Exploring Advanced Manufacturing Technologies. Steve Krar
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