US20030104763A1

US20030104763A1 - Tough and weak crystal mixing for low power grinding

Info

Publication number: US20030104763A1
Application number: US09/988,244
Authority: US
Inventors: Erik Einset
Original assignee: Individual
Current assignee: Diamond Innovations Inc; GE Superabrasives Inc
Priority date: 2001-11-19
Filing date: 2001-11-19
Publication date: 2003-06-05
Also published as: TW200300377A; AU2002350018A1; WO2003043784A1

Abstract

A resin bond grinding element is composed of a resin bond matrix containing superabrasive particles. The superabrasive particles are an at least 1:1 volume mixture of tough and weak particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The corresponding method involves grinding a workpiece with a resin bond grinding element composed of a resin bond matrix containing an at least 1:1 volume mixture of tough and weak superabrasive particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The mixture of tough and weak superabrasive particles also can be used in metal bond and vitreous bond grinding elements.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates generally to the manufacture of abrasive cutting and grinding elements and more particularly to the use of mixed abrasive crystals of different shapes and toughness to achieve overall lower power consumption in grinding operations.

Grinding wheels of various shapes, sizes, and composition are well known in the art. Wheels containing superabrasive materials (e.g., diamond or cubic boron nitride, CBN) in the edge or outer periphery of a circular grinding wheel or grinding cup also are well-known in the art in sawing, drilling, dressing, grinding, lapping, polishing, and other abrading applications. For these applications, the grit typically is surrounded in a matrix of a metal, such as Ni, Cu, Fe, Co, Sn, W, Ti, or an alloy thereof, or of a resin, such as phenol formaldehyde or other thermosetting polymeric material. By attaching the matrices to a body or other support, tools may be fabricated having the capability to cut through such hard, abrasive materials as steels, superalloys, ceramics, and cermets.

To minimize the heat generated in the grinding operation, which can thermally damage the workpiece, it is generally desired to minimize the power required to remove material. This power is consumed by a combination of friction caused by the actual cutting of material by the superabrasive grains, and friction caused by rubbing of the wheel bond material with the workpiece. By minimizing the total friction in the material removal operation, the working temperature can be minimized, thereby minimizing the chance of thermal damage to the workpiece. The present invention describes a way of minimizing the power consumption.

BRIEF SUMMARY OF THE INVENTION

A resin bond grinding element is composed of a resin bond matrix containing superabrasive particles. The superabrasive particles are an at least 1:1 volume mixture of tough and weak particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The mixture of tough and weak superabrasive particles also can be used in metal bond and vitreous bond grinding elements.

The corresponding method involves grinding a workpiece with a resin bond grinding element composed of a resin bond matrix containing an at least 1:1 volume mixture of tough and weak superabrasive particles, wherein there is at least about 10% difference in toughness between the tough particles and the weak particles. The tough/weak superabrasive particle mixture lowers the power required in grinding, thereby minimizing the temperature of grinding and ultimately minimizing the chance of thermal damage to the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which: [0008]
FIG. 1 displays the power consumption, as measured by specific energy as a function of abrasive concentration in the wheel for data reported in Example 1; [0009]
FIG. 2 displays the grinding power or specific energy as a function of toughness of the wheel as reported in Example 2; and [0010]
FIG. 3 is a schematic illustration of the protrusion of the weak and tough crystals above the bond wheel.[0011]
The drawings will be described in detail below. [0012]

