Note: Descriptions are shown in the official language in which they were submitted.
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Metal Cutting With High Pressure Coolant
Technical Fleld
The present invention relates to machining,
most particularly the way in which coolant is applied
to the vicinity of a cutting tool.
Background
The present invention is principally concerned
with the machining of metals using single point
cutting tools. With such tools, as with most metal
cutting in general, a coolant is applied to the
vicinity where the c~ltting takes place in order to
improve tool life and enhance the quality of the
surface finish. Generally, the coolan-t also has
lubricating properties; a combination of water soluble
oil and water is often favored because of cost and
good heat transfer characteristics. In certain
instances mineral oils and other coolants are
utilized.
The dynamics of what occurs at the interface be-
tween the edge of the cutting tool and the workpiece
have been extensively investigated, according to the
literature. This is attributable to the significant
economic impact which can result from improvements
in cutting tools and cutting methods. In recent
years, a major development has been the pro-
longation of -tool life by the use of sintered metal
carbides instead of hardened alloy steels. But
carbide tools still degrade with time and it is of
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great interest that this be lessened. Benefits
which accrue from extending tool life are much
greater than the replacement cost of the tool~ They
include the avoidance of down time, cost of re-
calibra-ting the tool location, and avoidance of
possible damage to the workpiece which can occur when
a tool breaks unexpectedly.
In machining generally, the chip which is cut
from the workpiece strikes the rake surface of the
cutting tool and thereby both causes mechanical wear
and transfersheat to the cutting tool. Damage to
this surface and the cutting edge itself will
ultimately cause the tool to break away, unless the
tool is preventively replaced. Thus, it has been
logicall~ sensed that the tool cuttiny edge and the
top surface of the tool where the chip strikes are
the regions most in need of coolant. Therefore,
earlier inventors took steps to specifically
direct the coolant to such a location, as is shown
for instance in U.S. Pat. No. 2,744,451 to Lee.
Generally, such "flood coolant" systems are most
often used even today. Coolant is circulated by low
pressure pumps, with outputs of the order of
70-105 kPa. In flood cooling it is only required
to draw the coolant from the machine sump, lift it
to a certain height above the tool and to then
generally discharge it in the vicinity of the work-
piece and cutter. Of course, the coolant which lands
on the workpiece or cutter, whichever may be moving,
is thrown about. Shields are used to contain the
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coolant which flies from the vicinity of the cut.
Generally, it has been undesired to add to this
spray of coolant by having excess pressure at the
point of discharge of the coolant. Thus, the
circulating pumps used in applying coolant have been
low pressure pumps and high coolant discharge
nozzle pressures have been avoided. In fact, "mist
coolingl', comprising the use of a stream of air and
water droplets, is commonly used in less severe
situations.
Even as long as 90 years ago it was recognized
that there could be certain improvements in the
manner in which the coolant is delivered to the
vicinity of the tool cutting edge. For example,
Chouteau in U.S. Pat. No~ 522,588 shows coolant
directed along channels in the rake surface of a
tool bit. More recently, Pigott in U.S. Pat. No.
2,683,303 shows how coolant is directed from a
manifold directly transverse to the cutting face of
a milling cutter. Onsrud in U.S. Pat. No.
2,524,232 shows coolant delivered to the cutting
face of a cutter by means of channels drilled in the
body of the cutter, which channels discharge fluid
immediately ahead of the cutting face. Jennings in
U.S. Pat. No. 3,176,330 shows a carbide insert held
in place by a chip breaker which has channels in it.
The coolant is supplied to the chip breaker and dis-
charges through the channels toward the cutting face.
While the foregoing art is representative of
certain efforts which have been made, the aforementioned
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inventions have not been widely utilized. Most
probably there are several reasons for this, in-
cluding that insufficient improvement has been
realized from them, that it is undesirable to cut
channels in the surface of a cutter and that it i5
inconvenient to deliver coolant to nozzles that have
to be mechanically integrated with the cutting
tool. It has been simply both easier and sufficient
to use a flood coolant procedure.
In the making of the present invention, low
pressure coolant was ini-tially directed specially
at the cutting face of the tool, more or less along
the lines taught by the Jennings patent. However,
using this procedure only gave a certain inadequate
tool life in the machininy grooves on the outsi~e
diameter of titanium and nickel alloy cylinders.
Both significant wear and premature breakage were
encountered. To increase tool life further research
was undertaken, and as a result the invention was
made.
