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
