High-speed steel cutting tools

It is difficult to give precise values of rake angles, due to the number of variables encountered during machining. The values in Table 7.1 are offered as a guide for high-speed-steel cutting tools. 

Titanium nitride is the material of choice for the coating of high-speed steel cutting tools. It is usually applied by physical vapor deposition (reactive sputtering or evaporation by electron-beam heating). These processes are preferred over CVD since the deposition temperature is below the autenitizing temperature of the steel and the tool is not dimensionally distorted. 

Table 7.1 Typical rake angles for high-speed-steel cutting tools...

A 300 mm diameter wheel is run at about 2000 rev/min, giving a speed at the diameter of almost 1900 m/min. Compare this with a piece of steel of the same diameter being cut with a high-speed-steel cutting tool on a lathe at 30 m/min. 

High-speed-steel cutting tools in a centre lathe can be used to turn plastics materials. Clearance angles should be increased to around 20 . A rake angle of 0° can be used on the more brittle plastics, while a 15° rake used on softer plastics aids the flow of material over the tool face. 

Tool Steels. Oxidation is a well-established process used for high-speed steel cutting tools. Increases in tool life of up to 100% are achieved, mostly due to a decrease in friction, because of the hard oxide coating and the ability of the porous oxide to entrap lubricant and draw it to the tool/workpiece interface. Steam oxidation of a finished tool is accomplished either by exposing it to steam at a temperature of about 565 °C (1050 °F) or by treating it in liquid sodium hydroxide and sodium nitrate salts at approximately 140 °C (285 °F) for 5 to 20 min. These treatments result in a black oxidized layer that is less than 5 p-m (0.2 mil) thick and will not peel, chip, or crack, even when the tool is bent or cut. Tool life improvements due to steam oxidation are listed in Table 5. 

High Speed Steels. Toward the latter part of the nineteenth century, a new he at-treatment technique for tool steels was developed in the United States (3,17) that enabled increased metal removal rates and cutting speeds. This material was termed high speed steel (HSS) because it nearly doubled the then maximum cutting speeds of carbon—low alloy steels. Cemented carbides and ceramics have since surpassed the cutting speed capabiUties of HSS by 5—15 times. 

It machines in a similar manner to soft copper, and high-speed-steel tools with high cutting speeds are most satisfactory. Trichloroethane is recommended as a cutting medium and the work must be kept well flooded at all... 

Relative Cutting Performance of Coatings for High-speed Steel Tools... 

It is olaimed that this metal is to high-speed steel what high-speed stool is to ordinary oarbon steel that is, it will allow of increasing the rate of cutting on the lathe from 20 per oent. to 50 per cent, and minimises the time consumed m sharpening tools (J. Soc. Ohern. Ind., 1917. 36, 441). 

At the Paris World Fair, the Bethlehem Steel Company presented the first high speed steel (HSS) cutting tools. For several decades after that, steel was the most prominent timgsten consumer. 

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