use, the fraction of the exposed surface form by depressions decreased during use.
Such a decrease of the total cross sectional area of the depressed portions may be provided by providing depressions of different depths and/or by providing one or more depressions that have a dimension in a direction parallel with the grinding surface which dimension decreases with increasing distance from the grinding surface. For example, the depressions may be provided with lateral side walls that are not orthogonal to the grinding surface. For example, the depressions may be wedge shaped depressions, pyramidical depressions, hemispherical depressions, elongated channels that have a cross section (in a plane orthogonal to longitudinal direction of the channels) which is triangular, trapezoidal, elliptic, parabolic and/or the like.
Some embodiments of the abrasive disc disclosed herein provide a grinding surface which has predetermined porosity properties.
In particular, in some embodiments, the bonding matrix has a porosity of between about 30% and about 55% such as between about 35% and about 50% such as between about 40% and about 45%. The high porosity causes small tunnels to be exposed by wear of the grinding layer during the grinding process. Ground-off material and used abrasives are thus efficiently flushed away, thereby keeping the upper surface clean and sharp. In addition a high porosity facilitates cooling of the active grinding sites.
When the grinding layer has an open-pore structure, an efficient network of small tunnels is provided in the grinding layer, thus further improving the effectiveness of the cleaning, sharpening and cooling of the grinding site. For the purpose of the present description, the term open-pore structure refers to structures having a high open porosity, in particular an open porosity higher than 15%. Hence, an open-pore structure generally has a high surface area/volume ratio. In some embodiments, the open porosity of the grinding layer is between about 15% and about 55%, such as between about 20% and about 45% such as between about 25% and about 40%, e.g. between about 25% and about 35%, e.g. about 30%. The term "open porosity" relates to the porosity of a material caused by a pore structure in which fluid flow can effectively take place, as opposed to closed porosity which is caused by dead-end pores or non-connected cavities. The term effective porosity is often used to denote open porosity. The porosity including closed as well as open pores will be referred to as total porosity or simply porosity.
The porosity of the grinding layer may be controlled during the production process of the disc, e.g. via the choice of binder and/or abrasive, or it can be obtained via organic additives, added to the binding mixture before pressing and which evaporate during heat treatment or by additives that can be dissolved by a suitable solvent.
An open-pore structure provides improved cleaning mechanisms, thereby ensuring that the grinding rate is kept high and constant without the need to clean or dress the disc, i.e. the disc is self dressing.
In some embodiments, the grinding layer has an apparent density that varies along a direction parallel with the exposed grinding surface.
The surface pattern of depressed portions may be provided during the production process of the abrasive disc, e.g. during a pressing step, for example by imposing a surface pattern on the grinding surface of alternating elevated and depressed portions. Such a processing step may thus further result in a varying apparent density. In some embodiments, the varying apparent density may be provided and/or enhanced by additional or alternative measures. For example material can be distributed non- homogenously into the pressing form e.g. in a specific pattern. Alternatively or additionally, the disc can be pressed in sequential layers, each layer containing patterns of density distribution.
Embodiments of the abrasive disc described herein provide a slow but constant wearing of the surface which effectively eliminates any build-up of foreign material (clogging), while ensuring a high and constant grinding rate without the need to clean or dress the disc, i.e. the disc is self dressing.
The surface properties of embodiments of the abrasive disc described herein may be adapted to different needs e.g. different materials, e.g. by varying the amount of abrasive grains per unit volume, the type of abrasive grains, the type of binder, etc. The bonding hardness of the matrix may be customized so as to allow a very fine wearing of the disc, continuously opening the structure for new abrasive grains, while retaining an excellent wear resistance of the disc. This balance ensures that the grinding surface is used and worn down concurrently with the grinding of the sample material giving the disc self sharpening properties. Even as abrasives are worn down, become rounded and lose their edge, these "old" abrasives become increasingly exposed as the binder is worn away, resulting in abrasive fracture or eventually complete dislodgment from the disc. The new edges of fractured abrasives combined with fresh abrasives that become exposed as the disc wears, result in a continuous supply of new sharp edges to maintain grinding efficiency throughout disc lifetime.
