Читать книгу Exploring Advanced Manufacturing Technologies - Steve Krar - Страница 21
Оглавление(Dr. George C. Ku, Professor – Central Connecticut State University)
PART 1 CARBIDE TOOLING
Among all the manufacturing processes that can be applied to shaping and forming of raw materials into useful products, the machining process has always been one of the most important operations. The fundamental cutting processes in machining, those of bringing the work into contact with the cutting tool, are still very much in evidence and should remain mainstays of the industry.
One of the most important components in the machining process is the cutting tool; its performance will determine the efficiency of the operation. Consequently, much attention has been directed to the selection of the tool materials, the cutting tool angles and their coating materials. Recent demands for high productivity combined with closer tolerance to machine heat resistant materials have made carbide tooling an important aspect of manufacturing technology.
CEMENTED CARBIDES
Cemented carbides are produced by a powder metallurgy process. These cutting tools are largely composed of tiny powder particles of tungsten carbides (carburized tungsten), carbon powder, and cobalt (the binder), that are sintered together at temperatures between 2550 and 2730°F (1400-1500°C). The process for manufacturing cemented-carbide tools involves the steps shown in Fig. 3-2-1.
▪Blending: mixing the right amount of carbide powders and cobalt together for the type of cemented-carbide required.
▪Compacting: molding the green powder into size and shape in a press.
▪Presintering: heating the green compacts to approximately 1500°F (815°C) in a furnace to hold their size and shape.
▪Sintering: the final heating process, 2550 to 2730°F (1400 to 1500°C), to cement the carbide powders into a dense structure of extremely hard crystals.
The powder metallurgy process produces a wide variety of hard metals. Powdered metals such as titanium (Ti), columbium (Cb), tantalum (Ta), and niobium (Nb), are also used in manufacturing cemented carbide to provide cutting tools with various characteristics. Cutting tools made of cemented carbide can increase cutting speed about three to five times faster than those used for high-speed steel tools.
Table 3-2-1 shows the Valenite, Inc., system of uncoated carbide grades used for metal-removal operations on various applications, types of materials, cutting tool characteristics, and machining conditions. Although the coding system may vary from manufacturer to manufacturer, the characteristics and use of each carbide grade will be the same.
CARBIDE TOOL IDENTIFICATION
There are four basic groups of cutting tool materials used in metal-removal operations: tungsten carbide, cermet, ceramic, and polycrystalline. There are two basic tool types used in carbide cutting - indexable insert tool and standard brazed on insert tool. Indexable insert tools, Fig. 3-2-2, are the most economical and those commonly used all types of metal-cutting operations. The inserts provide a number of low cost, indexable cutting edges. After all cutting edges are worn, it is generally more economical to replace the insert than to have someone regrind the insert. Various methods are used to lock the insert in position. A cam-type lock pin, or a clamp, or combination of lock pin and clamp are among the techniques used.
Standard brazed-on tools, low in initial cost, can be used for special and some general-purpose machining operations. They are rarely used in manufacturing today because of the time it takes to recondition and reset a tool once it has become dull.
Fig. 3-2-1 The powder metallurgy process of manufacturing cemented-carbide tools. (Carboloy, Inc.)
Table 3-2-1 Suggested uncoated carbide grades for metalcutting. (Valenite, Inc.)
Fig. 3-2-2 A variety of indexable carbide inserts. (Hertel Carbide, Ltd.)
Indexable Carbide Tool Identification
Numerous manufacturers throughout the world make cemented carbide indexable inserts in a wide variety of shapes and sizes. The more inserts available, the more difficult it is for those in industry to select the correct tool insert consistently. The ANSI and ISO coding systems, Table 3-2-2, that identify codes for inserts are recognized as standards throughout the world.
Turning Inserts
Most turning inserts today are .187 in. (5mm) thick, .50 in. (13mm) IC (inscribed circle) radius with 35°, 55°, 60°, and 80° geometries. The 60° triangular insert, with six cutting edges, is a good choice for general-purpose machining, Fig. 3-2-3.
▪For turning steel, the Sandvik PR inserts have been developed to achieve good chip control especially during roughing cuts. These inserts have a positive insert geometry with the top rake increased to 22° to enable high productivity where toughness is required, Fig. 3-2-4.
