TOOL MATERIALS USED IN CNC MACHINE TOOLS

Tool Materials used in CNC Machine Tools

CUTTING TOOL MATERIALS

         Various cutting tool materials have been used in the industry for different applications

High Speed Steel

• High-speed steel (HSS) tool materials have significant quantities of tungsten, (W), molybdenum, chromium and vanadium. The complex carbides of tungsten, molybdenum and chromium distributed through out the metal matrix provide very good hot hardness and abrasion resistance. The major alloying elements, which contribute to the hardness is tungsten and molybdenum. Tungsten is expen­sive, while molybdenum is cheap but has higher toughness. For the same hardness, less amount of molybdenum needs to be added, however more care need to be exercised in hardening as decarburising takes place in molybdenum steels. Also they have narrow temperature range for heat treatment. Molybdenum tool steels are more popular.
• The main advantages of high-speed steels is their high hardness, hot hardness, good wear resist­ance, high toughness and reasonable cost. Toughness of high-speed steels is highest among all the cutting tool materials. Thus they are quite extensively used in interrupted cutting such as in milling. The hardness of HSS falls rapidly beyond 650°C and thus they are limited to lower cutting speeds of the order of 0.5 to 0.75 m/s.
• The physical coating process (PVD—Physical Vapour Deposition) allows the HSS tools to be coated with hard nitrides of titanium and aluminum. With much favorable cutting geometries and the hard coatings the cutting performance and tool life of HSS tools has improved substantially. The PVD coatings are generally done at low temperatures as a result the adherence of coating is a problem, which is solved by improved cleaning and etching techniques. There are efforts to further improve the cutting performance by improving the coating characteristics by combining various nitrides.

Cemented Carbides

• Cemented carbides are produced by the cold compaction of the tungsten carbide powder in a binder such as cobalt, followed by liquid-phase sintering. These have a very large number of advantages compared to the other cutting tool materials. The following guidelines would be useful for selecting a carbide grade.
• (a) Choose a grade with the lowest cobalt content and the finest grain size consistentwith adequate strength to eliminate chipping.
• (b) Use straight tungsten carbide grades if cratering, seizure or galling are not
• Experienced in case of work materials other than steels.
• (c)  To reduce cratering and abrasive wear when machining steel, use gradesContaining titanium carbide.
• (d) For heavy cuts in steel where high temperature and high pressure deform the cutting edge plasti­cally, use a multi carbide grade containing W-Ti-Ta and/or lower binder content.
• As the cobalt content increases, toughness and impact strength of cemented carbide increase while hardness, Young's modulus and thermal conductivity decrease. Fine grain carbides are harder com­pared to coarse grain carbides. Multi-carbide grades increase chemical stability, hardness and hot hardness.
• Since tungsten and cobalt are expensive, some special cemented carbide having predominantly tantalum carbides with Ni and Mo as binder has been developed for auto industry application for finish machining of steels and malleable cast irons. These are some times called cermets. These are relatively brittle and easy to chip. These are relatively cheap and should find wide spread use in future.
• Cemented carbides being expensive are available in insert form in different shapes such as triangle, square, diamond and round. Each of the edge would act as a cutting edge. After the use of a single edge, the tip would be indexed in the cutting tool holder and thus these are called indexable bits. After all the edges are utilised, the tools are thrown out and a new bit is used in the tool holder. Thus these are also called throwaway bits. Because of their brittleness, generally small negative rake angles are used with the bits. However, in view of the developments in the processing methods and composi­tions, a number of grades are being offered by the various manufacturers, which can have a positive rake angle also. 

Coated Carbides

• Since late 1960s thin (about 5 //m) coating of Tin has been used on cemented carbide tools. The life of the coated tools is often two to three times that of the uncoated, also these can be used at higher cutting speeds, thus increasing productivity. These coatings such as titanium carbide, titanium nitride, aluminum oxide, hafnium nitride and hafnium carbide or multiple coatings of the above are depos­ited generally on the carbide tool bits by the chemical vapor deposition (CVD) process. Multiple coating generally provides higher tool life and offers more broad use for machining differing work materials. By virtue of the general applicability of a single grade for a spectrum of machining situations, the shop needs to maintain an inventory of small number of varieties. Coated carbides are being increasingly used in the industry in comparison to the uncoated varieties.

Ceramics

• Ceramics are essentially alumina based high refractory materials introduced specifically for high speed machining of materials, which are difficult to machine such as cast iron. These can withstand very high temperatures, are chemically more stable and have higher wear resistance than the other cutting tool materials. In view of their ability to withstand high temperatures, they can be used for machining at very high speeds of the order of 10 m/s. The main problems of ceramic tools are their low strength, poor thermal characteristics and the tendency to chipping. They are not suitable for intermittent cutting or for low cutting speeds.
• Apart from the pure alumina based ceramics, sometimes other materials such as Titanium carbide are added to enhance the transverse rupture strength. Some yittria may also be added as a sintering agent. Other ceramics of relatively recent origin are alumina-titanium diboride, alumina-zirconia-tungsten compound and silicon-aluminum-oxygen-nitrogen (Si-Al-O-N) complex compound. These are less hard than alumina ceramics, but are tougher.
• Ceramic tools should be used with very high cutting speeds on steels. They are neither suitable for low cutting speeds nor for intermittent cutting. Cutting fluid if applied should be in flooding with copious quantity of fluid to thoroughly wet the entire machining zone, as ceramics have very poor thermal shock resistance. It can also be machined with no coolant. Ceramic tools are used for machining work pieces, which have high hardness such as hard castings, case hardened and hardened steels. Ceramic tools cannot machine some materials such as aluminum, titanium, since they have strong affinity towards them, as a result of which chemical reactions are expected.
• Among other things, some of the vital requirements when machining with ceramics are the follow­ing.

• Use the highest cutting speed recommended and preferably select square or round

1. Inserts with large nose radius.
2. Use rigid machine with high spindle speeds and safe clamping angle.
3. Machine rigid work pieces.
4. Ensure adequate and uninterrupted power supply.
5. Use negative rake angles so that less force is applied directly to the ceramic tip.
6. The overhang of the tool holder should be kept to a minimum, i.e. not more than 1.5 times the shank thickness.
7.  Large nose radius and side cutting edge angle on the ceramic insert to reduce the tendency of chipping.
8. Always take a deeper cut with a light feed rather than a light cut with heavy feed as ceramic tips are capable of cuts as deep as one-half the width of the cutting surface on the insert.
9. Avoid coolants with aluminium oxide based ceramics.
10. Review machining sequence while converting to ceramics and if possible introduce chamfer or reduce feed rate at entry.

• The recommendations and characteristics of various cutting tool materials have been summarized in Table 3. These can act as guidelines, however many of the cutting tool manufacturers such as Sandvik, Widia, Seco, Kennametal provide detailed literature to help in choosing cutting tools. These along with the Metal Cutting Handbook should be used for finalizing the tool material selection.