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ABSTRACT
In machining of industrial components, surface quality is one of the most
specified customer requirements. Major indication of surface quality on machined parts is
surface roughness. Dry machining is making inroads onto many shop floors, as it
eliminates or greatly reduces coolant use, often cutting costs and providing a healthier
working environment.
Finish hard turning using Cubic Boron Nitride (CBN) tools allows manufacturers
to simplify their processes and still achieve the desired surface roughness. There are
various machining parameters which influence the surface roughness, but the effect of
machining process parameters on surface roughness during hard turning have not been
adequately quantified.
In order for manufacturers to maximize their gains from utilizing finish hard
turning, accurate predictive models for surface roughness and tool wear must be
developed.
During the hard turning powerful interactions among process parameters like
surface roughness, speed and depth of cut affect surface roughness of the product. It is
not possible to discover interactions by changing only one factor at a time. Proper design
of experiments (DOE) will reveal interactions that can help to achieve breakthrough
improvements in process efficiency and product quality. Factorial designs would enable
to study the joint effects of the factors on a response.
An L27 orthogonal array is used for experimental layout. The necessary
experimentation is carried out using hard turning concept and the surface roughness for
all experiments was recorded. The results are further analyzed using statistical techniques
to find out the influence of selected process parameters in controlling the desired surface
finish. In the present investigation ‘MINITAB’ statistical program is used for developing
and analyzing the experimental results.
1
CHAPTER 1
1 INTRODUCTION
1.1 HARD TURNING
As the title indicates, this work focuses on machining hardened steels with Cubic
Boron Nitride (CBN) cutting tools. The name itself implies “hard turning” is the
process if turning hard material.
Rotational turning” may sound like a redundancy, because turning already involves
rotation. The work piece spins. However, rotational turning is a process that adds a
brand new rotating element. A special tool pivots to sweep its long cutting edge
across the work piece surface. The result is turning so smooth that it can compete
with grinding and polishing. Rotational turning is a dry process that does not require
the costs associated with coolant. Meanwhile, rotational turning’s cycle times tend to
be twice as fast as those of hard turning.
Hard materials are defined somewhat arbitrarily, but a consistent threshold of 45HRC
(Rockwell C scale hardness) seems to exist. Typical materials for which hard turning
may be a potential machining process include heat treatment and case hardened
steels. These steels constitute an important class of engineering materials due to
improve strength and wear resistance compared with other materials. Due to stringent
dimensional and surface requirements, these materials have traditionally been
machined to finished geometries by abrasive processes such as grinding. However
2
recent improvements in machine tool technology (specifically the rigidity and
precision of modern CNC lathes and the advent of ceramic cutting tools have made it
possible to remove material from hardened steel components by turning and milling).
Turning is a type of machining process which is shown in the figure 1.1 on a manual
lathe and again from a side view in figure 1.2.In this process, material is removed by
sliding a hard cutting tool through a softer material and forming a chip. The main
process variables in this case are the cutting speed, the feed rate and the depth of cut.
Cutting speed is either given as the rotational velocity of the work piece or the linear
tangential velocity of the work piece at the tip of the cutting tool, feed rate is defined
as the linear distance that tool traverses during one rotation of the work piece and
cutting depth is the radial engagement between the cutting tool and the work piece.
Hard turning differs from conventional turning of softer materials in several key
ways. Because the material is harder, specific cutting forces are larger than in
conventional turning and thus the engagement between the cutting tools and the
workpiece must be limited. At the small cutting depths required, cutting takes
place on the nose radius of cutting tools, and the tools are prepared with chamfered or
honed edges to provide the stronger edge geometry that is prone to premature
fracture. Cutting on a chamfered or honed edge.
3
(fig 1.1) Example of turning process(from tvent 1977)
equates to a large negative effective rake angle, when neutral or positive rake angles
are typical in conventional machining. The large negative rake angles yield increased
cutting forces compared to machining with positive rake angles, and also induce
larger compressive loads on the machining surface. Higher temperatures are also
generated in the cutting zone, and because cutting is typically done without coolant,
hard turned surfaces can exhibit thermal damage in the form of micro structural
changes and tensile residual stresses.
4
(fig 1.2) Side view of the turning process
Cryogenic Hard Turning:
Cubic boron nitride (CBN) and polycrystalline cubic boron nitride (PCBN) inserts
have traditionally been the tools of choice for hard turning applications. However,
many shops contemplating hard turning are turned off by their high price. A new
extreme-temperature coolant method has been developed to offer longer insert life,
faster cutting rates and more affordable hard turning insert options.
The liquid nitrogen raises insert hardness, which significantly reduces the thermal
softening effect that an insert may experience as a result of hard turning’s inherent
high cutting temperatures. The steep temperature gradient between the chip/tool
interface and insert body also helps remove heat from the cutting zone. In addition,
the significant cooling maintains insert edge integrity to prevent “smearing” a part’s
hot, compressed surface layer, thus providing a quality surface finish.
5
Unlike CBN and PCBN, ceramic inserts tend to wear
unevenly and are prone to fracturing when hard turning
dry or with water- or oil-based coolants. Increased
fracture toughness resulting from low-temperature liquid
nitrogen cooling provides more predictable, gradual flank
wear for ceramic inserts, as well as increased cutting
speeds up to 200 percent.
