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Selection of Cutting Tool Material

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Selection of Cutting Tool Material

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									INDEX


                                                            INDEX

  1. Introduction to Ceramic Cutting Tools

  2. Tool Life

  3. Selection of Cutting Tool Materials

  4. Aluminum Oxide or Titanium Carbide Composite Cutting Tool

  5. Cermet Cutting Tools

  6. Alumina-Silicon Carbide Whisker Composite Tools

  7. Phase Transformation Toughened Materials for Cutting Tool

  8. Silicon Nitride Cutting Tools

  9. Aluminum Oxide Coatings for Cemented Carbide Cutting Tool

10. Polycrystalline Diamond and Cubic Boron Nitride

11. The New Diamond Technology and its Application in Cutting

12. Advanced Processing Concepts for Increased Ceramic Reliability




file:///D|/PG2004/TOOL/Anup/Ceramic%20tools/Index.htm [8/16/2004 3:34:15 PM]
 3
 Selection of Cutting Tool Materials



                     John D. Christopher
                    Machining Research, Inc.
                      Florence, Kentucky


         There are many pathways taken to select a specific cutting
tool for a machining process to manufacture a part. Unfortunately,
many of these paths are those of least resistance, or more likely,
least effort on the part of the decision maker. Without realizing the
long range impact on the cost of the machined part or the rate at
which parts are produced and delivered, purchasing agents, shop
foremen, and manufacturing           engineers often indiscriminately
choose cutting tools.      The choices are driven by many forces
including eye-catching      advertisements,    friendly salesmen, and
curiosity. Many companies purchase cutting tools based soley on
low bids, with little regard for performance.          Nationwide, poor
productivity, excessive tool-changing downtime, and unacceptable
part quality often result due to the performance of cutting tools
purchased with cost as the primary determining factor.


INTRODUCTION

         Figure 3-l provides a longitudinal perspective (not a cutting
speed recommendation)        on the development       of cutting tool
materials.     As metallurgical/material   technology advanced, the
improvement      in the performance      of cutting tools followed.
Advancements in cutting tools depend entirely on improving the
chemical composition and/or the manufacturing process of the tool
material. Invariably, improvements in the metallurgical quality of


                                   28
                                 Selection of Cutting Tool Materials                 29




           1300


           1200                                        'UC dXIDES


           1100


           1000


           900


           800
 UJlTING
  SPEED    700                                         LUNG&-
 FT./MN                                                v1mixRl31

           600
                                                       SIDEjLl-
           500


           400


           300
                                                       X&HSS
           200
                             i-m                           :&
                             ;     SrEEL
           100           n                             b   i
                             i       /

             0       -   -a-
                  1680 1710 1740 1770 1800 1830      1860 1890 1920 1950 1980 2010
                                           YIBROF INlTCU3CTICti




Figure 3-1. Cutting tool development.


a cutting tool material results in longer tool life or more
importantly, higher cutting speeds. Since the cutting speed is the
dominant influence on the cutting temperature of the machining
process, it naturally follows that tools that provide higher speeds
have more tolerance for higher temperatures. Figure 3-2 illustrates
this relationship.   Tool materials with higher hot hardness will
permit machining at higher productivity rates due to the higher
allowable cutting speeds.
30            Ceramic Cutting Tools




   RDCWELL
                                                                               ROCKWELL
 IMRDNESS “C”
                                                                                     "A"
                                                                              NllRDNESS
                                                                                          95




     65



     60



     55



     50



     45

          0           400               800           1200             1600     2000
                                        TEMPERATURE   (F)

                    OCARBON     STEEL    +IiSS               *COBALT    HSS

                    'XCARBIDE            'XCERAMIC




Figure 3-2. Hot hardness of various cutting tool materials.


