CA1067885A - Metal grinding and cutting aids and methods of manufacturing and using the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A grinding and cutting process and articles em-ploying an effective amount of a grinding aid, typical of which is sodium nitrite which, when applied to the abradent or cutting edge of a tool, accelerates the modification of a metal workpiece and prolongs the useful life of the tool.
The grinding aids of this invention are compounds free of sulfur and/or halogen and having a melting point in the range of 70°F. to 1000°F., a decomposition temperature at least 100°F. above the melting temperature and a latent heat of melting greater than 10 cal/gm.

Description

~ ` ~06'7~3~5 :.

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BACKGROUND OF THE_INVENTION

Field of the Invention:
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This invention relates to an aid for use in, and a process for, the machining of metals, and more particularly, to an aid which can be applied to cutting tools, drills, cut-of~ wheels, grinding wheels and coated abrasive products to accelerate the machining process~

_escr_ption of Prior Art.

Abrasive products and cutting tools employed to remove metal stock generally fail, i~e., they lose cutting effectiveness, after varying periods of use, especially when they are employed to modify high temperature metal alloys.

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678~35 ..
In the majority of cases involving high temperature alloys, one of the most prominent causes of failure resides in the fact that the freshly exposed or cut alloy surface is highly reactive and this "nascent" area is subject to the formation of a weld juncture which exerts an extremely high shear force against the abradant or cutting material. It is also quite clear that the welding becomes far more acute under conditions of high temperature. Although this inherent problem is en-countered in all forms of grinding and cutting, it is parti-cularly troublesome in the case of coated abrasives`where thesurface is not renewed as in grinding wheels and such. Since coated abrasives essentially rely on only a single layer of abradin~ particles, it has been found, to date that little can be done to improve their efficiency beyond a specific point.
It is also well-known that cutting tools, grinding wheels and coated abrasives become capped with a metal swarf, i.e. t loaded. In addition to the problems of welding and loading, there exists what is known as "glazing", wherain the cutting edges become deformed by the extremely hi~h tem-peratures to which the cutting points and edges of cutting tools, grinding wheels, abrasives and the like are exposed in grinding and cutting, causing plastic deformation o~ the cutting points and edges.
It appears that there is a direct inter-relationship between the foregoing factors and temperature. Factors which normally tend to elevate the temperature at the work surface (interface) also promote weldiny, chemical reactions, glazing j ~L()67~38S

.

generation of internal workpiece stresses, as well as burning of the workpiece surface, which adversely affect the metal-lurgical structure at the surface. These factors are present in all cutting techniques but they are substantially more severe in the super alloys due to their high temperature and low-thermal conductivity characteristics.
Attempts to externally improve grinding and cutting ability have included the application of grease sticks, oils and other lubricants to the workpiece surface during the grinding and/or cuttin~ operation. Also, attempts have been made ~y various manufacturers to incorporate aids into abra-sive belts and grinding wheels in a permanent fixed manner during fabrication.
These aids include solids, liquids and gases which serve generally to improve conditions within the restricted cutting or grinding area. Another common approach has been to incorporate within the metal to be machined quantities of sul~urj selenium and/or lead to provide improved machinability.
A similar result can be attained by the use of grinding aids -containing sulfur, halogen5 (e.g., fluorine, chlorine~ andphosphorus. The most commonly used grinding aids are in the form of liquids and include water, soluble oils, mineral and fatty straight cutting oils, as well as those that are sul-furized and chlorinated. The latter, as stated above, may be effective for certain metals but are not entirely usefuI
or desirable for certain super alloys and titanium due to chemical reactions between these chemicals and the metal surface being machined or ground. Greases and hard waxes .. .

7~S

are not effective except in reducing the loading of rela-tively soft metals such as aluminum, brass, etc. Other lubri-cants such as chlorinated and fluorinated hydrocarbons have ; been used to reduce heat generation in the area of the work-piece and grinding tool interface.
As a class, the presently employed aids are more or less toxic and their use and the surrounding environmen~
must be strictly controlled so as to minimize any danger to the health of the operatorO In the case of lead, bismuth, sul~ur, mercury or halogen containing aids, gases genera-ted during use can affect the workpiece and/or be toxic and care must be exercised in prolonged use with continual ins2ection and testing. In this regard, it should be observed that various specifications by the government an~ major aer~- -space ~anufacturers preclude the use of certain hal~en mate-rials in proximate relation with the woxkpiece as well as the operator.
The described aids have been used on standard materials with varying degrees of success but have been limi-ted in the field o~ space-age super alloys to safeguard the surface integrity of th~ workpiece. Further, the enactment and enforcement of laws protecting the health of factory workers now requires warning labels when certain of these aids are included ~or example as a supersize coat on coated abrasives.
I have discovered that the problems mentioned with respect to the prior art grinding and cutting aids can be over-come and that any grinding or cutting process on any metalor . ~,.

)678~35 other workpiece can be accelerated while also prolonging the useful life of the tool performing said process by bringing the workpiece into relatively moving contact with a grinding or cutting edge in the presence of an effective amount of a grinding or cutting aid comprising at least 10% by weight of a solid compound free of sulfur and/or halogen and having a melting point in the range of 70F. to 1000F., a decomposition temperature at least 100F. above the melting temperature and a latent heat of melting greater than 10 cal/gm. A typical compound having the above characteristics is sodium nitrite.
Generally, inorganic compounds are preferred because of their lower cost of manufacture.
U.S. Patent No. 3,595,634, issued to Sato on July 27, 1971, discloses the employment of 3 to 10% by weight of sodium nitrite as one of the initial ingredients of his formulation and p'rocess for making grindstones. Sato teaches the use of a highly effective and superior anticorrosive chemical compound, namely, amine nitrite, which, Sato teaches, is the reaction product of amines (120 to 250% of the equivalent weight of the epoxide) with the 3 - 10% of sodium nitrite in presence of heat and pressure when mixed with epoxy. According to Sato, there is no sodium nitrite in the final product produced by his process.
U.S. Patent No. 2,529,722, issued to Chester on November 14, 1950, relates to a buffing and polishing com-position for soft base metals which uses iron tailings as abrasive elements with alkali metals in the form of salts or complex oxides. To the foregoing, Chester adds a minute quantity of an electrolyte. Sodium nitrite is mentioned ,~

~67~ 5 among other suitable materials as an electrolyte and only in ~inute proportions, namely, 1/16 to 1/4% by weight as a rust inhibitor to prevent oxidation of the iron tailings in water. This amount would be insignificantly inadequa-te to perforlll the heat absorption function required of the aid of this invention.
U.S. Patent No. 3,607,161, issued to Monick on September 21, 1971, discloses a scouring composition which comprises a cationic surface~active compound and a water-soluble abrasive. Monick lists in excess of 60 water-solubla salts which act as abrasives, one of which is sodium nitrite.
Sodium nitrite in crystalline form is equated to an abrasive, and not taught to be an aid for some other abradant in lower-ing the grinding temperatures.

