US 3518068 A
Description (OCR text may contain errors)
June so, 1970 K. H. GILLIS 3,518,068 PROCE FOR UFACTURINC- GRINDING WHEELS TAIN COPPE OATED GRAINS Filed Dec. 196'! INVENTOR diw/viiw h. 6/11/5 BY'JMW ATTORNEY United States Patent 3,518,068 PROCESS FOR MANUFACTURING GRIND- ING WHEELS CONTAINING COPPER- COATED GRAINS Kenneth Gillis, Southfield, Mich., assignor to General Electric Company, a corporation of New York Filed Dec. 4, 1967, Ser. No. 687,807 Int. Cl. B24d 11/00 US. Cl. 51-295 4 Claims ABSTRACT OF THE DISCLOSURE An abrasive tool for use in grinding operations made by mixing together metal-coated diamond abrasive particles, a granulated organic matrix material, and a filler material and then compacting this mixture together at an elevated temperature While the pressure is maintained constant throughout the compacting cycle until densification of the abrasive tool is completed.
This invention relates to an abrasive tool in which the abrasive is diamond particles, and to a process of preparing such an abrasive tool.
In the preparation of a conventional resin-bonded diamond grinding wheel, a mixture of granulated resin, diamond particles and filler is normally placed in a mold, a molding pressure is applied which may vary from 250 to as high as 10,000 pounds per square inch but which is most frequently several thousand p.s.i., and the mold is heated to an elevated temperature suflicient to make the resin granules deform plastically at the pressure used. Normal practice is to use a stop in the mold which maintains the volume at nearly that level established by the cold volume of the resin at the initial pressure. As molding proceeds, relaxation of the mixture ordinarily occurs, so that the pressure initially applied to the mixture will gradually decrease during the compaction when the stop is reached, irrespective of the force applied to the mold. In other types of molding operations, the pressure is deliberately maintained at a given level at the beginning of the compacting operation and then at a second and lower level during the remainder of the compacting operation. In some cases, the pressure is adjusted in the inverse direction but is accompanied by a decrease in temperature.
I have now discovered that vastly improved results in grinding performance may be obtained if the resin-bonded abrasive tool contains diamond abrasive which has been coated with a metal and molded at a pressure which is deliberately maintained at a substantially constant level throughout the compacting cycle until densification of the tool is completed. In view of the fact that yielding of the molding mixture normally occurs during the corn pacting cycle, the maintenance of constant pressure means that the volume must be continuously readjusted during the molding cycle to maintain the pressure at a given constant level. In its preferred form, the invention contemplates the use of a constant pressure during the molding cycle which is as high or higher than the highest pressures conventionally used in the preparation of diamond grinding wheels. In practical terms, this will translate into a preferred constant pressure during the molding cycle of 10,000 p.s.i. or greater. By constant pressure, it is of course understood that this refers to the pressure level after full pressure is achieved.
The utilization of the present process with metal-coated abrasive particles has resulted in the case of dry grinding operations in improvements of a magnitude of as much as ten times over presently known wheels, as determined by measuring the grinding ratio, or the ratio of volume of workpiece removed to the volume of wheel wear. Grinding ratio is normally used .in connection with diamond grinding wheels to measure the performance or efficiency of the wheel. In its most preferred aspect, this invention involves the utilization of constant pressures during densification or compaction of a diamond grinding wheel in which the diamond has been coated with from 30-80% by weight of copper metal.
The invention will be more clearly understood when considered in connection with the accompanying drawing in which the single figure is a schematic representation in cross section of a molding apparatus suitable for preparing the grinding wheel in accordance with the invention.
The manufacture of a typical grinding wheel used in grinding operations is illustrated by the drawing. Grinding wheels for use in dry grinding are frequently of the flaring-cup design illustrated in cross section. A grinding wheel core 1, for example of aluminum, is placed in the steel die 2 having a central portion 3. A mixture 4 of diamond particles, resin granules and filler powder is then placed in a mold cavity 5, the mixture being distributed around the mold cavity as uniformly as possible. A press ring 6 is placed in mold opening 5, and the entire unit is then placed in a hydraulic press between heated platens (not shown). The volume of mixture is sufficient so that, even at full densification, the upper edge of press ring 6 does not penetrate beyond the vertical portion of cavity 5. The temperature is raised to :a level depending upon the particular resin used, sufficient to allow plastic deformation and plastic flow of the resin granules and to effect the thermal curing of the resin into a cohesive mass. Normally this temperature will range from about 200 C. The platens of the press are brought together with a force sufficient so that the pressure on the projected normal area of the wheel rim (of resin, diamond and filler) is at the desired level. The force on the platens is maintained so that the platens follow the compacting mixture and a constant pressure is maintained on the resin-diamond-filler mixture until curing of the resin is completed and full densification of the wheel rim has occurred. The temperature is maintained at (or above) its initial value during the entire cycle. The duration of the compaction cycle and therefore the time during which the pressure and temperature are maintained at a constant level will vary considerably, but normally will be from a few minutes to one hour. It should of course be understood that maintenance of the pressure at a constant level throughout the molding cycle refers to the pressure level after maximum pressure is achieved. There is a relatively brief time during which pressure buildup must occur.