(fig 1.3)
Hard turning has many potential advantages compared to grinding, and several are a
direct result of the way in which material is removed by the two processes. A
significant advantage of the turning process is that cutting is done with tools that have
a geometrically defined cutting edge. This allows many different parts to be machined
with same cutting tool by changing the relative path between the tool and the work
piece. On modern CNC lathes, many tools are mounted in the turret of the machine,
and the computer-controlled machines can produce a variety of part geometries with
only a handful of different tools. In grinding, where the individual grit geometries are
random, the overall shape of the grinding wheel must be modified to produce
different parts. This is typically addressed either by stocking a different wheel for
Liquid nitrogen insert
cooling extends insert
life and allows greater
use of low-cost ceramic
inserts for hard turning
operations.
6
each part, or by reshaping the wheel into a new in to a mew geometry each time a
new part is required. Both methods are time consuming, either due to changeover
time or dressing time (the process of reshaping the wheel. The small sizes of typical
grits also reduce the potential engagement between the grits and work piece thus
limiting the size of the chips that can be removed. This generally leads to small
material removal rates compared to turning, although some grinding operations are
done with wide grinding wheels to improve the volumetric removal rate.
By eliminating the need to change or redress grinding wheels, hard turning offers
increased flexibility by significantly reducing the setup time. For a typical setup, all
that is required is changing of the collet or chuck in the machine and loading a
different computer program in the control. The combination of increased flexibility
and improved material removal rates is becoming more important as manufacturers
tend toward production strategies that minimize the inventory and batch sizes.
Furthermore, because the process is more flexible and productive, fewer machine
tools are required. The cost of lathes is substantially lower than grinders (as an
estimate, the cost of a good CNC lathe and associated tooling maybe 1/2 to 1/10 the
cost of modern CNC grinder). Turning is also a more efficient cutting process than
grinding, so less energy is required to remove the same volume of material. This is
due to the small engagements between abrasive grits and the work piece in the
grinding, and the associated plowing that contributes to significant energy losses.
Finally, hard turning has the additional benefit of environmental friendliness because
cutting is done typically dry. This eliminates the cost and the environmental impact
7
of using cutting fluid and disposing of grinding sludge. Given the potential
advantages of hard turning is not being used more extensively.
First, it is a relatively new technology. It is only over the last couple of decades that
improvements in machine tools and development of ceramic inserts have made this
technology a viable alternative. Furthermore, because hard turning is so young
compared to grinding will take some time to overcome. Also, there are applications
where hard turning is not currently good enough to replace certain grinding
operations. A good example of this centre less grinding, which is a process for
grinding a cylindrical work piece where the part is not constrained to a geometric
center of rotation (which typically results when holding in a chuck or collet).The
process allows improved roundness, and existing fixturing methods on lathes will not
allow hard turning to complete with center less grinding when roundness tolerances
are tight (especially for compliant parts). Aside from special cases, the biggest
limitations on further implementation of hard turning are concerns about surface
quality and un acceptable life of expensive cutting tools.
Due to these concerns, there is a need to develop a better understanding of the effects
of process conditions on the wear behavior of cutting tools and the resultant surface
quality that can be obtained by hard turning. The potential advantages of hard turning
are attractive to many manufacturers’ tools to make more educated decisions about
selecting process conditions and enabling further implementation of this technology.
8
A properly dialed-in hard turning process can
deliver surface finish of 0.00011 inch,
roundness of 0.000009 inch and diameter
tolerance of ±0.0002 inch. Such performance can
be achieved on the same machine that "soft"
turns the part prior to hardening, maximizing
equipment utilization.
(fig 1.4)
Part—Though 45-Rc material is hard turning's starting point, hard turning is
regularly performed on parts that are 60 Rc and higher. Commonly hard-turned
materials include tool, bearing and case-hardened steels. From a metallurgical
standpoint, materials with a small hardness deviation (less than two Rc points)
throughout the cutting depth allow the best process predictability.
In some cases, a part's size or geometry simply does not lend itself to hard turning.
Parts that are best suited for hard turning have a small length-to-diameter (L/D) ratio.
In general, an L/D ratio for unsupported work pieces should be no more than 4:1, and
The goal in hard turning is to
deliver at least 80 percent of the
heat out with the chip in order
to maintain part thermal
stability.
9
it should be no more than 8:1 for supported work pieces. Despite tailstock support for
long, thin parts, high cutting pressures would likely induce chatter.
Machine—The degree of machine rigidity dictates the degree of hard turning
accuracy. Most machines made in the last 15 to 20 years have sufficient rigidity to
handle some hard turning applications. In many cases, a machine's overall condition
is more of a factor than its age. Even an old, well-maintained manual lathe can be a
candidate for hard turning. However, as required part tolerances get tighter and
surface finishes get finer, machine rigidity becomes more of an issue.
Maximizing system rigidity means minimizing all overhangs, tool extensions and part
extensions, as well as eliminating shims and spacers. The goal is to keep everything
as close to the turret as possible
Process—Because hard turning delivers the majority of cutting heat out in the chip,
examining the chips during and after the cut will reveal whether or not the process is
well-tuned. During a continuous cut, the chips should be blazing orange and flow off
like a ribbon. If cooled chips essentially disintegrate when crunched by hand, then
that demonstrates that the proper amount of heat is being carried out in the chip.