SUMMARY OF INVENTION

As indicated above, my invention consists in the discovery that grinding or cutting processing of metal wor~-pieces can be accelerated while prolonging the useful life of the tool performing the process by bringing the workpiece into relatively moving contact with a yrinding or cutting ; tool having an abradant or cutting edge in the presence of an effective amount of a grinding or cutting aid as described above and as typified by sodium nitrite (NaNO2). For purposes of illustration, the principals of the present invention will be discussed below with particular reference to sodium nitrite.
According to a broad aspect, the invention relates to a grinding tool for use in physically modifying a metallic workpiece, sai,d tool comprising abrasive particles admixed with a grinding aid consisting of a compound free of sulphur and/or halogen, stable under conditions of use, and having a melting point in the range of , ~L~67~35 70 F to 1000 F a decomposition temperature at least 100 F
above the melting temperature and a latent heat of melting greater than 10 gal/gm.
According to another broad aspect the invention relate~
to a process for lmproving the efficiency of a grinding or cuttin tool which comprises contacting said tool or workpiece with an aid consisting of a compound free of sulphur and/or halogen, sta-ble under conditions of use and having a melting point in the range of 70DF to 1000 F, a decomposition temperature at least 10 : 10 above the melting temperature and a latent heat of melting grea~
ter than 10 gal/gm, said contact being in a manner which will result in the formation of a coating or said aid on the cutting edges of said tool during use thereof.
Preferably, the grinding aid is selected from the group consisting of sodium nitrite, potassium nitrite, sodium ~' ~ nitrate, potassium nitrate, lithium nitrate, lithium nitrite, ; potassium dichromate and mixtures thereof.

-7a--~067~385 It was discovered that sodium nitrite unaergoes phase.transitions when subjected to elevated temperatures.
In such a phase transformation, the energy necessary to accomplish the transition is in the form of heat absorbed and is derived from surrounding objects.
Sodium nitrite, ~ecause of its excellent therma.l properties in heat transfer, is used as the principal com-ponent in the ~ormation of a grinding or cutting accelera-tor according to this invention. The quantity of sodium nitrite used is optimized to absorb, as much as possible, the frictional heat generated durlng the abrasive machining and/or cutting of metals and it was found that when sodium nitrite was applied, e.g., to a coated abra~ive ~elt via a solid vehicle in the form of a cerate, the sensible sur~ace temperature of the abraded metal can be reduced by 500F.
The reason for this is that a relatively large amount of heat generated by the abrading process is absorbed as the latent heat of melting of soaium nitrite. ~
The thermal properties of sodium nitrite were determined by differential.thermal analysis. Examinatlon of this data indicated the existence of peaks in specific heat with respect to temperature. There is an absorption of ex-cess heat at 164C. (327F.), where a second order transition change occurs in the solid state. A second peak occurs at the melting point, namely 280C. 1536F.). Further, sodium nitrite does not decompose at this temperature as taught in the technical literature, but will remain molten up to 675F. (360C.) before it decomposes~

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:~LOE;~13135 In addition to the foregoing, the heat absorption of sodium nitrite was evaluated in terms of specific enthalpy vs. temperature. The second order solid transition at 1~4C.
provides a heat absorption~from room temperature of approxi-mately 42 cal/gr. At melting point, the latent heat of meltingis about 55 cal/gr. For each gram of sodium nitrite applied to the surface of a cutting tool or an abrasive article such as a coated abrasive belt (to provide an interfacè betwèen the abrasive grains and the metal workpiece), the sodium nitrite will absorb approximately 140 calories as its temperature is rai~ed from room temperature to its peak molten temperature of 360C. ~675F.).
O~ basic and significant impoxt is the fact that the solid state transition and the solid to liquid phase change of the sodium nitrite are reversible. This characteristic permits the continual use of the sodium nitrite aid on an abrading or cutting tool without the necessity of replenishment, since abrasion occurs at only one point along the travel of the tool, e.g., the belt or a grindstone, and during the rest of its travel while it is not in contact with the metal being abraded, allows sufficient exposure and time for the sodium nitrite to rephase, i.e., to change ~rom liquid back to solid. Prior investigators in thls field have been completely unaware of this primary knowledge and discovery and, therefore, have never contemplated the use of sodium nitrite in the area of high temperature grinding and/or cutting except possibly as an anti-corrosion agent. However, as an anticorrosive or lubricating agent, the nitrite must be used in an aqueous solution whlch would turn to steam at 212F. ;(100C.), far below the melting , ~ .

-~67~s point of sodium nitrite.
Due to its physical characteristics, sodi~n nitrite has the property and the ability of being an excellent heat-sink over a comparatively wide range of temperatures. ~his thermodynamic feature, plus a good high temperature (above 536F.~ lubricity and thermal conductivity of liquid sodium nitrite increases its effectiveness as an aid capable o~ has-tening metal removal and extending the working life of the abrasive, since such high temperature phenomena, as the forma-tion o metal swarfs, welds, and glazing are minimized.
To further illustrate my discovery, measurementswere made of the surface temperatures in Waspaloy, the high-temperature alloy, which was abraded with and without ~n aid containing 50% by weight of sodium nitrite~ The results indi-cate that the metal-temperature was lowered by about 400-F.
because the sodium nitrite absorbed a sùbstantial amount of the frlctional heat generated in the abrasion. The reasons for these findings can be rationalized by calculations from thermal data, making use of some simplifying concepts.
The heat transferred to the Waspaloy was calculated as follows:

~ 50C.
; A h= J m cp dt where m = .35 g/min metal removal rate Cp = .12 cal/gr/C. - Average specific heat of aid dt = change in sensible temperature of metal in abrasion in C.
h = (0.35)(0.12)(525) = 23 cal/min.