In accordance with the practice of the invention, the abrasive is coated with a metal prior to the molding operation. Diamond has been coated with a metal for a variety of purposes for many years. The coating of diamond particles with a metal such as nickel. increases the performance, as measured by grinding ratio, of the diamond abrasive, particularly when used in a resin-bonded wheel used in wet grinding operations, i.e., operations which utilize a liquid coolant, commonly water, during the grinding operation. However, equivalent results have not generally been obtained in dry grinding operations where coolant is not uitlized. The present invention achieves its greatest performance improvement when used in such dry grinding operations in which the dry grinding wheel contains diamond coated with from 30-80% by weight of a metal, preferably copper, and the wheel is molded in accordance with the constant pressure cycle above described.
In carrying out dry grinding operations, the operating temperatures are normally higher than wet grinding. The resin bond of the grinding wheel or other abrasive tool accordingly thermally degrades and the efiiciency of performance in dry grinding operations is much lower than in wet grinding. In using metal-coated diamond in the formulation of grinding wheels, the amount of filler 1s necessarily adjusted downward to compensate for the addition of metal via the metal coating to give the molding mixture sufficient plasticity to mold at the relatively low pressures used. Filler is needed primarily to strengthen the resin bond at elevated temperatures. For dry grinding wheels, a reduction of filler content cannot be made because of the demanding hot-strength requirement. This has, therefore, acted as a limitation on the use of metal coatings on the abrasive in dry grinding applications. It has been found that the use of a constant high pressure molding cycle does not require a molding mixture of as much plasticity. This permits the use of optimum resin/ non-resin (filler plus abrasive) ratio and therefore ameliorates this problem with respect to the use of metalcoated diamond in the manufacture of dry grinding wheels. Furthermore, the higher pressure molding technique of the invention overcomes the stiffening effect of the filler which resists the flow of the mix to achieve full densification.
Heat damage to the resin is the chief mechanism of wheel wear when used for dry grinding. The heat conductivity of a filled resin system is improved significantly when full density is achieved. The last few percent density improvement has been found most important and accounts, at least in part, for the dramatic improvement achieved by the present process.
In addition, the improvement in grinding ratios obtained with metal-coated diamond has been heretofore somewhat dependent upon the quantity of metal for specific mesh size diamonds and specific applications. The use of the constant higher pressure molding technique of this invention extends the improvement in grinding ratio over a broader range of metal coating quantities and therefore reduces the sensitivity of a specific amount of coating for a specific application.
Any of a number of methods may be used to place the metal coating on the diamond. Typical known coating methods are electrolytic, electroless or chemical precipitation. For purposes of the present disclosure, electroless coating procedures will be illustrated, although it will be obvious that other coating methods may be used. The metal coating should itself comprise from about 80% by weight of the total weight of the coated diamond abrasive. The thickness of the metal coating, as contrasted with the percent by weight, will normally range from about 1-50 microns, although this thickness will vary with the mesh size of the abrasive, the specific matrix selected and the ultimate application intended. The abrasive will normally be between about and 325 US. standard sieve size. Preferably, the percentage of metal coating will vary from the lower end of the 3080% range for the larger size abrasive to the upper end of the range for the smaller size abrasive. A preferred range of coating weight is from -60% by weight for so-called 100 concentration diamond grinding wheels. A 100 concentration wheel contains 25% by volume diamond and is about the maximum common concentration. The diamond used in the grinding wheel of the invention may be either natural or synthetically produced, although the latter is preferred as it normally produces superior performance in grinding wheels.
