DE69901438T2 - GRINDING TOOLS - Google Patents

Abstract

Abrasive tools suitable for precision grinding of hard brittle materials, such as ceramics and composites comprising ceramics, at peripheral wheel speeds up to 160 meters/second are provided. The abrasive tools comprise a wheel core (2) attached to an abrasive rim of dense, metal bonded superabrasive segments (8) by means of a thermally stable bond (6). A preferred tool for backgrinding ceramic wafers contains graphite filler and a relatively low concentration of abrasive grain (4).

Description

The invention relates to grinding tools suitable for precision grinding of hard brittle materials, such as ceramics and composites with ceramics, at peripheral wheel speeds of up to 160 m/sec, and suitable for surface grinding of ceramic wafers. The grinding tools comprise a wheel core or hub attached to a metal-bonded superabrasive rim with a compound that is thermally stable during grinding operations. These grinding tools grind ceramics at high material removal rates (e.g., 19-380 cc/min/cm) with less wheel wear and less damage to the workpiece than conventional grinding tools.

An abrasive tool suitable for grinding sapphire and other ceramic materials is disclosed in U.S.-A-5,607,489 to Li. As described, the tool contains metal-clad diamond bonded in a ceramic matrix comprising 2 to 20 volume percent solid lubricant and at least 10 volume percent porosity.

An abrasive tool having diamond bonded in a metal matrix comprising 15 to 50 volume percent of selected fillers, such as graphite, is disclosed in U.S.-A-3,925,035 to Keat. The tool is used for grinding cemented carbides.

A cutting wheel with metal-bonded diamond abrasive grain is disclosed in US-A-2,238,351 to Van der Pyl. The bond consists of copper, iron, tin and, optionally, nickel, and the bonded abrasive grain is sintered onto a steel core, optionally with a brazing step to ensure sufficient adhesion The best bond is said to have a Rockwell B hardness of 70.

An abrasive tool having fine diamond grain (bort) bonded in a relatively low melting temperature metal bond, such as a bronze bond, is disclosed in U.S. Re. 21,165. The low melting temperature bond serves to prevent oxidation of the fine diamond grain. A grinding rim is formed as a single annular grinding segment and then attached to a central disk of aluminum or other material.

None of these grinding tools have proven to be completely satisfactory for precision grinding of ceramic components. These tools fail to meet stringent specifications for part shape, size and surface quality in service at commercially viable grinding rates. Most commercial grinding tools recommended for use in such operations are resin or vitrified bonded super grinding wheels, which are designed to operate at relatively low grinding performance levels to avoid damage to the surface and subsurface layer of ceramic components. Grinding performance is further reduced due to the tendency of ceramic workpieces to clog the wheel face, requiring the wheel to be constantly reworked and dressed to maintain precision shapes.

As the market demand for precision ceramic components in products such as motors, refractory equipment and electronic devices (e.g. wafers, magnetic heads and display windows) has increased, The need for improved grinding tools for precision grinding of ceramics has grown.

When finishing high performance ceramic materials such as aluminum titanium carbide (AlTiC) for electronic components, surface grinding or "grinding" operations require a smooth, high quality surface finish in low force and relatively low speed grinding operations. When grinding these materials, grinding performance is determined by the workpiece surface quality and control of the applied force, as well as high material removal rates and wear resistance of the grinding wheel.

The invention relates to a surface grinding tool, comprising a core with a minimum specific strength of 2.4 MPa-cm³/g, a core density of 0.5 to 8.0 g/cm³, a circular outer boundary and a grinding edge defined by a plurality of grinding segments; wherein the grinding segments contain, in amounts that add up to a maximum of 100% by volume, from 0.05 to 10% by volume of superabrasive grain, from 10 to 35% by volume of crumbling filler and from 55 to 89.95% by volume of metal bond matrix with a fracture toughness of 1.0 to 3.0 MPa·m1/2. The specific strength is defined as the ratio of the lower value of the yield strength or the fracture stress of the material divided by the density of the material. The crumbling filler is selected from the group consisting of graphite, hexagonal boron nitride, hollow ceramic spheres, feldspar, nepheline syenite, pumice, calcined clay and glass spheres, and combinations thereof. In a preferred embodiment, the metal bond matrix comprises a maximum of 5 volume % porosity.

Description of the drawings

Fig. 1 shows a continuous edge of grinding segments connected to the outer perimeter of a metal core to form a Type 1A1 grinding wheel.

Fig. 2 shows a discontinuous edge of grinding segments bonded to the outer perimeter of a metal core to form a grinding cup.

Fig. 3 shows the relationship between the amount of material removed and the normal force when grinding an AlTiC workpiece with the grinding wheels of Example 5.

The grinding tools of the invention are grinding wheels having a core with a central bore for mounting the wheel on a grinding machine, the core being designed to hold a rim of superabrasive with a metal bond along the periphery of the wheel. These two parts of the wheels are held together with a bond which is thermally stable under grinding conditions and the wheel and its components are designed to tolerate stresses generated at wheel peripheral speeds of up to at least 80 m/sec, preferably up to 160 m/sec. Preferred tools are Type 1A wheels and grinding cups such as Type 2 or Type 6 wheels or bell-like grinding cups of Type 11V9.