DETAILED DESCRIPTION OF THE INVENTION

It is well known that a lower concentration of crystals in a bonded wheel leads to lower power consumption in operation. This can be explained by understanding that the majority of power is consumed in the friction generated in the cutting operation (crystal creating a chip in the workpiece) and in the frictional rubbing of the wheel against the workpiece which does not lead to material removal. The sum of these frictional processes causes the grinding operation to be a high temperature process. FIG. 1 shows the power consumption (as measured by the specific energy) as a function of abrasive concentration in the wheel, as reported in Example 1. As the abrasive concentration goes down, it is known that the grinding power will decrease (lower rate of “scratching” of the workpiece), but each diamond crystal will have to remove a bigger “chunk” of material for a given overall material removal rate, so the surface finish becomes poorer. [0013]
It also is known that grinding power or specific energy decreases with crystal toughness. This is demonstrated in FIG. 2, which shows that a stronger or tougher crystal also leads to a lower grinding power, as reported in Example 2. Combine the tougher crystals with weaker crystals (acting almost as a “filler” to the bond system), and the operator should see the effect. That is, if the operator mixes a weaker crystal into the mix, the weak crystals will break down and not have a big impact on power, but they should impact the finish by “smoothing” out the poor finish created by the lower concentration of tougher crystals. As a result, a satisfactory finish will be realized with lower overall power. [0014]
The present invention, then, uses the foregoing two effects to create an abrasive blend, which will grind with lower power. Lower power grinding is accomplished by blending a large amount of high toughness crystals with a smaller amount of low toughness crystals to give an overall abrasive blend. The high toughness crystals will do most of the cutting while the low toughness crystals will tend to fracture and not work very hard. The effective concentration of the mixture, then, will be lower than the equivalent concentration of either high or low toughness crystals and, more importantly, the power will be lower. In addition, since the high toughness crystals will be dominating the work, they will lead to lower grinding power. Finally, the low toughness crystals will act to “smooth” out the surface finish leading to a good finish at low power consumption. [0015]
In general, the maximum ratio of strong and weak crystals should be on the order of about 1:1 in crystal volume; so if the weak crystals are smaller, there would be more of them. However, a range of 10:1 to 1:1 (by volume) of strong to weak crystals should produce the desired affect. [0016]
As for toughness, the “TI” test (toughness index) is an arbitrary test. The toughness index (TI) is measured at room temperature. In many cases, the tougher the crystal, the longer the life of the crystal in a grinding or machining tool and, therefore, the longer the life of the tool. This leads to less tool wear and, ultimately, lower overall tool cost. For the present invention, the difference between “Tough” and “Weak” crystals should be greater than about a 10% difference on the TI scale, with something more like about 30%-90% difference being more common. [0017]
With respect to high and low toughness crystals, it is generally known that inclusions in the particles lower toughness or strength, while nitrogen in the lattice improves the strength. See for example, Jackson, et al., “Influence of substitutional nitrogen in synthetic saw-grade diamond on crystal strength’, [0018] J. Mater. Res., Vol. 12, No. 6, p. 1646 (1997).
The diamond particles can be natural or synthetic. Synthetic diamond most often is used in grinding operations. Synthetic diamond can be made by high pressure/high temperature (HP/HT) processes, which are well known in the art. The particle size of the diamond is conventional in size for resin-bond or other grinding wheels. Generally, the diamond grit can range in particle size from about 400 mesh (37 microns) upwards to 40 mesh (425 microns). Narrow particle size distributions can be preferred according to conventional grinding technology. [0019]
Cubic boron nitride (CBN) also can be used in accordance with the present invention, as CBN crystals too can be classified as tough or weak according to the TI scale. A blend of tough and weak CBN crystals likewise will produce the desired affect. CBN also can be made by HP/HT techniques, as is well known by those skilled in the art. [0020]
The coating of diamond and cubic boron nitride (CBN) with nickel, nickel-phosphorous alloys, cobalt, cobalt-phosphorous alloys, copper, and various combinations thereof is a standard procedure in the industry for enhancing retention of the abrasives in resin and other bonded tools, and for enhancing the grinding operation. [0021]
The patent literature is replete in the coating field. See, for example, U.S. Pat. Nos. 2,411,867; 3,779,727; 3,957,461; 3,528,788; 3,955,324; 4,403,001; and 4,521,222; British Pat. No. 1,344,237; and German Pat. No. 2,218,932. U.S. Pat. Nos. 4,024,675 and 4,246,006 form aggregates of diamond grit in a metal matrix that includes silver and U.S. Pat. No. 4,239,502 dips diamond or cubic boron nitride (CBN) in a molten silver/manganese/zirconium brazing alloy. Some attempts have been made to enhance the adhesion of the abrasive-coating interface by deposition of a carbide-forming element under the Ni, Co, or Cu coating. (U.S. Pat. Nos. 5,232,469 and 5,024,680). Some attempts have also been made at improving the coating-resin interface, but all of these involve increasing the mechanical forces by roughening the surface of the coating (see for example U.S. Pat. Nos. 3,650,714 and 4,435,189; and Irish Patent No. 21,637). The coatings enhance the retention of the crystals in the bond by providing greater surface texture (also help with heat dissipation, lubrication, other minor factors). The advantages of the invention are realized whether or not the crystals are coated. [0022]
The resin most frequently used in resin bond grinding wheels is a phenol-formaldehyde reaction product. However, other resins or organic polymers may be used, such as, for example, melamine or urea formaldehyde resins, epoxy resins, polyesters, polyamides, and polyimides. Such grinding wheels are made from the abrasive mixture by their mixing with resin powders and other additives (SiC, Cu powders), pressing the mixture in a mold, and heating to cure the resin. [0023]
The tough and weak abrasive particle mix also can be combined with vitreous matrix composite materials. The mixture then can be sintered or hot-pressed following procedures common in the vitreous bond art. For vitreous bond grinding wheels, for example, the abrasive particle mix is mixed with SiO[0024] ₂, B₂O₃, Na₂O, CaO, MgO, or other similar glass forming material(s), and hot pressed.
The tough and weak abrasive particle mix further can be incorporated into a metal bond grinding wheel. For making these wheels, the tough and weak abrasive particle mix is added to a matrix of a metal, such as Ni, Cu, Fe, Co, Sn, W, Ti, or an alloy thereof, and processed conventionally. [0025]
Concentration of coated diamond and fabrication of all of the foregoing wheels is conventional and well known in that art. Broadly, such concentrations range from about 25 to 200 (100 concentration conventionally being defined in the art as 4.4 carats/cm[0026] ³with 1 carat equal to 0.2 g, wherein the concentration of diamond grains is linearly related to its carat per unit volume concentration). Preferably, the concentration of diamond grit ranges from about 50-100.
Grinding wheels can be disc shape or cup shape and can contain a secondary distribution of silicon carbide or other secondary abrasive particles without detrimentally affecting the performance of the grinding element containing the tough/weak mixed abrasive particles. In a typical preparation of a resin bond grinding wheel, for example, a mixture of granulated resin, abrasive particle mixture, and optional filler is placed in a mold. A pressure appropriate to the particular resin, usually several thousand pounds per square inch (several tens of thousands of Kilo Pascals, KPa), is applied, and the mold is heated to a temperature sufficient to make the resin plastically deform (and cure when the resin is heat-curable). Techniques for fabricating metal and vitreous bond wheels also are well known in the art. [0027]
While the invention has been described with reference to a preferred embodiment, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts and percentages are by weight, unless otherwise expressly indicated. Also, all citations referred herein are expressly incorporated herein by reference. [0028]