▪For heavy interrupted cuts or similar roughing operations, sturdier inserts such as the square or round inserts are recommended.
Fig. 3-2-3 The triangular 60° insert with six cutting edges is used for general-purpose machining. (Sandvik, Inc.)
Milling Inserts
Carbide milling inserts are available in a wide variety of geometric shapes and sizes. The Sandvik CoroKey geometry insert codes are illustrated in Fig. 3-2-5.
Code: L or light operations: Extra positive geometry for low cutting forces, suitable for small machines and sticky materials
M or medium operations: Positive geometry for general use, has high edge toughness and is the basic insert for most materials
H for heavy operations: Reinforced cutting edge that allows the highest possible feed rate, reliable in operations where the recutting of chips may occur
Table 3-2-2 ANSI and ISO codes for inserts. (Hertel Carbide Ltd.)
In the midst of all these insert styles, the simple round insert is emerging as the first choice milling insert for a growing number of machinists, programmers, and engineering. Round carbide inserts are strong, versatile and economical. Despite the simple shape, there are complex and sophisticated cutting tools. Subtle changes in geometry, depth of cut, lead angle and rake can make a big difference.
Carbide Tool Advantages
▪Excellent wear resistance
▪Greater hardness than high-speed steel tools
▪Ability to maintain a cutting edge under high-temperature conditions
▪Inserts can be replaced quickly when they are dull and therefore minimize the amount of machine downtime
▪Higher cutting speeds and feed rates can be used, increasing productivity
Fig. 3-2-4 The positive 22° rake on the insert provides high productivity where toughness is required. (Sandvik, Inc.)
Fig. 3-2-5 The CoroKey insert codes cover various machining conditions. (Sandvik, Inc.)
PART 2 INSERT SELECTION SYSTEMS
INSERT SELECTION
Dramatic advances in coating technology, chipbreaker design, and carbide metallurgy in the last few years have reduced the number of choices users face when selecting an insert for a particular application. Major suppliers offer a variety of coated carbides and chipbreaker geometries to machine almost all ferrous alloys effectively. Molded inserts and even molded chipbreakers, as precise as traditional ground carbides, provide a new generation of carbide tool family. Use Table 3-2-3 as a general guide for selecting the proper tooling to suit various machining applications.
The selection of inserts to suit a machining operation must consider five factors: the part material, insert geometry, selection of the chipbreaker type, grade of the carbide tool, and optimum cutting speed and feed.
Table 3-2-3 The tooling and selection guides for inserts and machining conditions. (Carboloy, Inc.)
1. Part Material
Most carbide manufacturers produce tool inserts to suit each of the following material groups:
▪Steel and cast steel, unalloyed to high alloy, case hardening and heat treatable steels, carbon steel, and structural steels.
▪Stainless and acid-proof steel, high heat-resistant alloys based on nickel and cobalt content.
▪Cast iron, grey cast iron, malleable cast iron, and chilled iron.
2.Insert Geometry
The shape of the insert selected must suit the type of machining operation to be performed (roughing or finishing), the contour of the form to be machined, the shape and hardness of the part, and the surface finish required. The following are some general guidelines to follow when selecting the shape of the insert.
▪Square inserts have the strongest structural shape (90° point angle). They are used where a lead angle is desirable and for chamfering tools.
▪Triangular inserts have a 60° point angle that allows producing square shoulders, cutting contour forms, chamfering, and plunge cutting.
▪Round inserts provide shallow feed marks at high speeds on finishing passes. They are excellent for roughing cuts, especially on cast iron.
▪Rectangular inserts are used for heavy-duty machining.
▪Diamond inserts with an 80° point angle are used as combination turning and facing tools; 55° nose angle inserts are used for machining contour forms.
▪Threading and grooving inserts are available for specific thread forms and tool widths.
MANUFACTURER CODING SYSTEMS
The cutting tool insert suppliers have reduced the number of carbide grades and chip groove geometries necessary for good performance. Furthermore, the easy-to-use, color code systems enable users to select exactly the right insert for the job. Special machining requirements that fall outside of the color-coded systems are addressed with specific and customized recommendations on a case-by-case basis. The carbide insert selection systems developed by the four leading carbide tool manufacturers are explained as a guide to assist in the selections of carbide insert tools.