The shape of the core is substantially circular. The core may comprise any material having a specific strength of at least 2.4 MPa-cm³/g, preferably 40-185 MPa-cm³/g. The core material has a density of 0.5 to 8.0 g/cm³, preferably 2.0 to 8.0 g/cm³. Examples of suitable materials include steel, aluminum, titanium and Bronze and their composites and alloys and combinations thereof. Reinforced plastics having the minimum specific strength mentioned can be used to construct the core. Composite and reinforced core materials typically comprise a continuous phase of a metal or plastic matrix, often in powder form, to which fibers or grains or particles of a harder, more compliant and/or less dense material are added as a discontinuous phase. Examples of reinforcing materials suitable for use in the core of the tools of the invention are glass fiber, plastic fiber, aramid fiber, ceramic fiber, ceramic particles and grains and hollow filler materials such as glass, mullite, alumina and Zeolite® spheres.

Steel and other metals having densities of 0.5 to 8.0 g/cm3 can be used to make the cores for the tools of the invention. In making the cores for use in high speed grinding (e.g., at least 80 m/sec), lightweight metals in powder form (i.e., metals having densities of about 1.8 to 4.5 g/cm3) such as aluminum, magnesium and titanium and alloys thereof and mixtures thereof are preferred. Particularly preferred are aluminum and aluminum alloys. Metals having sintering temperatures between 400 and 900°C, preferably 570 to 650°C, are selected when a co-sintering composition process is used to make the tools. Low density filler materials can be added to reduce the weight of the core. Porous and/or hollow ceramic or glass fillers such as glass beads and mullite beads are suitable for this purpose. Also suitable are inorganic and non-metallic fiber materials. If indicated by the processing conditions, an effective Amount of lubricant or other processing aids known in the art of metal formation and superabrasives added to the metal powder prior to pressing and sintering.

The tool should be strong, durable and dimensionally stable to withstand the potentially destructive forces that arise during high speed operation. The core must have a minimum specific strength to operate grinding wheels at the very high angular velocity necessary to achieve a tangential contact speed between 80 and 160 m/s. The minimum specific strength required for the core materials used in the present invention is 2.4 MPa-cm3/g.

Specific strength is defined as the ratio of the yield strength (or ultimate stress) of the core material divided by the core material density. For brittle materials where the ultimate strength is less than the yield strength, specific strength is determined by applying the lesser number, which is the ultimate strength. The yield strength of a material is the minimum force applied in tension, increasing the elongation of the material without further increasing the force. For example, ANSI 4140 steel hardened to over about 240 (Brinell scale) has a tensile strength of over 700 MPa. The density of this steel is about 7.8 g/cm3. Thus, its specific strength is about 90 MPa-cm3/g. Similarly, certain aluminium alloys, for example Al 2024, Al 7075 and Al 7178, which are heat treatable to a Brinell hardness above about 100, have tensile strengths of more than about 300 MPa. Such aluminium alloys have a low density of about 2.7 g/cm³ and thus exhibit a specific strength of more than 110 MPa-cm³/g.

Titanium alloys and bronze composites and alloys manufactured with a density of not more than 8.0 g/cm³ are also suitable for use.

The core material should be tough, thermally stable at temperatures reached in the grinding zone (e.g., about 50 to 200ºC), resistant to chemical reaction with coolants and lubricants used in grinding, and resistant to abrasive wear due to movement of cutting waste in the grinding zone. Although some aluminas and other ceramics have acceptable failure values (i.e., above 60 MPa-cm3/g), they are generally too brittle and fail structurally during high-speed grinding due to fracture. Therefore, ceramics are not suitable for use in the tool core. Metal, particularly hardened tool quality steel, is preferred.

The grinding segment of the grinding wheel for use in the present invention is a segmented or continuous rim attached to a core. A segmented grinding rim is shown in Fig. 1. The core 2 has a central bore 3 for attaching the wheel to a shaft of a machine drive (not shown). The grinding rim of the wheel comprises superabrasive grains 4 embedded (preferably in uniform concentration) in a metal matrix bond 6. A plurality of grinding segments 8 make up the grinding rim shown in Fig. 1. Although the illustrated embodiment shows 10 segments, the number of segments is not critical. A single grinding segment, as shown in Fig. 1, has a truncated rectangular ring shape (an arc shape) characterized by a length 1, a width w and a depth d.

The grinding wheel embodiment shown in Fig. 1 is representative of wheels that can be successfully used in accordance with the present invention and should not be considered limiting. The numerous suitable geometric variations for segmented grinding wheels include cup-shaped wheels as shown in Fig. 2, wheels with openings through the core and/or gaps between successive segments, and wheels with grinding segments of varying width relative to the core. Openings or gaps are sometimes used to provide paths to direct coolant to the grinding zone and to direct cutting debris away from the zone. A wider segment relative to the core width is sometimes used to protect the core structure from wear caused by contact with grinding debris as the wheel radially penetrates the workpiece.

The wheel may be manufactured by first forming individual segments of predetermined dimensions and then attaching the preformed segments to the periphery 9 of the core with a suitable adhesive. Another preferred manufacturing process involves forming segment precursor units from a powder mixture of abrasive grain and a binder, molding the composition around the periphery of the core and applying heat and pressure to create and attach the segments in situ (in place) (i.e. co-sintering the core and the rim). A co-sintering process is preferred for making surface grinding cups for use in abrading wafers and chips of hard ceramics such as AlTiC.

The grinding edge component of the grinding tools of the invention can be a continuous edge or a discontinuous edge, as shown in Fig. 1 2, respectively. The continuous grinding rim comprises at least 2 grinding segments which are separately sintered in molds and then individually attached to the core with a thermally stable bond (i.e., stable bond at the temperatures encountered during grinding at the portion of the segments away from the grinding surface, typically about 50 to 350°C). Discontinuous grinding rims, as shown in Fig. 2, are made from at least 2 such segments, and the segments are separated by slots or gaps in the rim, rather than fitting end to end along their lengths, 1, as in the segmented continuous grinding rim wheels. The figures show preferred embodiments of the invention and are not intended to limit the types of tool designs of the invention; for example, discontinuous rims may be used in 1A wheels and continuous rims in grinding cups.

