INDEX INDEX 1. Introduction to Ceramic Cutting Tools 2. Tool Life 3. Selection of Cutting Tool Materials 4. Aluminum Oxide or Titanium Carbide Composite Cutting Tool 5. Cermet Cutting Tools 6. Alumina-Silicon Carbide Whisker Composite Tools 7. Phase Transformation Toughened Materials for Cutting Tool 8. Silicon Nitride Cutting Tools 9. Aluminum Oxide Coatings for Cemented Carbide Cutting Tool 10. Polycrystalline Diamond and Cubic Boron Nitride 11. The New Diamond Technology and its Application in Cutting 12. Advanced Processing Concepts for Increased Ceramic Reliability file:///D|/PG2004/TOOL/Anup/Ceramic%20tools/Index.htm [8/16/2004 3:34:15 PM] 3 Selection of Cutting Tool Materials John D. Christopher Machining Research, Inc. Florence, Kentucky There are many pathways taken to select a specific cutting tool for a machining process to manufacture a part. Unfortunately, many of these paths are those of least resistance, or more likely, least effort on the part of the decision maker. Without realizing the long range impact on the cost of the machined part or the rate at which parts are produced and delivered, purchasing agents, shop foremen, and manufacturing engineers often indiscriminately choose cutting tools. The choices are driven by many forces including eye-catching advertisements, friendly salesmen, and curiosity. Many companies purchase cutting tools based soley on low bids, with little regard for performance. Nationwide, poor productivity, excessive tool-changing downtime, and unacceptable part quality often result due to the performance of cutting tools purchased with cost as the primary determining factor. INTRODUCTION Figure 3-l provides a longitudinal perspective (not a cutting speed recommendation) on the development of cutting tool materials. As metallurgical/material technology advanced, the improvement in the performance of cutting tools followed. Advancements in cutting tools depend entirely on improving the chemical composition and/or the manufacturing process of the tool material. Invariably, improvements in the metallurgical quality of 28 Selection of Cutting Tool Materials 29 1300 1200 'UC dXIDES 1100 1000 900 800 UJlTING SPEED 700 LUNG&- FT./MN v1mixRl31 600 SIDEjLl- 500 400 300 X&HSS 200 i-m :& ; SrEEL 100 n b i i / 0 - -a- 1680 1710 1740 1770 1800 1830 1860 1890 1920 1950 1980 2010 YIBROF INlTCU3CTICti Figure 3-1. Cutting tool development. a cutting tool material results in longer tool life or more importantly, higher cutting speeds. Since the cutting speed is the dominant influence on the cutting temperature of the machining process, it naturally follows that tools that provide higher speeds have more tolerance for higher temperatures. Figure 3-2 illustrates this relationship. Tool materials with higher hot hardness will permit machining at higher productivity rates due to the higher allowable cutting speeds. 30 Ceramic Cutting Tools RDCWELL ROCKWELL IMRDNESS “C” "A" NllRDNESS 95 65 60 55 50 45 0 400 800 1200 1600 2000 TEMPERATURE (F) OCARBON STEEL +IiSS *COBALT HSS 'XCARBIDE 'XCERAMIC Figure 3-2. Hot hardness of various cutting tool materials. Selecting the correct cutting tool material for a specific machining operation is the first step in creating the most effective process plan for manufacturing a part. The cutting tool material is dependent on the work material to be machined and the operation to be performed. Often, there are several possible choices of tool materials that will successfully (but not cost-effectively) produce parts. Additional factors then must be considered and these include: Selection of Cutting Tool Materials 31 + machine tool horsepower, speed range, rigidity, + productivity demands, + tooling budget limitations, + machine tool burden rate and + labor and overhead rate. Generally, the higher the combined hourly rate for the machine tool and operator, the greater the demand for higher productivity to reduce the cutting time per part. However, the size, performance capacity, and general condition of the machine tool may limit the productivity available from that particular machine. Whatever the limiting factor or factors may be, the wise process planner or programmer uses a valid cost analysis to determine his choice of either maximum productivity or minimum cost. Decisions made without an economic analysis will likely produce less than the maximum available output at a higher part cost than necessary. WORK MATERIAL/ALLOY The most important consideration in selecting the correct cutting tool is the work material and its hardness. The material may be metallic or nonmetallic, ferrous or nonferrous. The majority of materials machined in the United States are ferrous materials, carbon, alloy, stainless steels, or cast irons. The cast irons may be gray, ductile, or malleable. There are usually two or three levels of tensile strength within many grades of alloy and stainless steels, as well as the three types of cast iron. Higher tensile