CARBOLOGY’S SECOLOR 3 X 3 MATRIX SYSTEM
Secolor is a system that simplifies the selection of the correct inserts for most machining operations. It is based on color coding in accordance with the ISO standard for the application of cemented carbides. The blue color is for steels, yellow is for stainless steels, and red is for cast iron.
Each color has a different shade for finishing (F), medium-rough machining (M), and rough machining (R), Fig. 3-2-6.
Carbide Grades:
▪TP100 is used for high-speed machining in steel and is the first choice for cast iron.
▪TP200 is the first choice for medium-to-rough turning of steel and is a good choice cast iron, excellent for stainless steel.
▪TP300 provides the toughness and reliability needed for general turning of steel and stainless steel and is first choice for severe interrupted turning conditions of steel.
Carboloy’s three first-choice chip breakers are:
1.MF2 is designed for finishing cuts and has a modified groove that can control chips at depths of cut as low as .010 in., Fig. 3-2-6.
2.M3 is the most versatile chip breaker and is adapted for near-net shape forging and castings.
3.MR7 is designed for more demanding operations at high feed rates, interrupted cuts, and other combinations when edge strength is needed.
This selection method does not apply to turning very difficult-to-machine materials, such as titanium, nickel, cobalt or iron-based high temperature alloys. Carboloy has selection charts for polycrystalline cubic boron nitride (PCBN) inserts used for turning hardened alloy steels and cast iron.
KENNAMETAL GRADE SYSTEM
The Kennametal grade system consists of four basic groups of workpiece materials, Fig. 3-2-7. Each group contains a variety of insert grades to suit various metalcutting conditions.
▪Tungsten carbide: This group consists of uncoated, chemical vapor deposition (CVD) coated and physical vapor deposition (PVD) coated inserts. Each coated grade consists of various substrates of unalloyed (straight WC/Co) and alloyed (WC/TaC/NbC/Co) compositions.
Fig. 3-2-6 The coding system for three carbide grades and chipbreakers that cover a wide working range. (Carboloy, Inc.)
▪Cermet: Cermets consist mostly of titanium carbide (TiC) and titanium nitride (TiN) with a metallic binder.
▪Ceramic: The ceramic cutting tools can be divided into two basic families; alumina-base (aluminum oxide) ceramics, and silicon nitride-base (sialon) ceramics.
▪Polycrystalline: These inserts are divided into two basic families; polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN).
The six basic workpiece material groups are:
1.Steel, ferritic and martensitic stainless steels.
2.Hardened steels and hardened irons.
3.Austenitic stainless steels, free-machining and low carbon steels.
4.Cast irons
5.Nonferrous material.
6.Heat-resistant alloys and titanium.
Fig. 3-2-7 The insert tooling grade system. (Kelmar Associates)
Selection Procedures:
The steps in selecting the correct insert from the Kennametal color-coded system are to choose the insert geometry, grade, and cutting speed for the type of material being cut and the machining operation. It is always wise to use the manufacturer’s selection guide when selecting inserts. Table 3-2-4 lists the suggested grades and machining conditions for free-machining and low carbon steels.
SANDVIK’S CoroKey
The CoroKey system developed by Sandvik, Inc., converts the selection of inserts into turning, milling and drilling applications. The ISO standard divides workpiece materials into three major areas with appropriate color coding:
▪ISO P (Blue) is designed for long chipping materials including steels, steel castings, and martensitic/ferritic stainless steels, Fig. 3-2-8.
▪ISO M (Yellow) covers austenitic stainless steels, superalloys, and titanium, Fig. 3-2-9.
▪ISO K (Re) is designed for short-chipping materials including cast iron, hardened materials and nonferrous materials, Fig. 3-2-10.
Table 3-2-4 Suggested grades and machining conditions for free-machining and low carbon steels. (Kennametal, Inc.)
Fig. 3-2-8 Suggested insert for finishing operations. (Kennametal, Inc.)
Fig. 3-2-9 Suggested insert for general purpose medium machining. (Kennametal, Inc.)
Fig. 3-2-10 Suggested insert for roughing operations. (Kennametal, Inc.)
Designations: F, M, R for finishing, medium machining, and roughing, respectively, Fig. 3-2-11.
The various applications in turning and milling can be grouped into three main machining categories to include: finishing (F), medium machining (M), and roughing (R) for turning, light medium, and heavy machining for milling.