~067~35 .
Under the same abrasion conditions, but with the use of the aid of this invention, it was found, by weighing, that 0.1 gr. of sodium nitrite was consumed during 1 minute of abrasion with a two foot length of belt. Accordingly, using 140 calfgr. as the heat absorbed by sodium nitrite from FIG
URE 9, the calculated rate of heat absorbed by the sodium ni-trite was ~ h = 0.1 (140) = 14 cal/min. Thus, the sodium nitrite in this accelerator could account for dissipation o~
about 60% of the heat generated in the unaided belt. The fact that sodium nitrite is such an effective heat-sink accounts for the lower surface temperatures in the metals being cut or abraded. Furthermore, the temperature of the cutting edge of the abrading grain is kept cooler and because of this, the metal cutting efficiency ~ontinues. Further, as the grain is kept cooler, it can fracture along crystallographic cleavage planes, rather than plastically deform, and thereby present freshly renewed cutting edges to the abraded metal. Cutting efficiency and be1t-life are thereby enhanced. However, if frictional heat is allowed to develop, the frictional heat results in plastic flow at the cutting point of the abradant grain, which blunts the grainr leading to loss of cutting efficiency and generation of more heat caused by the blunt grain pushing or plowing through the workpiece.
The basic principle of this invention resides in the application to the cutting paint or edge of a tool, of an amount of a grinding aid, free of sulfur or halogen, which will change phase and melt without decomposition when exposed to an elevated temperature and by virtue of its high . .

latent heat of melting, a~bsorb excess thermal energy, thereby reducing temperatures of the cutting point or edge and the metal workpiece surface. The process is reversible since after passing the point of metal contact, the crystal, granule, or grain of grinding aid again cools and returns to its stable solid state.
Additional experimental investigations have re-vealed that good results can be attained with sodium nitrite when it is employed and uniformly distributed on the sur-face of the cutting tool in a matrix of a hard wax such asa paraffin wax or grease-like cerate and stearic acid, or in a soft wax such as microcrystalline waxes or slack waxl in an amount of between 10 to 70% by weight. "Soft" waxes are broadly defined to include tacky, sticky or gummy wax-like materials which provide a vehicle which independently will adhere to a rough moving surface, such as a coarse coated abrasive (grit size and larger than 50 grit) on a coated abrasive moving at a rate in the order of 500~ SFPM.
Such materials are well-known.
It has been found desirable in fabricating the product of this invention, that the wax or grease cerate be first heated 20F~ above its melting point and, while in such melted state, heated grinding aid e.g., sodium nitrite in crystalline, granular or micropulverized form added thereto and uniformly dispersed therein~ By preheating the aid to the same temperature as the wax prior to introduc-tion, the temperature of the melt will not be prematurely lowered, thereby assuring proper uniform distribution of the . ~,.

~L067~385 aid throughout the wax matrix. The use of thic~eniny or . suspension agents to control the viscosity (e.g., CABOSIL*-1~ by weight) prevents the salt particles from settling while the mass is cooling.
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The principles of the present invention can also be employed in ordinary machining processes such as drill-; ing, milling and lathing by conducting the machining pro-cess in a manner such that the interface of the cutting edge and the workpiece immersed in a liquid or waxy vehicle con-10 taining at least 10% of the grinding aid. The vehicle may r be a hard or soft wax as noted above, or a liquid such as water or preferably a natural or synthetic oily material such as a liquid hydrocarbon, or a Carbowax containing wet-ting and suspending agents to aid in the formation of a 15 stable suspension of the aid. The effect of using sodium nitrite as a cutting aid is remarkable. For example, when 304 stainless steel is machined in the presence of normal cutting oils, the drilling pressure can be such that the metal is removed in the form of small burned chi2s and the 20 effect of burning is obvious. However, at the same pressure, when an effective amount of sodium nitrite is added to the ., .
cutting oil, the metal is removed in the form or a cool, continuous, springy ribbon and the workpiece does not evidence any damage.
The grinding aid may be incorporated into and/or applied to the cutting or grinding edges in other vehicles and forms. It may be applied as a coating with or without a top coating to act as a vapor barrier to prevent ~ -13-* trade mark ~i ~067~38S

pickup of water. It may be impregnated into porous gxind-ing tools simply by soaking or pressure impregnating them in a nitrite molten or solvent solution which may contain a wet-ting agent to provide proper wetting and penetration. In addition, the aid can be incorporated directly into known grinding andfor cutting tools, as for example, during fabri-cation, e.g., in the size and/or make coat of a coated abrasive, and in the bonding resin of a bonded grinding wheel or cut off wheel. When applied as a coating, it can be admixed with a suspending agent and an inert liquid ve-hicle and can be applied by brush, doctor or roll coating, or even through an aerosol spray. It can also be applied as a 100% solid by using pressed or melted salts in a bar ; form or molten salt may itself be sprayed on the workpiece or the cutting tool or abrasive. For example, molten salts can be directed to the grinding interface when grinding o~
scarfing stainless steel billets with coarse grit resinoid wheels.
As noted above, the grinding aid can be incorpora-ted into bonded or aoated abrasive products by admixing it with the resins or adhesives which are used in formation of the product. Such materials may include glue, phenolics, urea formaldehydes, melamines, epoxies and the like. In the case of bonded resin products such as resinoid grinding wheels, the aid may be incorporated throughout the wheel or just in the radially external portions of the wheel. In the case of coated abrasive products, the aid may be used in the make coat and/or in the size coat, or in a super ~0~;7~85 size coat. The resin-grinding aid mixtures may be joined by simply mixing the materials to obtain a uniform disper-sionO The mixtures may -then be admixed with or coated on abradant particles. A minimum amount of aid (i.e., from 10% to 60% based on the total weight of the coating) should be present in order to obtain the benefits of the present invention.
In the event it is desired $o prevent intimate contact between the resin and aid as for example when the resin and the aid are reactive with each other, this can be easily accomplished by encapsulation of aid particles in a resinous or oily material using known encapsulation tech-niques, or by absorbing the nitrite into a porous mineral material such as vermiculite, perlite, alumina, koalin, etc. Alternatively, the pH o~ the resin may be modified to prevent reaction with the aid.
Accordingly, it is an object of the presen~ in-vantion to provide a relatively inexpensive and highly e~-ficient aid for metal cutting and abrading processes.
Another object is to provide a metal cutting and abrading aid which may be externally applied to or ,abri-cated directly in, the cutting and abrading tool and which will maintain a relatively lower temperature during opera-tion, thereby preventing the undesirable results associa-ted with excessively high temperatures. .
Still another object is to provide an aid for the abrading of high temperature, high strength and low ~hermal conductivity metals and alloys which will hasten metal re-.- ~,.

~1~67~3~35 moval over an extended uniform period and prolong the use-ful life of the cutting and abrading tool used to carry out these processes.
Other objects and many of the attendant advantages of this invention will be more readily apparent from the following specification, claims and drawings.
.

. BRIEF DESCRIPTION OF DR~WINGS

FIGURE 1 is a sectional view of a typical cutting point or abrasive grain in a grinding wheel or on the sur-face of a coated abrasive, FIGURE 2 is a sectional view showing the effectof plastic deformation caused by excess heat generated - during the grinding operation on the outting point of the grain, FIGURE 3 is a section view showing the capping of the grain due to hot metal, FIGU~E 4 is a sectional view of a grain overlayed with the accelerator of this invention, FIGURE 5 is a sactional view of a coated abrasive prior -to use, illustrating the fact that not all the grains are the same size or at the same height.
FIGURE 6 is a sectional view of the coated abrasive after glazing has occurred, where the abrasive i5 no longer cutting but generating high temperatures in the workpiece and at the tips of the glazed abrasive gr~ins.