For high speed grinding, especially grinding of workpieces with a cylindrical shape, a continuous rim, Type 1A wheel, is preferred. Segmented continuous grinding rims are preferred compared to a single continuous grinding rim formed as a single piece in ring shape due to the easier achievement of a truly round planar shape during the manufacture of a tool from multiple grinding segments.

For grinding at lower speeds (e.g. 25 to 60 m/sec.), particularly for grinding surfaces and finishing flat workpieces, discontinuous grinding edges (e.g. the grinding cup shown in Fig. 2) are preferred. Since surface quality is critical in low-speed surface finishing operations, slots may be formed in the segments or some segments may be left out of the edge to assist in the removal of waste material that could scratch the workpiece surface.

The abrasive edge component contains a superabrasive grain held in a metal matrix bond, which is typically formed by sintering a mixture of metal bond powder and the abrasive grain in a mold designed to provide the desired size and shape of the abrasive edge or abrasive edge segments.

The superabrasive grain used in the grinding edge can be selected from diamond, in natural and synthetic form, CBN, and combinations of these abrasives. The grain size and type selected depend on the type of workpiece and the type of grinding process. For example, when grinding and polishing sapphire or AlTiC, a superabrasive grain size in the range of 2 to 300 microns is preferred. For grinding other alumina, a superabrasive grain size of about 125 to 300 microns (60 to 120 grit; Norton Company grit) is generally preferred. For grinding silicon nitride, a grain size of about 45 to 80 microns (200 to 400 grit) is generally preferred. Finer grit sizes are preferred for surface finishing and larger grit sizes are preferred for cylinder, profile or ID grinding operations where larger amounts of material are removed.

As a volume percentage of the grinding edge, the tools contain 0.05 to 10 volume percent of superabrasive grain, preferably 0.5 to 5 volume percent. A minor amount of a crumbling filler material having a hardness less than that of the metal bond matrix may be used as a bond filler. added to increase the bond removal rate. A volume percentage of filler of 10 to 35 volume percent, preferably 15 to 35 volume percent, may be used as the volume percentage of the edge component. Suitable friable filler materials must be characterized by suitable thermal and mechanical properties to withstand the sintering temperature and pressure conditions used in the manufacture of the abrasive segments and to assemble the wheel. Graphite, hexagonal boron nitride, hollow ceramic spheres, feldspar, nepheline syenite, pumice, calcined clay and glass spheres and combinations thereof are examples of useful friable filler materials.

Any metal bond suitable for bonding superabrasives and having a fracture toughness of 1.0 to 6.0 MPa m1/2, preferably 2.0 to 4.0 MPam1/2, may be used herein. Fracture toughness is the stress intensity factor at which a crack initiated in a material will propagate through the material and cause the material to fracture. Fracture toughness is expressed as K1C = (σf)(π1/2)(c1/2), where K1C is the fracture toughness, σf is the stress applied at fracture, and C is half the crack length. There are several methods that can be used to determine fracture toughness, and each includes an initial step of creating a crack of known dimension in the test material and then applying a stress load until the material fractures. The stress at fracture and the crack length are substituted into the equation and the fracture toughness is calculated. (For example, the fracture toughness of steel is about 30-60 MPa·m1/2, of aluminum oxide about 2-3 MPa·m1/2, of silicon nitride about 4-5 MPa·m1/2 and of Zirconia 'or zirconium dioxide is about 7-9 MPa·m1/2).

To achieve optimum wheel durability and grinding performance, the bond removal rate should be equal to or slightly higher than the abrasive grain removal rate during grinding operations. Fillers such as those mentioned above can be added to the metal bond to reduce the wheel wear rate. Metal powders that tend to form a relatively dense bond structure (i.e., less than 5 volume percent porosity) are preferred to achieve higher material removal rates during grinding.

Materials useful in the metal bond of the edge include, but are not limited to, bronze, copper and zinc alloys (brass), cobalt and iron, and their alloys and mixtures thereof. These metals can optionally be used with titanium or titanium hydride or another superabrasive reactive (i.e., active binder components) material capable of forming a carbide or nitride chemical bond between the grain and the binder at the surface of the superabrasive grain under the selected sintering conditions to strengthen the grain-binder bonds. Stronger grain/binder interactions limit premature grain loss, as well as workpiece damage and shorter workpiece life as a result of premature grain loss.

In a preferred embodiment of the grinding edge, the metal matrix comprises 55 to 89.95 volume percent of the edge, more preferably 60 to 84.5 volume percent. The crumbling filler comprises 10 to 35 volume percent of the grinding edge, preferably 15 to 35 volume%. The porosity of the metal matrix bond should be kept to a maximum of 5 volume% during the manufacture of the grinding segment. The metal bond preferably has a Knoop hardness of 2 to 3 GPa.

In a preferred embodiment of a grinding wheel of type 1A, the core is made of aluminum and the rim contains a bronze bond of copper and tin powders (80/20 by weight), and, optionally, with the addition of 0.1 to 3.0 weight percent, preferably 0.1 to 1.0 weight percent, of phosphorus in the form of a phosphorus/copper powder. In the manufacture of the abrasive segments, the metal powders of this composition are mixed with diamond abrasive grain of grit number 100 to 400 (160 to 45 microns), formed into abrasive edge segments and sintered or compacted in the range of 400 to 550°C at 20 to 33 MPa to give a dense abrasive edge, preferably with a density of at least 95% of theoretical density (i.e. with no more than about 5% porosity by volume).