▪Finishing (F) and Light (L): machining operations at light depths of cut (DOC) and feedrates to produce surface quality.
▪Medium (M): the majority of all applications, general purpose to light roughing, which cover a wide range of depth of cut and feedrate combinations.
▪Roughing (R) and Heavy (H): machining operations for maximum stock removal and severe conditions that include high depth of cut and feedrate combinations.
CoroKey insert recommendations cover the most common types of turning and rotating types of machining, with recommended first choice for versatility and two complementary choices for increased productivity or added security. See various Sandvik guides and catalogs for more details on specific insert selections, material applications, and machining operations.
VALENITE’S SPECTRA TURN SYSTEM
Valenite Inc. developed a complete version of its Spectra System™ Turn Application Guide whose purpose is to allow the user to hold tighter dimensional tolerances. When the correct insert is used on the proper application, Table 3-2-5, the Spectra Turn inserts should extend tool life by almost 400%.
Valenite has developed a Spectra Turn Application Guide, a color-coded guide that covers 95% of turn applications and the nine grades and thirteen chipbreakers that make up its Spectra Turn line of turning inserts. The guide differs from traditional ISO groupings and bases its recommendations on the similarity of machinability intended for workpiece materials that share common failure modes. The Color-coding groupings are:
Fig. 3-2-11 Insert grades for finishing, medium machining, and roughing operations. (Kennametal, Inc.)
▪Blue for carbon, alloy, and tool steels.
▪Yellow for stainless steels, titanium, and high temperature alloys.
▪Red for gray and ductile irons, aluminum, and non-ferrous materials.
The insert grade application range, Fig. 3-2-12, shows the applications, failure modes, and suggested speed ranges that can be used as a guide when selecting an insert to suit a particular application. The typical failure modes are identified for applications that include heavy roughing, roughing, semifinishing, and finishing.
Insert failure analysis is a method of determining how close an insert and application match the optimum insert life span, as prescribed by the manufacturer. The idea is to look at the primary failure mode for various workpiece materials and select an insert that minimizes this primary failure mode. For example, some alloys of stainless steel may produce a build-up edge on the insert that may result in premature, catastrophic failure of the insert.
Selecting an insert having the features designed to reduce build-up edge will extend insert life. Failure mode analysis is a method to help a shop specify an insert with substrate, coating, chipbreaker, edge preparation, and coolant designed to reduce build-up edge enough for the insert to wear out at a consistent and predictable rate.
Applying failure mode analysis to insert selection enables the user to reduce or eliminate the primary causes of insert failure.
▪When insert selection is approached from a failure mode, workpiece material groupings are changed: steel is blue; stainless are yellow and gray; and ductile irons and aluminum are red.
▪Materials are classified based on their machinability characteristics rather than chip formation type.
Failure mode analysis looks at two types of insert failure: primary and secondary. Primary, the initial or underlying problem will eventually result in secondary failure. For example, the build-up edge can result in chipping or breaking away edge of the tool, a secondary failure mode that would constitute a catastrophic failure of the insert edge.
Fig. 3-2-12 Application speed, grade ranges, and applications. (Valenite, Inc.)
Table 3-2-5 Spectra System application guide for insert grades. (Valenite, Inc.)
PART 3 COATINGS COATED CARBIDE INSERTS
Research shows that cemented-carbide tools coated with a film of titanium carbide, titanium nitride, or aluminum oxide can increase tool life, improve material-removal rates as much as 30%, and produce freer-flowing chips. The coating acts as a permanent lubricant, greatly reducing cutting forces, heat generation, and tool wear, Fig. 3-2-13. This permits higher speeds to be used during the machining process, particularly when a good surface finish is required. The lubricity and antiweld characteristics of the coating greatly reduce the amount of heat and stress generated when making a cut.
Fig. 3-2-13 Coatings act as a lubricant, reduce tool wear and cutting forces, and heat generation. (Balzers Tool Coating, Inc.)
The use of hard, wear-resistant coatings of carbides, nitrides, and oxides to carbide inserts have greatly improved the performance of carbide-cutting tools. Inserts are available with a combination of two or three materials in the coating to give the tool special qualities. Strong wear-resistant titanium carbide forms the innermost layer. This layer is followed by a thick layer of aluminum oxide, which provides toughness, shock resistance, and chemical stability at high temperatures. A third, very thin layer composed of titanium nitride is applied over the aluminum oxide. This provides a lower coefficient of friction and reduces the tendency to form a built-up edge.
