Trent E.M., Wright P.K. Metal Cutting

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PERFORMANCE OF LAMINATED AND COATED TOOLS 213

layers. The coefficient of thermal expansion of the coatings is generally higher than that of the

substrate. Cooling from the coating temperature therefore introduces tensile stresses into the

layers and polishing of coating surfaces generally reveals a network of very fine cracks. These

penetrate no deeper than the coating, but transverse rupture tests show a drop in strength com-

pared with un-coated carbide.

There is no evidence that this influences the performance of

clamped-tip tools, the main type of tooling for which coated tools are used.

7.9.3 Performance of coated tools

A large proportion of indexable inserts for cutting iron and steel is now coated. The success

of coatings is based on their proven ability to extend tool life by a factor of 2 or 3 by reducing

the rate of wear in high speed turning of cast iron and steel. Many users consider that more

important economies are made by increasing cutting speed by 25 to 50% without reduction in

tool life. Figure 7.34 is a typical set of laboratory test results comparing the rate of flank wear

on coated and uncoated carbide tools. Coated tools cannot be universally applied even in turn-

ing; operations involving severe interruptions of cut may fracture and flake away the coatings.

Improvement in the bonding has made them more suitable for milling operations.

27-30

Research has indicated that the wear resistant qualities of TiC, TiN and alumina coatings are

mainly because of their high resistance to diffusion wear on the parts of the tool surface where

seizure occurs.

Evidence from quick-stops demonstrates that the coatings do not prevent sei-

zure when cutting steel at high speed

(though they may eliminate the built-up edge at low

speed). The heat source raising the tool temperature is still a flow-zone and the temperatures

appear to be only slightly lower with coated than with uncoated tools. For this reason the rate of

metal removal, particularly when cutting high strength materials, is limited by the ability of the

substrate to withstand the stress and temperature at the cutting edge. These very thin coatings on

cemented carbide substrates may be worn-through locally at the positions of most rapid wear,

such as the center of the crater or on the flank just below the edge, but the coating will continue

to be effective in reducing the rate of wear as long as it remains intact at the cutting edge.

15,28

FIGURE 7.34 Tool wear vs time for coated and uncoated cemented carbide tool tips. (1) Uncoated steel

cutting grade; (2) coated WC-Co alloy and (3) coated steel cutting grade

214 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

7.9.4 Comparisons between different types of coating

There are differences between the different coatings in respect of their resistance to flank and

crater wear. Most reported results on steel cutting suggest that TiN is more resistant than TiC to

crater wear and that TiC is more resistant to flank wear. The crater and flank wear rates of alu-

mina-coated tools appear to be not greatly different from those of carbide and nitride coated

tools. There is evidence that the crater wear of alumina coatings is caused by superficial plastic

deformation, rather than by a diffusion mechanism.

The greatest difference in wear rate is

reported to be that in the notch at the outer edge of the chip (Figures 6.23 and 6.24) where slid-

ing rather than seizure occurs. Wear at this position was minimal with alumina coatings. TiN

coatings were nearly as good as alumina, but notch wear on TiC coatings was very much greater

and not greatly different from that on uncoated tools.

7.9.5 Multiple layer and nanolayer coatings: a design goal

It is clear that there is a place for more than one type of coating, and therefore multi-layer

coatings may provide a type of tool which will cope well with a variety of operations. Colloqui-

ally speaking, could research work discover a tooling combination that would “give the best of

all worlds”? It might consist of a tough carbide core - probably one of the WC-Co, “K-grades” -

coated with several layers: one that would bond reliably to the core; another that would buffer

thermal-expansion discrepancies; another that would provide insulation; another that would

buffer diffusion wear; another on the very outside that would have ultra-low friction characteris-

tics.

Will such an “all-purpose”, multi-layer “sandwich” eventually be discovered over the next

few years? This is the new challenge to cutting tool developers. Some progress is reviewed in

the next two sections.

7.9.6 Multiple layer coatings

For such multiple-layer “sandwiches” the first layer is usually TiN or TiCN created by the

PVD process. These can be created on straight WC-Co substrates, or on mixed carbide sub-

strates of varying cobalt content. TiN or TiCN minimize or eliminate the formation of -phase

(Co

) at the interface with the substrate. It is also common to create the last, or outer, layer

of the “sandwich” with TiN using PVD. This induces beneficial residual compressive stresses on

the complete structure.

In today’s industrial inserts of this type, multiple layers of only TiN may in fact be successful.

Multiple layering allows the final thickness of the “sandwich” to be as much as 10-15 m. This

is greater than can be achieved by trying to create the whole layer in one operation. When used

in turning and milling operations, the successive layers of very thin coats, rather than one heavy

coat, give a more stable structure. In other, triple-layer “sandwiches” the middle layer is often

TiCN as shown earlier in Figure 7.33.

