Cutting Tool Materials Part 3 pps

Moreover, ceramic tools at this juncture, were only re￾ally employed for turning operations and in particular,

in ‘stable machining’, where interrupted/intermittent

cutting operations did not occur.

With the recent advances in powerful and very

rigid CNC machine tools, this has opened-up the pos￾sibility of utilising ceramic tooling, either in a purely

sintered monolithic tooling insert, or more recently as

a multi-coated variant – more will be said on this topic

shortly. Returning to the monolithic ceramic cutting

tool materials, they have normally been available in

three distinct grades, which will now be mentioned.

These cutting inserts consist of:

• ‘Pure’ ceramic – this is the traditional tooling in￾sert material, consisting of aluminium oxide. The

alumina is white in colour and is produced by cold

pressing powder in the desired insert geometry

dies

, with subsequent sintering, the fused alumina

particulates are sintered together, thereby signifi￾cantly decreasing porosity. These ceramics, have

been known in the past as ‘pure oxide’, or ‘cold￾pressed’ ceramics. The major disadvantage of such

ceramics is their low thermal conductivity, making

them highly susceptible to thermal shock (i.e. the

hot and cold thermal cycles that can occur when in￾terrupted cutting takes place). These thermal shock

 These consolidated cutting inserts produced in compound,

or ‘floating’ die sets from the admixed powders, are termed

‘green compacts’ and are friable, that is having very limited

mechanical strength and must be gently handled, prior to sin￾tering – thereafter the desired mechanical strength occurs.

Figure 11. A typical ‘super-glide’ coating of molybdenum disulfide (MoS2) applied to a hard-coating on a tool’s sub￾strate – weak bonds between crystal layers allow easy movement of the planes. [Courtesy of Guhring]

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Cutting Tool Materials 21