strength levels invariably produce a higher hardness and a more difficult to machine material. A very thorough definition of the work material is a valuable aid in making an intelligent selection of the tool material. The purpose of this chapter is to survey the major categories of cutting tool materials with comments concerning unusual properties or limitations of each group, and the normal applications of each group. The various tool materials that will be discussed include: 32 Ceramic Cutting Tools + high speed steel, + uncoated carbides, + coated carbides, + ceramics, + cermets, + polycrystalline diamond and + polycrystalline cubic boron nitride. HIGH SPEED STEEL The earliest version of tool material used in machining was high carbon tool steel. This material was generally unalloyed steel and could be heat treated to a hard but shallow case. The addition of various alloying elements, particularly tungsten, chromium, and vanadium, added hardenability to the materials as well as much higher hot hardness. Table 3-l shows the chemical composition of selected high speed steels. These improvements in the chemical Table 3-1. HSS - Chemical Composition. c w MO Cl- v co M-l 0.80 1.50 8.00 4.0 1.0 -- M-2 0.85 6.00 5.00 4.0 2.0 -- M-7 1.oo 1.75 8.75 4.0 2.0 -- M-4 1.30 5.50 4.50 4.0 4.0 -- M-42 1.10 1.50 9.50 3.7 1.2 8.0 T-15 1.50 12.0 -- 4.0 5.0 5.0 composition of the steel provided increased performance from higher cutting speeds than those available with the carbon steels. These increases in cutting speed introduced the tern1 “high speed” steel, usually abbreviated HSS. Today’s HSS tools are available in the normal ingot cast version and as the particle metallurgy (PM) (a patented process) version. The ingot cast materials, while capable of doing a satisfactory job in most applications, are limited by the permissible Selection of Cutting Tool Materials 33 composition and/or heat treatment to achieve higher wear resistance (hardness) also produces a lower toughness and therefore a more brittle HSS. The PM steels, “mechanically” alloyed in the dry or particle condition, do not have the same limitations as those blended in the molten state. The result of this freedom in alloying is a series of high speed steel grades that can be heat treated to higher levels of hardness (68-70 Rc) without a severe reduction in toughness. These tools can provide longer tool life due to increased wear resistance without the risk of chipping or breaking from reduced toughness. As expected, this increased performance carries a higher cost. Generally, the effects of the various alloying elements are predictable. Table 3-2 illustrates the effects of various elements on the performance properties of high speed steels. Higher levels of carbon increase the hardness as they combine with other elements, particularly vanadium. The more popular grades, Ml, M2, M7, and Ml0 have l-2% vanadium. Vanadium carbides which form in Table 3-2. Effects of Various Elements on the Performance of HSS. MO: Acts as a substitute for W C: Provides high hardness Cr: Increases hardenability v: Increases wear resistance co: Increases hot hardness the microstructure of steels, are a major contributor to the wear resistance of the grade. Grade M4 (4% vanadium) is very wear resistant, compared to the previous four grades. The addition of cobalt (Co) to the chemical composition increases the hot hardness of the steel and elevates it to a “premium” category of higher performance and price. The most wear resistant grade in the premium category is T15, which has 1.5% carbon, 5% vanadium, and 5% cobalt. Figures 3-3 through 3-5 show the relative hot 34 Ceramic Cutting Tools hardness, toughness, and wear resistance, respectively for nine grades of HSS. Hard coatings can be applied to the surfaces of finish ground HSS tools to improve their performance, particularly in machining ferrous alloys. The physical vapor deposition (PVD) process applies a single coating usually of titanium nitride (TiN). The coating is applied as the vapor solidifies on the tools within a vacuum chamber. Other coatings of titanium carbonitride (TiCN), zirconium nitride (ZrN), and chromium nitride (CrN) are also available. The PVD process operates at a temperature lower than the tempering temperature of the HSS and therefore does not degrade the hardness of the steel. While coating increases the cost of the tool, it has in many instances provided substantial improvements in performance. A typical example is found in tapping, where only a small amount of wear on the tap will produce an undersize and therefore unacceptable thread. The cost of the coating is offset by the increased tap life. However, as the coating thickness is only about 0.0002-0.0003 inches in thickness, it does not “armor coat” the tool, and will not always survive when machining abrasive materials or steels much harder than 40 Rc. UNCOATED CARBIDES This tool material was first developed with tungsten carbide (WC) and cobalt as the binder. This material was satisfactory for machining gray cast iron, which was very common in the 1920’s and 1930’s. As the metallurgy of steel progressed, and its usage increased, the inability of these “straight” grades of carbide to successfully machine steels became obvious. The lack of alloying caused the tools to fail by cratering. The addition of titanium carbide and tantalum carbide to the composition of tungsten carbide and cobalt produced a series of grades that were crater resistant in turning and milling carbon and alloy steels. This improvement in carbide tool material increased the production rate of machining steels to that of cast iron in the 1940’s, 50’s, and early 60’s. Selection of Cutting Tool Materials 35 Figure 3-3. Relative hot hardness for various HSS tool materials. Figure 3-4. Relative toughness for various HSS tool materials. 36 Ceramic Cutting Tools 120 - - PERCENT 60 - -1 - - I 40 20 0 - T15 H42 Ml0 T5 HZ T4 Ml HSS GRADE Figure 3-5. Relative wear resistance for various HSS tool materials. A comparison between HSS and a C6 carbide turning an alloy steel is shown in Figure 3-6. The increase in speed from using carbide is approximately seven times. Selection of Cutting Tool Materials 60 50 ‘: 40 i ............... __i__ TOOL LIFE 30 MINUTES HSS 20 ......... j.. ... ... .; .. 10 ................. 0 I- 0 50 100 150 200 250 300 350 400 450 CUTTING SPEED-FEET/MINUTE Figure 3-6. Cutting speed vs. tool life for HSS and carbide tools (300 BHN steel). The relative percentages of carbide to cobalt determine the wear resistance or toughness of the grades. In the United States the Cl to C8 designations are still commonly used by both producers and users of carbide tools. The Cl to C4 categories contain only the WC and Co, and are suitable for roughing applications or interrupted cutting. The higher numbers are harder with more carbide and less binder, providing higher wear resistance, lower toughness, and are more suitable for semi- finishing and finishing cuts. 38 Ceramic Cutting Tools The C5 to C8 grades contain the additional components of TiC and TaC for machining materials that generally produce a continuous (often work hardened) chip formation that causes tool cratering. Tool cratering can occur without severe flank and nose wear that would produce parts out-of-tolerance. The result of severe tool cratering is catastrophic tool failure and perhaps a damaged part. The relationship of hardness and toughness is the same for the grades, C5 to C8 as for Cl to C4. Lower numbered grades are tougher, while the higher numbered grades are harder and more wear resistant. The effect on tool life and cutting speed using various grades of carbide is shown in Figure 3-7. Uncoated carbides are still quite widely used in the machining industry. Virtually all of the carbide tipped tools, drills, reamers, milling cutters, saws, etc. use uncoated carbide grades (Cl-C4). Many machine tools, unable to fully utilize the higher- performance, more expensive coated carbides, due to lack of speed or horsepower, are able to cut with uncoated carbide tools. Materials that are very abrasive or high in hardness, are not ideal applications for coated carbides and are often machined with uncoated carbides. The differences in cast irons can be critical to the correct selection if the cutting tool material is uncoated carbide. Ductile (spheroidal graphite) and malleable (quenched and tempered) irons are machined with “steel cutting grades” of uncoated carbide (C5- CS). Gray (flake graphite) cast iron, producing a discontinuous chip, is machined with the Cl-C4 grades of uncoated carbide. COATED CARBIDES The most common tool wear on uncoated carbides is diffusion-related wear. The temperatures and pressures associated with the normal cutting parameters on ferrous alloys cause the cobalt binder on the surface of the carbide tools to diffuse out of the matrix with the hot chips produced during the cutting process. As the binder diffuses from both the top and side surfaces, the grains of carbide are displaced gradually, leaving wear scars on the flank and nose of the tool and the crater on the rake face. Selection of Cutting Tool Materials 39 40 25 TOOL LIFE 20 MINUTES 15 10 5 -... 0 i- 200 400 600 SO0 1000 1200 1400 CUTTING SPEED-FEET/MINUTE Figure 3-7. Cutting speed vs. tool life for various grades of carbide tool materials (200 BHN malleable iron). Attempts were made to reduce this diffusion process, all with limited success, until the development of the chemical vapor deposition (CVD) process. This process deposits various vaporized compounds on the surfaces of the carbide tools in a vacuum chamber. The first successful coating was titanium carbide (TIC). In addition to TIC, titanium nitride (TIN) and aluminum oxide (Al,O,) are now used in various combinations, with TIC serving as the base coat. The familiar gold colored exterior coating is usually TIN, although hafnium nitride (HfN), similar in appearance, is used by some manufacturers. 