Trends in Cutting Tool Technology

8/13/2019 Trends in Cutting Tool Technology

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affect performance in operations involving interruptions of cut and inconsistency of workpiece

microstructure such as found in some nodular irons. Recently developed mediumtemperatureCVD

(MTCVD) coatings have shown a reduced tendency to formation of eta phase. MTCVDcoated tools offer

increased resistance to thermal shock and edge chipping compared to conventional CVDcoated tools.

The result is greater tool life as well as increased toughness compared to hightemperature CVD

coatings.

Sharp for Steel . . . and Others

Physical-vapor-deposition (PVD) coatings also offer advantages over CVD coatings in certain operations

and/or workpiece materials. Commercialized in the mid1980s, the PVD coating process involves

relatively low deposition temperatures (approximately 500C), and permits coating of sharp insert

edges. (CVDcoated insert edges are usually honed before coating to minimize the effect of eta phase.)

Sharp, strong insert edges are essential in operations such as milling, drilling, threading and cutoff, and

for effective cutting of longchipping materials such as lowcarbon steels (Figure 4, at below). In fact, a

wide range of problem materialssuch as titanium, nickelbased alloys, and nonferrous materialscan

be productively machined with PVD coated tools. From a workpiece structure point of view, sharp

edges reduce cutting forces, so PVD coated tools can offer a true advantage when machining

thinwalled components.

The first PVD coatings were titanium nitride (TiN), but more recently developed PVD technologies

include titanium carbonitride (TiCN) and titanium aluminum nitride (TiAlN), which offer higher

hardness, increased toughness, and improved wear resistance. TiAlN tools in particular, through their

higher chemical stability, offer increased resistance to chemical wear and thereby increased capability

for higher speeds.

Recent developments in PVD coatings include soft coatings such as molybdenum disulfide (MoS2) for

dry drilling applications. Combination soft/hard coatings, such as MoS2 over a PVD TiN or TiAlN, also

show great potential, as the hard (TiN or TiAlN) coating provides wear resistance while the softer,

more lubricious outer layer expedites chip flow.

Running Dry

Government mandates also can affect cutting tool development. In some countries, increasingly strict

environmental regulations governing the disposal of cutting fluids are resulting in increased use of dry

machining. While dry machining is not appropriate for every process and workpiece material, in some

cases careful selection of cutting tool material can enable a user to minimize or avoid the use of

coolant. A cutting tool with a thick alumina coating can allow increased feed rates in the machining of

steel, reducing contact time of the insert with the workpiece and minimizing exposure of the tool to

high cutting temperatures, and thereby enabling productive dry machining (Figure 5, below). In

addition, advanced coatings such as PVD TiAlN can provide good performance in dry machining or in

minimal coolant systems. As mentioned previously, lubricious PVD MoS2 coatings can also facilitate dry

drilling and tapping. A focus on dry machining will spark further effort to develop cutting tools with

high resistance to thermal load.


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Cermets For NearNet

Cermet cutting tools (also effective in dry machining applications) are one facet of the cutting tool

industrys response to nearnetshape manufacturing trends. These trends entail efforts to lower

manufacturing costs by casting and forging components to near their final (net) shape, thereby

reducing the number of machining operations necessary to complete a part. Fewer heavy roughingoperations are required, and the need for tools engineered for semifinishing to finishing duty expands.

Development of cermet tools is one way tool manufacturers are addressing this need. Cermets,

comprised mostly of titanium carbonitride (TiCN) with a nickelcobalt binder, are hard and chemically

stable, leading to high wear resistance. Cermets work best in materials that produce a ductile chip,

such as steels and ductile irons. Their increased speed capability enables them to machine carbon,

stainless steels and ductile irons at high speeds while producing excellent surface finishes.

Recently developed cermets combine excellent resistance to deformation and chemical wear with a

degree of toughness that enables them to be used in semifinishing as well as finishing operations. PVD

coatings further enhance the performance of cermets on a wide variety of workpiece materials.

Machining The Hard Way

Both environmental/governmental factors (disposal of coolant/swarf) and economic concerns (the high

cost of grinding) are accelerating the replacement of grinding by machining in the processing of

hardened workpieces. The cutting tool industry is constantly developing and evaluating tools

engineered to provide maximum productivity in hard-machining operations. These tools include

superhard materials such as polycrystalline cubic boron nitrides, as well as ceramic tools.

Coatings, which reduce frictional heat and promote longer tool life, are among the new concepts being

utilized in tools for hard turning (Figure 6, at left).

In field tests, coated superhards have outlasted other PCBN tools by 20 to 100 percent. Coatings have

also proven effective on ceramic tools engineered for hard turning. In situations where the hardened

workpiece doesnt have roughness or other interruptions, coated ceramics offer more cutting edges

and lower cost, and can be a costeffective alternative to PCBN tools in hard turning.

Updated Ceramics For Difficult Materials

Development efforts in ceramic tool technology are enabling these hightech tools to move into new

areas of application. While recently developed silicon nitride tools offer improved fracture resistance

compared to their predecessors, their relatively low resistance to chemical wear has limited their use

in the machining of nodular cast irons (Figure 7, below). However, wearresistant CVD alumina coatings

have expanded the application range of siliconnitridebased tools to include these difficult tomachine

irons.

Regarding alumina (A1203based) ceramics, the addition of silicon carbide whiskers offers increased

productivity in the machining of Inconel and similar highstrength, hightemperature alloys in the

aerospace industry. Singlecrystal whiskers deflect cracks in the alumina matrix and thereby improve

fracture toughness of the tool.


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Geometric Progression

Perhaps the common thread through all manufacturing is the drive for increased productivity and

reliability. As metalcutting operations become increasingly finetuned, the relationship between cutting

tool micro (cutting edge preparation) and macro (rake face topography) geometry is becoming more

and more important. Chip control, tool life, workpiece finish and accuracy can be greatly improved byapplying the proper combination of micro and macro geometries in conjunction with the proper

substrate and coating. Control of the chip, dissipation or deflection of heat via restricted contact

topographies, and reduced cutting forces as a result of positive rake surfaces all lead to the improved

performance of todays modern molded cutting insert geometries. Advances in tool manufacturing

technology are making possible more precise matching of macro geometries and hones to specific

machining applications.

Productivity First

True breakthroughs in cutting tool technology occur, but they are rare. Most tool development comes

from development, refinement and innovative combinations of existing tool materials. The directionfor this development begins with the analysis of the characteristics of the materials being machined,

includes the demands of specific operations, and involves ongoing communication between toolmaker

and end user.

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Trends in Cutting Tool Technology