1.2 WET MACHINING
Metal working fluids in manufacturing processes is viewed as undesirable for both
economic and environmental reasons. Every year manufacturers consume millions of
gallons of metalworking fluids. Metalworking fluids have an considerable affect on
manufacturing costs and environment. Even more important is the fact that OSHA
10
(Occupational safety and health administration) and the EPA (Environmental
protection agency) consider metal working fluids to be detrimental to the
environment. These fluids contaminate the air causing maintenance and the employer
health problems. Also, at the end of fluids useful life it must be disposed properly.
Machining parts with metal working fluids puts an enormous burden on
manufacturing companies and environment Manufacturing companies need to realize
the costs and environmental issues involved
with the use of metal working fluids and move to environmentally and cost conscious
manufacturing practices.
Due to increasingly strict environmental laws aimed at controlling the health hazards
and pollution, the costs of metal working fluids use in manufacturing processes is
rising substantially. Therefore, the elimination of metal working fluids in
manufacturing processes can be significantly economic incentive
11
Wet machining(fig 1.5}
1.3 DRY MACHNING
Dry machining is no longer a utopian dream in the metal working industry.
Manufacturing companies all over the world are currently examining the question
whether metal working fluids are really needed in machining process and if so, to
what extent. While the need for dry machining may be apparent, issues including the
perceived as inability to cut dry and change over cost, resulting dry machining being
perceived as impractical by most manufacturers. How ever, this is not the case, high-
speed dry machining is possible with most manufacturing process.
12
(fig 1.6) Test cut with PCBN cutting tool integrate correctly, manufacturing can realize improve work piece accuracy, reduced
manufacturing cost, and other related benefits associated with high speed dry
machining.
Recent research reveals that trend in manufacturing is to minimize or eliminate the
use of metal working fluids in manufacturing processes. Dry machining has the
potential to reduce environmental pollution, health hazards, and costs associated with
the use of metal working fluids. However, to pursue dry machining, one has to
13
compensate for several beneficial effects of metal working fluids without using them.
Removal of metal working fluids in manufacturing processes can use a variety of
machining problems related to heat, tool life, and chip removal. In dry machining, the
functions of metal working fluids must be assumed by alternative methods. The
challenge of heat dissipation without coolant requires a completely different approach
to manufacturing. Special tooling utilizing high performance coatings, heat resistant
materials and through spindle air are required. By examining the manufacturing
processes capable of dry machining, it becomes apparent that the key is a balance
between advanced metal cutting strategies, special tooling and the machine tool
specifications.
1.4 WET OR DRY MACHINING
For a number of years, manufacturing professionals have explored the potential of dry
machining. Driving the interest in dry machining is often related to the costs and
health issues associated with the use of coolants in most manufacturing operations.
The total cost of coolant is upwards of five times the cost of cutting tools.
There a number of factors that needs to be considered before moving to a dry
machining environment or even MQL machining (Minimum Quantity Lubricant).
What if we look at the use of coolant from the point of view of the tool? Instead of an
all or nothing decision regarding the application of coolant in a shop or on a specific
machine, what happens if we consider eliminating the use of coolant on certain types
of tools in order to improve that tools performance?
14
The key to exploiting the potential of turning off the coolant for tools is based on
recognizing what factor heat is having on the operation. Simply put heat is the friend
of tools, but too much heat is an enemy. Though a combination of energy (generated
primarily from the spindle/cutting speed) and tool geometry, the cutting zone reaches
a temperature where the yield strength of the work piece material is approached and
separation, or a chip, can be efficiently formed. The more heat in the cutting zone, the
easier the chip will separate. Too much heat though and not only does the work piece
want to separate, but the tool as well.
Here are suggestions about when to use coolant and when to cut dry:
Threading: Single point threading tool producing standard thread profiles often run
with significantly longer tool life when they are run without coolant.
Grooving: shallow grooving operations such as snap ring grooves are frequently
improved with out coolant.
Index able milling tools: because of the frequent thermal cycling due to being in and
out of cut nearly all index able milling tools preferred to run without coolant when
possible.
Short contact time finish turning operations : when the tools in contact time is short
and the material is not especially sticky tools will generally run longer when produce
improved surface finishes when they are run dry.
15
1.5 Problems of wet machining:
1) Environmental hazards: Used coolant is from machining processes is always
harmful to both environment and human health. Chemical substances that provide
the lubrication function used in the machining process are toxic to the
environment if the coolant and cutting fluids are released into soil and water.
2) Health hazards: the chemical substances used in coolant cause serious health
problems to workers who are exposed to the coolant in both liquid and mist form.
3) Contamination: Some cutting fluids stain or contaminate the workpiece thus
affecting the surface finish.
4) Increasing costs: The cost of using coolant is increasing as the number and the
extensiveness of environmental laws and regulations are increasing.
5) Management costs: The maintenance and the management costs are also
increasing because of chemical disintegration of some coolants.
6) Disposal: This is another main problem concerned with the coolants. Disposing
into environment leads to many hazards as discussed above.
7) Treatment costs: The used coolant should be treated and then released into the
environment. This treatment process is an additional burden. According to a
survey coolant, coolant management & coolant treatment costs account for 16 to
20% of the manufacturing costs.
