GB2136011A - Grinding wheel containing cubic boron nitride - Google Patents

Abstract

A resin bonded cubic boron nitride wheel in which the resin bond contains by volume 15-35% of a fluoride salt grinding aid, 0-16% of a dry film lubricant filler and 5 to 40% silver, the resin content of the bond being at least 30% by volume. Up to 50% of the silver may be substituted by nickel. The resin may be a phenolic resin or a polyamide. The dry film lubricant filler may be MoS2, hexagonal boron nitride, graphite or a fluorinated polymer.

Description

SPECIFICATION Grinding wheel containing cubic boron nitride This invention relates to resin bonded grinding wheels containing cubic boron nitride abrasive parti cles.

The development of diamond grinding wheels began in the 1930's, and, until the late 1960's, diamond wasthe sole "super" abrasive, much harderthan the conventional aluminum oxide and silicon carbide abrasives. Concurrent with the development of synth eticdiamond by the high pressure-high temperature method taught in General Electric patents issued in the 1960's, the high density cubic form of boron nitride (CBN) having a hardness similar two diamond was discovered, by applying high pressure to hexagonal boron nitride at high temperature and in the presence of a catalyst, as disclosed in U.S. Patent 2,947,617.

While not superior to diamond in the grinding of carbides (e.g. cemented tungsten carbide) cubic boron nitride (CBN) has been found to be superiorto diamond when ferrous metals, particularlytoolsteels are being ground. Thus CBN wheels can be superior to diamond wheels in the grinding oftools which combine hard carbides and steels.

British Patent 834,351, published in 1960 teaches the use of metal sulfides together with alkali-metal salts such as sulfates as fillers in conventional abrasive wheels for grinding alloy steels. While the patent refers to abrasives generally, and lists diamond, such fillers, so far as is known, have never been commercially employed in diamond wheels or in CBN wheels.

German Patent P2230701, to Winter & Sohn discloses employing zinc sulfide with cryolite as a grinding aid in stickform to be applied to the workpiece. No suggestion of incorporating the grinding aids in the wheel bond appears in the patent.

Copending U. S. Application S.N. 06/245,855, filed March 1982, discloses the use of grinding aids such as fluorides in a CBN resinoid wheel togetherwith a solid lubricant.

U. S. Patent 3,779,727 issued December 1973, to Sioui and Carver, discloses the use of a metal powder such as silver, togetherwith a dry film lubricant in diamond or in CBN wheels.

U. S. Patent 3,820,290, directed to a particular resin bond and a method of curing it, includes a broad disclosure offillers and abrasives, the fillers including fluoride salts and other grinding aids and the abrasives including CBN and diamond.

The present invention resides in the discoverythat the use of a particular combination of known prior art additives orfillers in a grinding wheel containing CBN abrasive grits results in unpredictably high grinding performance in terms of the ratio ofthe volume of work removed to the volume of wheel wear (G-ratio), and in very low power requirements compared to priorartwheels. The invention requires the presence of a powdered metal filler, particularly silver, a grinding aid, particularly a fluoride salt, and preferably a dryfilm lubricant powder, particularly molybdenum disulfide.

A preferred bond composition of phenolic resin, metal filler, grinding aid, and dryfilm lubricant, in terms of volume % of each ingredient is as follows: Material Volume % KAIF4 powder 25 Ag powder 20 MoS2 powder 10 Phenolic resin (a novalac including 2.0 volume % of lime based on total bond volume) 45 Nickel powder may be used to substitute for up to 50% of the silver; other fluorides which can be used are KAl3F6, Na3AIF6, KBF4, and CaF2; other solid lubricants are hexagonal boron nitride, graphite, and polytetrafluoroethylene (powder) orotherfluorocarbon polymers.

The "normal" amount of abrasive to include in wheels containing CBN is 25% by volume of the bond plus abrasive (called 100 concentration). As little as 10% and as much as 50% may be used in special circumstances. Diluent abrasives can be employed to a limited extent.