40 Ceramic Cutting Tools It was noted by metallographic failure analysis studies on coated carbides that occasionally a damaged region (heat affected zone) at the interface of the carbide and the first coating was caused by the high temperatures associated with the CVD process. Machinability testing confirmed that the presence of this damaged region caused a weakening of the cutting edge manifested in heavy and/or interrupted cutting. The PVD coating process, commonly used on HSS tools, was tested on carbide and found to consistently produce a damage free interface. Subsequent cutting tests with PVD coated tools confirmed that an increase in tool life resulted over the CVD coated tools. This work has been publicly documented at SME cutting tool clinics by the Kennametal Company. A special group of PVD coated carbides are recommended for heavy or interrupted cutting applications. The substrates for the coated carbides are usually not cutting grades such as Cl or CS, but special compositions that are tailored to the use of the coated tool, having high toughness and deformation resistance. The wear resistance of the tool is usually dependent on the coating and not on the substrate. The presence of the coating will often increase the metal removal rate over an uncoated carbide 50% to 150%. Figure 3-8 shows the improved performance of the coated carbide over the uncoated tool when turning gray cast iron. These inserts are available in a wide variety of chip control geometries in all standard insert configurations. There is little doubt that the coated carbides are the closest product to an all-purpose cutting tool material for ferrous alloys. Their success is the reason that coated carbides account for approximately 60% of all sales of indexable inserts. CERAMIC TOOLS Cold Pressed Alumina Early generations of ceramic tools mnaufactured in the late 1940’s and early 1950’s were primarily cold pressed aluminum oxide (Al,O,). While these tools were chemically inert and had good hot hardness compared to tungsten carbide, they were Selection of Cutting Tool Materials 90 80 70 60 Tool Life 5. minutes 40 30 -i- 20 10 -i- 0 0 500 1000 1500 2000 2500 3000 Cutting Speed-feet/minute Figure 3-8. Performance improvement of coated vs. uncoated carbide tools (class 35 gray cast iron). notoriously low in toughness. This deficiency caused the tools to easily chip and break catastrophically, creating a poor image for early ceramic tools. The ceramic tools available today are of consistent high quality, and when correctly applied, are capable of delivering a cost-effective performance on finish cuts (light feed and depth of cut) of low hardness cast iron and medium hardness steels. Hot Pressed Alumina/Tic The development of the hot pressing process (without excessive grain growth) was a major step forward in producing 42 Ceramis Cutting Tools high quality ceramic tools. This process allowed the addition of TIC to aluminum oxide, producing the HP Al,OJTiC, an excellent all purpose ceramic tool. This grade is available in a wide variety of standard insert configurations at an affordable price (20-25% higher than coated carbides). Although other ceramic materials may be better for specific applications, HP AI,O,/TiC is acceptable for most machining situations where ceramics are applicable. It is an excellent material for turning tool steels as hard as 60-63 Rc, capable of holding diameters to a tolerance off 0.00025” and producing surface finish values of less than 5 micro-inch. This versatile ceramic material also has excellent thermal stability and is capable of cutting dry or with a water base cutting fluid. Whisker-Reinforced Alumina Silicon carbide (Sic) whiskers added to an A&O, matrix in random orientation, produces a ceramic tool material with very high toughness. This tool material is used in turning nickel-based alloys. These alloys which work harden when machined, cause a notching wear scar, usually at the depth of cut area on the side cutting edge of the tool. Notch wear can lead to the tool nose breaking off the insert, particularly when repeated passes are made at the same depth of cut. The high toughness of the whisker- alumina tool, along with the use of round inserts rather than nose radius style inserts, gives this ceramic material good success for high metal removal rate cuts on nickel base alloys. Several intelligent ramping techniques are recommended for these tools as an alternative to repeated passes at the same depth of cut. Varying the depth of cut minimizes the development of the severe notch at the same location on the cutting edge of the insert. Although the whisker-alumina tool is capable of providing a good performance in a variety of applications, other ceramic materials may be more cost effective. This ceramic tool material costs over twice the price of the hot pressed alumina/titanium carbide. Selection of Cutting