Coolant/lubricant for machining operations

A non-toxic coolant/lubricant is provided which is specifically designed for use in extremely high-load, high-stress machine operations, such as broaching. The composition of this coolant/lubricant includes about 8 to 15 wt % of molybdenum disulfide (MoS.sub.2) powder; about 2 to 6.6 wt % of soap flakes; about 6 to 12 wt % of a liquid polytetrafluoroethylene suspension; and about 66.4 to 84 wt % water. The liquid polytetrafluoroethylene component, which is a water-based suspension of polytetrafluoroethylene, serves as a replacement for toxic CCl.sub.4, which has been used to increase lubricity in coolant/lubricants comprising molybdenum disulfide. The replacement of CCl.sub.4 with liquid polytetrafluoroethylene in the present composition results in a non-toxic but still highly effective coolant/lubricant.

CROSS-REFERENCE TO RELATED APPLICATION 
The present application is related to application Ser. No. 08/083,244, 
filed Jun. 24, 1993, which discloses and claims a broaching tool, a method 
of forming a finishing hole using the broaching tool, a broaching method, 
and a lubricant and coolant composition for use with the broaching tool. 
The present application is directed to an improved lubricant and coolant 
composition for machining operations using cutting tools including 
broaching. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to the art of machining, and, more 
particularly, to a coolant/lubricant for forming small, deep holes with 
high precision and surface finish. The new coolant/lubricant is especially 
formulated for use with high toughness and high strength metal alloys. 
2. Description of the Related Art 
Machine operations involving cutting processes such as milling, drilling, 
broaching, and the like require a coolant/lubricant to aid in the 
machining. Of particular interest are improved lubricants/coolants for 
applications involving (1) machining of high strength, high toughness 
metal alloys and (2) high load and high stress machining operations. The 
following description is directed to the broaching process; however, it 
will be understood that this is merely an exemplary process in the use of 
lubricants/coolants. 
Machining of small, deep holes with high precision and surface finish is a 
problem which has persisted in the art. A small, deep precise hole can be 
defined as having a diameter of less than 12 millimeters, an aspect 
(depth/diameter) ratio of at least 5, a precision of ISO standard H6-H7, 
an angular tolerance of H6, a surface roughness of 0.2 to 0.4 micrometer, 
and a bore out-of-roundness, cylindrical out-of-roundness and taper which 
are within 1/3 to 1/2 of the tolerance. 
Prior art methods for machining small, deep holes include drilling and 
expanding followed by rough and fine reaming, rough and fine boring, or 
boring and grinding. Other methods include honing and electron discharge 
machining (EDM). These prior art methods suffer from the drawbacks of 
multiple complex machining processes, extreme difficulty in obtaining 
satisfactory precision, surface finish and exchangeability, low 
productivity, poor quality control, high reject rate, and often conical 
deformation at the exit ends of the holes. 
Broaching is a process for machining holes, slots, and grooves with high 
productivity compared to the methods described above. Broaching can be 
used for forming holes in numerous metals including low-carbon steel, 
low-carbon alloy steel, phosphor bronze, pure aluminum, stainless steel, 
titanium alloys, and other materials. 
A broaching tool generally includes an elongated body on which a number of 
parallel cutting teeth are formed or attached. The diameters of the teeth 
progressively increase from the front end to the rear end of the tool by 
an increment known as the "rise", such that each tooth cuts slightly 
deeper than the previous tooth. 
A basic broaching tool and method are described in U.S. Pat. No. 1,945,535, 
entitled "BROACHING TOOL", issued Feb. 6, 1934 to B. Schlitz. A method of 
fabricating a basic broaching tool is described in U.S. Pat. No. 
4,498,361, entitled "BROACH MANUFACTURING METHOD", issued Feb. 12, 1985 to 
W. Grace. 
Broaching as practiced conventionally has not achieved its potential for 
forming small, deep holes with high precision and surface finish. This is 
due to a number of fundamental problems which have remained unsolved. 
As the broaching tool is forced through the workpiece, high friction and 
specific pressure between the front face of each cutting tooth and the 
compressed material ahead of the tooth generate a large amount of heat 
which results in the formation of a layer of material which clings to the 
front face of the tooth and is known as a "built-up edge". 