1.6 Advantages of dry machining:
1) Increases tool life by eliminating thermal shocks created by flood coolant
16
2) Eliminates coolant purchase and disposal costs.
3) Tool life increases because coated carbide, ceramics, cermets, cubic boron nitride
(CBN), and polycrystalline diamond (PCD) are all brittle, they are susceptible to the
chipping and breaking caused by thermal stresses—especially those found in face-
turning and milling operations—that can be aggravated by the introduction of coolant.
4) For continuous cuts, the high tool tip temperatures that occur in dry turning serve
to anneal (soften) the pre-cut area, which lowers the hardness value and makes the
material easier to shear. This phenomenon is why it is beneficial to increase the
speeds when cutting dry.
5) A chip formed through a properly configured hard turning process takes with it 80
to 90 percent of the heat generated (cutting zone temperatures can reach 1,700°F). If
such a blazing chip would contact straight-oil coolant with a low flash point, the
process could literally go up in flames.
Parameters Conventional
wet
High speed dry Improvement
Surface feet/min 250 5000 2000%
Revolutions/min 160 3200 2000%
Inches/min 32 80 250%
No of inserts 10 5 50%
17
Tool life 1600 6000 375%
(Tab 1.1}Improvement in performance for conventional wet turning to high
speed dry turning operation.
CHAPTER-2
2 LITERATURE SURVEY
2.1 Turning to hard turning:
The vast majority of hard components used in the automotive industry are machine to
final geometrical form after hardening. Currently, grinding is the pre dominant
18
method for finishing these parts, which includes bearings, Gears, shafts, and pinions.
However, thanks to improvements in machine tool rigidity and development of CBN
and PCBN cutting tools, hard turning is gaining ground as a cot effective alternative
to grinding.
Hard turning is performed on the materials in the 45 to 60 HRC range using the
variety of tipped of solid cutting inserts. Since its production in the mid 80’s, the
process has dramatically increased in popularity and the sales of CBN cutting tools
are dealing in hundreds of millions annually. Clearly, more and more manufacturers
are recognizing the advantages of hard turning. But due to the cost of CBN cutting
tools, many continue to view it as an expensive process.
2.1.1 Is hard turning more expensive??????????
While CBN cutting tools ca cost up to 10 to 20 times more than conventional tools,
studies have shown them to be 10 to 300 times more effective in terms of overall
productivity and tool life. In part, these finding are based on the tool cost per parts
analysis. For a better understanding of economic benefits of hard turning, it helps to
consider a few factors that are some times overlooked by the accounting department.
These include tool change time, setup time, cycle time, machine maintenance, part
quality and original machine cost.
19
Part of the cost effectiveness of the hard turning may be attributed to the machine tool
itself. A grinder is a much larger investment than a CNC Lathe, it is typically one half
to one third of a grinders cost. Also, CNC Lathes are much more flexible in terms of
machining capabilities. Tool changes can be made in less than 2 mins , without the
production time losses necessary for a wheel change. This flexibility allows fast, cost
effective production of small batches of parts.
2.1.2 Benefits of hard turning:
1) Low maintenance is also a benefit , as worn CBN tools may be quickly removed
and replaced with new inserts, and do not require truing or dressing to maintain the
cutting profile.
2) CNC lathe also takes less floor space than grinders, do not require flume systems.
3) It does not require coolant.
4) Since hard turning removes metal “peeling” a softened chip from the work piece,
coolant is generally not recommended. This helps to keep costs down while
eliminating the environmental damage caused by coolant use.
5) Dry machining also reduces the time and money spent on government regulated
chip disposal and reclamation process.
6) Although grinding is known to produce good surface finishes at relatively high
feed rates, hard turning using CBN inserts can produce better surface finish and
significantly higher metal removal rates.
7) Although the process consists of small depth of cuts and feed rates, estimates of
reduced machining time are as high as 60% for conventional hard turning.
20
8) Studies have shown that by using the right combination of insert nose radii, feed
rate, or the new wiper technology, hard turning can produce a better surface finish
than grinding.
9) The fact that multiple hard turning operations may be performed in single
chucking rather than multiple grinding setups also contributes to high accuracies.
However, there is still much debate surrounding the overall surface integrity of hard
turned parts.
2.2 Hard turning vs. Grinding:
1) Material removal rates are higher in hard turning than in grinding.
2) The experimental results showed that intermittent hard turning can produce
surface integrity which is good enough for replacing the grinding process.
3) Machining time is reduced in Hard turning compared to Grinding.
4) The fatigue life of hard turned surfaces is better than that of ground surfaces.
5) The turned surface has a longer life than the ground one with equivalent surface
finish.
6) Hard turning generates less heat in the workpiece than grinding, due the CBN’s
ability to put most of the heat into the chips -- not into the workpiece.
7) Traditional grinding, in contrast, creates extreme heat that requires coolant and
may cause surface imperfections.
8) Shorter cutting time, less tool change time make the hard turning process a faster
process than grinding.
21
HARD TURNED SURFACE GROUND SURFACE
(Magnified 2000 times) (Magnified 2000 times)
(Fig 2.1) Surface obtained by hard turning and grinding.