Coatings increase tool life and manufacturing productivity, while reducing machining costs, Fig. 3-2-14. Some of the coatings used for cemented carbide tools that have been successful are titanium carbide (TiC), titanium nitride (TiN), aluminum oxide (Al2O3), and titanium carbonitride (TiCN):
▪Titanium Nitride (TiN), a gold-colored coating, is an excellent general purpose coating for protecting a wide variety of tools from wear. TiN coated tools are used for machining high alloy steels and low alloy steels at medium and high cutting speeds. Tool life is three to five times longer than uncoated HSS and carbide end mills.
▪Titanium Carbonitride (TiCN), a blue-gray colored coating, is a high performance coating for milling cutters used for machining stainless steel at low cutting speeds, machining alloy steels, and when increased speed and feed rates are desired.
▪Chromium Nitride (CrN) is a silver-gray colored coating that resists adhesive wear, corrosion, and oxidization. It is used for machining copper alloys, bronze, aluminum bronze, nickel silver titanium, and titanium alloys.
▪Chromium Carbide (CrC), a silver-gray colored coating, has high temperature oxidization-resistant properties used for aluminum and magnesium die-castings.
▪Titanium Aluminum Nitride (TiAIN) is a violet-gray colored multi-layer coating used for machining cast iron, stainless steel, nickel-base high temperature alloys and titanium alloys. This coating is used for high-speed dry and semi-dry machining operations.
▪Tungsten Carbide/Carbon (WC/C) is a black-gray colored coating of hard tungsten carbide particles in a soft amorphous carbon matrix. It is used for precision components with abrasive and adhesive wear, seizure problems (poor lubrication) and for dry machining applications.
Fig. 3-2-14 Coatings on inserts help to increase productivity. (Niagara Cutter, Inc.)
▪Polycrystalline Diamond (PCD), a layer of diamond fused to the cutting tool, is used for machining abrasive non-metallic, non-ferrous materials, graphite, plastics, green compacts, and composites.
For a list of properties and applications for thin wear-resistant coatings refer to Table 3-2-5.
Principles and Background Information
▪Chemical Vapor deposition (CVD) coatings adhere well, but high temperatures 1760-2425°F (800-1100°C) can damage substrates.
▪The physical vapor deposition uses a lower coating temperature 440-1100°F (200-500°C). PVD coatings prove more useful for milling, parting, grooving, and drilling, while CVD coatings perform better in turning.
▪The medium-temperature chemical vapor deposition 1760-1870°F (800-850°C) produces smoother, less-brittle coatings with lower residual stress.
▪The combined result of cobalt-enriched substrate and the medium-temperature chemical vapor deposition produces a very hard and tough cutting edge that wears well and is crater resistant.
▪Chemical vapor deposition (CVD) physical vapor deposition(PVD), and more recent medium-temperature chemical vapor deposition (MTCVD) constitute the primary processes for 80% of all coating tools.
▪Multi-layer coatings (three to five layers) are used to combine with thermally-resistant materials such as AL203 and/or abrasion-resistant layers, Fig. 3-2-15.
Table 3-2-6 Properties and applications of various thin-film wear-resistant coatings. (Balzers Tool Coating, Inc.)
Fig. 3-2-15 Multi-layer coatings are used to combine thermal-resistant materials with wear-resistant materials. (Carboloy, Inc.)
Multi-layering improves adhesion and allows a wider range of substrate/coating combinations.
The CVD coatings allow the combined advantages of various coatings and can be used into an optimum sequence to handle specific applications.
PART 4 CERAMIC CUTTING TOOLS
CERAMIC CUTTING TOOLS
The strength of ceramic cutting tools has nearly doubled, their uniformity and quality have been greatly improved, and they are now widely accepted by industry. Ceramic cutting tools are used successfully in the machining of hard ferrous materials and cast iron. As a result, lower costs, increased productivity, and better results are being gained. In some operations, ceramic tools can be operated at three to four times the speed of carbide tools.
Manufacture of Ceramic Tools
Most ceramic or cemented-oxide cutting tools are manufactured primarily from aluminum oxide.