In a typical co-sintering process for making a disk, the metal powder of the core is placed in a steel mold and cold pressed at 80 to 200 kN (about 10 to 50 MPa pressure) to form a green compact part having a size of about 1.2 to 1.6 times the desired final thickness of the core. The green compact core part is placed in a graphite mold and a mixture of the abrasive grain (2 to 300 microns grit number) and the metal bond powder mixture is added to the cavity between the core and the outer edge of the graphite mold. A fixation ring may serve to compact the abrasive and metal bond powders to the same thickness as the core preform. The contents of the graphite mold are then hot pressed at 370 to 410°C under 20 to 48 MPa pressure for 6 to 10 minutes. As in As is well known in the art, the temperature may be ramped up (for example, from 25 to 410ºC over 6 minutes; held at 410ºC for 15 minutes) or increased gradually before pressure is applied to the contents of the mold.

Following hot pressing, the graphite mold is removed from the part, the part is cooled, and the part is finished by conventional methods to obtain a grinding edge that has the desired dimensions and tolerances. For example, the part can be finished to size using ceramic grinding wheels on grinders or carbide cutters on a lathe.

When the core and rim of the invention are sintered together, only a small amount of material is required to give the part its final shape. Other methods of forming a thermally stable bond between the grinding rim and the core may require machining of both the core and rim before a cementing, bonding or diffusion step takes place to ensure a suitable surface for the parts to fit and bond together.

In creating a thermally stable bond between the rim and the core using segmented grinding rims, any thermally stable adhesive with the strength to withstand peripheral wheel speeds of up to 160 m/sec can be used. Thermally stable adhesives are stable to grinding process temperatures expected in the region of the grinding segments away from the grinding surface. Such temperatures typically range from about 50 to 350ºC.

The adhesive bond should be mechanically very strong to withstand the destructive forces encountered during wheel rotation and grinding. Two-part epoxy resin binders are preferred. A preferred epoxy binder, Technodyne® HT-18 epoxy resin (obtained from Taoka Chemicals, Japan) and the modified amine hardener thereof can be mixed in a ratio of 100 parts resin to 19 parts hardener. Fillers such as fine silica powder can be added in a ratio of 3.5 parts per 100 parts resin to increase the binder viscosity. Segments can be applied around the entire circumference of the grinding wheel cores or a partial circumference of the core with the binder. The perimeter of the metal cores can be sandblasted to obtain a roughness level prior to application of the segments. The thickened epoxy binder is applied to the ends and bottom of the segments, which are arranged around the core substantially as shown in Figure 1, and are mechanically held in place during curing. The epoxy binder is allowed to cure (e.g., at room temperature for 24 hours, followed by 48 hours at 60°C). Dewatering of the binder during curing and movement of the segments is minimized during curing by adding sufficient filler to optimize the viscosity of the epoxy binder.

The adhesive bond strength can be tested by spin testing at an acceleration of 45 rpm as used to measure the burst velocity of the disc. The discs demonstrated burst values corresponding to at least 271 m/s tangential contact velocities to be qualified for service under currently applicable safety standards of 160 m/s tangential contact velocity in the United States.

The grinding tools of the invention are particularly designed for precision grinding and finishing of brittle materials, such as advanced ceramic materials, glass, and components containing ceramic materials and ceramic composite materials. The tools of the invention are preferred for grinding ceramic materials including, but not limited to: silicon, mono- and polycrystalline oxides, carbides, borides and silicides; polycrystalline diamond; glass; and composites of ceramic in a non-ceramic matrix; and combinations thereof. Examples of typical workpiece materials include, but are not limited to, AlTiC, silicon nitride, silicon oxynitride, stabilized zirconia, aluminum oxide (e.g., sapphire), boron carbide, boron nitride, titanium didoride, and aluminum nitride, and composites of these ceramics, as well as certain metal matrix composites such as cemented carbides and hard brittle amorphous materials such as mineral glass or inorganic glass. These improved grinding tools can grind either single crystal ceramics or polycrystalline ceramics. For each type of ceramic, the quality of the ceramic part and the efficiency of the grinding process increase with an increase in the peripheral wheel speed of the wheels of the invention up to 80 to 160 m/s.

Ceramic parts improved by use of the abrasive tools of the invention include ceramic engine valves and rods, pump seals, ball bearings and fittings, cutting tool inserts, wear parts, drawing tools for metal forming, fire resistant components, display windows, flat glass for windshields, doors and windows, insulators and electrical parts and ceramic electronic components including, but not limited to silicon wafers, AlTiC chips, read-write heads, magnetic heads and substrates.

Unless otherwise indicated, all parts and percentages in the following examples are by weight. The examples are illustrative of the invention only and are not intended to limit the invention.

example 1

Grinding wheels according to the invention were manufactured in the form of Type 1A1 metal bond diamond wheels using the materials and processes described below.

A mixture of 43.74 wt.% copper powder (Dendritic FS grade, particle size +200/-325 mesh, obtained from Sintertech International Marketing Corp., Ghent, Nt; 6.24 wt.% phosphorus/copper powder (Grade 1501, +100/-325 mesh particle size, obtained from New Jersey Zinc Company, Palmerton, PA); and 50.02 wt.% tin powder (Grade MD115, +325 mesh, 0.5% maximum, particle size, obtained from Alcan Metal Powders, Inc., Elizabeth, New Jersey) was prepared. Diamond abrasive grain (320 grit synthetic diamond, obtained from General Electric, Worthington, Ohio) was added to the metal powder mixture and the combination was mixed until uniformly blended.