Beyond this, Komanduri

reports that many “sandwich” combinations are being investigated

today including the following: {TiN-Al

-TiC-TiCN}, or {TiN-TiCN-TiC}, or {TiN-A1

TiCN}, or {A1

-TiC}, or {A1

-TiC-TiCN}, or {TiN-A1

-TiC}, or {TiN-A1

-TiN-

-TiN-A1

-TiCN}. In addition, Komanduri reports that one grade of TiN-A1

on a

PRACTICAL TECHNIQUES OF USING CEMENTED CARBIDES FOR CUTTING 215

substrate is also available, but that the technology to coat crystalline A1

by PVD is not

feasible today.

7.9.7 Multiple nanolayer coatings

Nanotechnology also involves sputtering, i.e. deposition, by PVD. It is being used today in a

wide variety of industries. These include CMOS semiconductor wafer manufacturing, and the

manufacture of the wafers that become the read-write heads for computer disc drives. Magne-

tron sputtering with many types of targets can be employed and the wafers are rotated with

respect to the target. The sputtering rate and the rotational speed of the turntable that holds the

wafers, determine the thickness of the coated layer.

Such coating technologies are also being investigated for cutting tools because unusual com-

binations of layers can be built up. The key concept is to deliberately mix alternating hard and

tough layers. Shaw

and colleagues report that a crack that might be initiated in a hard, brittle

layer is arrested when it meets the tough layer thereby increasing the overall fracture toughness

of the whole ensemble. Almost any refractory hard material can be used as the hard nanolayer;

almost any compatible metal can be used as the tough nanolayer. An example of the {hard car-

bide + metal} nanolayered system might be {B

C + W}. Or perhaps the B

C might be replaced

by {HfC, SiC} and the W by {Al, Cr, Ti, Si, Mg, Zr}.Oxidation resistant layers and low-friction

layers of molybdenum disulphide (MoS

) can also be mixed in. When using introducing nano-

layers of MoS

, the adjacent, non-MoS

nanolayers interrupt the formation of columnar grains

of MoS

which are ineffective as lubricants. This interruption produces a uniform equiaxed

structure of MoS

throughout the coating, which are very effective as lubricants.

Whereas each layer of the multiple-layer “sandwich” may only be a few nanometers thick,

the total thickness can be in the range of 2-5 m. Komanduri

reports that while today, nano-

coatings for cutting tool applications are not being used commercially, the number of potential

combinations of material systems that can be used for nanolayer coatings is virtually unlimited.

Thus the technology is expected to grow in the future.

32-34

7.10 PRACTICAL TECHNIQUES OF USING CEMENTED

CARBIDES FOR CUTTING

7.10.1 Early methods using brazed-on carbide tips

High speed steel tools are normally made in one piece, but from their earliest days cemented

carbides were made in the form of tool tips: small inserts which were brazed into a seat formed

in a steel shank. Except for very small tools, it was not economical to make the whole tool of

cemented carbide (which typically costs 20 times as much as high speed steel, weight for

weight) and there was normally no advantage in terms of performance. A brazed carbide was

used until worn and was then reground. Most grinding of carbide tools required the use of

bonded diamond grinding wheels, and more skill was required and the cost was greater than for

grinding high speed steel tools.

216 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

7.10.2 The transition to throw-away tool tips or indexable inserts

Because cemented carbides are less tough than high speed steel, the tool edge must be more

robust. With steel tools the rake angle may be as high as 30°, so that the tool is a wedge with an

included angle as small as 50° (Figure 2.2). The rake angle of carbide tools is seldom greater

than + 10° and is often negative (Figure 2.2). A tool with an included angle of 90° set at a nega-

tive rake angle of - 5° is particularly useful for conditions of severe interruption of cut, or when

machining castings or forgings with rough surfaces.

FIGURE 7.35 Indexable insert tool tips of cemented carbide

This common usage of tools with low positive rake, or negative rake angles led to the concept

of throw-away tool tips. These are small ‘tablets’ of carbide, which are usually square or triangu-

lar in plan, though other shapes may be used (Figure 7.36). Each tip has a number of cutting

edges. Positive rake, triangular tips have three cutting edges. Negative rake square tips have

eight cutting edges.

In use, the tips are mechanically clamped to the tool shank using one of a number of different

methods. When a cutting edge is worn, and the tool no longer cuts efficiently, the tip is

unclamped and rotated to an unused corner, and this is repeated until all the cutting edges are

worn. Then, the tip is discarded rather than reground. Used in this way, the tips are called index-

able inserts. Very little of the tool material is worn off when the tool tip is thrown away, but, in

spite of this, in many operations very considerable economies are achieved compared with the

possible use of a brazed tool which would have to be reground. The high cost of regrinding is

eliminated, the time required to change the tool is greatly reduced, and the cost of brazing is

avoided.

The powder metallurgy process is well adapted to mass production of carbide tool tips. These

are made by pressing on automatic presses, followed by a sintering operation, which produces

tips of high dimensional accuracy, so that little grinding is required. The introduction of throw-

away tooling has made possible important new lines of development in tool shapes and materi-

als. The tool edges can be given considerable protection against chipping caused by mechanical

PRACTICAL TECHNIQUES OF USING CEMENTED CARBIDES FOR CUTTING 217

impact, by slightly rounding the sharpened edge. Many manufacturers supply edge-honed tips,

on the cutting edges of which there is a small radius, usually about 25 to 50 μm (0.001-0.002

in). Such tips can give much more consistent performance in many applications, without the rate

of wear being increased.