EXAMPLE 1

This example demonstrates the increase in specific energy with increasing wheel abrasive concentration. The measurements were made in the surface grinding of M-2 tool steel (M2 HSS, HRC 60-62) using a 1A1 grinding wheel. The wheel was dressed by plunging into a soft aluminum oxide dressing stick. The grinding conditions included: 6000 sfpm wheel speed, 50 fpm table speed, downfeed of 0.0015″ per pass, and a coolant flow of 5 gpm at 20 psi. The measured specific energy is in units of W-hr/cm[0029] ³. The abrasive was a 60% Ni coated CBN (GE brand “Type-II”, GE Superabrasives, Worthington, Ohio) of mesh size 120/140 and the wheel was a phenolic resin type bond matrix. The only variable changing in the tests was the abrasive concentration, which is defined as the volume fraction of abrasive to wheel. A 100 concentration wheel is 24 volume-%, while a 50 concentration wheel is 12 volume-%. The data recorded is set forth below and illustrated in FIG. 1:

TABLE 1

CONCENTRATION Specific Energy

100 14.3

80 12.8

65 11.6

50 10.3
The data recorded shows that higher wheel concentrations lead to a higher specific energy in grinding. This may be interpreted as follows. A higher concentration of abrasive translates into a larger number of crystals per volume of wheel and, therefore, a larger number of cutting points. Each cutting point is a contributor to the grinding power. As the number of cutting points increases, the required power, or energy of grinding, increases. But the finish on the part is better with a higher concentration on the wheel, because the wheel has more cutting points, each of which takes a smaller “scoop” of material out of the workpiece. This is true generally, because most grinding operations are run at a fixed material removal rate. So, the tradeoff is a good surface finish for a higher grinding power (and, therefore, higher temperature exposure of the workpiece). This interpretation is independent of whether you have a resin, metal, or vitreous bonded wheel. [0030]