         Selecting the correct cutting tool material for a specific
machining operation is the first step in creating the most effective
process plan for manufacturing a part. The cutting tool material is
dependent on the work material to be machined and the operation
to be performed. Often, there are several possible choices of tool
materials that will successfully (but not cost-effectively) produce
parts.    Additional factors then must be considered and these
include:
                        Selection of Cutting Tool Materials       31

       +   machine tool horsepower, speed range, rigidity,
       +   productivity demands,
       +   tooling budget limitations,
       +   machine tool burden rate and
       +   labor and overhead rate.

        Generally, the higher the combined hourly rate for the
machine tool and operator, the greater the demand for higher
productivity to reduce the cutting time per part. However, the size,
performance capacity, and general condition of the machine tool
may limit the productivity available from that particular machine.
Whatever the limiting factor or factors may be, the wise process
planner or programmer uses a valid cost analysis to determine his
choice of either maximum          productivity   or minimum    cost.
Decisions made without an economic analysis will likely produce
less than the maximum available output at a higher part cost than
necessary.


WORK MATERIAL/ALLOY

         The most important consideration in selecting the correct
cutting tool is the work material and its hardness.     The material
may be metallic or nonmetallic,       ferrous or nonferrous.    The
majority of materials machined in the United States are ferrous
materials, carbon, alloy, stainless steels, or cast irons. The cast
irons may be gray, ductile, or malleable. There are usually two or
three levels of tensile strength within many grades of alloy and
stainless steels, as well as the three types of cast iron. Higher
tensile strength levels invariably produce a higher hardness and a
more difficult to machine material. A very thorough definition of
the work material is a valuable aid in making an intelligent
selection of the tool material.
         The purpose of this chapter is to survey the major
categories of cutting tool materials with comments concerning
unusual properties or limitations of each group, and the normal
applications of each group. The various tool materials that will be
discussed include:
32     Ceramic Cutting Tools

               +   high speed steel,
               +   uncoated carbides,
               +   coated carbides,
               +   ceramics,
               +   cermets,
               +   polycrystalline diamond and
               +   polycrystalline cubic boron nitride.


HIGH SPEED STEEL

        The earliest version of tool material used in machining was
high carbon tool steel. This material was generally unalloyed steel
and could be heat treated to a hard but shallow case. The addition
of various alloying elements, particularly tungsten, chromium, and
vanadium, added hardenability to the materials as well as much
higher hot hardness. Table 3-l shows the chemical composition of
selected high speed steels. These improvements in the chemical

Table 3-1.   HSS - Chemical Composition.

               c       w       MO      Cl-     v      co
M-l            0.80    1.50    8.00    4.0     1.0    --
M-2            0.85    6.00    5.00    4.0     2.0    --
M-7            1.oo    1.75    8.75    4.0     2.0    --
M-4            1.30    5.50    4.50    4.0     4.0    --
M-42           1.10    1.50    9.50    3.7     1.2    8.0
T-15           1.50    12.0    --      4.0     5.0    5.0



composition of the steel provided increased performance from
higher cutting speeds than those available with the carbon steels.
These increases in cutting speed introduced the tern1 “high speed”
steel, usually abbreviated HSS.
         Today’s HSS tools are available in the normal ingot cast
version and as the particle metallurgy (PM) (a patented process)
version.     The ingot cast materials, while capable of doing a
satisfactory job in most applications, are limited by the permissible
                        Selection of Cutting Tool Materials       33

composition and/or heat treatment to achieve higher wear resistance
(hardness) also produces a lower toughness and therefore a more
brittle HSS. The PM steels, “mechanically” alloyed in the dry or
particle condition, do not have the same limitations as those
blended in the molten state. The result of this freedom in alloying
is a series of high speed steel grades that can be heat treated to
higher levels of hardness (68-70 Rc) without a severe reduction in
toughness.     These tools can provide longer tool life due to
increased wear resistance without the risk of chipping or breaking
from reduced toughness. As expected, this increased performance
carries a higher cost.
         Generally, the effects of the various alloying elements are
predictable. Table 3-2 illustrates the effects of various elements on
the performance properties of high speed steels. Higher levels of
carbon increase the hardness as they combine with other elements,
particularly vanadium.     The more popular grades, Ml, M2, M7,
and Ml0 have l-2% vanadium. Vanadium carbides which form in


Table 3-2. Effects of Various Elements on the Performance          of
HSS.