22
2.3 A surface caveat:
Hard turning affects the surface microstructure by generating residual stress patterns
and over hardened surface zones, also known as white layers. These thin, rehardened
layers are typically followed by an over tempered region just below the white layer.
Due to the structural alteration of the material, the rehardened layer appears white in
an optical micrograph, and the tempered region appears dark.
Research has confirmed the existence of white layers on both hard turned and ground
surfaces .Although they are commonly associated with residual tensile stress, white
layers may also indicate residual compressive stresses. Either way, the cause of
white layers and the effects they have on finished workpiece is not completely
understood.
Some studies suggest that the use of cutting coolant helps in eliminating white layers,
while others indicate that it has no effect. There is also some evidence that tool
conditions affect the white layer formation. If a white layer forms during hard
turning, it is typically because a dull insert causes too much heat to be delivered into
the part. It is most commonly formed on bearing steels and is most problematic for
parts like bearing races that receive high contact stresses. Over time, the white layer
can delaminate and lead to bearing failure.
23
Overall, new tools are more likely to produce undamaged surfaces, with the white
layer increasing with tool wear. This may result from the heat generated by friction
between the tool and the workpiece as the flank land increases, or by higher plastic
deformation caused by increased friction.
In recent years, the unknown surface integrity of hard turned parts has also caused
some reluctance to use hard turning as a finishing operation on critical surfaces.
However recent advances in cutting tool technology are eliminating these
perceptions. In particular, CBN inserts have proven to produce as good or better
tolerances than conventional grinding processes and reduce machining time up to
90%.
24
CHAPTER-3
CUTTING TOOL MATERIALS FOR MACHINING PURPOSE
1) CUBIC BORON NITRIDE (CBN)
2) CERAMICS
3) CARBIDES
4) CERMETS
5) POLYCRYSTALLINE CUBIC BORON NITRIDE (PCBN)
3.1 CBN Cutting Tools:
Cubic boron nitride cutting tools (CBN) have CBN, which is a synthesized material
exceeded in hardness only by diamond. However, unlike diamond cutting tools, CBN
cutting tools are stable at temperatures up to 1,800 degrees F for machining hardened
ferrous or super alloy materials of Rc 45 or higher and for machining some cast irons.
Polycrystalline cubic boron nitride cutting tools (PCBN) feature PCBN blanks, which
are manufactured from CBN crystals using a high-temperature, high-pressure process.
CBN cutting tools have crystals, which are either sintered with a binder phase or
25
integrally bonded to a tungsten-carbide substrate. The binder phase, usually a metallic
or a ceramic matrix provides chemical stability that enables the qualities of the PCBN
to be used in high-speed machining operations. By varying the binder phase and the
percentage of CBN crystals, PCBN cutting tools can be made in a wide range of
grades and in a variety of shapes and sizes for turning, boring, facing, forming,
milling, grooving, reaming, parting, and fly cutting applications. Because PCBN
cutting tools maintain their cutting edge, they impart excellent surface finishes to
parts, while maintaining close tolerances and high productivity rates.
Higher CBN content in specific grades of cutting tools provide increased fracture
toughness and resistance to abrasion. Cutting tools that have grades with more than
70 percent CBN content exhibit high thermal conductivity, excellent abrasion
resistance, and exceptional toughness. High-content CBN cutting tools have grades,
which are primarily used for machining case-hardened and through-hardened steels,
super alloys, chilled cast iron, sintered metals, pearlitic gray cast iron, hard coatings,
hard facing materials and work hardening materials.
CBN cutting tools with grades less than 69 percent CBN offer low thermal
conductivity, low diffusion wear and chemical inertness. Cutting tools with these
grades are often used for finish cuts on hardened steels even with interruptions, hard
powdered metals, nitride steels, and hardened cast iron.
CBN cutting tools can run dry for clean machining processes to save coolant,
maintenance, and disposal costs while reducing the potential for environmental and
health impacts.
26
CBN cutting tools are available as solid, full-face, or tipped inserts. Tipped CBN
cutting tools use CBN inserts, which can be economical and reliable for a wide range
of roughing and finishing applications, and some applications require a solid or full-
faced insert.
As discussed earlier CBN cutting tools machine hardened steels with apparent ease
because, using relatively high surface speeds, heat is generated at the point of cutting,
so the CBN cutting tool cuts locally softened material. The heat is carried away by the
swarf, which becomes brittle and harmless and CBN cutting tool, which has high
thermal conductivity. If a light cut is required, however, a tool with high CBN content
conducts too much heat away from the shear zone and the condition for efficient
machining are not achieved.
Low CBN tools can therefore keep sufficient heat at the cutting point to enable the
optimum cutting conditions to be achieved when a light cut is taken. In most cases,
even when very light cuts are required, low CBN tools employ negative rake
geometry to provide a strong edge. Due to the nature of cutting, however, cutting
forces are still very small. Low CBN can be used to provide a productive and cost
effective alternative to grinding. Tolerances achieved are comparable but machining
time can be dramatically reduced.
27
CBN Inserts
Mini Tips Flip Tips Standards
Inexpensive.
In many
shapes and
grades for
shallow cuts
Cost-effective. Two cutting
edges for finish cuts. Flip
Tips is a trademark of J&M
Diamond Tool, Inc.