1.Bauxite (a hydrated alumina form of aluminum oxide) is converted into a denser, crystalline form called alpha alumina.
2.Ceramic tool inserts are produced by either cold or hot pressing.
▪In cold pressing, the fine alumina powder is compressed into the required form and then sintered in a furnace at 2912 to 3092°F (1600 to 1700°C).
▪Hot pressing combines forming and sintering, with pressure and heat being applied simultaneously.
3.Certain amounts of titanium oxide or magnesium oxide are added for certain types of ceramics to aid in the sintering process and to retard growth.
4.After the inserts have been formed, they are finished with diamond-impregnated grinding wheels.
Types of Ceramic Grades
Ceramic cutting tools can be divided into two grades or families: alumina-base ceramics and silicon-base ceramics.
▪Alumina-base ceramics offer superior wear resistance and chemical wear stability, and are used for high velocity semi-finishing and finishing of ferrous and nonferrous materials.
•The addition of silicon carbide whisker reinforcements has improved the reliability of some alumina-based ceramics, especially when machining nickel-base alloys.
•Alumina-base ceramics should be considered primarily for semi-finishing and finishing operations.
▪Silicon nitride-base ceramics offer increased toughness and thermal shock resistance over alumina-base ceramics and therefore are considered high-velocity ceramics. They retain the toughness and thermal shock properties of conventional ceramics but offer superior abrasion resistance.
Characteristics of Reinforced Ceramic Inserts
When using reinforced ceramic inserts, high temperature is needed ahead of the cutting tool to soften or plasticize the workpiece material and assist its removal. The ideal cutting temperature in nickel alloys is in the area of 2200°F (998°C). This cutting temperature is beyond the upper limit for sintered carbide inserts. At this temperature carbide will soften, deform, and fail. Successful cutting with reinforced ceramic inserts require high surface speed along with balanced feed rates.
Fig. 3-2-16 A variety of indexable ceramic tool inserts. (Kennametal, Inc.)
Ceramic Insert Tools
The most common ceramic cutting tool is the indexable insert, Fig. 3-2-16, which is fastened in a mechanical holder. Indexable inserts are available in many styles, such as triangular, square, rectangular, and round. These inserts are indexable; when a cutting edge becomes dull, a sharp edge can be obtained by indexing (turning) the insert in the holder. The common shapes are in descending order from strongest to weakest: round, 100° diamond, square, 80° diamond, triangle, 55° diamond and 35° diamond. It is always good practice to use the strongest insert shape possible that suits the machining operation.
Cemented ceramic tools, Fig. 3-2-17, are the most economical, especially if the tool shape must be altered from the standard shape. The ceramic insert is bonded to a steel shank with an epoxy glue. This method of holding the ceramic inserts almost eliminates the strains caused by clamping inserts in mechanical holders.
Fig. 3-2-17 A variety of ceramic inserts bonded to various styles of steel shanks. (Hertel Carbide Ltd.)
Ceramic Tool Applications
The most common applications of ceramic inserts are in the general machining of steel where there are no heavy, interrupted cuts and where negative rakes can be used. This type of cutting tool has the highest hot-hardness strength of any cutting-tool material and produces excellent surface finish. No coolant is required with ceramic tools since most of the heat goes into the chip and not into the workpiece. Table 3-2-7 lists some of the most common applications for ceramic cutting tools.
Ceramic tools can be used to replace carbide tools that wear rapidly in use, but they should never replace carbide tools that are breaking. Ceramic tools are successfully used for:
▪High-speed, single-point turning, boring, and facing operations, with continuous cutting action
▪Finishing operations on ferrous and nonferrous materials
▪Cutting hard steels between 45-65 Rc hardness where other cutting tools have failed
▪Machining materials where other tools break down because of the abrasive action of sand, inclusions, or hard outer scale
▪Light interrupted cuts on steel or cast iron, heavy interrupted cuts on cast iron if the tool and machine are rigid enough
▪Any operation where the size and finish must be accurately controlled and where other tools have failed
Advantages
Many of the advantages of the grinding process - high heat tolerance, excellent surface finish, and long tool life - can be found in the use of ceramic inserts. When ceramics tools are used properly, on the correct application, they can offer the following advantages:
▪Ceramic inserts work best on hard ferrous metals and nickel base alloy; they are not effective on ferrous metals below 42Rc.