Certain "sticky" materials such as stainless steel are particularly prone 
to the formation of built-up edges due to their high elasticity, 
percentage elongation, and plastic deformation characteristics. The 
frictional forces and pressures between the chips generated during 
broaching, the broaching tool, and the workpiece are especially high for 
these materials, causing chips to break away from the workpiece that cause 
scaling of the surface of the hole and further enabling the built-up edge 
to grow to an undesirably large size. This causes the diameter of the hole 
to progressively increase, and creates a "nibbled" surface finish with a 
high degree of roughness. 
If the built-up edge grows to a large size and then fractures off, the hole 
will have a surface with band-shaped scaling. Because cooling and 
lubrication are relatively ineffective in the lower portion of a deep hole 
which is being formed by vertical broaching, the scaling bands generally 
appear in the lower half of the hole. 
There are four aspects of a coolant/lubricant to consider: 
(1) It lubricates the cutting edge/chip/workpiece interfaces so that the 
chips will slide over the cutting tool surfaces with a minimum of friction 
and therefor generate a minimum of frictional heat and cutting tool 
abrasion. The coolant/lubricant also prevents built-up at cutting edges 
and extends useful life of the cutting tool. 
(2) It conducts away heat generated by the separation of the chips from the 
workpiece and also the heat generated by the cutting edge's trailing edge 
which slides over the workpiece surface. 
(3) It must penetrate and adhere at all the interfaces between the cutting 
tool and parts. To achieve this, sufficient volume of the 
coolant/lubricant fluid and pressure is required. 
(4) It must not be corrosive to surfaces. 
Satisfying these requirements, a well-formulated coolant/lubricant adds 
greatly to the production of smooth cutting surfaces and long cutting tool 
life. 
Ineffective cooling and lubrication not only result in poorer quality holes 
due to occurrence of build-up edges, but also fail to protect the 
broaching tool itself from wear. The large heat concentration at the 
cutting point of the tool causes loose chips to fuse to the cutting tool 
edge, eventually blunting the tool. Once begun, the deterioration of a 
blunted broaching tool accelerates with use because a blunted tool 
generates more heat and results in more loose chips for fusion to the tool 
edge. Consequently, the service life of the broaching tool is shortened by 
inadequate lubrication and cooling, and the holes machined by the worn, 
blunted broaching tools have rough surface finishes and are generally of 
poor quality. 
Prior art lubricants and coolants, including conventional cutting oils such 
as engine oil, spirdel oil, sulfurizing oil, and emulsions, are incapable 
of adequately preventing built-up cutting edges and reducing the 
frictional forces, temperatures, and pressures created during broaching 
small, deep holes. This lack of effective lubrication and cooling for 
broaching operations has limited the precision and surface finish of holes 
formed by broaching and has shortened the service life of broaching tools. 
A coolant/lubricant specifically designed for the rigors of the broaching 
process is described in a pending application entitled "HIGH PRECISION, 
HIGH SURFACE FINISH BROACHING METHOD, TOOL, AND COOLANT/LUBRICANT", 
application Ser. No. 08/083,244, filed Jun. 24, 1993, in the name of 
Lin-Sen Yuan, one of the present inventors. The coolant/lubricant of that 
application comprises a molybdenum disulfide power dispersed in a liquid 
suspension of soap and water. In an alternative embodiment claimed within 
that application, the liquid suspension also includes kerosene, 
chloroparaffin, and carbon tetrachloride (CCl.sub.4). The use of CCl.sub.4 
is to minimize sticking and prevent built-up cutting edges. 
While the coolant/lubricant of that application overcomes the limitations 
of prior art lubricants and coolants described above regarding the 
broaching process, its use of the toxic chemical CCl.sub.4 is a concern 
for environmental and safety reasons. Moreover, the use of CCl.sub.4 is 
heavily regulated due to its toxicity, so that the cost of using CCl.sub.4 
is effectively increased. 
Further, the two classes of lubricants/coolants mentioned above function 
well only for different types of metals. The moly/soap/water formulation 
functions well for non-alloy metals, such as carbon steel, copper, etc., 
while the coolant/lubricant using CCl.sub.4 functions well for alloy 
metals, such as stainless steel. 