~067~3S

FIGURE 7 is a sectional view of the coated abrasive when the grinding aid of this invention is employed illustra-ting the fact that the abrasive grains continuously renew their cutting surface and all the abrasive is used for grinding.
~ IGURE 8 is a graph of the differential ~nermal : analysis of sodium nitrite.
FIGURE 9 is a graph showing the spec.ific enthalpy change of sodium nitrite vs. temperature.

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DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in detail wherein like numerals indicate like elements throughout the several views, the illustration of FIGURE 1 shows a typical abrasive .. . .
grain 10 firmly support~d within what is designated as a : 15 holder 11 that may be:in the form of resin, glue, glass, ; ceramic, metal or any suitable material to hold the grain : during grinding. Holder 11 may be the size coat of a coa.~ed abrasive ("sandpaper"). or the body of a grinding wheel.
Although the grain extends substantially above the ~urface 12 of the holder, the top or cutting edge 13 perfor~s the abrading function. This configuration is selected merely to represent a general type of cutting tool in the field of cutting and abrading and is employed for simple illustra-tive purposes.
In the process of abrading or grinding, the cutting tool, namely the abrasive grain 10, is continually . ~

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~0~788S

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fractured by the mechanical forces induced in the process.
In this manner, as the sharp cutting edges of the grain are worn away by attrition by its contact with the metaIlic work-piece, new, sharp cutting surfaces are ~ormed. This "re-newing" only takes place under conditions wherein the grainwill -~xacture. The grain will fracture (undergo attritious wear~ under grinding conditions, provided, (1~ it is in physical contact with the workpiece and t2) the temperature is below some critical value. The inclusion and use of a grinding fluid or solution in the process serves to maintain ~ a continuous temperature below the critical fracture value, ; thus, in most instances, assuring the continual ~ormation - of cutting edges.
On the other hand, as illustrated in FIGURE 2, when the grain temperature is permitted to rise sufficiently high, the grain becomes "plastic" and its heated sur~ace deforms. Under the grinding action in the direction 14 and the generated pressure along 15 between the grain and the metallic workpiece, the sharp cutting edges of the grain are blunted as at 16 and rounded out so that the grain in-stead of "chipping" away the metal is actually forced to "plow" therethrough. In so "plowing" the grain pushes a quantity of metal in front of it creating a furrow. This ackion, in turn, generates heat, raises the surface tem-~5 per~ture and further plasticly deforms the grain, causinga substantial loss in abrading efficiency. I permitted to continue, the grinding effect will disappear and the workpiece surface will become discolored and scored while ~8678~3~

the grain abrading surfa~e will assume a condition known as "glazed".
In addition to the foregoing, with certain ~etals, a metal swarf at elevated temperatures may melt and the por-tion so softened may deposit on the surface of the grainsas shown in FIGURE 3. The exposed surface of the grain will be capped with a metal swarf 17 to fonm an interface between the grain and the workpiece so as to preclude operational contact thereb~tween. Under these conditions no abrading action will occur even when the temperature is lowered.
By mixing or distributing uniformly an effective amount of a grinding aid of this invention, e.g., sodium ; nitrite, in a wax or grease matrix or cerate and daubing or applying the resultant material to the surface of the grains, they are provided with a uniform aid coating 18 as shown in FIGURE 4 which will hasten metal removal from the surface of a workpiece.
; FIGURE 5 illustrates the fact that the abrasive grains 10 on a coated abrasive having a backing 18a, a ~-make coat 11 and a size coat 18, are not all the same shape or size nor the same height. FIGURE 6 illustrates a coated abrasive after glazing occurs on surfaces 16 such as in the grinding of space-age materials. As will be noted, many abrasive grains such as low grains lOa do no cutting at all and whereas much of the abrasive remains, it is unable to perform its grinding function. FIGURE 7 shows the effect of the grinding aid in allowing the abrasive grains to per-form their normal grinding function by forming renewed .

~)67~38~
cutting edges 16a. Interestingly, when the grinding aid is applied to a glazed abra~sive as in FIGURE 6, it will be re-stored to useful lîfe and in the presence of the grinding aid will perform its grinding function and appear as in FIGURE 7.
The thermal properties of sodium nitrite were inves-tigated by di~ferential thermal analysis and the results are ; illustrated in FIGU~E 8. Examination of the plotted data in-dicates the existence of peaks in the specific heat with re-spect to temperature. There is an absorption of heat at 164C.
(327F.) at the first peak 23, where a second order solid state transition change occurs. The second peak 19 occurs at`~
the melting point, namely 280C. (536F.3 which is at a low enough temperature to provide cooler surface temperatures, thereby insuring metallurgical surface integrity during abrasion while heat is being absorbed by the sodium nitrite.
Further, sodium nitrite will not decompose at this tempera-ture as taught in the technical literature, but will remain molten up to 675F. (360C.) before it decomposes.
In addition to the foregoing, the heat absorption of sodium nitrite was evaluated in terms of specific enthalpy vs. temperature. The resultant plot thereof is shown in FIGURE 9. The second order solid transition 21 occurs at approximately 164C. with a heat absorption from room tempera-ture of approximately 42 cal/gr. At the melting point 22, the total absorption ~rom room temperature is about 130 cal/gr.
with the latent heat of melting about 55 cal/grams.

3~67~3~35 From the foregoing data, it follows that theore-tically, for each gram of sodium nitrite applied to the sur-face of a cutting tool, such as an abrasive coated belt (to provide an interface between the abrasive grains and the metal workpiece), the sodium nitrite will absorb approxi-mately 140 calories as its temperature is raised from room temperature to its peak molten temperature of 360C. (675F.).
Measurements were made of the surface temperatures in Waspaloy, a high-temperature alloy, which was abraded without and with the grinding accelerator containing 50%
by weight of sodium nitrite. The results thereof, indicate that the metal temperature was lowered by about 400F. be-cause the sodium nitrite absorbed a substantial amount of the frictional heat generated in the abrasion.
The following are illu5trative examples of the ; grinding aids made and used in accordance with the princi-ples of the invention.

