Note: Descriptions are shown in the official language in which they were submitted.
~ ` ~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
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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
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. ~,.
)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
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~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
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~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.
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~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
, ~ .
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~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
25C
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.
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~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
. .
67885
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.
, .
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
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~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
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~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.
.
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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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
-18-
~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
--19--
.
~)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.
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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
-21
~ 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
-22-
* 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
- BELT SPEED - 3600 SFPM
PRESSURE - 16 psi
ABRASIVE - 60 grit ~LOX R.B.
*
VEHICLE - Mobil 412 paraffin
* trade mark
-23-
~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
-24-
. ~ . .
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.
EXAMPLE 2
~ 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
-25-
,
'~
~067~385
: ' ,
EXAMPLE 3
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.
.
- 20
- EXAMPLE 4
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.
EXAMPLE 5
It has also been found that good results can be
-26-
, ~
.. ..
~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
-28-
ii
.
)67~5
- EXAMPLE 7
. _
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
-29-
~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
. . . .
EXAMPLE 9
.
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.
-30-
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.
-31-
' 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~
EXAMPLE 12
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
-33-
~6'~8~
EXAMPLE 13
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.
-34-
~''
~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
57
24
28
19
~ '
; ~ 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
-37-
* 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,
-38-
. . ~ .
~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
-39-
~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.
-40-
.
