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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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Many of the tool materials discussed in section 2.1 are very expensive, many are also brittle; for these reasons alone it is impractical to make the entire tool out of the cutting tool material. Many turning tools and milling cutters, particularly the larger sizes, consist of a plain Carbon Steel shank or body with H.S.S. or Carbide tips brazed or welded to them (see Fig 2.2.1.) Fig 2.2.1.
Such tools require removal from the machine for re-sharpening as the cutting edge dulls and wears, with consequent re-setting wasting valuable production time, just as if the tool were made from solid. A fairly recent development (in the last twenty years or so) has been the introduction of inserted tip tools or indexable insert tools (also known as throw away tips). On an insert tool the shank (usually a tough H.C. Steel) has a mechanical means of clamping an insert made of the cutting tool material – usually Carbide or Ceramic. Fig 2.2.2. shows two methods of clamping commonly used. In general it is desirable all clamping components are held captive to prevent loss, in fact many insert holders these days rely only on tension in the toolholder and cutting forces driving the insert tighter into the toolholder (see Fig 2.2.3.)
Fig 2.2.2.
Fig 2.2.3.
When the tip becomes worn and needs replacing it can be easily replaced without removing the tool from the machine – improving productivity. When using qualified tooling the tips are guaranteed to repeat within certain tolerances after changing or indexing. Fig 2.2.4.
Fig 2.2.4. illustrates how using insert tools with negative rake (a) such as used for machining hard, brittle materials can result in twice the amount of usable cutting edges than is possible when using positive rake tools.
Inserts are not limited to turning tools, many face mills and larger diameter end mills, even drills, utilise this principle. |
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