In one embodiment the grinding layer has a larger diameter than the base layer, i.e. the grinding layer has a circumferential rim portion radially protruding outwards beyond a circumferential rim of the base layer, thereby avoiding that the materialographic sample is brought into contact with the base layer during treatment due to wear of the circumferential rim of the grinding layer. The base layer may comprise a substantially plane interface surface, opposite to the mounting surface, onto which interface surface the grinding layer is applied. The base layer may be made of any type of material which can withstand a grinding process e. g. metal, impregnated woven or non- woven tissue, cardboard, plastic, such as plastic with incorporated glass fibres or metal particles etc. The mounting surface of the base layer is adapted to be applied onto a rotatable disc, table, abutment or other type of rotatable support member of a grinding apparatus. For this application, it is desirable that this mounting surface of the base layer is plane. In order to facilitate a temporary fixation of the abrasive disc to a magnetized rotatable disc of a grinding apparatus, the base layer may comprise ferromagnetic material. The ferromagnetic material may be in any form and be placed anywhere in the base layer, e. g. in terms of ferromagnetic granules incorporated in a polymer liner, as a separate layer, or the like. In one embodiment a metal foil may be used as a base layer, e. g. a foil having a thickness of around 0.05-2 mm. A base layer made of a thin, magnetic sheet provides resilience to the porous abrasive layer and allows rapid changing of the abrasive disc.
The grinding layer is made of a composite material comprising a bonding matrix having abrasive particles embedded therein. The matrix may further have one or more non-grinding admixtures embedded therein, and/or comprise further additives, e.g. lubricating agents and/or soluble particles for providing increased porosity.
Even though other materials, such as a ceramic bonding matrix, epoxy, polyurethane, polyester, acryl, may also be used as a bonding matrix, an organic resin such a phenolic resin, e.g. Bakelite, has been found to yield particularly good grinding results in connection with soft and ductile materials. Phenolic resin systems provide a high degree of flexibility in terms of controlling the desired porosity and binding hardness.
Soft and ductile materials may generally be defined as materials with low to medium hardness, e.g. less than 400 HV as determined by the Vickers hardness test, and which can suffer significant plastic deformation, e.g. more than 10% strain, without fracture.
The abrasive grains may be grains of one or more suitable materials e.g. SiC, AI2O3, Diamond, and/or the like. Preferred choices of abrasive grains may depend on the specific type of sample material to be ground.
In some embodiments, the grinding layer is a thickness of between 0.5mm and 4mm, e.g. between about 1 mm and about 3mm, e.g. between about 1.5mm and about 2.5mm such as about 2mm. Hence, even though the grinding layer is only around one or a few millimetres thick, it has turned out that it can be used for several hours of grinding without significantly degrading the grinding quality. Furthermore, the limited thickness of the grinding layer prevents any unevenness of the grinding layer caused by non- uniform wear from ever reaching critical levels. Thus the disc can be used without truing throughout its entire lifetime.
The present invention relates to different aspects including the abrasive disc described above and in the following, corresponding methods, and uses, each yielding one or more of the benefits and advantages described in connection with the above-mentioned abrasive disc, and each having one or more embodiments corresponding to the embodiments described in connection with the above-mentioned abrasive disc.
BRIEF DESCRIPTION OF THE DRAWINGS The above and other aspects will be apparent and elucidated from the embodiments described with reference to scaling of two-dimensional images represented by pixels arranged in scan lines and columns and with reference to the drawing in which:
Figs. 1 a-b show an embodiment of an abrasive disc.
Figs. 2a-b show other embodiments of an abrasive disc.
Fig. 3 shows a cross-sectional view of yet another embodiment of an abrasive disc.
Fig. 4 illustrates the relation between porosity of an abrasive disc, and the relative amounts of abrasives and binder in the grinding layer of embodiments of an abrasive disc as described herein.
Fig. 1 a shows a top view of an embodiment of an abrasive disc, while fig. 1 b shows a cross-sectional view of the abrasive disc of fig. 1 a along the line M-Il.