               MO:    Acts as a substitute for W
               C:     Provides high hardness
               Cr:    Increases hardenability
               v:     Increases wear resistance
               co:    Increases hot hardness


the microstructure of steels, are a major contributor to the wear
resistance of the grade. Grade M4 (4% vanadium) is very wear
resistant, compared to the previous four grades. The addition of
cobalt (Co) to the chemical composition increases the hot hardness
of the steel and elevates it to a “premium” category of higher
performance and price.     The most wear resistant grade in the
premium category is T15, which has 1.5% carbon, 5% vanadium,
and 5% cobalt. Figures 3-3 through 3-5 show the relative hot
34      Ceramic Cutting Tools

hardness, toughness, and wear resistance, respectively for nine
grades of HSS.
         Hard coatings can be applied to the surfaces of finish
ground HSS tools to improve their performance, particularly in
machining ferrous alloys. The physical vapor deposition (PVD)
process applies a single coating usually of titanium nitride (TiN).
The coating is applied as the vapor solidifies on the tools within a
vacuum chamber. Other coatings of titanium carbonitride (TiCN),
zirconium nitride (ZrN), and chromium nitride (CrN) are also
available. The PVD process operates at a temperature lower than
the tempering temperature of the HSS and therefore does not
degrade the hardness of the steel.
         While coating increases the cost of the tool, it has in many
instances provided substantial improvements      in performance.     A
typical example is found in tapping, where only a small amount of
wear on the tap will produce          an undersize     and therefore
unacceptable thread.     The cost of the coating is offset by the
increased tap life. However, as the coating thickness is only about
0.0002-0.0003 inches in thickness, it does not “armor coat” the
tool, and will not always survive when machining              abrasive
materials or steels much harder than 40 Rc.


UNCOATED       CARBIDES

        This tool material was first developed with tungsten carbide
(WC) and cobalt as the binder. This material was satisfactory for
machining gray cast iron, which was very common in the 1920’s
and 1930’s. As the metallurgy of steel progressed, and its usage
increased, the inability of these “straight” grades of carbide to
successfully machine steels became obvious. The lack of alloying
caused the tools to fail by cratering.      The addition of titanium
carbide and tantalum carbide to the composition of tungsten carbide
and cobalt produced a series of grades that were crater resistant in
turning and milling carbon and alloy steels. This improvement in
carbide tool material increased the production rate of machining
steels to that of cast iron in the 1940’s, 50’s, and early 60’s.
                        Selection of Cutting Tool Materials          35




Figure 3-3. Relative hot hardness for various HSS tool materials.




Figure 3-4.   Relative toughness   for various HSS tool materials.
36         Ceramic Cutting Tools




               120




                                      -      -
     PERCENT   60                                     -                    -1
                                                                -      -




                                                                       I
               40




               20




                0                                               -
                     T15       H42           Ml0      T5   HZ   T4    Ml
                                          HSS GRADE




Figure     3-5.     Relative   wear   resistance      for various    HSS   tool
          materials.


A comparison between HSS and a C6 carbide turning an alloy steel
is shown in Figure 3-6. The increase in speed from using carbide
is approximately seven times.
                                               Selection of Cutting Tool Materials




             60




             50




                       ‘:
             40                i
                       ............... __i__



     TOOL
     LIFE    30
   MINUTES



                                              HSS

             20        .........    j.. ...   ... .; ..




             10               .................




             0    I-

                  0                50          100          150     200   250    300    350   400   450
                                                          CUTTING   SPEED-FEET/MINUTE



Figure 3-6. Cutting speed vs. tool life for HSS and carbide tools
       (300 BHN steel).