EXAMPLE I
,:
A petroleum wax having a melting temperature of approximately ~65~. was first melted and held at a tempera-ture of 2~F. abo~e the meltin~ point. To the molten wax a selected proportion of small dry granulPs of sodium nitrite was added. The sodium nitrite was at the temperature of . .
the molten wax and uniformly distributed therein.
The resultant product was permitted to cool in a metal mold to provide a stick or bar-like configuration in order to facilitate handling and surface application. A

~ 067~
number of such bars were, fabricated, each with a different proportion by weight of sodium nitrite, including a control bar having no sodium nitrite. The specific bars fabrica-ted included percentages'of sodium ni-trite from 10~ to 70-~.
Additional specimens were made with different carrier matrices or vehicles and each with the above-described percentages of sodium nitrite, e.g., ~obil T~7ax 412, 2305r tallow, lard, grease, stearic acid, beeswax, commercially avallable slack wax, belt dressings and grease sticks.
A typical, representative nickel-based super alloy referred to as WASPALOY was selected as the wor.~piece. It was in the form of a 1/4 inch diameter rod. The abrasive selected as representative was a resin bonded 60 grit alu-minum oxide coated abrasive belt which was mounted on a contact wheel whose speed was 3600 surface feet p~r minu~e.
The workpiece was firmly mounted so as to provide a~ infeed pressure through dead weight loading oE 16 pounds per square inch.
Initially, an untreated, as received, belt ~Jas evaluated by first abrading the workpiece to the e~tent that 3/8 inch thereof was removed and the time required to accom-plish this was recorded. A second abrading run ~as then made removing an additional 3/8 inch of the rod, ma:~ing a total o~ 3/4 inch. Two independent passes were made to pro-vide more accurate and meaningful results.
The same procedure was followed in evaluating all vehicles containing various percentages of sodium nitrite, r * trade mark ~'i .

~067885 including the vehicle without the nitrite ~0~). Prior to each run or pass the surface of the 60 grit aluminum oxide resin bonded abrasive belt was uniformly coated by hand appli-cation of the specimen being evaluated and the time to remove a total of 3/4 inch of the WASPAIOY*noted. After comple-tion of these extensive experiments, it was found that the results were almost entirely (within experimental error) in-dependent of the vehicle used. That is, the time required to abrade the workpiece for any one of the specimens, having the same percentage of sodium nitrite, was about the same.
The following table was arrived at from the data recorded using Mobil 412*paraffin as the vehicle carrying the sodium nitrite. The data was converted into percentage of time saving attributed to the use of sodium nitrite as opposed to an untreated belt or a belt having thereon a vehicle containing no sodium nitrite. It was found that the average time required to remove 3/4 inch of the workpiece (4.5 grams) for either the untreated belt or the belt coated with only the vehicle was 15.5 minutes.

. TABLE I

WORKPIECE - WASPALOY

PRESSURE - 16 psi ABRASIVE - 60 grit ~LOX R.B.
*

VEHICLE - Mobil 412 paraffin * trade mark ~j '. ~' ~L067813~i AID TIME RE~UIRED TO RATE
% NaN02 REMOVE 3/4" - g/min.Increase - MINUTES (4.5 gr.) 0 -15.5 .25 5 10 13.2 .34 36%
11.2 .40 60 ; 30 ~.o .50 100 6.7 .66 164%
5.0 .90 260%
10 60 4.3 1 0~ 316%
- 70 3.7 1.21 384%

It can be concluded from the foregoing that even relatively small amounts~ on the order o 10%, begin to show some improvement in rate of metal removal through the use of sodium nitrite. Far greater improvement is evident for ; proportions above 20~, although it has been found that where ; abrasive powders and other materials (within the vehicle) are necessary for a particular finishing operation, small percentages of the aid, upwards of 10%, are useful.
The aid, as disclosed in this example, was applied to other metal alloy~ with the same results. Metal removal rates were first determined for a standard commercially available externally untreated belt and these w~re t~en com-pared to the rates of belts to which the aid was applied.
The results where the workpiece was a titanium alloy (Ti-6Al-4V) and the abrasive was a 60 grit resin bond sili-con carbide operated at a speed o~ 3600 SFM and at pressures of 4 and 8 psi are as follows.
- The percentage increase in metal removal using a treated belt with an aid having as a vehicle 50 grams of paraffin, admixed with 50 grams of sodium nitrite (NaNO2), operated at 4 and 8 psi respectively, over the untreated . ~ . .

10678~i belt werc 57% and 98%, r~spectively.
All the vehicles enumerated as being suitable possess one necessary physical characteristic, i.e., the ability to adhere to the surface of the fast moving abra-sive surface. Although discernible differences in resultswere found among the various vehicles used, they all showed some improvement and therefore any vehicle capable of ad-hering to the belt surface by direct application and having a suitable melting point could be used, provided the aid can be dispersed therein. Such vehicles are well-known.

~ ttempts were made to prepare specimens wherein the sodium nitrite exceeded 70%, but the resulting bar lacked structural strength and, therefore, pure sodium nitrite was melted, and poured into a brass mold to form a bar. The bar was daubed or rubbed on to the abrasive sur-face of a moving belt and it was visually observed that only the smaller particles clung to the surface whil~ the larcJer-particles were readily dislodged by centrifical force. To assure retention, the surface of the belt with the nitritc -~ thereon was coated or sprayed with shellac or Krylon. Follow-ing the same test procedure, it was found that the percent time saving was only slightly better than that of the 70%
sodium nitrite composition evaluated hereinbefore.

* trade mark , '~

~067~385 : ' , It has been found that molten sodium nitrite has a viscosity approaching ~hat of water, and in addition, exhibits good wetting properties. To this end, a porous ceramic or vitrified grinding wheel heated to the melting temperature o~ sodium nitrite was entirely immersed into molten sodium nitrite, then removed and permitted to cool~
and dry. The grinding wheel was then used to grind tool steel on a surface grinder without the use of a grinding fluid. A $imilar grinding operation was performed with an 100 ~ sodium nitrite bar applied to the surface. The trea-ted surface and the immexsed wheel exhibited in excess of a two-fold increase in grinding ratior e.~., ratio of the weight of metal used to abrasive used.

Visual examination and weight measurements of the wheel before and after immersion revealed that the sadium nitrite had filled the interstices of the porous grinding wheel, thereby providing a continuous supply of sodium nitrite during the grinding operation.
.

SLmilar results as in Example 3 were obtained by immersing a preheated porous grinding wheel in a super-saturated aqueous solution of sodium nitrite at 265F.

It has also been found that good results can be , ~

.. ..

~067~85 obtained by forming an aqueous and preferably saturated solution of sodium nitrite and wiping or brushir,g the liquid onto the abrading surface. However, under these ; conditions an aqueous or any other thin liquid will not readily adhere to a moving surface and therefore a thicken-ing or thixotropic agent should be added to the solution.
One such widely employed material is sold by God~rey L.
Cabot, Inc. under the mark CAB-O-SIL. This thixotropic agent is a colloidal silica prepared in a hot gaseous en-vironment by a vapor-phase hydrolysis of a silicon compound.
It should be noted that a wide variety of suitable thixo-~ tropic agents are readily available on the market and can ; be used in place of CAB-O-SlL, provided they do not create a health hazard and do not degrade or affect the workpiece.
All that is necessary is that a sufficient quantity of the agent be added to the solution so that -the resultant liquid admixture adheres to the moving abrading surface when it is ~ .
applied thereto as by wiping or brushing the liquid on the surface to provide a thin coat. The mix-ture can be conti-nually or intermittantly applied as desired. A typicalexample is as follows:

WORKPIECE - Greek Ascoloy RC-32, 3/8 inch rod ABRADANT - 60 grit aluminum oxide resin bond coated abrasive belt.