The abrasive disc is a circular disc having a circular grinding surface 7 and a mounting surface 4 opposite to the grinding surface. The circular disc defines an axis 8 of rotation through the centre of the abrasive disc and protruding through the grinding and mounting surfaces. The abrasive disc comprises a grinding layer 2 disposed on top of a base layer 3 such that the grinding layer provides the grinding surface 7 and the base layer provides the mounting surface, and the grinding and base layer define an interface surface 15 between the base and grinding layers.
Generally, the abrasive disc is a relatively thin planar structure that defines a plane of the disc, and the grinding surface is parallel to that plane. The abrasive disc may be circular. The diameter of the abrasive disc may be between 150-500 mm, such as between 200-400 mm; however, it will be appreciated that the abrasive disc may have any size as long as it is possible to rotate the sheet on a rotatable disc. Embodiments of the abrasive disc may have a thickness of between 1 mm and 10mm, e.g. between 1 -8mm, such as between 1.8mm and 3mm, such as 2mm.
The grinding layer is made of a bonding matrix 6 of a phenol resin having embedded in it abrasive particles, as illustratively designated by reference numeral 5. Alternatively, other materials may be used as a bonding matrix. The grinding layer 2 has a larger diameter than the base layer such that a circumferential rim 14 of the grinding layer protrudes radially outwards beyond the rim of the base layer.
The abrasive particles/grains employed may e.g. be diamonds, aluminum oxide or silicon carbide of different fineness for the different steps. It will be appreciated that combinations of different types of abrasive particles may be used in the same abrasive disc. The size of the abrasive particles may be chosen according to the desired grinding results, e.g. particles having an average diameter of between about 22μm and about 200μm, corresponding to a grit size of between about #80 and about #800. In some embodiments even smaller particles may be used, e.g. particles having an average diameter of between about 14μm and about 22μm, corresponding to a grit size of between about #800 and about #1200. It will be appreciated that particles of different sizes may be used in the same disc.
The base layer 3 should have a sufficient stiffness in its own plane to prevent crumpling of the abrasive disc under the influence of the tangential grinding force, and besides it should be sufficiently flexible perpendicularly to its own plane to be urged into plane contact with the surface of the rotatable disc of a grinding apparatus under the influence of the axial grinding pressure. The base layer may consist of metal, e.g. tinplate, Cu, Al, with a thickness of the order of 0.05-2.0 mm, e.g. between 0.1 -1.0 mm, but it is also possible to use a non-metallic sheet fulfilling the specified conditions, e.g. consisting of fibre, hard plastics, cardboard, impregnated paper, impregnated tissue, e.g. non- woven. In one embodiment, the base layer is a plate having a thickness of about 0.3mm, optionally with an anti-slip coating in the back side having a thickness of about 0.09mm.
When using soft iron or another material with high magnetic permeability, in combination with a permanently magnetized rotatable disc of a grinding apparatus, the exchange of grinding and/or polishing discs between different steps of the treatment can take place by peeling-off the abrasive disc from the rotatable disc of the grinding apparatus and replacing it by another, and each of the abrasive discs can be separately and repeatedly re-used until they are worn out, and can then be discarded.
If abrasive discs with base layers without magnetic properties are used, the base layer may be provided on its mounting surface with an adhesive layer with a detachable protective layer.
It will be appreciated that, even though the base layer is shown as a single layer, the base layer may itself be a layered structure comprising a plurality of non-grinding layers, e.g. a magnetic and a non-magnetic layer. The base layer may also comprise an adhesive layer at the interface 15 between the base layer and the grinding layer for providing a bond between the grinding and base layers. Examples of suitable adhesives include glue, such as epoxy and/or acrylic glue, and tape such as double-sided adhesive tape. Generally, the mounting surface is an uninterrupted plane surface.