The mixture was placed in a graphite mold and hot pressed at 407ºC for 15 minutes at 3000 psi (2073 N/cm2) until a matrix with a target density of over 95% of theoretical was formed (for example, for disc #6 in Example 2: > 98.5% of theoretical density). The Rockwell B hardness of the segments produced for disc #6 was 108. The segments contained 18.75 volume % Abrasive grain. The segments were ground to the required arcuate geometry to mate with the periphery of a machined aluminum core (7075 T6 aluminum, obtained from Yarde Metals, Tewsbury, MA), resulting in a disk with an outside diameter of approximately 393 mm and segments 0.62 cm thick.

The abrasive segments and the aluminum core were assembled using a silica-filled epoxy binder system (Technodyne HT-18 adhesive, obtained from Taoka Chemicals, Japan) to produce grinding wheels with a continuous rim consisting of multiple abrasive segments. The contact surfaces of the core and segments were degreased and sandblasted to ensure proper adhesion.

To characterize the maximum operating speed of this new type of disc, full-size wheels were intentionally rotated to destruction to determine burst strength and rated maximum operating speed according to the Norton Company's Maximum Operating Speed Testing Procedure. The table below summarizes the burst test data for typical examples of the 393 mm diameter metal bonded test discs. Bursting strength data of metal bonded disks from the experiment

According to these data, the experimental grinding wheels using this design are allowed to operate at speeds of up to 90 m/s (17,717 ft/min). Higher operating speeds of up to 160 m/s can be easily achieved by making some further modifications to the manufacturing process and wheel designs.

Example 2 Evaluation of grinding performance

Three experimental metal bonded segment wheels of 393 mm diameter, 15 mm thickness, 127 mm central bore (15.5 in x 0.59 in x 5 in) manufactured according to the procedure of Example 1 above (#4 with segments having a density of 95.6% of theoretical, #5 with 97.9% of theoretical and #6 with 98.5% theoretical density) were tested for grinding performance. The initial test at 32 and 80 m/s showed that wheel #6 was the wheel with the best Grinding performance from the three wheels was acceptable, although all of the experimental wheels were acceptable. Wheel 46 was tested at three speeds: 32 m/s (6252 sfpm), 56 m/s (11,000 sfpm), and 80 m/s (15,750 sfpm). Two commercially available, state-of-the-art grinding wheels recommended for grinding advanced ceramic materials served as control wheels and were tested along with the wheels of the invention. One was a vitrified bonded diamond wheel (SD320-N6V10 wheel obtained from Norton Company, Worcester, MA) and the other was a resin bonded diamond wheel (SD320-R4BX61SC wheel obtained from Norton Company, Worcester, MA). The resin wheel was tested at all three speeds. The ceramic wheel was only tested at 32 m/s (6252 sfpm) due to speed tolerance considerations.

Over one thousand profile grinds of 6.35 mm (0.25 inch) width and 6.35 mm (0.25 inch) depth were made on silicon nitride workpieces. The grinding test conditions were as follows:

Grinding test conditions

Machine: Studer Grinder Model S40 CNC

Disc specifications: SD320-R4BX619C, SD320-N6V10,

Size: 393mm diameter, 15mm thickness and 127mm hole.

Disc speed: 32.56 and 80 m/s (6252, 11000 and 15750 sfpm)

Coolant: Inversol 22 @60% oil and 40% water

Coolant pressure: 270 psi (19 kg/cm²)

Material removal rate: varies, starting at 3.2 mm³/s/mm (0.3 in³/min/in)

Working material: Si₃N₄ (rods made of NT551 silicon nitride, obtained from Norton Advanced Ceramics, Northboro, Massachusetts) 25.4 mm (1 in) diameter · 88.9 mm (3.5 in) long

Working speed: 0.21 m/s (42 sfpm), constant

Diameter at start of work: 25.4 mm (1 inch)

Diameter at working end: 6.35 mm (0.25 inch)

For operations where dressing and finishing is necessary, the conditions suitable for the metal bonded wheels of the invention were as follows:

Dressing process

Disc: 5SG46IVS (obtained from Norton Company)

Disc size: 152 mm diameter (6 inches)

Wheel speed: 3000 rpm (rpm); at a ratio of +0.8 relative to the grinding wheel

Guide: 0.015 in (0.38 mm)

Compensation: 0.0002 in

Post-processing process:

Rod: 37C220H-KV (SiC)

Type: Hand Stick Dressing

Tests were conducted in a cylinder OD profile mode while grinding the silicon nitride rods. To maintain the best stiffness of the work material during grinding, the 88.9 mm (3.5 in) samples were held in a chuck with approximately 31 mm (1-1/4 in) exposed for grinding. Each series of profile grinding tests began at the other end of each rod. First, the wheel made a 6.35 mm (1/4 in) wide and 3.18 mm (1/8 in) radial profile depth to complete a test. The work rate in revolutions per minute (rpm) was then readjusted to compensate for the loss in work speed due to a reduced work diameter.

Two more similar profile grinds were made on the same location to reduce the working diameter from 1 in (25.4 mm) to 1/4 in (6.35 mm). The wheel was then moved laterally 1/4 in (6.35 mm) closer to the chuck to make the next 3 profile grinds. Four lateral movements were made on the same side of a sample to complete the twelve profile grinds on one end of a sample. The sample was then turned over to expose the other end for another twelve grinds. A total of 24 profile grinds were made on each sample.