SPEED - 3600 sur~ace feet per minute PRESSURE - 7.3 pounds per square inch * trade mark ~ -27-~, ...
~i ~67~

AID - a saturated aqueous so]ution of sodi~n nitrite at 140F.
to which approximately*8~
by weight of CAB-O-SIL was added.

An untreated belt was first employed to remove stock from the workpiece and the workpiece was weighed at equal time intervals to ascertain the total removed. The same procedure was followed for the same type of bel~ ex-cept a thin li~uid coat of the aid was applied prior to abra-sion. A~ter 10 minutes 6.73 grams was removed by the "as received" or untreated belt while the belt to which the aid was initially applied removed 9.70 grams for the identi-cal time interval, the percentage increase in value being 45~ -Although the percent of the thixotropic agent canbe substantially varied, it is economically sound to employ the least proportion that will provide satis~actory results.
Further, the thickened aid can be applied to the abrading tool and then permitted to dry or placed in an oven for that purpose.

.
EXP~IPLE 6 Similar results are obtained when the solution described in Example 5 is dispensed from a manually operated spray can or bottle as well as when nitrite was directly incorporated in-to an aerosol systeln.

.

* trade mark ii .

)67~5 . _ It has been found that sodium nitrite is hygro-scopic r and although this does not severely inhibit the aid's characteristics, the resultant wetting of the tool or work-piece surface is undesirableO This problem can be overcome by providing an overlaying protective resinous film coatingwhich embodies the particles of the aid and acts as a water ~apor barrierO
Various coatings including phenolic, acetate, cellulose and urea resins can provide moisture barriers or shields which additionally serve to extend the shelf life and storage of the finished product. In the case of porous vitrified grinding wheels ~abricated under high temperatures where the aid would be vaporized, the aid.is applied by : 15 immersing the fabricated wheel in either molten salt or in a solution which may include therein any well-known wet~ing : agent to provide increased absorption into the pores of the , grinding wheel. .:
The sodium nitrite can be incorporated into the resin used in the size coat of a coated abrasive, with equally ~ good results.

: .
. EXAMPLE 8 - A suitable quantity (72% by weight) of abrasive grains, e.g., alumina, is wet with furfural in a mixing 25 chamber. In a separate mixing vessel, 9.35% of phenol-formaldehyde resin, 16.5% of sodium nitrite, about 2.0% of ~0~7~31!35 , ~
lime and hexamethylene tetramine are blended to a homo geneous dry powder mass. The dry mixture is added slowly to the furfural wetted abrasive grains with mixing, until a uniform granular mix is obtained. The mixture is put into a mold, pressed and cured at approximately 350F. in the mold.
The resultant grinding wheel has improved grind-ing properties as compared with a similar wheel made without sodium nitriteO

. . . .

.
The use of an effective amount more than 10%
sodium nitrite in other cutting aid fluids results in im-proved cutting speed, tool life and workpiece protection as shown in the following example:
About 40% by weight of finely pulverized sodiurn nitrite was added to a conve~ional cutting oil and used to ! - lubricate a 1/2" drill in the drilling of 304 Stainless Steel aboutl/2" thick. The conventi-onal cutting oil was applied and the pressure increased to the point when the me-tal chips were blue and the drilled holes scored. The use of the cutting oil with the aid at the same pressure and speed did not discolor the metal nor score the holes. The metal removed with the aid sho~ed no discoloration due to overheating.

EXAMPL~ 10 It was demonstrated that NaNO2 could be incorporated in a supersized coating on a regular coated abrasive belt, resulting in flexible coats which showed improved grinding characteristics.
The coat was made by mixing a phenolic resin and a neoprene rubber blend vehicle (l/l) with a quantity of finely-ground NaNO2 and a solvent, so that the coattng could be brushed on the belt uniformly. Upon drying, at 200F.
for 2 minutes, and at room temperature for one day, the con-centration of NaNO2 was 77.5% by weight of the dried super-sized coat and 0. 07 g/in.2.
T~sts in triplicate, under identiGal conditions were conducted with Waspaloy abraded on an as received belt, a supersized belt prepared as shown above, an~ a commercially available premium priced belt containing fluoride in the .
supersi2e coating. Comparisons made, after 6 minutes of abra-sions, showed that the supersized coatings, containing NaNO2 on regular belts resulted in a 140~ increase in meta} re-moved over the regular as-received belts. Furthermore, this supersized coating out-performed, by 45~, a commercially available premium-priced belt containing fluorides in its supersized coat.

' DC, .. . .. ~. ..

~67~itS
~X~MPLE 11 The effect of particle size of Na~O2, incorporated in a grinding aicl externally applied to abrasive belts, was evaluated for two average sizes. In -the free-flo~ing con-dition of the NaNO2, normally obtained from commercial sources,the averac3e particle size was about 250f1_ (microns), as de-termined by a sieving test. To make a grinding aid bar with a uniform suspension, a mixture of 54% salt, 45~ molten micro-crystalline wax, and 1% of thickening agent ("Cab-O-Sil'') was prepared. This batch was cast into bars in a brass mold.
Upon ball-milling the NaNO2 so that it had an average parti-cle size of about 100/~ (microns) a solid bar grinding aid ~as made as above-described, except that it ~as not neces-sary to add a thickening agent. Due to the finer size of the particles, it was found that there was no perceptible settling in the molten wax and that this solid bar grinding aid showed improved adhesion onto a running belt of-60X Al-Ox R~s at 3600 SFM.
Comparative metal abrasion tests were conductecl on Waspaloy under identical conditions. The improvement in metal removal in 5 minutes using the aid with the coarser par-ticle size (250~) of NaN02 was 36~ over the as-received belt. As a result of using the aid with a finer particle size (100~) of NaNO2, the improvement was 49% over the as-received belt. The very fine sized~particles in this test had about sixteen times yreater surface area than the coarser ones and increased the efficiency of the grinding aid by removincj 25~o more metal during the time lnterval of this test.