The abrasive disc of figs.1 a-b has a structured grinding surface 7. In particular, the grinding surface 7 comprises a pattern of alternating elevated portions 11 and depressed portions 10. In the embodiment of figs. 1 -b, the surface pattern only covers a central portion of the grinding surface 7 leaving a rim portion 12 without a pattern, thus leaving the rim of the abrasive disc without edges resulting from transitions between elevated and depressed portions, as such edges would cause the abrasive disc to more easily break at the rim. In the example of figs. 1 a-b, the elevated portions 11 have a hexagonal shape, and they are arranged on a regular grid, leaving depressed portions 10 in the form of channels between respective elevated portions. Even though other geometrical shapes may be used, depressed and/or elevated portions in the form of polygons further facilitate the grinding process due to the plurality of sharp edges. In some embodiments between about 25% and about 50% of the surface area covered by the surface pattern corresponds to depressed portions. It will be appreciated, however, that the elevated portions and/or the depressed portions may have different shapes and/or may be arranged in a different way across the grinding surface. In one embodiment, the top surfaces of the elevated portions 11 together define a plane grinding surface, i.e. the top surfaces of all elevated portions 11 have the same height measured from the interface 15 between the grinding layer and the base layer.
The depth c/ of the depressed portions 10 measured from the surface 7 defined by the elevated portions is smaller than the thickness D of the grinding layer. The depth c/ may be measured as the depth of the deepest point of a depression relative to the lowest point of an elevated surface adjacent to the depression. In one embodiment all depressed portions have the same depth. When the depth of the depressed portion is between 5% and 60%, e.g. between 5% and 50%, e.g. between 10% and 60%, e.g. between 10% and 50%, such as between 25% and 40% of the thickness of the grinding layer, the abrasive disc provides a good initial grinding performance without the need to "open" the abrasive disc, while providing an uninterrupted grinding surface once the elevated portions have bee worn off during the initial grinding. For example, the pattern may be imprinted in the upper 0.3-0.8 mm of the grinding layer, e.g. corresponding to about 1/3 -1/6 of the total thickness of the grinding layer In use, embodiments of the abrasive disc described herein are placed on a rotatable disc of a grinding apparatus such that the mounting surface is in contact with the rotatable disc. A matehalographic sample is then brought into contact with the exposed grinding surface while the abrasive disc is caused to rotate around its axis 8 by the rotatable disc on which it is placed, i.e. the sample is pressed against the grinding surface in a direction parallel to the axis of rotation. Typically, the disc is rotated at between 100-500 rpm, e.g. between 200rpm-400rpm. Optionally, the sample holder may also be rotated, e.g. at between about I OOrpm and about 200 rpm around an axis parallel to the axis 8 around which the rotatable disc rotates. The resulting relative speed of the sample across the grinding surface may be between 0.3m/s and 5m/s; a typical speed being 3 m/s. During the grinding process, a lubricant/coolant may be applied. Water may be used as a lubricant/coolant, thus providing a simple setup, compatible with existing grinding machines.
Generally, an abrasive disc may be produced by providing the grinding layer and the base layer, and by attaching the grinding layer to the base layer, e.g. by means of a suitable adhesive.
The grinding layer may be produced by blending a mixture of abrasive particles, bond material, and dispersoid particles. Alternatively or additionally to the use of dispersoid particles, the porosity may be controlled by the particle sizes of the abrasive and/or bonding particles, by the bonding type, and/or by the applied pressure during pressing. In one embodiment, the mixture comprises between about 70 weight% and about 90 weight% abrasive particles and between about 10 weight% and about 30 weight% binder. The mixture may then be filled into a mould of a suitable shape and pressed into a disc-shape composite which then may be thermally treated/baked e.g. at pressures between about 200 and about 400 bar, and at temperatures between about 170 0 C and about 200 0 C e.g. for about 40 hours in the case of a Bakelite bonding and at between about 800 0 C and about 1300 0 C for a similar period in case of ceramic bonding.
The thermally treated disc may then be immersed into a solvent such as e.g. water in which the dispersoid particles are soluable but not the bonding material and the abrasive particles. When the thermally treated disc is immersed in the solvent for a period of time suitable to dissolve substantially all of the dispersoid particles the process results in a porous abrasive disc. To avoid the extra production process of dissolving the dispersoid particles, these could instead be soluble in the grinding lubricant i.e. they are dissolved as the disc is used, e.g. as described in WO 99/08837 in connection with organic acid crystals (e.g. citric acid) as dispersoids.