The coating metal may be any of a variety of metallic materials, including but not limited to copper, nickel, gold, silver, aluminum, cobalt, titanium, tantalum, molybdenum, vanadium, chromium, niobium and alloys of these metals. In the preferred practice of the invention, the metallic coating is copper. This results from the fact that the invention has its greatest advantages in the case of dry grinding applications. Dry grinding applications normally involve greater thermal stresses and require more 4 elficient heat conductivity in the grinding wheel. Copper possesses a number of unique properties, making it particularly suitable for use in the present invention, including its very high thermal conductivity, its wide and relatively cheap availability and its ease of plating on the abrasive.
The filler acts to improve the thermal conductivity of the resin, stiffen the resin matrix, and improve the abrasive resistance of the resin. It may or may not perform all of these functions in a given wheel. A preferred filler material for use in this invention is silicon carbide because of its high thermal conductivity. However, other filler materials which may be used include boron carbide, aluminum oxide, silicon dioxide and metallic powder, as for example copper.
The resinous bonding material most commonly used in resin-bonded grinding wheels is a phenol-formaldehyde reaction product, a typical example of which is a Bakelite resin sold by the Union Carbide Corporation under the designation BRP 5727. However, other resinous or organic polymeric bonding materials may be used, as for example rubber, shellac, melamine or urea-formaldehyde resins, epoxy resins, polyesters, polyamide and polyimide resins. Typically, the grinding wheels of the invention will contain by volume 25% diamond abrasive and about 25- 40% each of filler and resin. If the abrasive is coated, the filler and resin contents will be proportionately adjusted downward to the lower end of the ranges.
The following is a typical example of the preparation of a diamond grinding 'wheel in accordance with the practice of the invention.
synthetically produced diamond having a mesh size of /120 was coated with copper by an electroless deposition process which is known in the art. Briefly, this in volved activation of the diamond surface by chemical precipitation of a very thin layer of a catalyst metal such as palladium. The diamond was then added to a formaldehyde reduced electroless copper bath and permitted to plate until the solution was depleted of copper. This cycle was repeated until the thickness of copper was built up to produce a diamond having 55% by weight of coating.
The copper-coated diamond was then mixed with silicon carbide as filler and a granulated phenol-formaldehyde (BRP 5727) resin. The proportions of the three ingredients were as follows:
Vol. percent This mixture was then placed in a mold of the type shown in the drawing, an effort being made to distribute as evenly as possible the mixture in the mold cavity 5. Press ring 6 was then placed in the opening in the mold, the entire unit was placed in a hydraulic press between heated platens, the temperature raised to C., and the platens of the press pressed together with a force so that the pressure on the projected normal area of the Wheel rim was 10,000 p.s.i. As heat is conducted from the platens of the press into the mold, the mixture of abrasive, resin and filler normally compacts. The platens must therefore be constantly adjusted so that the pressure on the projected normal area of the wheel rim is maintained at 10,000 p.s.i. The heat and pressure were maintained for approximately 45 minutes, after which the wheel was removed from' the mold and placed in an oven for final curing at C.
The grinding ratio of (1) wheels made in accordance with the above example was compared with (2) wheels prepared from copper-coated diamond pressed and compacted in the same manner, except that a pressure of 10,000 p.s.i. was applied to the wheel during the first six or seven minutes of the molding cycle, and a pressure of 2,000 p.s.i. was applied during the last 38 or 39 minutes of the molding cycle. This pressure cycle was selected because it is typical of standard practice. For comparative purposes, the foregoing wheels were also compared with (3) wheels containing uncoated diamond pressed at 10,000 p.s.i. and then 2,000 p.s.i. (variable pressure), as in sample 2. The wheels were otherwise identical in every respect.
TABLE Wheel Coating on Grinding sample Molding pressure diamond ratio 1 (1) Constant 10,000 p.s.i Copper. 429 (2) 10,000 p.s.i. then 2,000 p.s.i do 131 (3) do Non 37 and 200 C. a mixture of diamond abrasive particles, each of said particles containing from 30-80% by weight of a copper coating, and a granular phenol-formaldehyde resin at a pressure of at least 10,000 pounds per square inch constant level throughout the compacting cycle until densification of the mixture is completed.
2. The process of claim 1 in which the temperature is maintained at or above approximately the same level throughout the compacting cycle.
3. 'The process of claim 1 in which the grinding wheel also contains a filler.
4. The process of claim 3 in which the filler is silicon carbide.
References Cited UNITED STATES PATENTS 2,193,265 3/1940 Benner et al. 51-309 2,367,404 1/1945 Kott 51-309 3,383,191 5/1968 Thomas 51-298 FOREIGN PATENTS 1,142,688 9/1957 France.
DONALD J. ARNOLD, Primary Examiner US. Cl. X.R.