A wide
assortment of
shapes and
grades
Large Tips Solid CBN Full Face
For deeper
cuts. -
Available on
most standard
inserts
Cutting edges on both
sides. Indexable.
Cutting edges.
Full top surface
Indexable.
28
(fig 3.1) Figures of different CBN inserts.
Materials recommended for cutting with CBN (Tab 3.1)
Alloy steel (45-68 RC) Ni HardDie steel (45-
68 RC)Rene
Carbon tool steel (45-68 RC)Forged
steelStellite
Ductile
iron
Moly chrome steel rollsNodular
ironColmonoy Incoloy
High speed steel (45-68 RC) Grey iron Waspoloy Monel
Chilled cast ironInconel
600
Meehanite
iron
Grade Application
1000 Most cast iron
2500
Continuos cutting hardened
steel and slight interrupted
cuts.
3000 Hardened steel ( severely
interrupted cut )
29
5000 Nodular iron - Ductile Iron
6000 Super alloy, Ni/Co base alloys
8200Continuos cutting hardened
steel and slight interruptedcuts
30
Recommended Use of CBN
Grades
Material CBNGrades
Alloy
steels ( 45-
68 RC )
2500, 3000,
8200
Carbon
tool steels (
45-68 RC )
2500, 3000,
8200
Die steel
( 45-68 RC
)
2500, 3000,
8200
High speed
steel ( 45-
68 RC )
2500, 3000,
8200
Chilled
cast iron1000
Nodular
cast iron5000
Ni Hard 1000, 6000
Forged
steel2500, 8200
Moly
chrome
steel rolls
6000
Inconel
6006000
Rene 6000
Incoloy 6000
Monel 6000
Recommended Speeds and Feeds
MaterialSpeed
(SFM)
Feed
Rate
(IPR)
Depth
of
Cut
(inche
s)
Carbon
Steel
200 -
500
.008
Max
.020
Max
Bearing
Steel
200 -
500
.008
Max
.020
Max
Alloy Steel200 -
500
.008
Max
.020
Max
Die Steel160 -
350
.008
Max
.020
Max
Tool Steel160 -
350
.008
Max
.020
Max
High
Tensile
Cast Iron
200 -
500
.060
Max
.020
Max
Chilled
Cast
Iron
130 -
260
.032
Max
.020
Max
Grey Cast
Iron
2000
-
4000
.020
Max
.020
Max
Powdered
Metal
500 -
650
.016
Max
.020
Max
Inconel500 -
650
.006
Max
.020
Max
Rene 42500 -
650
.006
Max
.020
Max
31
(Tab 3.2) (Tab 3.3)
3.1.1 Advantages & Cost effectiveness of CBN cutting tool:
“Hard turning” tool steels 45Rc to 60Rc
1) Speeds of 200sfm to 600+sfm (surface feet per minute).
2) Chips load of .002 to .020 IPR (inch per revolution)
3) Depth of cut .002 to .200 inches
4) Good surface finishes.
5) Interrupted cuts –no problem
6) Solid top CBN inserts eliminate the problem of Mimi-Tip melt off.
Chilled Cast Irons & Grey Cast Irons
1) 300% increased productivity Vs. carbide
2) Wear life 5 to 10 times TiN coated carbide
3) 200% wear life in finishing cuts Vs. ceramics
4) Operations include turning, milling and boring.
3.2 CERAMICS IN MACHNING:
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There are many applications for ceramic materials due to hardness and increased wear
resistance resulting from ionic or covalent bonding. A growing area for application of
ceramic materials is in machining, where cutting operations are increasingly replacing
traditional abrasive processes for finishing hard metals, ceramics, and inter metallic
compounds.
Machining processes, which are defined as shaping of a part by the removal of
material, are used to produce metal parts in countless applications. The definition of
machining encompasses three subsets of processes that can be called abrasive
processes, cutting processes, and non traditional machining processes.
Design engineers continually desire new materials with improved strength, hardness,
thermal characteristics, and wear resistance. However, the same properties that make
these materials desirable also make them difficult to machine. Two examples are
hardened steel alloys that provide strength and wear resistance in automotive
applications, and nickel-based super alloys that maintain high temperature strength in
aerospace applications. Because these materials are often used in applications where
some degree of precision is required, at least one of the machining processes are
required after casting or forging to achieve desirable dimensional accuracy and
surface finish. As an example of a typical application with a steel alloy, processing of
a ball bearing for the automotive industry will be discussed. To withstand repetitive
contact stresses while achieving acceptable fatigue lives, bearings are typically made
from steel alloys with significant amounts of carbon to allow hardening by heat
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treatment and chromium to provide corrosion resistance. Typically processing steps
include: casting or forging to obtain a cylindrical shape, rough turning the steel in the
soft state to approximate dimensions, heat treating the part to obtain the desired
properties (hardness and toughness), grinding to finished dimensions, and possibly
super finishing to improve surface quality.
Ceramic cutting tools have made it possible to combine rough turning and grinding
into a single turning process after heat treatment, which offers substantial cost
benefits. There are many types of ceramic tools that have been developed for
machining hard materials, but the tools can be much more expensive than traditional
steel tools, and the life of ceramic tools can be prohibitively short if the wear behavior
is not understood. To provide supporting information relative to this research, the
processing, applications, and wear of several ceramic cutting tool materials will be
discussed.