In an alternative production process that does not require the use of dispersoid particles, a powder mixture (e.g. of Bakelite and abrasives) is dried for about 1 -2 days. A mould is filled with a predetermined amount of the dried powder, and the powder is evenly distributed in the mould. The powder is pressed and the resulting disc is heat treated for about 40 hours. A plurality of discs may be heat treated together, e.g. by stacking them with metal sheets separating the individual discs. Finally, the resulting discs are glued on a backing layer.
Generally, the porosity of a sample of the grinding layer may be measured using a volume/density method which determines the pore volume as the apparent volume minus the material volume. The volume measurements may be performed by any known measurement technique, e.g. as described in ISO standard 1014 in connection with the determination of the density of coke using this principle. The total porosity may be measured by measuring the apparent volume of a sample of the grinding layer and the volume of the sample after having been crushed, e.g. in a mortar. The open porosity may be determined from the apparent volume of a sample of the grinding layer and from the volume of displaced fluid when the sample is submerged into a fluid.
The apparent volume of a grinding layer or a predetermined part thereof may be determined by use of a caliper on rectangular cut samples of the grinding layer of an abrasive disc. Weighing the cut samples further allows determination of the apparent density of the cut samples.
The open porosity of the samples is subsequently determined using a density measurement kit. The cut samples are weighed in air and then submerged in alcohol (95%), a low surface tension liquid, and weighed again while being submerged. The difference between these two weight measurements gives the weight of the displaced volume of liquid. The open pore volume (i.e. the effective porosity pore volume) can be calculated from the displaced volume of liquid divided by the density of the liquid. This in turn allows the calculation of the open porosity as the open pore volume divided by the apparent volume. The open pore porosity values disclosed in the present description refer to measurements performed according to the above process. However, other suitable methods for measuring the open porosity may be used as well.
By using a crushed sample (e.g. using a mortar) in the above procedure the porosity is measured i.e. including closed as well as open pores. The true material density of the grinding layer can be calculated accordingly.
The inventors have found that it is desirable to minimize the fraction that isolated (or closed) pores constitute of the total porosity of the discs thus the effective porosity almost equals the total porosity.
The measured true material density can also be used to estimate the relationship between abrasive and binder, due to the fairly large difference in specific gravity between the two. The surface pattern of alternating elevated and depressed portions may be provided during the production process, e.g. the production process described above. For example, a stamp or punch with a corresponding negative pattern for pressing the composite material may be used during the pressing step. Alternatively, the composite material may be filled in a mould having a bottom surface having a corresponding negative pattern. By pressing the composite material in such a way before the thermal treatment, the final disc will thus have the desired surface pattern. Furthermore, this process results in the abrasive disc to have an apparent density that varies in a plane parallel to the grinding surface, even at a depth larger than the depth c/ of the depressions. Consequently, even after the initial use during which the elevated portions of the grinding layer have been worn off, the exposed grinding surface still has an apparent density the varies across the exposed grinding surface. It has turned out that this varying apparent density further improves the grinding action provided by the abrasive disc.
Figs. 2a-b each show an alternative embodiment of an abrasive disc. The abrasive discs of figs. 2a-b are similar to the abrasive disc of figs. 1 a-b.
The abrasive disc of fig. 2a differs from the abrasive disc of figs. 1 a-b in that the depressed portions have different depths. In the example of fig. 2a, the grinding surface comprises three types of depressed portions 10a, 10b, and 10c, respectively. The depressed portions 10b have a larger depth than the depressed portions 10a, but a smaller depths than the depressed portions 10c. During use of the abrasive disc, the top part of the abrasive layer is gradually removed. When the abrasive layer has been removed up to a thickness equal to the depth of the depressed portions 10a, the specimen contact area is increased by an amount corresponding to the size of the depressed portions 10a. After further removal of abrasive material down to a thickness equal to the depth of depressed portions 10b, the specimen contact area is further increased by an amount corresponding to the size of the depressed portions 10b. Finally, after further removal of abrasive material down to a thickness equal to the depth of depressed portions 10c, the specimen contact area is further increased by an amount corresponding to the size of the depressed portions 10c. Hence, the abrasive disc of fig. 2a provides a stepwise increase of the specimen contact area during use of the disc. While the abrasive disc of fig. 2a has depressed portions with three different depths, it will be appreciated that alternative embodiments may be provided with a different number of different depths. Furthermore, it will be appreciated that some or all of the depressed portions may have different dimensions in a directiona parallel to the plane defined by the grinding surface.