The initial comparison tests for the metal bonded wheels of the invention and the resin and ceramic wheels were conducted at 32 m/s peripheral speed at three material removal rates (MRR') from about 3.2 mm3/s/mm (0.3 in3/min/in) to about 10.8 mm3/s/mm (1.0 in3/min/in). Table 1 shows the performance differences, as indicated by G-ratios, between the three different wheel types after 12 profile grinds. The G-ratio is the unitless ratio of the volume of material removed to the volume of wheel wear. The data showed that the Grade N ceramic wheel had better G-ratios than the Grade R resin wheel at the higher material removal rates, suggesting that a softer wheel will perform better when grinding a ceramic workpiece. However, the harder metal bonded wheel (#6) of the experiment was far better than the resin wheel and the ceramic wheel at all material removal rates.

Table 1 shows the estimated G-ratios for the resin wheel and the new metal bonded wheel (#6) at all material removal rate conditions. Since after twelve grinds at each material removal rate for the metal bonded wheel did not give measurable wheel wear, a symbolic value of 0.01 mil (0.25 µm) radial wheel wear was taken for each grind. This gave the calculated G-ratio of 6051.

Although the metal bonded wheel of the invention contained a diamond concentration of 75 (about 18.75 volume percent abrasive grain in the grinding segment) and the resin and ceramic wheels had a concentration of 100 and 150, respectively (25 volume percent and 37.5 volume percent, respectively), the wheel of the invention still demonstrated better grinding performance. At these relative grain concentrations, one would expect better grinding performance from the control wheels with a higher volume percent abrasive grain. Therefore, these results were unexpected.

Table 1 shows the surface finish (Ra) and waviness (Wt) data measured on samples ground with the three wheels at the low test speed. The waviness value, Wt, is the maximum peak-to-valley height of the waviness profile. All surface finish data were measured on surfaces created by cylindrical profile grinding without sparking. These surfaces are typically rougher than surfaces created by transverse grinding.

Table 1 shows the difference in grinding energy consumption at different material removal rates for the three types of wheels. The resin wheel had a lower energy consumption than the other two wheels; however, the experimental metal bonded wheel and the ceramic wheel showed comparable energy consumption. The experimental wheel consumed an acceptable amount of energy for ceramic grinding operations, especially in the In view of the favorable G-ratio and surface finish data observed with the wheels of the invention. In general, the wheels of the invention demonstrated power consumption proportional to the material removal rates. Table 1

When measuring grinding performance at 80 m/s (15,750 sfpm) in an additional grinding test under the same conditions, the experimental resin wheel and metal wheel exhibited comparable energy consumption at a material removal rate (MRR) of 9.0 mm³/s/mm (0.8 in³/min/in). As shown in Table 2, the experimental wheels operated at increasing material removal rates without a loss of performance or unacceptable current loads. The power consumption of the metal bonded wheel was approximately proportional to the material removal rate. The highest material removal rate achieved in this study was 47.3 mm³/s/mm (28.4 cc/min/cm).

The data of Table 2 are averages of 12 grinding passes. The individual values in terms of energy remained remarkably consistent for the experimental wheel within each material removal rate for each of the twelve passes. Normally, one would expect an increase in energy as successive grinding passes are performed and the abrasive grains in the wheel gradually become dull or the face of the wheel becomes loaded with workpiece material. This is often observed as the material removal rate is increased. However, the consistent power consumption levels observed within each MRR during the twelve grinding passes unexpectedly indicate that the experimental wheel maintained its sharp cut points throughout the entire length of the test at all material removal rates.

Furthermore, during this entire test, with material removal rates ranging from 9.0 mm³/s/mm (0.8 in³/min/in) to 47.3 mm³/s/mm (4.4 in³/min/in), it was not necessary to dress or rework the experimental disk.

The total cumulative amount of silicon nitride material ground without any disc wear was equal to 271 cm3 per cm (42 in3 per inch) of disc width. In contrast, the G-ratio for the resin disc with a concentration of 100 at 8.6 mm3/s/mm (0.8 in3/min/in) material removal rate was about 583 after twelve profile grinds. The wheel of the invention showed no measurable disc wear after 168 profile grinds at 14 different material removal rates.

Table 2 shows that the samples ground with the metal-bonded wheel of the experiment showed a Material removal rates maintained consistent surface finishes between 0.4 um (16 uin) and 0.5 um (20 uin) and exhibited waviness values between 1.0 um (38 uin) and 1.7 um (67 uin). The resin wheel was not tested at these high material removal rates. However, the ceramic rods ground with the resin wheel had slightly better but comparable surface finishes (0.43 versus 0.5 um) at approximately 8.6 mm³/s/mm (0.8 in³/min/in) and exhibited poorer waviness (1.73 versus 1.18 um).

Surprisingly, there was no obvious deterioration in surface finish when the ceramic rods were ground with the new metal bonded wheel as the material removal rate increased. This is in contrast to the generally observed deterioration in surface finish with an increase in cutting rates with standard wheels, such as the control wheels used herein.