Thus, a need remains for a non-toxic coolant/lubricant that can provide 
substantially the same level of lubrication and cooling in the harsh 
environment of machinging operations involving cutting currently achieved 
by molybdenum disulfide power dispersed in a liquid suspension of soap, 
CCl.sub.4, and water. The coolant/lubricant should also be convenient to 
store and transport. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a non-toxic coolant/lubricant is 
provided which is specifically designed for use in extremely high-load, 
high-stress machine operations, such as broaching. The present 
coolant/lubricant includes the use of a water-based suspension of 
extremely fine Teflon (polytetrafluoroethylene) liquid (60% solids) mixed 
in a water-based slurry of MoS.sub.2 and soap. (Teflon is a trademark of 
E. I. dupont de Nemours, Wilmington, Del.) The water-based suspension of 
Teflon, also termed "liquid Teflon", serves as a replacement for toxic 
CCl.sub.4, which has been used to increase lubricity in previous 
coolant/lubricants comprising molybdenum disulfide, as described above. 
The coolant/lubricant of the present invention is substantially more 
effective in the harsh environment of broaching operations than 
conventional cutting oils such as engine oil. The superiority of the 
present coolant/lubricant is particularly evident in the lower portion of 
broached holes, where extreme temperatures and pressures have been 
problematic for conventional cutting oils. The exceptional lubricity and 
cooling provided by the present composition enable the production of 
precision holes with highly polished surfaces, and low surface tension and 
strong capillary action of the composition extend the service life of 
broaching tools by removing loose chips from the cutting edge. 
Thus, the benefits of a coolant/lubricant comprising molybdenum disulfide 
are realized in the practice of the invention without resorting to the 
inclusion of toxic chemicals such as CCl.sub.4. Rather, the replacement of 
CCl.sub.4 with liquid Teflon in the present composition results in a 
non-toxic but still highly effective coolant/lubricant. 
Use of the present composition is also economically prudent. The resulting 
reduction in rejection rate for manufactured parts as well as the 
extension of broaching tool service life translate to decreased 
manufacturing costs. Further, the present coolant/lubricant is inexpensive 
to produce, in that it simply involves mixing readily-available components 
in a simple mechanical mixer. It requires no special transportation or 
storage arrangements, since it may be transported in concentrated paste 
form and is chemically stable at temperatures ranging from -25.degree. to 
70.degree. C. in its final hydrated form. Moreover, cleaning up this 
aqueous-based coolant/lubricant is easily accomplished using ordinary 
water. This coolant/lubricant has potential application in other 
high-load, high-stress machining operations aside from broaching. For 
example, the benefits of the present invention extend to such operations 
as high-speed cutting, drilling, milling, the making of gears, turning, 
reaming, coring, legging, drawing of wires, drawing of tension bars, 
drawing of tubes, and making screws. 
These and other features and advantages of the present invention will be 
apparent to those skilled in the art from the following detailed 
description, taken together with the accompanying drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The discussion which follows is directed to one specific example of a 
cutting tool, namely, a broaching tool. However, it will be appreciated 
that the present invention is not limited to the use of broaching tools, 
but can be used for machining operations with many different types of 
cutting tools. The broaching tool is discussed below merely as an example 
to aid in the understanding of the present invention. 
A pull broaching tool 10 benefited by the lubricant/coolant of the present 
invention is illustrated in FIG. 1 for broaching small, deep precision 
holes with high surface finish. The present tool 10 is capable of 
broaching holes having a diameter of approximately 5 to 50 millimeters, 
aspect (depth/diameter) ratio of approximately 1 to 25, precision of ISO 
standard H6 to H7, angular tolerance of H6, surface roughness of 0.1 to 
0.4 micrometer, and bore out-of-roundness, cylindrical out-of-roundness 
and taper which are within 1/3 to 1/2 of the tolerance. 
The tool 10 includes a body 12 having a front end 12a and a rear end 12b, 
and is intended to be pulled leftwardly as indicated by an arrow 14 
through a hole for broaching. The left end portion of the body is formed 
into a pull shank 16 to enable it to be gripped by the jaws of a 
conventional broaching machine (not shown). 
A cylindrical front pilot 18 is formed on the body 12 rearward (rightward) 
of the pull shank 16. The front pilot 18 has a diameter which is equal to 
or slightly smaller than the initial diameter of a hole to be broached for 
smoothly guiding the tool 10 into the hole. 