The abrasive disc of fig. 2b differs from the abrasive disc of figs. 1 a-b in that the depressed portions have a dimension in a direction parallel to the plane defined by the grinding surface 7 that decreases with increasing distance from the exposed top surface of the elevated portions, i.e with increasing depth. Consequently, during use of the abrasive disc and due the the resulting gradual removal of abrasive material from the exposed grinding surface 7, the specimen contact area is gradually increased until the specimen contact area reaches a maximum value, when abrasive material down to a thickness equal to the depth of depressed portions 10 has been removed. In the example of fig. 2b, the depressed portions are provided as wedge shaped channels that have a triangular cross section. However, other shapes of depressions may be used that provide a gradual increase in specimen contact area.
For example, depressed portions 10 as the ones shown in 2b may be provided by a stamp having a suitably shaped stamp surface. It will be appreciated that, in yet another embodiment, depressed portions with depths-depending size on of different depths may be provided, i.e. by combining the embodiments shown in figs. 2a and 2b.
Fig. 3 shows a cross-sectional view of yet another embodiment of an abrasive disc. The abrasive disc of fig. 3 is similar to the disc of figs. 1 a-b, but where the rim portion 14 of the grinding layer that radially extends further outward than the base layer has a larger thickness than the central portion of the grinding layer, i.e. the side of the grinding layer opposite the grinding surface includes a recess of a size and depth suitable for receiving the base layer and to provide a plane mounting surface of the abrasive disc over its entire width. The base layer and the grinding layer are attached to each other at the interface 15 by a suitable adhesive. Even though shown with a grinding surface similar to the one shown in figs. 1 a-b, it will be appreciated that the abrasive disc of fig. 3 also may be provided with depressions as described in connection with figs. 2a-b.
It will be appreciated that the abrasive discs described in connection with figs. 2a-b, and 3 may be produced with the same material composition, sizes, etc, and by the same production processes as described in connection with figs. 1 a-b.
Fig. 4 illustrates the relation between total porosity of an abrasive disc, and the relative amounts of abrasives and binder in the grinding layer of embodiments of an abrasive disc as described herein. Lines parallel to line 23 are lines of constant binder content, while lines parallel to line 24 are lines of constant abrasive content, and lines parallel to line 25 are lines of constant porosity. Each point in the diagram specifies a specific combination of abrasive, binder, and porosity. Abrasive and binder refer to the volume percent of abrasive and binder, respectively, in the grinding layer of the disc. In fig. 4a, the area inside line 21 corresponds to abrasive discs with parameter combinations of abrasive, binder and porosity that have been found particularly useful for grinding of soft and ductile materials. This area corresponds to grinding layer materials with between about 25 vol.% and about 50 vol. % of abrasive particles such as SiC, between about 10% and about 25% binder such as Bakelite, and a total porosity of between about 35% and about 55%.
Generally, it has been found that an increased abrasive content provides an improved removal rate, while increased binder content results in an improved durability, i.e. life time, of the disc, and that an increased porosity results in an improved grinding quality of the sample surface. Thus a selection of these parameters provides a trade-off between the above factors and allows the design of suitable abrasive discs for a variety of applications.
In fig. 4b, the area inside line 26 corresponds to abrasive discs with parameter combinations of abrasive, binder and porosity of an alternative embodiment. This area corresponds to grinding layer materials with between about 25 vol.% and about 50 vol. % of abrasive particles and an total porosity of between about 20% and about 45%
Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.
For example, the abrasive disc may have a shape and/or size different from the ones shown and described herein. For example, alternative embodiments of the abrasive disc may have a central hole, e.g. having a diameter of 2-7 cm. It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
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