1) Machining operations - cutting tool materials (1)
Engineering - Machinery ·The recommended cutting feeds and speeds in the accompanying tables are given for high-speed steel, coated and uncoated carbides, ceramics, cermets, and polycrystalline diamonds. More data are available for HSS and carbides because these materials are the most commonly used. Other materials that are used to make cutting tools are cemented oxides or ceramics, cermets, cast nonferrous alloys (Stellite), singlecrystal diamonds, polycrystalline diamonds, and cubic boron nitride.
| Carbon Tool Steel | High-Speed Steel | ||||
|---|---|---|---|---|---|
| make the less expensive drills, taps, and reamers | 63≤$R_c$1≤65 for ordinary high-speed steels. | ||||
| seldom used to make single-point cutting tools | up to 70 $R_c$ for the so-called super high-speed steels. | ||||
| some have a small amount of vanadium and chromium added to improve their hardening quality | They can retain sufficient hardness at temperatures up to 1,000 to 1,100°F to enable them to cut at cutting speeds that will generate these tool temperatures, and they will return to their original hardness when cooled to room temperature. | ||||
| The cutting speed to use for plain carbon tool steel should be approximately one-half of the recommended speed for high-speed steel | High-speed steels can be made soft by annealing so that they can be machined into complex cutting tools such as drills, reamers, and milling cutters and then hardened. | ||||
2) : This designates a number of steels having several properties that enhance their value as cutting tool material. They can be hardened to a high initial or room temperature hardness ranging from 63 $R_c$1 to 65 $R_c$ for ordinary high-speed steels and up to 70 $R_c$ for the so-called super high-speed steels. They can retain sufficient hardness at temperatures up to 1,000 to 1,100°F to enable them to cut at cutting speeds that will generate these tool temperatures, and they will return to their original hardness when cooled to room temperature. They harden very deeply, enabling high-speed steels to be ground to the tool shape from solid stock and to be reground many times without sacrificing hardness at the cutting edge. High-speed steels can be made soft by annealing so that they can be machined into complex cutting tools such as drills, reamers, and milling cutters and then hardened.
The principal alloying elements of high-speed steels are tungsten (W), molybdenum (Mo), chromium (Cr), vanadium (V), together with carbon (C). There are a number of grades of high-speed steel that are divided into two types: tungsten high-speed steels and molybdenum high-speed steels. Tungsten high-speed steels are designated by the prefix T before the number that designates the grade. Molybdenum high-speed steels are designated by the prefix letter M. There is little performance difference between comparable grades of tungsten or molybdenum high-speed steel.
The addition of 5 to 12 per cent cobalt to high-speed steel increases its hardness at the temperatures encountered in cutting, thereby improving its wear resistance and cutting efficiency. Cobalt slightly increases the brittleness of high-speed steel, making it susceptible to chipping at the cutting edge. For this reason, cobalt high-speed steels are primarily made into single-point cutting tools that are used to take heavy roughing cuts in abrasive materials and through rough abrasive surface scales.
The M40 series and T15 are a group of high-hardness or so-called super high-speed steels that can be hardened to 70 $R_c$; however, they tend to be brittle and difficult to grind. For cutting applications, they are usually heat treated to 67–68 $R_c$ to reduce their brittleness and tendency to chip. The M40 series is appreciably easier to grind than T15. They are recommended for machining tough die steels and other difficult-to-cut materials; they are not recommended for applications where conventional high-speed steels perform well. Highspeed steels made by the powder-metallurgy process are tougher and have an improved grind ability when compared with similar grades made by the customary process. Tools made of these steels can be hardened about 1 $R_c$ higher than comparable high-speed steels made by the customary process without a sacrifice in toughness. They are particularly useful in applications involving intermittent cutting and where tool life is limited by chipping. All these steels augment rather than replace the conventional high-speed steels.
3) Cemented Carbides: They are also called sintered carbides or simply carbides. They are harder than high-speed steels and have excellent wear resistance. Information on cemented carbides and other hard metal tools is included in the section CEMENTED CARBIDES.
Cemented carbides retain a very high degree of hardness at temperatures up to 1400°F and even higher; therefore, very fast cutting speeds can be used. When used at fast cutting speeds, they produce good surface finishes on the workpiece. Carbides are more brittle than high-speed steel and, therefore, must be used with more care.
Hundreds of grades of carbides are available and attempts to classify these grades by area of application have not been entirely successful.
There are four distinct types of carbides: 1) straight tungsten carbides; 2) crater-resistant carbides; 3) titanium carbides; and 4) coated carbides.
This is not the completed documents.
- References
Erik oberg, 28th ed., Machinery’s Handbook