Disclosure of the Invention
An object of the invention is to improve the
machining of difficult workpiece materials and con-
figurations to reduce tool bit wear, and to avoid
tool bit breakage especially in the machining of
undercut grooves.
According to the invention a high pressure and
flow of liquid coolant is discharged from port onto
the rake surface of a tool bit. The coolant flows
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toward the cutting edge with great Eorce and causes
the chip being machined from the workpiece to
fracture into small pieces. When an undercut groove
is being machined this alleviates a tendency of the
ductile chip to yather and pack in the groove,
thereby causing breakage, a cause which we discovered
in the course of our work.
The pressure and flow are critical and must be
rather high to accomplish the desired result. When
machining an undercut groove in a titanium alloy,
wear was reduced initially as the pressure was
raised above 690 kPa and beyond that ordinarily
utilized in flood cooling. But, still there was
produced a continuous chip which packed in the
groove and caused tool bit breakage. When the
pressure was further increased to 1400 kPa and above,
the chip was continuously fractured and the problem
of tool breakage was alleviated. When machining a
stronger nickel base alloy even hisher pressures
of the order of 2200 kPa were required. Aqueous
base coolants are preferred because of their high
mass, low ViSCQSity~ and general utility.
The foregoing and other objects, features and
advantages of the present invention will become
more apparent from the following description of the
preferred embodiments and accompanying drawings.
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Brief Description of Drawings
Flgure 1 shows generally a tool configuration
used in the practice of the invention.
Figure 2 shows how an undercut groove is
machined on the outside circumference of a cylindrical
workpiece in the practice of the invention.
Figure 3 is a detail view of part of Figure 2
showing the tool bit within a machined groove just
as the groove is finished.
Figure 4 shows in end view how a cylindrical
workpiece is machined at its outer diamter surface
by a single point tool in the practice of the in-
vention.
Figure 5 shows how the wear chamfer at the
cutting edge is characterized and measured.
Figure 6 shows how wear of the cutting edge of
a tool bit is relatively improved by increased
coolant pressure and resultant flow, using a tool
like that shown in Figure 1.
Figure 7 is analogous to Figure 6 and shows how
-tool failure rate due to packing of chips within
a groove is reduced when pressure is increased
sufficiently.
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sest ~ode for Carrying Out the Invention
The invention is descrlbed in terms of machininq
the cornmercial titanium alloy known as AMS 4928
(by weight percent 6 Al, 4 V, balance Ti), wherein
an undercut shape circumferential groove is machined
on the outside diameter of a cylindrical structure,
such as a compressor rotor for a gas turbine engine.
As discussed below, the machining of such a con-
figuration presents substantial problems. ~owever,
it will be appreciated from the description that
the invention will be useful for machining other
part configurations and other materials.
Figure 1 shows a tool bit 20 in a holder 28
while Figure 2 shows how the same tool bit is used
to machine a groove 26 on the outside diameter 22
of a cylinder 24 (shown in partial cross section)
having a radius R. In Figure 2 the tool bit is
shown within the virtually finished undercut groove
26. The tool bit is held in a holder 28 which in
turn is mounted in the cross slide 29 of a vertical
turret lathe (not shown) which lathe rotates the
part in the direction indicated by the arrow 27.
Figure 4 shows a conventional tool bit 50 with which
the invention is more simply applied for the re-
moval of material from the circumference of another
cylinder 66.
Referring to Figure 1 again, a C-4 grade
carbide tool bit 20 is held on a tool holder 28 by a
clamp 30 (the conventional fastening details for
which are not shown in the Figure). The holder has
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a support part 33 extending under the tool bit. The
intersection of the relief surface 42 and the rake
surface 36 form the arcuate cutting edge 32 which
enables the tool bit to cut in directions com-
prising 180 about the y-z plane. The tool bit has
zero rake angle and about 7 relief angle. A
rectangular cross section tube 34 is captured within
the clamp 30 and is aimed along the top or rake
surface 36 so that liquid issuing from its exit
port 38 is aimed toward the portion of the cutting
edge 32 which lies directly along the y axis. The
tool bit is defined herein as being undercut in
that the shank part ~0 is substantially narrower in
width (z axis dimension) than the portion 41 which
defines the cutting edge.