The wheels can befabricated from the above ingredients by wetting the abrasive with furfural and blending it with the mixture of bond and fillers.

Sufficient of the mix is then placed in a mold of the desired shape and hot pressed to form. The tool may then be removed from the mold and the cure completed in an oven. Various conventional shapes of wheels such as cup wheels or cylindrical (raj) wheels may be formed by cementing or otherwise attaching the abrasive shape to a suitable core or center. Typical molding conditions are a pressure of 5 tons per square inch, a temperature of 1 600C, and a molding time of 20 minutes. The final cure can be carried out in an air atmosphere oven for 24 hours at 175 C, as is known in the art, the time and temperature of cure being effective to control the grade or hardness of the wheel depending upon the specific application.

Otherthermally stable resins than phenolics, such as polyamide may be employed with appropriate changes in the processing conditions.

A series of mixes were prepared containing various levels of silver and fillers.

Using standard core materials each mixture was fabricated into several 11 V9 shape cup wheels, 3-3/4" by 1-112" by 1-114" with a rim thickness of 1/16".

The wheels were then tested under the following conditions: Machine: Norton 6 x 18S-3 Surface Grinder Spindle Speed: 3600 r.p.m.

Table Traverse: 100 inches per minute Unit Infeed: 3.0 mils, 5 mils, and 7 mils.

Total Infeed: 45 mils Material Ground: M2 Steel surfaces 1/4"x 1/2" As a result of the testing of over 50 different mixes thefollowing equations for G-ratio (material removed, divided by wheel wear in volume units) and power in watts were computer generated four a nominal CBN content of 18.75 volume %; where:AG=volume % silver, MO=volume % MoS2, KA=volume % KAIF4, and 1N=infeed in mils: G = 116.49 + 0.271 (AG) + 0.06404 (AG)(MO) + 0.04095 (AG)(KA) - 37.384 (IN) + 2.420 (IN)2 - 0.0346 (to)2 P = 675.33 - 22.67 (MO) - 19.35 (KA) + 0.3909 (KA)2 + 0.7076 (AG)(MO) + 74.44 (IN) From these results, and other considerations, the following broad and preferred limits were derived: Broad Limits Preferred MoS2 0-16% 8-16% KAIF4 15-35% 20-30% Ag 5-40% 10-30% Resin 30-50% 40-50% The preferred composition, listed above, gave the following results, where G is the volume ratio of metal removed to wheel wear.

3 Mil Infeed 5 Mil Infeed G Power-Watts G Power-Watts 77.4 600 27.7 825 Priorartwheels, one containing silver and polytetrafluoroethylene in optimum amounts, as taught by U.S. Patent 3,779,727, designated wheel "A", and one containing active fillers and graphite, in optimum amounts, designated wheel "B" performed as follows: 3 Mil Infeed 5 Mil Infeed G Power-Watts G Power-Watts Wheel A 36 600 3 1,000 Wheel B 35 500 5 650

Claims (7)

1. A resin bonded grinding wheel containing cubic boron nitride abrasive grits in which the resin bond contains byvolume 15to 35% of a fluoride salt grinding aid, Oto 16% of a dryfilm lubricant filler, and Sto 40% of silver, the resin content ofthe bond being atleast30% byvolume.

2. A grinding wheel according to claim 1, in which a minor portion ofthe silver is substituted by nickel.

3. A grinding wheel according to claim 1 or 2, in which thefluoride salt is KAIF4.

4. A grinding wheel according to any one of the preceding claims, in which the dry film lubricantfiller is MoS2, graphite, hexagonal boron nitride, ora fluorinated hydrocarbon polymer.

5. A grinding wheel according to claim 4, in which thedryfilm lubricant is MoS2.

6. A grinding wheel according to claim 5, in which thedryfilm lubricantfillercontentis8to 16%.

7. A resin bonded grinding wheel containing cubic boron nitride abrasive grits substantially as herein described with reference to the Example.