Tool Materials 43 Silicon Nitride Silicon nitride is produced by a variety of processes with different microstructures. Some versions have a binder material, others do not. At least one material is a complete matrix of silicon nitride whiskers. Variations in processing are the most likely explanation for the wide range in performance from one producer to another. While capable of several ceramic applications, S&N, is possibly the most ideal tool material for machining gray cast iron in turning, boring, and face milling operations. A “good” grade of silicon nitride will machine common grades (used in the automotive industry) of gray iron at cutting speeds of 4000-5000 feet per minute (fpm). Most of the silicon nitride tools are capable of continuous machining at 3000 fpm, which is considerably faster than the productivity obtainable with coated carbides. Laboratory tests in face milling class 30 gray cast iron have been performed at 7000 fpm with tool life values up to one hour of cutting time. Rotary tools, end mills and drills, are now manufactured by CNC grinding from solid blanks. These tools have enjoyed selective applications with great success over HSS, carbide, and coated carbide rotary tools of similar geometry. Ceramic Summary The major deficiency associated with the use of ceramic tools in production machining is low toughness. This results in chipping and breakage of the tools rather than wear. To alleviate this problem, several techniques have evolved to strengthen the cutting edge and produce wear rather than chipping or breaking. These include increasing the thickness or nose radii (round inserts have been produced) of the tool that can result in improved performance. However, the most recent and effective improvement in ceramic inserts is in the development of the edge preparation. There are three types of edge preparations that eliminate the perfectly sharp edge where the sides of the insert intersect with the rake face. The earliest technique was the hone, performed carefully by hand with a fine grain diamond hone. This operation 44 Ceramic Cutting Tools is now automated. A radius is formed at the intersection of the face and side of the insert. The size of the radius can be varied to accommodate the application. The most common edge preparation is the T-land, which is a chamfer ground to a specific angle and width of the land. The angles vary from 10” to 35”, while the width of the land varies from 0.002-0.030 in, and occasionally higher. The most common combination is 20” x 0.004-0.006 in. The width of the land varies somewhat with the feed rate of the operation and is usually wider as the feed increases. The angle of the land is subjective, but generally decreases if the width of the land is very high. Although this technique is very successful in protecting the cutting edge, it is not an exact science. As optimization efforts proceed in the field and the data base increases, a better definition of the exact T-land for a specific material/operation will ultimately emerge. Inserts with ground T-lands can also have a subsequent honing operation which rounds the intersections of the land and the original faces of the insert. This added process eliminates any sharp intersections which may chip within the cutting zone. TiUTiN Cermets These tool materials derive their name from the use of ceramic materials with a metallic binder. Today’s cermets are usually titanium carbide and titanium nitride with a binder material. They are an effective material for machining steels as they are both wear and crater resistant to the continuous chip formation of steels. Cermets are available in an assortment of insert shapes with chip control grooves and edge preparations. This tool material is capable of providing a performance equal to or greater than coated and uncoated carbides on steels in the soft to medium hardness range where other ceramics are usually ineffective. The popularity or acceptance of cermets is not as widespread in the United States as their performance deserves. In Japan, cermets represent about 30% of tool sales, as compared to about 5% in the U.S. There are grades of cermets available which have adequate toughness for milling and interrupted cutting on Selection of Cutting Tool Materials 45 steels up to approximately 40 Rc hardness. Inserts are also available in positive rake geometry to minimize cutting forces and resultant part deflection. There are some practical guidelines to the application of cermets. These inserts are normally offered with T-lands, usually lighter than those on ceramic tools. Gray cast iron is generally not recommended as silicon nitride is more effective. However, ductile cast iron which cuts more like steel than gray cast is a good application. Since this TiC/TiN material does not have the excellent thermal stability of other materials, it is safer to cut without a fluid and risk intermittent flow which may cause thermal cracking of the