         The relative percentages of carbide to cobalt determine the
wear resistance or toughness of the grades. In the United States
the Cl to C8 designations        are still commonly used by both
producers and users of carbide tools. The Cl to C4 categories
contain only the WC and Co, and are suitable for roughing
applications or interrupted cutting. The higher numbers are harder
with more carbide and less binder, providing            higher wear
resistance, lower toughness, and are more suitable for semi-
finishing and finishing cuts.
38      Ceramic Cutting Tools

         The C5 to C8 grades contain the additional components of
TiC and TaC for machining materials that generally produce a
continuous (often work hardened) chip formation that causes tool
cratering. Tool cratering can occur without severe flank and nose
wear that would produce parts out-of-tolerance.        The result of
severe tool cratering is catastrophic tool failure and perhaps a
damaged part. The relationship of hardness and toughness is the
same for the grades, C5 to C8 as for Cl to C4. Lower numbered
grades are tougher, while the higher numbered grades are harder
and more wear resistant. The effect on tool life and cutting speed
using various grades of carbide is shown in Figure 3-7.
         Uncoated carbides are still quite widely used in the
machining industry. Virtually all of the carbide tipped tools, drills,
reamers, milling cutters, saws, etc. use uncoated carbide grades
(Cl-C4).    Many machine tools, unable to fully utilize the higher-
performance, more expensive coated carbides, due to lack of speed
or horsepower,      are able to cut with uncoated carbide tools.
Materials that are very abrasive or high in hardness, are not ideal
applications for coated carbides and are often machined with
uncoated carbides.
         The differences in cast irons can be critical to the correct
selection if the cutting tool material is uncoated carbide. Ductile
(spheroidal graphite) and malleable (quenched and tempered) irons
are machined with “steel cutting grades” of uncoated carbide (C5-
CS). Gray (flake graphite) cast iron, producing a discontinuous
chip, is machined with the Cl-C4 grades of uncoated carbide.



COATED CARBIDES

         The most common tool wear on uncoated carbides is
diffusion-related wear. The temperatures and pressures associated
with the normal cutting parameters on ferrous alloys cause the
cobalt binder on the surface of the carbide tools to diffuse out of
the matrix with the hot chips produced during the cutting process.
         As the binder diffuses from both the top and side surfaces,
the grains of carbide are displaced gradually, leaving wear scars on
the flank and nose of the tool and the crater on the rake face.
                               Selection of Cutting Tool Materials            39




             40




             25



     TOOL
     LIFE    20
   MINUTES



             15




             10




              5   -...



             0    i-
                  200    400        600        SO0      1000    1200   1400
                                  CUTTING   SPEED-FEET/MINUTE



Figure 3-7. Cutting speed vs. tool life for various grades                    of
       carbide tool materials (200 BHN malleable iron).

Attempts were made to reduce this diffusion process, all with
limited success, until the development     of the chemical vapor
deposition (CVD) process. This process deposits various vaporized
compounds on the surfaces of the carbide tools in a vacuum
chamber. The first successful coating was titanium carbide (TIC).
In addition to TIC, titanium nitride (TIN) and aluminum oxide
(Al,O,) are now used in various combinations, with TIC serving as
the base coat. The familiar gold colored exterior coating is usually
TIN, although hafnium nitride (HfN), similar in appearance, is used
by some manufacturers.
40      Ceramic Cutting Tools

        It was noted by metallographic failure analysis studies on
coated carbides that occasionally a damaged region (heat affected
zone) at the interface of the carbide and the first coating was
caused by the high temperatures associated with the CVD process.
Machinability testing confirmed that the presence of this damaged
region caused a weakening of the cutting edge manifested in heavy
and/or interrupted cutting. The PVD coating process, commonly
used on HSS tools, was tested on carbide and found to consistently
produce a damage free interface.       Subsequent cutting tests with
PVD coated tools confirmed that an increase in tool life resulted
over the CVD coated tools.           This work has been publicly
documented      at SME cutting tool clinics by the Kennametal
Company.        A special group of PVD coated carbides are
recommended for heavy or interrupted cutting applications.
        The substrates for the coated carbides are usually not
cutting grades such as Cl or CS, but special compositions that are
tailored to the use of the coated tool, having high toughness and
deformation resistance. The wear resistance of the tool is usually
dependent on the coating and not on the substrate. The presence
of the coating will often increase the metal removal rate over an
uncoated carbide 50% to 150%. Figure 3-8 shows the improved
performance of the coated carbide over the uncoated tool when
turning gray cast iron. These inserts are available in a wide variety
of chip control geometries in all standard insert configurations.
There is little doubt that the coated carbides are the closest product
to an all-purpose cutting tool material for ferrous alloys. Their
success    is the reason       that coated    carbides   account    for
approximately 60% of all sales of indexable inserts.