▪About 80% of reinforced ceramic usage is on nickel alloys and aerospace alloys such as Inconel, Waspoloy, Hastelloy, and others.
▪Ceramic’s melting point is 3700°F (1678°C), higher than sintered carbide, allowing it to be used at higher speed rates on hard materials.
▪Machining time is reduced because of the higher speeds possible and the long tool life.
▪Accurate part size is possible because of the greater wear resistance.
▪The surface finish on machined parts is better than what is produced by other cutting tools.
▪Turning is an ideal operation for reinforced ceramic inserts; milling can be compared to interrupted machining in turning.
▪Hard milling operations require much higher spindle speeds to generate the heat equivalent of a single-point turning tool.
Table 3-2-7 The composition and application for various ceramic grades. (Kennametal, Inc.)
Disadvantages
A few of the disadvantages or cautions that a user should be aware of:
▪Ceramic insert are brittle and tend to chip if not set up or used properly.
▪Considerably more power and higher cutting speeds are required for ceramics to cut efficiently.
▪The initial cost of ceramics is higher than carbides; however this is offset by higher productivity.
▪The machine tool used must be more rigid than those using carbide tools.
PART 5 CERMET CUTTING TOOLS
Continued research aimed at improving the strength of ceramic cutting tools led to the development of cermet tools that are a combination of various ceramic and metallic materials. They combine the ceramic properties of hardness, wear resistance, temperature, and oxidation, with the properties of metals that include toughness, impact strength, and ductility. The multi-component alloy cermets, made up of different hard materials and binder elements, have high wear-resistance qualities that result in long tool life. The properties of cermet tools are shown in Fig. 3-2-18.
Types of Cermet Tools
There are two main types of cermet tools: those composed of titanium carbide (TiC) based materials and those containing titanium nitride (TiN) based materials.
Titanium carbide (TiC) cermets have a nickel and molybdenum binder and are produced by cold pressing and sintering in a vacuum. They are used extensively for finishing cast irons and steels that require high speeds and light-to-moderate feeds.
Titanium nitride (TiN) has been added to titanium carbide to produce titanium carbide-titanium nitride (TiCTiN) cermets. Other materials such as molybdenum carbide, vanadium carbide, zirconium carbide, and others may be added, depending on the application.
Because of their high productivity, cermets are considered a cost-effective replacement for coated and uncoated carbide and ceramic tools. However, cermets are not recommended for use with hardened ferrous metals (over 45 Rc) or nonferrous metals.
Fig. 3-2-18 The properties of cermet insert tools. (Kelmar Associates)
Characteristics of Cermet Tools
The main characteristics of cermet tools are:
▪They have great wear resistance and permit higher cutting speeds than do carbide tools.
▪Edge buildup and cratering are minimal, which increases tool life.
▪They possess high hot-hardness qualities greater than carbide tools, but less than ceramic tools.
▪Excellent chemical stability at conventional (carbide) speeds.
Advantages of Cermet Tools
Cermet tools have the following advantages:
1.The surface finish is better than that produced with carbides under the same conditions, which often eliminates the need for finish grinding.
2.High wear resistance permits close tolerances for extended periods, ensuring accuracy of size for larger batches of parts.
3.Cutting speeds can be higher than with carbides for the same tool life.
4.When operated at the same cutting speed as carbide tools, cermet tool life is longer.
5.The cost per insert is less than that of coated carbide inserts and equal to that of plain carbide inserts.
Use of Cermet Tools
Titanium carbide cermets are the hardest cermets and are used to fill the gap between tough tungsten carbide inserts and the hard, brittle ceramic tools. They are used mainly for machining steels and cast irons where high speeds and moderate feeds may be used. See Table 3-2-8 A to F for recommended cutting speeds for machining various material groups.
Titanium carbide-titanium nitride inserts are used for semifinish and finish machining of harder cast irons and steels (less than 45 Rc) such as alloy steel, stainless steel, armor plate, and powder metallurgy parts.
For more information on CUTTING TOOL TECHNOLOGY see the Websites: www.carboloy.com www.cormorant.sandvik.com/us www.kennametal com www.niagaracutter.com
Table 3-2-8 Cermet grade inserts with recommended material applications and suggested speed ranges. (Hertel Carbide Ltd.)