A cutting section 20 including a plurality of annular cutting teeth 22 is 
formed in the body 12 rearward of the front pilot 18. The cutting section 
20 can include a continuous set of cutting teeth of the same type, or can, 
as illustrated, include a roughing section 20a, a semi-finishing section 
20b and a finishing section 20c having teeth of different types. A rear 
pilot 24 including rings or smoothing teeth 26 is formed after the cutting 
section 20. 
A method of broaching using the tool 10 generally includes the steps of 
forming a hole through a workpiece, and then pulling the tool 10 through 
the hole to increase the diameter and improve the precision and surface 
finish of the hole. Preferably, a pilot hole will be formed by drilling or 
casting. The intended finished diameter D of the broached hole is larger 
than the diameter of the pilot hole by an amount .DELTA.D which is 
selected in accordance with the precision and surface finish of the 
secondary hole. 
In all cases, the broaching tool should be maintained as concentric with 
the hole as possible. The higher the concentricity, the higher the 
precision and surface finish that can be achieved. The diameter increase 
.DELTA.D to be accomplished by broaching and the precision of the finished 
hole are limited by the precision of the pilot hole, including geometric 
parameter such as straightness, ellipticity, and taper. 
Prior art lubricants/coolants based on conventional cutting oils such as 
engine oil, spirdel oil, and sulfurized oil, are not sufficiently 
effective to enable small, deep holes to be broached with high surface 
finish using conventional broaching tools, especially in the lower 
portions of the holes. Conventional cutting oils are also ineffective in 
preventing chips from sticking to the cutting teeth. Alternatively, a 
recently developed coolant/lubricant based on a molybdenum disulfide 
powder dispersed in a liquid suspension including soap, water, and carbon 
tetrachloride (CCl.sub.4) provides lubrication and cooling superior to 
that offered by conventional cutting oils, but suffers the disadvantage of 
toxicity due to the presence of CCl.sub.4. 
The present composition offers the superior lubrication and cooling 
properties of MoS.sub.2 -based products while eliminating CCl.sub.4 as a 
source of toxicity. The present coolant/lubricant includes soap paste, 
molybdenum disulfide (MoS.sub.2) powder dispersed in a suspension of the 
soap paste, "liquid Teflon", and water. More specifically, the composition 
of this coolant/lubricant includes about 8 to 15 wt % MoS.sub.2 ; about 2 
to 6.6 wt % soap; about 6 to 12 wt % liquid Teflon suspension; and about 
66.4 to 84 wt % water. 
Molybdenum disulfide (MoS.sub.2) is a powdery solid that offers good 
lubricity, adhesion, heat resistance, non-corrosivity, and low friction 
under high compressive force. In the practice of the invention, it is 
preferable that the purity of the MoS.sub.2 used be at least 98% and that 
the powder size be less than 1.5 .mu.m. Given that MoS.sub.2 is a powder, 
this component must be dispersed in a suitable liquid suspension to avoid 
precipitation in solution. The present invention uses a soap paste to 
encapsulate the MoS.sub.2 powder, thereby successfully enhancing its 
suspendability in water. 
The so-called "liquid Teflon" used in the practice of the invention 
comprises a dry polytetrafluoroethylene (also designated as (C.sub.2 
F.sub.4).sub.n or Teflon) powder suspended in water plus a surfactant, the 
dry Teflon powder being a fine amorphous powder representing about 58 to 
62 wt % of the total suspension. Liquid Teflon is commercially available 
from DuPont under the tradename Teflon 30 and from Shanghai Rubber under 
the tradename PTFE (Emulsified Polytetrafluoroethylene). As available from 
dupont, the surfactant may be present in an amount ranging from 0 to about 
5 wt % and comprises either octyl phenoxypolyethoxyethanol or nonyl 
phenoxypolyethoxyethanol. 
Liquid Teflon offers many desirable qualities to the coolant/lubricant of 
the present invention, including low friction coefficient, superior 
chemical stability, low surface tension, strong capillary force, low 
adhesion, and good penetration properties. Further, liquid Teflon is 
highly wettable. 
The soap is preferably a sodium fatty-acidulate having the chemical 
composition (C.sub.n H.sub.2n+1)-COONa, where n ranges from 8 to 18, 
including approximately 96% sodium fatty-acidulate soap and the balance, 
approximately 4%, water. The soap takes the form of dry flakes prior to 
its combination with water. 