CERAMIC TOOLS

Cold Pressed Alumina

       Early generations of ceramic tools mnaufactured in the late
1940’s and early 1950’s were primarily cold pressed aluminum
oxide (Al,O,). While these tools were chemically inert and had
good hot hardness compared to tungsten carbide, they were
                        Selection of Cutting Tool Materials




               90



               80



               70



               60


   Tool Life   5.
    minutes


               40



               30
                                            -i-

               20



               10
                                            -i-

                0

                    0   500     1000      1500      2000    2500   3000
                              Cutting   Speed-feet/minute


Figure 3-8. Performance improvement         of coated        vs. uncoated
       carbide tools (class 35 gray cast iron).

notoriously low in toughness. This deficiency caused the tools to
easily chip and break catastrophically,  creating a poor image for
early ceramic tools. The ceramic tools available today are of
consistent high quality, and when correctly applied, are capable of
delivering a cost-effective performance on finish cuts (light feed
and depth of cut) of low hardness cast iron and medium hardness
steels.

Hot Pressed Alumina/Tic

        The development  of the hot pressing process (without
excessive grain growth) was a major step forward in producing
42      Ceramis Cutting Tools

high quality ceramic tools. This process allowed the addition of
TIC to aluminum oxide, producing the HP Al,OJTiC, an excellent
all purpose ceramic tool. This grade is available in a wide variety
of standard insert configurations    at an affordable price (20-25%
higher than coated carbides).
         Although other ceramic materials may be better for specific
applications, HP AI,O,/TiC is acceptable for most machining
situations where ceramics are applicable. It is an excellent material
for turning tool steels as hard as 60-63 Rc, capable of holding
diameters to a tolerance off 0.00025” and producing surface finish
values of less than 5 micro-inch.     This versatile ceramic material
also has excellent thermal stability and is capable of cutting dry or
with a water base cutting fluid.

Whisker-Reinforced Alumina

          Silicon carbide (Sic) whiskers added to an A&O, matrix in
random orientation, produces a ceramic tool material with very
high toughness. This tool material is used in turning nickel-based
alloys. These alloys which work harden when machined, cause a
notching wear scar, usually at the depth of cut area on the side
cutting edge of the tool. Notch wear can lead to the tool nose
breaking off the insert, particularly when repeated passes are made
at the same depth of cut. The high toughness of the whisker-
alumina tool, along with the use of round inserts rather than nose
radius style inserts, gives this ceramic material good success for
high metal removal rate cuts on nickel base alloys.           Several
intelligent ramping techniques are recommended for these tools as
an alternative to repeated passes at the same depth of cut. Varying
the depth of cut minimizes the development of the severe notch at
the same location on the cutting edge of the insert. Although the
whisker-alumina tool is capable of providing a good performance
in a variety of applications, other ceramic materials may be more
cost effective. This ceramic tool material costs over twice the price
of the hot pressed alumina/titanium     carbide.
                        Selection of Cutting Tool Materials        43