The overall results indicate that the experimental metal wheel was able to grind effectively at an MRR that was over 5 times the MRR achievable with a standard commercially used resin bonded wheel. The wheel of the invention had a G-ratio over 10 times higher than the resin bonded wheel at the lower material removal rates. Table 2 14 MRRs tested at 80 m/s disc speed

When operated at wheel speeds of 32 m/s (6252 sfpm) and 56 m/s (11,000 sfpm) (Table 1), the energy consumption for the metal bonded wheel was higher than that of the resin wheel at all of the material removal rates tested. However, at the high wheel speed of 80 m/s (15,750 sfpm) (Tables 1 and 2), the energy consumption for the metal bonded wheel became comparable or slightly lower than that of the resin wheel. Overall, the trend showed that the energy consumption for both the resin wheel and the metal bonded wheel in the experiment decreased with increasing wheel speed when grinding at the same material removal rate. The energy consumption during grinding, with a A significant amount of this is transferred to the workpiece as heat, is less important in grinding ceramic materials than in grinding metallic materials, due to the greater thermal stability of the ceramic materials. As demonstrated by the surface quality of the ceramic samples ground with the wheels of the invention, the energy consumption did not affect the finished piece and was at an acceptable level.

For the metal-bonded disk of the experiment, the G-ratio was essentially constant at 6051 for all material removal rates and disk speeds. For the resin disk, the G-ratio decreased with increasing material removal rates at each constant disk speed.

Table 2 shows the improvement in surface finishes and waviness of ground samples at higher wheel speeds. In addition, samples ground with the new metal bonded wheel showed the lowest measured waviness of all wheel speeds and material removal rates tested.

In this test, the metal-bonded wheel showed better wheel durability compared to the control wheels. In contrast to the commercially available control wheels, it was not necessary to dress and rework the experimental wheels in the extended grinding tests. The experimental wheel was successfully operated at wheel speeds of up to 90 m/s.

Example 3

In a subsequent grinding test of the Experimental Wheel (#6) at 80 m/sec under the same operating conditions as those in the previous example, an MRR of 380 cc/min/cm was achieved, producing a surface finish (Ra) measurement of only 0.5 µm (12 uin) and using an acceptable level of energy. The observed high rate of material removal without damaging the surface of the ceramic workpiece achieved by using the tool of the invention has never been reported for any grinding operation of a ceramic material with any commercially available grinding wheel of any bond type.

Example 4

A cup-shaped grinding tool was created and tested for grinding sapphire on a vertical spindle machine of the "Blanchard type".

A cup-shaped wheel (diameter = 250 mm) was made from abrasive segments identical in composition to those used in Example 1, Wheel #6, except that (1) the diamond had a grain size of 45 microns (US mesh 270/325) and was present in the abrasive segments at 12.5 volume percent (50 concentration) and (2) the segment sizes were chord length of 46.7 mm (133.1 mm radius), width of 4.76 mm, and depth of 5.84 mm. These segments were bonded along the periphery of a side surface of a cup-shaped steel core to a central spindle bore. The surface of the core had grooves along the periphery, creating individual, shallow pockets with the same width and length dimensions as those of the Segments were formed. An epoxy binder (Technodyne HT-18 binder, obtained from Taoka, Japan) was added to the pockets and the segments inserted into the pockets and the adhesive was allowed to cure. The finished disk resembled the disk shown in Fig. 2.

The cup wheel was successfully used to grind the surface of a work material consisting of a solid sapphire cylinder of 100 mm diameter, yielding an acceptable surface flatness under favorable grinding conditions in terms of G-ratio, MRR and energy consumption.

Example 5

Type 2A2 cup grinding tools (280 mm diameter) suitable for grinding AlTiC or silicon wafers were prepared using the grinding segments described in Table 3 below. Except as noted below, the segments had sizes of 139.3 mm radius length, 3.13 mm width, and 5.84 mm depth. Diamond abrasives with bond batch mixtures sufficient to produce 16 segments per wheel in the proportions given in Table 3 were prepared by sieving the weighed components through a 140/170 U.S. mesh screen and mixing the components to evenly blend them. The powder required for each segment was weighed, placed in a graphite mold, leveled, and compacted. The graphite segment molds were hot pressed at 405ºC for 15 minutes at 3000 psi (2073 N/cm2). After cooling, the segments were removed from the mold.

Assembling a disc by adhering the segments to a machined 7075 T6 aluminum core was carried out as in Example 1. The segments were degreased, sandblasted, coated with adhesive and placed in molds which were machined to conform to the disk periphery. After the adhesive had cured, the disk was machined to size, leveled and speed tested. Table 3 Bond composition Table 4 Grinding segment composition volume %

a. All diamond grain used in the segments had a grain size of 325 mesh (49 microns), except for sample (1) where the grain was 270 mesh (57 microns) Diamond concentration levels are given below the volume % diamond value.

b. Porosity was estimated from observation of the microstructure of the segments. Due to the formation of intermetallic alloys, the density of the test samples was often greater than the theoretical density of materials used in the segments.

Example 6 Grinding performance evaluation

280 mm diameter, 29.3 mm thick, 228.6 mm center bore (11 in x 1.155 in x 9 in), low diamond concentration samples of experimental graphite-filled segment wheels made according to Example 5 were tested for grinding performance. The performance of these samples was compared to that of the control grinding wheel of Example 5 made with the high (75 concentration) diamond grinding segment composition of Example 1 (wheel #6) without graphite filler.