3.3 CARBIDES
To allow machining at higher cutting speeds (and increased production rates), carbide
tools were developed in 1930s (kalpakjian 1997). These tools now consume an
estimated 70% of the machining market. Because the tools are typically pressed and
sintered from ceramic powders (often with cobalt binder material), they are
sometimes called sintered carbides or cemented carbides. There are two basic subsets
of carbide tools : tungsten carbide (WC) and titanium carbide (TiC), WC tools being
the most prevalent.
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Pure WC is very hard, but also brittle. To improve toughness, WC powder is mixed
with 5-15% cobalt (weight percentage). Hardness and wear resistant can be improved
by reducing the grain size of the WC particles, which are typically in the range of 0.5-
5 micro meter. An optional grain size and cobalt percentage must be determined to
allow the hardness and toughness required for a particular cutting operation.
Even at relatively low cutting speeds around 150 ft/min (45 m/min), WC-Co tools
form significant craters behind the cutting edge because cutting temperatures can
exceed 1000 c, and steel work piece materials can absorb WC in solid solution. To
reduce cratering, 5-25% of titanium carbide (TiC) can be added to WC-Co tools. TiC
has very low solubility in iron, and thus acts as a barrier to cratering by the diffusion
of WC. TiC is also harder than WC, so addition of TiC improves abrasive wear
resistance in addition to improving the chemical stability.
3.4 CERMETS:
Since the 1920s, cermets have been an integral part of the metalworking industry.
However, in the past few years they have enjoyed a surge in popularity, thanks to new
technology, which has expanded possible cermets applications. Traditionally used
just for semi finishing to finishing operations, newer cermets have increased
toughness that makes them comparable to some carbides, while their good shock
resistance promotes good performance for some interrupted cuts. Origins The word
cermet is derived from the terms ceramic and metal. A cermet is a hard material based
on titanium carbide or titanium carbonitride cemented with a metal binder. The first
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series of cermets developed in the late 1920s were based on titanium
carbide/molybdenum carbide with a nickel binder, and were characterized by low
bending strength and high brittleness. However, in the 1960s improvements were
made with the addition of molybdenum to the binder.
This increased cermet toughness. A few years later, the addition of metal
carbonitrides contributed to improved wear resistance, thermal shock resistance and
decreased plastic deformation. In 2003, new micro grain cermet grades were
introduced, including a PVD-coated super micro grain cermet whose coating provides
greater thermal stability and the ability to handle higher cutting speeds. The micro
grain structure of these cermets contributes to a doubling in the bending strength,
while the fracture toughness remains comparable to other cermets. This new
generation of cermets also is capable of handling interrupted cutting operations that
before weren't feasible.
Charecteristics
Characteristics of cermets include high wear resistance, low reactivity with most
work pieces and long tool life. They produce excellent surface finishes, and maintain
tight tolerances over their life span. Higher cutting speeds may be used with cermets,
especially for semi finishing to finishing operations, because of their high wear
resistance.
High Wear Resistance
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Cermets' high wear resistance is contributed in part by their high hot hardness, which
is greater than most cemented carbides. Cermets also feature low reactivity, as they
are more chemically inert than tungsten carbide. The low affinity to the work piece
means that edge buildup and cratering are not significant for cermets, as with carbides
resistance than previous cermets, thereby allowing the use of coolant when necessary
for a wide range of applications. It is not mandatory to use coolant when machining
with cermets; however, coolant is generally applied for three reasons: cooling,
lubrication and chip evacuation. Due to their good wear resistance, welding resistance
and high hot hardness, cermets provide excellent surface finishes even when dry
machining. In addition, numerous different chip breakers promote smooth chip
evacuation without the use of coolant. Chip breakers are the molded or ground
patterns on an insert that break up the chips formed by the machining process into
manageable pieces. Long chips can damage the work piece, or be dangerous to the
operator.
Applications
Traditionally, ideal applications for cermets have included finishing and semi
finishing at higher speeds, lower feeds and cutting depths. Although turning on CNC
lathes is the most common cermet application, Swiss-type machining is also ideal for
cermets, since they hold their size and maintain tight tolerances.
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Grooving and threading applications also are compatible. Cermets also may be
considered for finishing milling operations, and they offer better finishes and
dimensional control than carbides. Significant tool cost reduction may be achieved
when milling with cermets as they can run at higher cutting speeds than carbides, and
their tool life is longer. In some cases, they can eliminate grinding or polishing
operations. In addition, the new generation of micro grain cermets can also handle
interrupted cuts when turning due to their superior toughness. When specifically
pertaining to the mold industry, cermets are used in milling molds and dies for
applications like vehicle transmission pieces, brake drums and rotors, rear differential
housing, body panel molds, etc.
Comparison of cutting material properties (fig 3.2)
Disadvantages of Cermets:
38
The cermets known to the state of the art have either various binder contents at the
surface, which can be recognized by their spotty appearance, or have a tendency of
attachment of the binder to the sintered substrate, which leads to changes of the
composition in the contact zone because of the reactions related thereto. Further
disadvantages of the cermets known to the present state of the art are a partially high
surface roughness, as well as poor attachment of the applied wear-resistant layers due
to the increased binder content in the surface. The mentioned disadvantages show
particularly clearly that the cermets can not be used as cutting inserts in machining
processes.