Referring to Figure 2, and the greater detail
of Figure 3, the circumferential groove 26 is cut in
the outside diameter of the cylinder by rotating
the cylinder and moving the tool first in the Xf
direction at 0.05 mm/revolution to make a radial
plunge cut. In all the cuts described herein, the
tool bit cutting surface speed is about 2.8 m/s, a
speed shown by experience to be optimum for economic
material removal. When the desired x axis depth is
reached, the tool holder and tool bit are translated
in the z+ direction at a relatively low feed rate of
0.013 mm/revolution until the desired axial dimension
of the upper side of the groove is achieved. Then
the tool is moved in the z- direction at the same
feed rate to produce the desired width of groove.
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Next the tool is restored to its basic z axis entry
positlon, whereupon it is withdrawn in the x-
direction.
It will be appreciated that when the tool bit
is making the foregoing cuts it is subiected to
substantial adverse forces. First, there is a
relatively large length of the cutting edge in con-
tact with the workpiece during the plunge; and,
second, there is a bending moment due the undercut
shape during the side cuts. It is highly un-
desirable that a tool bit might break within the
groove because the groove may thereby be damaged.
In aircraft engine components, any such damage re-
quires very special scrutiny to ensure if it is
benign. And if it is not benign then the part is
rendered useless, a condition with substantial
adverse economic consequences.
Before the invention was made, the tool bit
described was prone to breakage during the foregoing
operations. This breakage was not readily
explainable because the feeds were so light. Despite
careful attention eight of eleven tool bits were
broken during the aforementioned z- motion.
Naturally, a flood coolant was used and it was not
easy to ascertain what was happening. But then
there was discovered in the machining residue small
packets of compressed chips. It was deduced that
these were accumulating in the region 46 of the
undercut groove while the part 44 was being
machined. When this occurred the -tool bit would jam
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and break. Naturally, the machining parameters
were varied to try to eliminate this problem. How-
ever, AMS 4928 titanium, as a ductile material,
produced a continuous chip. (By this is meant that
the chip did not fracture as it was continuously
removed fro~ the workpiece and slid across the
rake surface. Of course, ultimately it got
to a great length, e.g., 1-3 m or more; whereupon
mechanical forces resulting from the chip tangling
with other parts of the machine would finally cause
it to break.) Usually, a chip breaker on the tool
bit would be used under such circumstances. But it
will be appreciated that the shape of the tool is
such that a chip breaker cannot be readily placecl
on the tool. We also tried packing the machined
portion 46 with rubber, wax and the like, all to
no avail.
Before we made our invention cutting was done
in the conventional mode and a flood coolant was
used. In this procedure a coolant such as water
soluble oil and water, such as one using Hocut
3210 (E.F. Houghton Co.) is directed liberally
in the vicinity of the cut by nozzles generally
aimed at the point where the cutting takes place.
In an effort to obtain more cooling, we mounted a
coolant nozzle of about 15 mm area in the manner
shown in Figures 1 and 4, so that the coolant was
directed along the length of the tool rake surface.
We used a conventional source of pressurized fluid
which was actually relatively high in pressure for
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ordinary use, having an output of about 690 kPa.
(All pressures herein are gage pressures measured on
a feed line between the pump and the tool bit. The
actual nozzle discharge pressure was somewhat lower
than indicated.) Although the coolant was directed
into the cut we continued to have the aforemen-tioned
problem of packing. Naturally cooling was improved
as taught by the Jennings Patent No. 3,176,330
mentioned in the Background. But then we discovered
that by raising the pressure to in excess of about
1400 kPa we were able to eliminate the tool bit
breakage. The force and volume of the coolant was
causing the continuous chip to fracture. And the
amount of wear was substantially reduced. Our
experiments were insufficient to get an exact series
of ~uantitative data but the effect was dramatic
and verifiable.
Wear on the tool bit was evidenced primarily
by degradation in the sharpness of the cutting edge;
there was less significantcratering of the rake
surface as well. Figure S shows how a chamfer 48
of length L is produced at the cutting edge, i.e.,
at the intersection of the relief surface 42a and
the rake surface 36a. This parameter was measured
on tool bits which did not fracture and become
lost in the chip debris.