Silicon Nitride

         Silicon nitride is produced by a variety of processes with
different microstructures.      Some versions have a binder material,
others do not. At least one material is a complete matrix of silicon
nitride whiskers.      Variations in processing are the most likely
explanation for the wide range in performance from one producer
to another.
         While capable of several ceramic applications, S&N, is
possibly the most ideal tool material for machining gray cast iron
in turning, boring, and face milling operations.      A “good” grade
of silicon nitride will machine common grades (used in the
automotive industry) of gray iron at cutting speeds of 4000-5000
feet per minute (fpm). Most of the silicon nitride tools are capable
of continuous machining at 3000 fpm, which is considerably faster
than the productivity obtainable with coated carbides. Laboratory
tests in face milling class 30 gray cast iron have been performed
at 7000 fpm with tool life values up to one hour of cutting time.
         Rotary tools, end mills and drills, are now manufactured by
CNC grinding from solid blanks.            These tools have enjoyed
selective applications with great success over HSS, carbide, and
coated carbide rotary tools of similar geometry.

Ceramic Summary

         The major deficiency associated with the use of ceramic
tools in production machining is low toughness.        This results in
chipping and breakage of the tools rather than wear. To alleviate
this problem, several techniques have evolved to strengthen the
cutting edge and produce wear rather than chipping or breaking.
These include increasing the thickness or nose radii (round inserts
have been produced) of the tool that can result in improved
performance. However, the most recent and effective improvement
in ceramic inserts is in the development of the edge preparation.
         There are three types of edge preparations that eliminate the
perfectly sharp edge where the sides of the insert intersect with the
rake face.     The earliest technique was the hone, performed
carefully by hand with a fine grain diamond hone. This operation
44       Ceramic Cutting Tools

is now automated.      A radius is formed at the intersection of the
face and side of the insert. The size of the radius can be varied to
accommodate the application.
         The most common edge preparation is the T-land, which is
a chamfer ground to a specific angle and width of the land. The
angles vary from 10” to 35”, while the width of the land varies
from 0.002-0.030 in, and occasionally higher. The most common
combination is 20” x 0.004-0.006 in. The width of the land varies
somewhat with the feed rate of the operation and is usually wider
as the feed increases.     The angle of the land is subjective, but
generally decreases if the width of the land is very high. Although
this technique is very successful in protecting the cutting edge, it
is not an exact science. As optimization efforts proceed in the field
and the data base increases, a better definition of the exact T-land
for a specific material/operation  will ultimately emerge.
        Inserts with ground T-lands can also have a subsequent
honing operation which rounds the intersections of the land and the
original faces of the insert. This added process eliminates any
sharp intersections which may chip within the cutting zone.


TiUTiN    Cermets

        These tool materials derive their name from the use of
ceramic materials with a metallic binder. Today’s cermets are
usually titanium carbide and titanium nitride with a binder material.
They are an effective material for machining steels as they are both
wear and crater resistant to the continuous chip formation of steels.
Cermets are available in an assortment of insert shapes with chip
control grooves and edge preparations.        This tool material is
capable of providing a performance equal to or greater than coated
and uncoated carbides on steels in the soft to medium hardness
range where other ceramics are usually ineffective.
        The popularity     or acceptance    of cermets is not as
widespread in the United States as their performance deserves. In
Japan, cermets represent about 30% of tool sales, as compared to
about 5% in the U.S. There are grades of cermets available which
have adequate toughness for milling and interrupted cutting on
                        Selection of Cutting Tool Materials        45

steels up to approximately      40 Rc hardness.      Inserts are also
available in positive rake geometry to minimize cutting forces and
resultant part deflection.
         There are some practical guidelines to the application of
cermets. These inserts are normally offered with T-lands, usually
lighter than those on ceramic tools. Gray cast iron is generally not
recommended as silicon nitride is more effective. However, ductile
cast iron which cuts more like steel than gray cast is a good
application.    Since this TiC/TiN material does not have the
excellent thermal stability of other materials, it is safer to cut
without a fluid and risk intermittent flow which may cause thermal
cracking of the insert.
         The cost benefits of cermets are usually higher productivity
through higher cutting speeds and longer tool life. Since cermets
are about 20% lower in cost than a coated carbide and exhibit
better performance, there is a great untapped potential for TiC/TiN
in the machining of steels.

