US 3528789 A
Description (OCR text may contain errors)
United States. Patent 3,528,789 LUBRICATING COMPOSITION APPLIED T0 BORON NITRIDE GRINDING WHEELS Wilfred F. Mathewson, '.lr., Franklin, Mich., and Robert S. Owens, Latham, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed May 15, 1968, Ser. No. 729,395 Int. Cl. B24b 29/00 US. Cl. 51295 3 Claims ABSTRACT OF THE DISCLOSURE Greatly improved grinding is accomplished by effecting relative motion between the workpiece and a cubic crystal boron nitride grain abrasive tool in contact with the workpiece while simultaneously supplying to the working surface of the abrasive tool a solid dry lubricant from the group consisting of hexagonal boron nitride, lubricants having the general lamellar crystal structure of cadmium iodide, molybdenum disulfide, and mixtures thereof. In the preferred embodiment the solid dry lubricant is supplied by contacting the working surface of the abrasive tool with a rod or other shaped body containing the solid dry lubricant such that the lubricant is rubbed onto the abrasive tool as the grinding proceeds. The preferred composition for the shaped lubricant body is a mixture of molybdenum disulfide, an active diluent such as sulphur, an adhesive material to provide better transfer of the lubricant to the working surface of the abrasive tool, and a resin binder.
This invention relates to a method and composition for grinding whereby stock is removed from a workpiece by abrading it with an abrasive tool such as a grinding wheel in which the abrasive is cubic boron nitride while simultaneously applying one or more of certain dry lubricants to the working surface of the grinding wheel.
Reference is here made to US. Pat. 2,947,617 issued Aug. 2, 1960, to Robert H. Wentorf, Jr., and assigned to the assignee of the present invention. The Wentorf patent discloses and covers cubic crystal structure boron nitride having a zinc blende cubic structure and having a hardness approaching the hardness of diamond. Such cubic crystal boron nitride grain is frequently referred to as borazon.
Because of its extreme hardness, borazon is a superb abrasive grain material for grinding wheels and similar abrasive tools. [It can be bonded and formed into grinding wheels and the like either by means of a metal bond, an organic resin bond or by a vitreous bond.
It is known that in any abrasive system, friction represents a major source of energy consumption. Several effects can result from this phenomenon. Friction is symtomatic of sliding contact which, in the case of grinding, occurs at the abrasive crystal-workpiece interface. This intimate contact, along with the heat generated by friction, can cause abrasive or adhesive wear and can result in a detrimental chemical reaction between the abrasive and the workpiece. 'In addition, the heat of friction can cause premature bond failure, thermal stress failure of the abrasive and undue workpiece damage. Hence, it is well known to apply lubricant to the grinding wheelworkpiece interface during grinding operations. Generally liquid lubricants such as emulsifiable oils are used which additonally serve as a cooling medium; however, it is also known that solid lubricants, for example graphite, can be used. For example, US. Pat. 3,321,287 discloses the use of a graphite lubricant with grinding wheels of various types and reports improvements in wheel life of about 25%.