The water used in the practice of the invention is preferably a soft water 
and is, most preferably, substantially deionized water. 
One gallon of the preferred coolant/lubricant composition comprises (from 
experimental evidence): 
______________________________________ 
Component Amount (pounds) 
Amount (kg) 
Wt % 
______________________________________ 
soap (chips) 
0.13 to 0.44 0.06 to 0.20 
2 to 6.6 
MoS.sub.2 (powder) 
0.60 to 1.1 0.27 to 0.50 
8 to 15 
liquid Teflon 
0.44 to 0.88 0.20 to 0.40 
6 to 12 
(60% solids) 
water 4.42 to 5.54 2.01 to 2.52 
66.4 to 84 
______________________________________ 
The coolant/lubricant of the present invention is prepared by producing a 
soap paste by mixing the soap flakes with water. All of the MoS.sub.2 is 
then added to the soap paste to form a MoS.sub.2 /soap paste, to which all 
of the liquid Teflon is added to form a MoS.sub.2 /soap/liquid Teflon 
paste. Finally, water is added to the MoS.sub.2 /soap/liquid Teflon paste 
to form a smooth, free-flowing liquid that represents the final 
coolant/lubricant product. A simple mechanical mixer may be used to mix 
the components. 
By way of example, the following procedure has been used to produce one 
gallon (3.875 liters) of a specific composition of the coolant/lubricant 
of the present invention: 
1. Mix 130 grams of soap flakes with 650 grams of water to form a soap 
paste; 
2. Mix 385 grams of MoS.sub.2 powder with the soap paste to form MoS.sub.2 
/soap paste; 
3. Mix 306 grams of liquid C.sub.2 F.sub.4 with the MoS.sub.2 /soap paste 
to form MoS.sub.2 /soap/liquid C.sub.2 F.sub.4 paste; and 
4. Add balance of water to the MoS.sub.2 /soap/liquid C.sub.2 F.sub.4 paste 
to form 3.875 liters (1 gallon) of coolant/lubricant. 
The coolant/lubricant is coated on the cutting tool prior to cutting. The 
coolant/lubricant may also be coated on the workpiece to be machined. With 
particular reference to the broaching tool shown in FIG. 1, the 
coolant/lubricant is coated on the broaching tool 10 prior to broaching, 
with care being taken to ensure that the slots 28 of the cutting teeth 22 
are completely filled with the coolant/lubricant. 
EXAMPLE 
The coolant/lubricant of the present invention was used to form a plurality 
of holes in a workpiece comprising steel SAE 4620 MOD. The composition of 
the coolant/lubricant comprised 4.3 wt % soap (Norfox 92, available from 
Norman, Fox & Company), 11.5 wt % MoS.sub.2, 9 wt % liquid Teflon (60% 
solids), 75.2 wt % water. 
FIG. 2 shows the statistical distribution of hole sizes in one typical 
experiment using the coolant/lubricant of the present invention. The Table 
below shows the improvement when compared to the use of 329 Soluble Oil, 
available from Castrol Industrial, which is an emulsion-type lubricant. 
______________________________________ 
Precision Surface Rejection 
(&lt;0.0002") 
Roughness Rate 
______________________________________ 
Emulsion-Type 
45% 1.3 to 3.2 .mu.m 
10% 
Lubricant 
Present Cool- 
83% 0.2 to 0.4 .mu.m 
1.3% 
ant/Lubricant 
______________________________________ 
The above Table shows that (1) the number of parts whose hole size 
precision that is better than (i.e., less than) 0.0002 inch improves from 
45% to 83% of the total parts, (2) surface roughness improved to only 0.2 
to 0.4 .mu.m, or about 1/10 that provided by the prior art lubricant, and 
(3) rejections of parts was a low 1.3%. 
While illustrative embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art without departing from the spirit and scope of 
the invention. 
For example, the weight percentages described above are preferred values 
and should not be considered as limiting the scope of the invention. These 
ratios can be varied within substantial ranges as required for particular 
applications. It will be further understood that the present 
lubricants/coolants are not limited to broaching and can be used for other 
cutting and machining operations. 
Accordingly, it is intended that the present invention not be limited 
solely to the specifically described illustrative embodiments. Various 
modifications are contemplated and can be made without departing from the 
spirit and scope of the invention, as defined by the appended claims.