EXAMPLE 2

This example demonstrates the decrease in specific energy with increasing crystal toughness. The tests were done with a 1A1 vitrified grinding wheel in the creepfeed grinding of M2 HSS. In this case, the diamond was uncoated (a coating is not needed for a vitreous bond wheel) and of mesh size 120/140. The wheel concentration was 150 (36 volume-%) and was dressed by the same method used in Example 1. The grinding conditions were: 6000 sfpm wheel speed, 10 ipm table speed (creepfeed mode), and depth of cut of 0.040″. The grinding was done with a water based coolant (24 gpm at 95 psi). The data recorded is set forth below and displayed in FIG. 2: [0031]

TABLE 2

Power (W/cm)

Toughness Run Repeated Run

49.3 4752 4748

40.8 5457 5190

64.9 3018 2961

84.8 2841 2833
These results can be explained as follows. A stronger crystal can maintain a higher protrusion above the bond in a grinding wheel. By the crystal sitting at a higher protrusion, there is more room for the cutting chips to be washed away from the workpiece, and there is more room for the coolant to get to the cutting area. Overall, these result in the overall friction generated by the chips and workpiece rubbing against the bond being decreased. An analogy is that a wheel with tough crystals which protrude high above the bond will be “sharper”, like a sharpened saw, than a wheel with weak crystals which are not protruding very high above the bond. This is why the overall grinding power decreases for higher strength (toughness) crystals. [0032]
The invention takes the value of both of these observations to give an overall lower grinding power while still maintaining a good surface finish. If the wheel has a mixture of tough and weak crystals, the weak crystals will protrude to a lower level than the tough crystals, thereby leading to a lower number of the highly effective cutting points at the grinding surface. However, there will be some cutting action of these weaker crystals, which will act to smooth out the surface finish of the workpiece. A schematic of the protrusion above [0033] bond 10 is illustrated in FIG. 3 for weak crystals 12 and tough crystals 14.
Thus, by replacing some of the tough crystals in a wheel with weak crystals, the invention demonstrates that the cutting friction will be lowered, but the overall finish of the part will be nominally the same. This is a method for lowering the power of grinding through crystal mixing. [0034]

Claims

1. In a bond grinding element composed of a bond matrix containing superabrasive particles, the improvement which comprises:

said superabrasive particles comprising an at least 1:1 volume mixture of tough and weak particles, wherein there is at least about 10% difference in toughness between said tough particles and said weak particles.

2. The grinding element of claim 1, wherein said bond matrix is one or more of a resin bond matrix, a vitreous bond matrix, or a metal bond matrix.

3. The grinding element of claim 1, wherein said abrasive particles are one or more of diamond or cubic boron nitride (CBN).

4. The grinding element of claim 1, wherein said volume ratio of tough to weak particles ranges from about 10:1 to 1:1.

5. The grinding element of claim 1, wherein there is a difference in toughness between said tough particles and said weak particles ranging from between about 30% and 90%.

6. The grinding element of claim 2, wherein said bond matrix is a resin bond matrix of one or more of a phenol-formaldehyde resins, melamine formaldehyde reins, urea formaldehyde resins, epoxy resins, polyesters, polyamides, and polyimides.

7. The grinding element of claim 6, wherein said abrasive particle is diamond.

8. In a method for grinding a workpiece with a bond grinding element composed of a bond matrix containing superabrasive particles, the improvement which comprises:

providing said grinding element to contain superabrasive particles comprising an at least 1:1 volume mixture of tough and weak particles,

wherein there is at least about 10% difference in toughness between said tough particles and said weak particles.

9. The method of claim 8, wherein said bond matrix is one or more of a resin bond matrix, a vitreous bond matrix, or a metal bond matrix.

10. The method of claim 8, wherein said abrasive particles are one or more of diamond or cubic boron nitride (CBN).

11. The method of claim 8, wherein said volume ratio of tough to weak particles ranges from about 10:1 to 1:1.

12. The method of claim 8, wherein there is a difference in toughness between said tough particles and said weak particles ranging from between about 30% and 90%.

13. The method of claim 9, wherein said bond matrix is a resin bond matrix of one or more of a phenol-formaldehyde resins, melamine formaldehyde reins, urea formaldehyde resins, epoxy resins, polyesters, polyamides, and polyimides.

14. The method of claim 13, wherein said abrasive particle is diamond.