What we have discovered is that in the case of a borazon grinding tool, if certain solid dry lubricants are supplied to the working surface of the abrasive article during the grinding operation, an increase in [grinding efficiency is accomplished of an order of magnitude far beyond what would be expected. For example, in grinding ferrous base metals with borazon grinding wheels, we have accomplished increases in grinding performance in excess of l000% by the use of a solid dry lubricant in accordance with this invention. The solid dry lubricants which provide the outstandingly excellent results with borazon are hexagonal boron nitride or a material having the lamellar crystal structure of cadmium iodide or molybdenum disulfide. Examples of the latter are molybdenum disulfide, tungsten disulfide and lead iodide. We do not purport to be able to explain with accuracy why such lubricant provides the unusual results with borazon as the grinding abrasive. However, it is believed to involve the fact that borazon, in an oxidizing atmosphere and in a heated condition, develops a thin oxide surface layer. The solid dry lubricant either minimizes any adverse effect from this phenomenon or coacts with it to provide the great increase in efliciency. Moreover, grinding employs relatively high velocities between abrasive grain and workpiece and, in addition, is a multi-chip rather than a singlechip operation. Hence, friction and other interactions between grain and workpiece can cause unusually high grinding wheel Wear with borazon because of its relatively high reactivity. An important aspect of the present invention is therefore believed also to reside in positioning the lubricant between the abrasive grain and the chip. It should be noted that graphite is not included within the scope of the invention, albeith graphite is perhaps the best known and most widely used dry lubricant and, by its nature, would appear to minimize oxidation of the boron nitride to the oxide. Yet, the fact is that graphite does not give results even closely comparable to the above solid lubricants. The solid lubricants that are included within the scope of the invention are, in addition to hexagonal boron nitride, those solid lubricants which have the general lamellar crystal structure of cadmium iodide or molybdenum disulfide. These crystal structures are described in more detail at pages 278, 280 and 397 of Structural Inorganic Chemistry, A. F. Wells, Clarendon Press, 2nd edition, 1950. Page 278 of this publication contains the following listing:
Cd I structure: CdIg, 'CaI MrgI Ph l- Mnl Fel C01 Ybl MgBr- FeBr C0Br CdBr- NiBr TiCl TiI The solid dry lubricant can be supplied to the working surface of the grinding wheel or other abrasive tool during the grinding operation in any of a number of ways. For example, the solid lubricant can be formulated into the grinding tool itself. This would appear to be the best mode of practicing the invention, particularly from the standpoint of convenience; however, we have found that in fact best results are achieved by applying the solid lubricant to the working surface of the abrasive tool with a solid easily abradable and therefore relatively soft body consisting of or containing the solid lubricant. \One advantage of this is that it enables the application of a far greater amount of the solid dry lubricant to the working surface than can be accomplished, as a practical matter, by including the solid lubricant in the abrasive tool itself. Hence, in the preferred embodiment of the invention the working surface of the [grinding tool, during its rotary or other motion in the grinding operation, is in contact with a rod of easily abradable material containing the solid dry lubricant such that as the working surface of the abrasive tool engages the workpiece, it has a thin deposit of the solid dry lubricant thereon. This places the lubricant between the abrasive grain and the chip. 'Ihere need be no other lubrication nor need cooling fluid be applied, as by the use of liquid lubricants, though such can be used if desired.
In the most preferred embodiment, the relatively soft, easily abradable body in contact wih the moving working surface of the borazon abrasive tool consists essentially of a resin-bonded mixture of the solid dry lubricant plus an active diluent such as sulphur plus a material which functions as an adhesive to better assure transfer and adherence of the solid lubricant onto the surface of the tool. The preferred organic resins for use as a bonding medium for the lubricant body are the thermosetting resins, especially epoxy and polyester resins. The thermoplastic resins, such for example as the acrylics and the vinyls, can also be used though they do not provide as strong a bond as does epoxy or polyester resin. Certain of the thermosetting resins, for example the phenolics, have the disadvantage, as compared with epoxy or polyester resin, of being somewhat brittle.
As the adhesive ingredient we prefer to use a silane, gamma aminopropyltriethoxysilane being aspecially excellent. Other materials which serve well as the adhesive are the higher alcohols such as octadecanol and triethanolamine. In general, solid or liquid organic compounds having amine or hydroxyl groups function as adhesives by improving the adherence of the solid dry lubricant to the grinding wheel. While the adhesive ingredient is generally desirable, it is not essential.