* trade mark -32-~0~7~

The effectivaness of using a eutectic mixture of ~N03 (55%) and NaN02 (45%) as the grinding aid was demonstrated by abrasion te~ts~ Two different preparation methods were used and evaluated.
; A simple mechanical mixture of the salts in the above proportion was ground in a mortar and pestle, and incorporated in a microcrystalline wax vehicle (55% salt~ and 45% vehicle and 1% Cab-0-Sil)*. The resultant aid was applied to the surface of an abrasive belt and used to grind Waspaloy./ The improvement, after 10 minutes of testing for this aid over .
the as-received belt was 44%.
A eutectic mixture of KN03 t55%) and Ma~02 (45%) was melted at about 300 FD then ca~t) cooled and ground in a mortar and pestle. When this was incorporated into a grinding aid bar, made as ju~t de~cribed, it was evaluated in abrasion under the same test conditions. The improvement ovex the as-received belt wa~ 8~%.
Since thl~ eutectic mixture melts at a temperaturP
below that o~ lead used in lead cored grinding wheels it may be used to impregnate vitrified grinding wheels without re-balancing simply by immersing the grinding wheels into the eutectic solution at 300F.
It wab also found that the eutectic mixture of 30dium nitrite (40%), potassium nitrate (53%) and sodium nitrate (7%) gave similar results, that this mixture ~emained in the liquid phase over a wide temperature range from about 290F. to about 1100F. and in the liquid phase had a high ~pecific heat about 0.35 calories per gram per C.

* Trademark ~6'~8~

The effects of reducing the sensible temperature ; on the surface o~ a metal during abrasion were measured ex-perimentally. Sensible temperature is defined as the tem-- 5 perature measured with 30 gauge chromel-alumel thermocouples imbedded at a constant location in a metal at the timè the abrasive grains cut through the couple. These were recorded on a L&N Azor instrument with a chart speed of 6'/minute and indicating the seebeck emf (converted by calibration to F.). In each case, $he thermocouple was positioned in the middle of a 1/4" round Waspaloy at .25" from the surface at the start of abrasion.
The surface conditions oE the belt were I - as-received condition; II - heated wi-th a grinding aid stick having 55~ NaNO2; III - a dried supersized coating painted on the belt which coating contained 77.5~ NaNO2 or .079 g/in. NaNO2; and IV - heated with a sot stick wax commer-cially sold as a grinding aid.
The results of these tests are given in the-following Table showing the significant decreases in sen-sible temperatures when using the grinding aids of this in-vention. For example, it was possible to show a decrease in sensible temperature by about 600F., during this test when the most concentrated amount of NaNO2 was existent on the belt surface.

~'' ~L~6~S

a O
~ ~ ~ ~ .
P~Z C~ _ ~.. , ,.._ ..._ .
, ~ ~1 Z }5 ~ ,~ u~ o~ . ~
~ H ~ ~ _ . ... _ . -- --~

: OP:: ~Q~ -¦ ¦
i . _ .IJ O ~ ,_ ~
~:: a) z ~ o\~ ~1 '~ ~' .~Z ~0~ ~
,1 ~ ~ ~ r r~ ~

o ~ ~ ~mo a: ~ . ~ ~ ~ o z; o ,.. _ _ .. __ ._ O H H H ~

~61!678~35 EX~MPLE 14 When coatings of the aid made with Mobil ~ax 412 and other paraffins, tallows, etc., were uniformly coated on 60 grit aluminum oxide coated abrasive, the same rate of metal remo~al was noted; however, it was observed that the aid made wi-th Mobil 412 paraffin, etc. was not as relatively effective on the coarser grits such as 36 grit aluminum ; oxide. Quantitative studies disclosed that the reason was that the Mobil Wax 412 paraffin was too hard and lacked suf-ficient adhesion and the large grains o~ the coarser grit belts chopped away at the relatively hard and brittle matrix and little of the aid attached itself to the 36 grit belt.
The following example shows the percentage of material applied, which actually adhered to the 36 grit alu-minum oxide belt with increasing nitrite content:

Example:

Belt: 36 grit aluminum oxide belt 4" x 132"
Speed: 3600 S.~.P.M.

Aid: 50~ Sodium Nitrite, 49% Mobil 412 Para~fin, 1% CAB-O-SIL in the form of a 5/8" x 1 5/8" bar applied by an air cylinder with 3 psi pressure for - 10 seconds.

* trade mark r.

B, ., .

~06713~35 Percent by Weight Percent by lleiyht of Bar NaNO2 adhering to Abrasive ~ ' ; ~ 10 Whereas the data reported in the immediately preceding Table indicated substantially high metal removal rates with increased percentages of nitrite, this example shows that less material adheres to the abrasive as the percentage of nitrite increases making the use of the aid very costly and impractical on the coarser yrits.
When Mobil 2305 microcrystalline wax, a tacky material, was substituted for Mobil 411, the percentage of adhesion increased twofold. Other soft wa~v materials were used such as slack wax with similar results. However, some of~
these other materials have limited application because they contain sulphur, which can be poisonous to certain space-age alloys, or insoluble gums and resins which are diffic~lt to remove from the metal after grinding and interfere with welding, electroplating, etc.
- One of the more satisfactory matrix ma~erials was refined petrolatum modified wax paraffin having higll adhesion to coated abrasive surfaces, minimal sulphur,yum and resin content and ready solubility at low temperature in commercially * trade mark . ~ .....

~06~781~5 available solvent type metal cleaners~
The aid suitable forapplication to coarse grit abrasive was applied in the grinding of numerous metals with uniformly favorable resu~ts on a variety of coarse and fine grit abrasive belting.
Numerous tests conducted with sodium nitrate showed results similar to, but not quite as favorable as, sodium nitrite. Further tests with potassium nitrate also proved effective, but not as favorable as sodium nitrite.
Tests conducted with mixtures of granules of these compounds produced favorable results with grinding time less than that for potassium nitrate but longer than sodium nitrite.
It was found that a mixture of 45% sodium nitrite with 55% potassium nitrate when melted together foxm a 1~ eutectic having a melting point of about 290F.
The cooled eutectic mixture was ground and i~s ; pe~formance as an ai~ closely approximates that of sodium nitrite.
The iower melting point eutectic is particularly useful to impregnate porous vitrified gxinding wheels at temperatures above 290F. The lowered temperature of the eutectic compound compared with 536F. for the sodium nitrite, permits the impregnation of manufactured grinding wheels equipped with lead cores without melting such cores.
The use of low melting point eutectics permits this invention to be utilized by industrial distributors of grinding wheels as a service to their customer and, also, . . ~ .