The Figures 6 and 7 show in relative terms the
results which we obtained. Figure 6 shows that as
coolant pressure (and of course the resultant flow)
was increased the wear of the tool bit decreased. For
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e~ample, the dimension L dropped from 0.18 mm to
0.05 mm when cutting a groove. This is attributable
to expectable phenomenon, as described in the
Jennings patent and elsewhere. Cooling and intimate
lubrication of the tool bit prolongs its life,
especially for titanium alloys. But reference to
Figure 7 shows that Eailure rate was still high
when this result was first obtained. (Failure was
evidenced by breakage and was due to chip packing ,
as we now know.) But when pressure was increased
substantially beyond that needed to obtain the re-
duced wear, tool failure rate was substantially
decreased. In fact 27 workpieces were machined wlth
the same parameters referred to above, without tool
bit failure. A pressure of about 1400 kPa was
sufficient for titanium an~ even higher pressures
did not give any further improvement. Data for the
commercial IN-100 type nickel superalloy are also
shown. This alloy also produces a continuous chip,
but it is substantially stronger. (AMS 4928 has a
room temperature ultimate tensile strength of about
lO00 MPa and 13-15~ elongation while modified IN-100
has about 1400 MPa strength and 12-1~% elongation.
But AMS 4928 has even lower relative high temperature
strength since IN-lO0 is a superalloy designed for
high temperature strength and it is not. The exact
temperature of the chip is uncertain but it is most
likely above 600C and may be as much as 1000C at
the point of cutting.) When the same type of undercut
groove was being machined (with a C-9 carbide at a
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lower surface cutting speed of about 1.3 m/s) sub-
stantially higher pressure of the order of 2250 kPa
was needed to obtain chip breakage and avoid packing
of modified IN-lO0 in the groove.
Examination of the chip debris showed that when
the pressure reduced failure rate the continuous
chip was converted to a discon-tinuous chip of 3-12 mm
length. Not only was chip packing reduced but wear
also was reduced, both as evidenced by the chamfer
48 and a reduced tendency for rake surface cratering.
As shown in Figure 1 the coolant was directed
along the y axis of the tool. Therefore, during the
plunge cut it directly impacts on thechips as they
are for~ed and start to slide across the rake
surface. But, when the tool cuts in the z axis
direction the coolant does not directly impact the
chips. However, owing to the configuration of the
groove and volume and pressure of the coolant,
there is deflection of the coolant sufficient to
break up the chip by secondary impact, we believe.
Thus, in the simpler configuration of single point
outside circumferential cutting shown in Figure 4,
somewhat lower pressure and volume may be effective
to convertthe continuous chip 64 into the dis-
continuous chips 64. While packing will not be a
problem in such a circumstance, there will be
benefits nonetheless in reduced cratering, chip dis-
posal, and possibly reduced cutting edge wear.
Naturally, other shapes of ports beyond the
rectangular port 38 shown in Figure 1 may be used and
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multiple ports may be used as well. Such an
arrangement would be appropriate with tool bits
which are operating in unconfined environments and
which are cuttlng for more than a short distance
along the cutting edge.
The coolant flows associated with the pressure
recited herein were of the order of 0.6-0.7 l/s~ The
general size of the undercut tool bit we refer to
herein was that the cutting edge part 41 had a z axis
width of about 7.3 mm while the shank part 40 had a
width of about 2.5 mm. The coolant we used was of
the aqueous type and this type is preferred because
of high specific gravity and low viscosity, com-
pared Eor instance to oils. It is the momentum of
the liquid stream which causes the fracturing of the
chip. Since momentum is a function of the mass flow
rate and the vPlocity of the coolant, it is dependent
on the density of the coolant (the velocity of course
being a function of the discharge pressure). There-
fore, if higher density coolants are used it will be
possible to use lower pressures within the practice
of our invention. Also, we used a relatively simple
nozzle and it is conceivable that better nozzle design
might enable reduction in applied pressure. The
materials we machined are ductile and it is to such
materials with their tendency to form continuous
chips in normal machining that the invention is
principally useful. Of course if weaker materials are~
machined than those we refer to, lower applied
pressures could be needed to carry out the invention.
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Conversely, if stronger materials or greater feed
rates (wlth resultant thicker chips) were en-
countered, higher pressures could be required.
Lastly, while we describe the invention in terms of
lathe turning, the principles will be applicable to
other metal cutting operations where a tool bit is
used.
Although this invention has been shown and de-
scribed wi-th respect to a preferred embodiment, it
will be understood by those skilled in the ar~ that
various changes in form and detail thereof may be made
without departing from the spirit and scope of the
claimed invention.