The precise proportions of ingredients used in the formulation of the body should be such as to supply to the surface of the abrasive tool a thin deposit of the solid dry lubricant and yet with the body having sufficient strength and hardness to assure against breakage or other rapid degeneration of the body during the grinding operation. We have found it desirable to use from about 30 to 60% by volume organic bonding resin and at least about 30% by volume solid dry lubricant. Where an adhesive is included, only about 2 to 6% by volume need be used. The following composition, expressed in percent by volume, has been found to be outstandingly excellent: Molybdenum disulfide about 32%, sulphur about 22%, gamma aminopropyltriethoxysilane about 4%, epoxy resin about 42%.
After thorough mixing, this composition is shaped into a cylindrical rod of say about .1 inch diameter and the epoxy resin then cured over a period of about 12 hours at room temperature with a 3-hour post cure at 90 C. in the practice of the invention, such a rod is consumed at the rate of about inches per hour at normal grinding speeds.
Specific examples of lubricant compositions made in accordance with the invention are as follows:
1) Molybdenum disulfide 32.2%, sulphur 21.6%, aminopropyltriethoxysilane 3.8%, epoxy resin 42.4%.
(2) Same as (1) except lead iodide substituted for the molybdenum disulfide.
(3) Hexagonal crystal powdered boron nitride 38.6%, aminopropyltriethoxysilane 5.4%, epoxy resin 56%.
Grinding efficiency can be determined and expressed in terms of the grinding ratio, i.e., the ratio between the rate of stock removal and the rate of grinding wheel wear. The higher the ratio, the greater the efiiciency. With the present invention, increases of as high as l000% in grinding ratio can be accomplished. To illustrate, in grinding M2 high speed steel with an unlubricated resin-bonded borazon grain grinding wheel, the grinding ratio was 33. Using an identical grinding wheel and with all other conditions being identical except that the grinding wheel was lubricated by a rod formed of composition 1 above, the grindng ratio was 400'.
Whereas it is much preferred to apply the solid dry lubricant to the working surface of the abrasive tool from a solid, easily abradable body, the lubricant can, if desired, be formulated into the abrading tool itself. Where this is done, it is desirable that the solid dry lubricant be formulated into the abrading tool in an amount of from about 5% to 25% by volume of the total constituents of the abrading tool composition. The following specific example will serve to illustrate, the percentages being by volume:
Percent Borazon (60 to mesh) 10 Molybdenum disulfide l5 These ingredients were homogeneously mixed, pressed into a grinding wheel and cured. The grinding ratio was 62 as compared with a grinding ratio of about 30 without the inclusion of the molybdenum disulfide.
1. A lubricating composition for application to cubic crystal boron nitride grain grinding wheels to provide lubrication at the surface of said wheels during grinding, said composition comprising a mixture of from about 30% to 60% by volume thennosetting organic bonding resin and at least about 30% by volume solid dry lubricant selected from the group consisting of hexagonal born nitride, molybdenum disulfide, or an inorganic metal halide having the lamellar crystal structure of cadmium iodide or molybdenum disulfide, and mixtures thereof, the balance of said mixture being composed of essentially non-abrasive materials.
2. A lubricating composition as claimed in claim 1 wherein the solid dry lubricant is selected from the group consisting of molybdenum disulfide, lead iodide, and hexagonal crystal powered boron nitride.
3. A lubricating composition as claimed in claim 1 containing about 32% by volume molybdenum disulfide, 22% by volume sulfur, 4% by volume gamma aminopropyltriethoxysilane, and 42% by volume epoxy resin.
References Cited UNITED STATES PATENTS 2,240,302 4/ 1941 I ones 51295 2,881,065 4/19'5'9 Renter 5 l--298 2,947,617 8/1960 Wentorf 51298 3,081,161 3 1963 Cantrell 5 1298 3,383,191 5/ 1968 Thomas 151-298 3,385,684 5/1968 Voter 51-298 "582,736 5/1897 Haas 51-295 2,904,419 9/1959 Couch et al. 51-298 3,27 6,170 10/ 1966 Crowe 5 1-295 DONALD J. ARNOLD, Primary Examiner U.S. Cl. X.R. 51298, 307