Over 70 cuts, each 114.3 mm (4.5 inches) wide and 1.42 mm (0.056 inches) deep, were made on AlTiC workpieces (AlTiC Grade 210, obtained from 3 M Corporation, Minneapolis, MN) of either 4.5 in (114.3 mm) or 6.0 in (152.4 mm) square dimensions, and the microns of material removed and the normal grinding force were recorded. The grinding test conditions were as follows:

Grinding test conditions

Machine: Strasbaugh Grinder Model 7AF

Grinding type: Vertical spindle profile grinding

Disc specifications: 280 mm diameter, 29.3 mm thickness and 229 mm hole

Disc speed: 1,200 rpm (revolutions per minute)

Working speed: 19 rpm (revolutions per minute)

Coolant: Deionized water

Material removal rate: Varies, 1.0 micron/sec to 5.0 micron/sec

The wheels were dressed and refinished using a 6 inch (152.4 mm) dress pad, specification 38A240-HVS, obtained from Norton Company, Worcester, MA. After the initial operation, dressing and refinishing were performed periodically as needed and when depth feed rates were changed.

The grinding test results (normal force versus material removed) for Example 5, Examples 2,4 and 1 are shown in Table 5 below and in Fig. 3. Table 5 Normal grinding force value versus removed material

a. 2a is sample 2 of Table 3 with a grinding segment edge width of 3.13 mm.

b. 2b is sample 2 of Table 3 with a grinding segment edge width of 2.03 mm.

These results demonstrate that a significant increase in normal force was required to remove larger amounts of material at higher MRRs (ranging from 1 to 3 to 5 microns/sec MRR) when grinding surfaces with the control wheel sample without graphite filler and a diamond abrasive concentration of 75. In contrast, the graphite-filled wheels of Example 5 of the invention with the low diamond concentration (Samples 2a, 2b and 4) required significantly less normal force during the grinding. The force required to remove an equivalent amount of material at an MRR of 2 microns/second for the invention wheel was equivalent to that required at an MRR of 1 micron/second for the control wheel sample. In addition, the wheel 2a samples required approximately equal normal forces to grind at either an MRR rate of 1 micron/second or an MRR of 2 microns/second. Inventive wheels 2a, 2b, and 4 of Example 5 also exhibited relatively stable normal force requirements as the amount of material ground increased from 200 to 600 microns. This type of grinding performance is highly desirable when grinding AlTiC wafers because these low force steady state conditions minimize thermal and mechanical damage to the workpiece.

The control wheel (Example 1) could not be tested at higher material removal levels (for example, above about 300 microns) because the force required to grind with these wheels exceeded the normal force capacity of the grinder, causing the machine to automatically shut down and preventing accumulation of data at the higher material removal levels.

While not wishing to be bound by any particular theory, it is believed that the improved grinding performance of the graphite-filled wheels of the invention is related to the low diamond concentration with the smaller number of individual grains per unit area of the grinding segment that come into contact with the surface of the workpiece at any one time during grinding. Although one skilled in the art would expect a lower MRR at a lower diamond concentration, the improvement in grinding power in the invention is unexpectedly achieved without a compromise in MRR. The wheel 2b, which has a The sample from Wheel 2b, which had a grinding segment width of 2.03 mm, required less force to grind at the same rates and amounts of material removal than did the sample from Wheel 2a, which had a grinding segment width of 3.13 mm. The sample from Wheel 2b has a smaller surface area and fewer grinding points in contact with the surface of the workpiece at any time during grinding operations than does the sample from Wheel 2a.

Claims (11)

1. Surface grinding tool comprising a core with a specific strength of maximum 2.4 MPa-cm³/g, a core density of 0.5 to 8.0 g/cm³, a circular outer boundary and a grinding edge defined by a plurality of grinding segments, the grinding segments containing, in amounts totaling 100% by volume, from 0.05 to 10% by volume of superabrasive grain, from 10 to 35% by volume of crumbling filler and from 55 to 89.95% by volume of metal bond matrix with a fracture toughness of 1.0 to 6.0 MPa m1/2. 2. Grinding tool according to claim 1, wherein the core contains a metallic material selected from the group consisting of aluminum, steel, titanium and bronze, composites and alloys thereof and combinations thereof. 3. The grinding tool of claim 1, wherein the grinding segments contain 60 to 84.5% by volume of metal bond matrix, 0.5 to 5% by volume of abrasive grain and 15 to 35% by volume of crumbling filler and the metal bond matrix has a maximum of 5% by volume porosity. 4. The abrasive tool of claim 1, wherein the crumbling filler is selected from the group consisting of graphite, hexagonal boron nitride, hollow ceramic spheres, feldspar, nepheline syenite, pumice, calcined clay and glass spheres, and combinations thereof. 5. Grinding tool according to claim 1, wherein the abrasive grain is selected from the group consisting of diamond and cubic boron nitride and combinations thereof. 6. Grinding tool according to claim 5, wherein the abrasive grain is diamond with a grain size of 2 to 300 micrometers. 7. The grinding tool of claim 1, wherein the metal bond contains 35 to 84 weight percent copper and 16 to 65 weight percent tin. 8. The abrasive tool of claim 7, wherein the metal bond further contains 0.2 to 1.0 weight percent phosphorus. 9. The grinding tool of claim 1, wherein the grinding tool includes at least two grinding segments, each having an elongated arcuate shape and an inner curvature selected to mate with the circular outer perimeter of the core, and each grinding segment has two ends configured to mate with adjacent grinding segments such that the grinding edge is continuous and substantially without gaps between the grinding segments when the grinding segments are connected to the core. 10. Grinding tool according to claim 1, wherein the tool is selected from the group consisting of Type 1A1 wheels and grinding cups. 11. The grinding tool of claim 1, wherein the thermally stable compound is selected from the group consisting essentially of an epoxy bonding compound, a metallurgical compound, a mechanical bond, and a diffusion bond, and combinations thereof.