Tips are also made with grooves formed in the rake face close to the edge which curl the

chips into shapes which can be readily cleared from around the tool. Clearance of chips from the

cutting area is of increasing importance for efficiency of machining operations on automated

CNC machine tools. The ability to cut for long periods without stopping to clear chips can have

a large effect on machining costs. Carbide tool manufacturers have put great effort into design-

ing chip grooves of optimum shape for operation in wide ranges of speed, feed, depth of cut and

work material. A large number of patents cover these edge shapes. The research of Jawahir and

colleagues

is also of particular interest in this regard.

Figure 7.36 shows chip grooves on indexable inserts marketed by one carbide tool producer

to meet requirements for chip control when cutting specific work materials at different speed,

feed and depth of cut.

FIGURE 7.36 Chip groove shapes on indexable inserts produced by one carbide tool manufacturer.

(Courtesy of Kennametal Inc.)

218 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

Chip-form Classification

FIGURE 7.37 ISO-based chip-form classification (Figures 7.37 to 7.42 are reproduced with the

permission of Professor I.S. Jawahir)

Figure 7.37 shows the most commonly known ISO-based chip-form classification.

Figure

7.38 shows a typical chip chart showing the different regimes of chip formation for various feeds

and depths of cut. The representation of chip-forms as a chip-chart enables immediate visual rec-

ognition of acceptable and unacceptable regimes of chip formation. The chip chart, thus, not

only provides the performance capability of a grooved cutting tool, but also serves as an impor-

PRACTICAL TECHNIQUES OF USING CEMENTED CARBIDES FOR CUTTING 219

tant tool in selecting cutting parameters (such as feed, depth of cut, cutting speed) for a particu-

lar operation.

FIGURE 7.38 A representative chip chart showing the regimes of chip formation for varying feeds and

depths of cut (Cutting speed = 100 m min

-1

, rake angle = 0 deg., nose radius = 0.8 mm, work material =

1045 steel)

One of the significant results from the research on chip-forms and chip breakability

37,38

has

been the establishment of chip breaking as a cyclic process. Recent work has shown the effect of

the cutting tool thermal conductivity on the tool-chip contact length and the chip-form in

orthogonal cutting with a grooved tool. Regions of sticking friction, sliding friction and the con-

tact at the groove backwall have been identified (Figure 7.39).

220 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

FIGURE 7.39 Tool-chip contact zones in machining with a grooved tool (Cutting speed = 100 m min

-1

feed = 0.3 mm/rev., work material = 1045 steel)

VB flank wear N nose wear

BW width of groove backwall wear NL

notch wear length on main cutting edge

BL length of groove backwall wear NW

notch wear width on main cutting edge

KT depth of groove backwall wear NL

notch wear length on secondary cutting edge

SW width of secondary face wear NW

notch wear width on secondary cutting edge

SD depth of secondary face wear

FIGURE 7.40 Measurable tool-wear parameters in a grooved tool

PRACTICAL TECHNIQUES OF USING CEMENTED CARBIDES FOR CUTTING 221

FIGURE 7.41 Variation of tool-wear patterns in three chip-grooves at varying depths of cut (cutting speed

= 274 m min

-1

, feed = 0.25 mm/rev., work material = 1037M steel)

222 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

The measurable tool-wear parameters

39,40

for a typical grooved tool are shown in Figure

7.40. Section A-A in the figure represents the approximate chip-flow direction for a given set of

cutting conditions. The influence of the chip-groove geometry can be understood from its effect

on the 3-D chip flow on the tool rakeface. The variations in cutting conditions (feed, depth of

cut, cutting speed) and the tool geometry (inclination angle, rake angle, nose radius, side-cutting

edge angle) also significantly affect the 3-D chip flow, and consequently the tool-wear parame-

ters. In many instances, the grooved tool fails due to unfavorable chip flow resulting from either

improper chip-groove design or inappropriate application of cutting conditions. Figure 7.41

shows the variation of the wear regions with depth of cut for three different grooved tool inserts.

Tool-life

varies significantly when using different chip-grooves and coatings. A new tool-

life relationship incorporating these two effects has recently been developed. This improved new

tool-life relationship is summarized as follows:

(7.1)

where V = cutting speed; T = tool-life; n = Taylor's tool-life exponent; T

= reference tool-life,

= reference cutting speed, W

= coating effect factor and W

= chip-groove effect factor.

(7.2)

where n

is the actual tool-life slope modified by the coating effect.

(7.3)

where f = feed; d = depth of cut; m = machining operation effect factor; n

, n

and k = empirical

constants. Figures 7.42a and b show the tool-life variations with feed and depth of cut respec-

tively for four different grooved tool inserts.

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