Polycrystalline   Diamond and Cubic Boron Nitride

        The hardest substances known are 1) natural single crystal
diamond, 2) polycrystalline diamond, and 3) cubic boron nitride.
Polycrystalline  tools are manufactured  using extremely      high
temperatures and pressures.

Polycrystalline   Diamond, PCD

        The random orientation of the PCD tools corrects one of the
major deficiencies of the natural diamond, the possible presence of
a cleavage plane within the single crystal. This plane creates a
natural failure site and can weaken the tool with a disastrous effect
on performance. Therefore, single crystal diamonds used as cutting
tools must be correctly oriented by a diamond expert.          Single
crystal diamond is an excellent special purpose tool for creating
super fine finishes on items such as optical components.      Usually
the diamond is bonded to a standard carbide insert as a single tip
on the insert. The insert is then ground (and perhaps polished) to
provide a very smooth finish on the diamond.
46      Ceramic Cutting Tools

        The PCD tool provides an excellent general purpose tool for
machining nonferrous and nonmetallic, abrasive materials.        The
most common applications for PCD tools include copper and
aluminum alloys machined at high cutting speeds. It is standard
practice for aluminum automobile wheels to be turned at 8,000 to
10,000 feet per minute with PCD tools. A deficiency of these
inserts is the lack of chip control, a problem on soft materials like
aluminum.       Another common application        for PCD tools is
machining nonmetallics such as hard fiber reinforced plastics and
materials such as granite and marble. Because PCD is much more
abrasion resistant than carbides, it provides higher cutting speeds
and/or longer tool life than carbides in the same machining
operation.
        PCD inserts cost lo-13 times more than carbide inserts and
have only one cutting edge compared to the multiple edges on the
carbides, this must be considered when deciding whether or not a
PCD tool is cost effective.      Often, there is no other choice for
producing quality parts on a reasonable production schedule. PCD
inserts are available in both positive and negative rake style. The
diamond section can often be reground to extend the life of the
insert and thus lower the cost per cutting edge.

Polycrystalline   Cubic Boron Nitride, CBN

         Cubic boron nitride tools are available in both tipped inserts
(like PCD) and also in solid CBN inserts. The solid inserts cost
about three times as much as the tipped insert, but offer multiple
cutting edges and a much tougher cutting material. CBN inserts
can be used successfully to turn nickel base alloys, but they have
a difficult time competing with the cost of the whisker/alumina
insert. A single tipped CBN insert can cost about 3 times as much
as the whisker/alumina insert which can have 4-8 as many cutting
edges.     Therefore, CBN is generally not cost effective for
production machining of nickel alloys.
         A second application for CBN is machining hard ferrous
alloys (65-68 Rc).         Parts with this hardness are usually
manufactured on a grinder rather than machined.          However, the
metal removal rate in machining may be 10 times as great as the
                        Selection of Cutting Tool Materials        47


removal rate in grinding. Once again, CBN must compete with
lower priced ceramics on steels in the 55-63 Rc range.          The
alumina/Tic    ceramic is about 10% the price of a single tipped
CBN insert and can have 4-8 cutting edges. It is wise to consider
the entire economic picture of any machining operation in order to
justify high performance/high  price cutting tool materials such as
CBN.
         A machining operation where solid CBN inserts can out-
perform other tool materials is face milling steels in the 2 60 Rc
range. Solid CBN inserts have incredible toughness and when
correctly applied can withstand the most severe interruptions
without chipping and breaking.


SUMMARY

        The choice of the cutting tool material should always be
made with as complete an economic analysis as the situation
permits. This analysis should be made after the built-in constraints
of machine tool capability (hp and speed), production schedules,
and most importantly, part quality are considered.        Selecting a
specific tool material on the basis of longer tool life when more
than one type will provide a satisfactory product is a safe choice
when the tool-change time is long and tool life is short. Fewer tool
changes reduce non-productive down time. If tool-change time is
very short, the selection should be a tool that will increase
productivity, reduce cycle time, and ultimately lower direct labor
and machine burden on the cost per part.

								
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