insert. The cost benefits of cermets are usually higher productivity through higher cutting speeds and longer tool life. Since cermets are about 20% lower in cost than a coated carbide and exhibit better performance, there is a great untapped potential for TiC/TiN in the machining of steels. Polycrystalline Diamond and Cubic Boron Nitride The hardest substances known are 1) natural single crystal diamond, 2) polycrystalline diamond, and 3) cubic boron nitride. Polycrystalline tools are manufactured using extremely high temperatures and pressures. Polycrystalline Diamond, PCD The random orientation of the PCD tools corrects one of the major deficiencies of the natural diamond, the possible presence of a cleavage plane within the single crystal. This plane creates a natural failure site and can weaken the tool with a disastrous effect on performance. Therefore, single crystal diamonds used as cutting tools must be correctly oriented by a diamond expert. Single crystal diamond is an excellent special purpose tool for creating super fine finishes on items such as optical components. Usually the diamond is bonded to a standard carbide insert as a single tip on the insert. The insert is then ground (and perhaps polished) to provide a very smooth finish on the diamond. 46 Ceramic Cutting Tools The PCD tool provides an excellent general purpose tool for machining nonferrous and nonmetallic, abrasive materials. The most common applications for PCD tools include copper and aluminum alloys machined at high cutting speeds. It is standard practice for aluminum automobile wheels to be turned at 8,000 to 10,000 feet per minute with PCD tools. A deficiency of these inserts is the lack of chip control, a problem on soft materials like aluminum. Another common application for PCD tools is machining nonmetallics such as hard fiber reinforced plastics and materials such as granite and marble. Because PCD is much more abrasion resistant than carbides, it provides higher cutting speeds and/or longer tool life than carbides in the same machining operation. PCD inserts cost lo-13 times more than carbide inserts and have only one cutting edge compared to the multiple edges on the carbides, this must be considered when deciding whether or not a PCD tool is cost effective. Often, there is no other choice for producing quality parts on a reasonable production schedule. PCD inserts are available in both positive and negative rake style. The diamond section can often be reground to extend the life of the insert and thus lower the cost per cutting edge. Polycrystalline Cubic Boron Nitride, CBN Cubic boron nitride tools are available in both tipped inserts (like PCD) and also in solid CBN inserts. The solid inserts cost about three times as much as the tipped insert, but offer multiple cutting edges and a much tougher cutting material. CBN inserts can be used successfully to turn nickel base alloys, but they have a difficult time competing with the cost of the whisker/alumina insert. A single tipped CBN insert can cost about 3 times as much as the whisker/alumina insert which can have 4-8 as many cutting edges. Therefore, CBN is generally not cost effective for production machining of nickel alloys. A second application for CBN is machining hard ferrous alloys (65-68 Rc). Parts with this hardness are usually manufactured on a grinder rather than machined. However, the metal removal rate in machining may be 10 times as great as the Selection of Cutting Tool Materials 47 removal rate in grinding. Once again, CBN must compete with lower priced ceramics on steels in the 55-63 Rc range. The alumina/Tic ceramic is about 10% the price of a single tipped CBN insert and can have 4-8 cutting edges. It is wise to consider the entire economic picture of any machining operation in order to justify high performance/high price cutting tool materials such as CBN. A machining operation where solid CBN inserts can out- perform other tool materials is face milling steels in the 2 60 Rc range. Solid CBN inserts have incredible toughness and when correctly applied can withstand the most severe interruptions without chipping and breaking. SUMMARY The choice of the cutting tool material should always be made with as complete an economic analysis as the situation permits. This analysis should be made after the built-in constraints of machine tool capability (hp and speed), production schedules, and most importantly, part quality are considered. Selecting a specific tool material on the basis of longer tool life when more than one type will provide a satisfactory product is a safe choice when the tool-change time is long and tool life is short. Fewer tool changes reduce non-productive down time. If tool-change time is very short, the selection should be a tool that will increase productivity, reduce cycle time, and ultimately lower direct labor and machine burden on the cost per part.
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