3.5 CUTTING MATERIALS
All cutting operations require tool materials that can with stand the difficult
conditions produced during machining. There are primarily three problems all cutting
tools face wear at cutting edge, heat generated during the cutting process, and thermo
mechanical stock. Characteristics that allow tool materials to stand up to cutting
process include hardness, toughness, wear resistance, and chemical stability. In
general, increased hardness improves wear resistance but is associated with decreased
toughness. Depending on machining conditions and work piece properties, different
degrees of hardness and toughness are required.
3.6 HIGH SPEED STEELS
39
High speed steels (HSS) are important to discuss due to their extensive use in metal
cutting and to demonstrate the need for ceramic tools. High speed steels have the
lowest hardness and highest fracture toughness for general use tools. The primary
reason high speed steel tools are not used more extensively is that they soften
significantly at temperatures above 500 c, as shown in figure. This softening behavior
limits high speed steels to relatively low cutting speeds on softer materials, and has
caused the need for carbide and ceramic cutting tools that maintain hardness at
elevated cutting temperatures.
(fig 3.3)Hot Hardness of several cutting tool materials (from Kalpakjian1997)
3.7 SURFACE INTEGRITY
If hard turning is to replace any grinding operation it must be capable of producing
surfaces of acceptable quality. This includes both the surface topography (surface
finish) and surface integrity, which is achieved when “the surface of a component
meets the demands of a specific stress system and environment”.
40
There are two major types of surface damage that can be caused by hard turning. The
first white layer, which has generally been assumed to result from temperatures
generated at the work piece surface that exceed the austenizing temperature of the
material, followed by rapid cooling . The second type of damage is the formation of
undesirable residual stress profiles at, and just below, the work piece surface .
Mechanical loading, plastic flow, and phase transformation can affect residual
stresses, but negative effects are primarily due to elevated temperature during
machining. Thus the two types of damage (white layer and tensile residual stresses)
are related and have generally been investigated together.
It is generally believed that generation of white layer requires both excessive heat at
the work piece surface and subsequent rapid cooling. Heat generation is attributed to
large amounts of energy generated in the shear region during chip formation and to
the frictional energy between the tool flank and work piece surface. However,
experimental results disagree about the source of rapid cooling. Tonshoff performed
hard turning experiments with and without coolant and found the white layer
magnitude was identical, indicating that work piece self cooling, and not coolant,
must be responsible for quenching of the work piece surface. This argument is
reasonable because the heat affected zone is small in hard turning, and because the
cutting velocities are large enough that the contact time between the tool and work
piece is minimal. Therefore, it is possible that the bulk piece material acts as a heat
sink and draws heat from the surface to create a self cooling effect. However, it has
41
been found that cutting with a worn tool produced white layer, but that a similar
cutting conditions with the application of coolant resulted in undamaged surfaces.
Many researchers have paid considerable attention to the generation of white layer
because it appears similar to thermal damage generated in grinding that is referred to
as “grinding burn”. To determine the structure of white layer, we used an X-ray
technique to determine the separate structures of bulk work piece material and the
white layer region. The results showed that for 16 MnCr 5 steel hardened to 60-62
HRC, the bulk material was composed of approximately 75% martensite and 25%
austenite. The whole layer consisted of only 30% martensite and almost 70
%austenite.
Griffiths (1987) reported three situations where white layers have been generated :
surface subjected to significant rubbing and wear), surfaces that see similar conditions
resulting from pin-on disk testing, and the surfaces that undergo certain machining
processes . In addition to machining conditions, material properties affect white layer
generation. Formation of all white layers to heating and quenching of the materials,
and concluded that chemical composition of the material affects the transformation.
Several publications have proposed that white layers may have increased hardness
relative to the bulk material. Others have reported nearly identical hardness in the
white layers compared with the bulk material.
42
To discuss the effects of hard turning on residual stresses, the surface influences of
hard tuning compared to grinding should be mentioned. Compared to grinding, the
force components are large, particularly the thrust force, which is generally larger
than cutting force in hard turning. If the tool loading is thought of as a Hertzian
contact, the maximum compressive stress induced in the work piece occurs at a depth
approximately 0.7 times the contact area of the tool. Because the contact area is larger
than grinding (a single grit) and load is increased, larger residual compressive stresses
that penetrate deeper below the work piece surface result in hard turned components.
As expected and the depth of residual stresses are a function of tool geometry and
process conditions.
Unlike residual tensile stress, reasonable levels of compressive stress are desirable.
Based on the residual stress caused by mechanical loading only, hard turned surfaces
should exhibit increased fatigue life compared to ground surfaces. However, the
undesirable tensile stresses generated by heat are super imposed on the compressive
stress. As tool flank wear increases, so does the frictional energy
Between the tool flank and work piece, as well as the depth of the compressive stress
induced by mechanical loading. Thus, increased tool wear results in larger tensile
stresses near the surface, followed by steep stress gradients with a larger compressive
stress further below the surface. The stress pattern with les overall change was
generated by a tool with very little flank wear compared to the other stress pattern,
which was generated with a significantly worn tool.
43