~0678~3~
by large industrial consumers of vitrified grinding wheels who may wish to impregnate vitrified grinding wheels on their premises without the need for manufacturing new cores, balancing, etc.
Since the salts are hydroscopic and will pick up atmospheric moisture, a spray of varnish Krylon or similar barrier coating keeps treated wheels dry.
Investigations were conducted using other chemical compounds free o~ halogens and sulphur having melting points above a temperature of 70F. and below 1,000F. and tempera-tures of dissociation at least 100F higher than the melting temperature, and relatively high latent heat of melting in ; excess of 10 cal/gram, with notable increase in me-tal removal rates and substantial lowering of the temperature of the workpiece as follows:
Waspaloy rod t3/8") weighing 2.65 g. was ground on a 60 grit belting and the cutting time was measured.
All the aid samples were prepared using a microcrystalline **
wax having 1% Cab-O-Sil and 45O of the salt. The grinding time and rate of metal removal are given in the following table:
TIME RATE - %
~ID (min.) (g./min.)INCREASE
.... .. .
As received3.86 .68 K2CrO4 1.53 1.73 154 r LiNo3 2.43 1.1 61 NaN03 1. 4 . 1. 9 179 KNO3 1.62 1.64 141 NaNO2* 1.2 2.2 223 *For Cornparison ** trade mark ~j ~L~67~3135 From these results and a review of the physical - properties of other chemical compounds it was concluded that other chemicals or mixtures might be substituted for, or mixed ; with, sodium nitrite as a grinding and cutting aid provided they are neither explosive nor inflammable under conditions of use and they meet the definitian in the preceding paragraph as to high latent heat of melting and relatively low melting temperature (70F. to 1000F.) with decomposition taking place at least 100F. above the melting point to permit the molten compound to function as a high temperature coolant and lu-bricant continuously as it is heated and cooled in the grind-ing process.
Experiments were also conducted to determine the effect of the particle size of the chemical compounds used - 15 in the aid.
The differences due to particle size are of rela-tively minor significance, however, since the presence of effective amounts of the aid produced very substantial im-provem~nt in grinding efficiency in the order of 50~ to 350%.
The improvements due to the cvntrol of the particle size are in the order of 10 to 25% of the total improvement.

.

Claims (29)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for physically modifying a workpiece which comprises the steps of bringing said workpiece into relatively moving contact with a grinding or cutting tool having an abradant or cutting edge respectively in the presence of a grinding or cutting aid composi-tion consisting of from 10% to 40% by weight of a grinding or cutting aid which is sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate or mixtures thereof, externally applied to the tool in a waxy vehicle other than paraffin, or in an inert, oily liquid vehicle other than paraffin.

2. The process of claim 1 wherein the aid is sodium nitrite.

3. The process of claim 1 wherein the aid is a eutectic mixture of potassium nitrate and sodium nitrite.

4. The process of claim 1 wherein the aid is a eutectic mixture of potassium nitrate, sodium nitrite and sodium nitrate.

5. Method for preserving the cutting or grinding characteristics of a cutting or grinding tool while maintain-ing the surface integrity of a metal workpiece at an accelerated rate of cutting or grinding comprising:
effecting contact and relative motion between the workpiece and a cutting or grinding edge of a cutting or grind-ing tool;
applying to the interface between the edge of the cutting or grinding tool and the workpiece by external applica-tion to the tool, an effective amount of a grinding or cutting aid composition consisting of from 10% to 40% by weight of a grinding or cutting aid which is sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate or mixtures thereof, in a waxy vehicle other than paraffin, or an inert, oily liquid vehicle other than paraffin;
whereby, the composition, upon exposure to the frictional heat generated at the interface, undergoes melting and reduces the surface temperature generated at the interface by the heat absorption due to heating the compound to the melting point, the latent heat of melting of the compound and the additional heat absorption of the molten compound, while simultaneously forming a lubricating liquid film at the interface.

6. Method as claimed in claim 5 wherein the compound is a eutectic mixture of potassium nitrite and sodium nitrate.

7. Method as claimed in claim 5 wherein the compound is a eutectic mixture of potassium nitrate, sodium nitrite and sodium nitrate.

8. A tool for physically modifying a workpiece, said tool having an edge for modifying said workpiece when brought into moving contact therewith, in combination with a grinding or cutting aid com-position consisting of from 10% to 40% by weight of a grinding aid which is sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate or mixtures thereof, applied to the interface between said tool and said workpiece in a waxy vehicle other than paraffin.

9. The article of claim 8 wherein the aid is sodium nitrite

10. The article of claim 8 wherein the aid is a eutectic mixture of potassium nitrate and sodium nitrite.

11. The article of claim 8 wherein the aid is a eutectic mixture of potassium nitrate, sodium nitrite and sodium nitrate.

12. A grinding or cutting aid composition comprising 10%
to 40% by weight of sodium nitrite, potassium nitrite, sodium nitrite, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate and mixtures thereof, an effective amount of a suspending agent, and an inert, oily liquid vehicle other than paraffin.

13. A composition of claim 12 containing an aerosol or spray propellent.

14. As an article of manufacture, a grinding tool for use in physically modifying a metallic workpiece, said tool com-prising abrasive particles admixed with a grinding aid consist-ing of a compound free of sulphur and/or halogen, stable under conditions of use, and having a melting point in the range of 70°F. to 1000°F., a decomposition temperature at least 100°F. above the melting temperature and a latent heat of melting greater than 10 cal/gm.

15. The tool of claim 14 wherein the abrasive particles are overcoated with a resin having the aid dispersed therein.

16. The tool of claim 14 wherein the abrasive particles are formed into a porous, vitrified mass and the aid is impregnated in the pores of said mass.

17. The tool of claim 14 wherein the abrasive particles are bonded together in a resin matrix and said aid is dispersed at least partially throughout said matrix.

18. The tool of claim 14 wherein the abrasive particles and the aid are deposited on a flexible substrate in the form of a coated abrasive backing material.

19. The tool of claim 14 wherein the aid is a member of the group consisting of sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate and mixtures thereof.

20. The tool of claim 19 wherein the aid is sodium nitrite.

21. A process of forming the tool of claim 16 which comprises impregnating a vitreous mass of abrasive particles with an aqueous solution of the aid.

22. The process of claim 21 wherein the aid is sodium nitrite.

23. The process of claim 21 wherein the aid is a eutectic mixture of sodium nitrite and potassium nitrate.

24. The process of claim 21 wherein the aid is a eutectic mixture of sodium nitrite, sodium nitrate and potassium nitrate.

25. A process according to claim 21 wherein the aid is in molten form.

26. A process according to claim 21 wherein the impregnated vitreous mass is overcoated with a resinous film vapor barrier.

27. A process for improving the efficiency of a grinding or cutting tool which comprises contacting said tool or work-piece with an aid consisting of a compound free of sulphur and/or halogen, stable under conditions of use, and having a melting point in the range of 70°F. to 1000°F., a decomposition temperature at least 100°F. above the melting temperature and a latent heat of melting greater than 10 gal/gm, said contact being in a manner which will result in the formation of a coating of said aid on the cutting edges of said tool during use thereof.

28. The process of claim 27 wherein the aid is a member selected from the group consisting of sodium nitrite, potassium nitrite, sodium nitrate, potassium nitrate, lithium nitrite, lithium nitrate, potassium dichromate and mixtures thereof.

29. The process of claim 27 wherein the aid is sodium nitrite.