To the onlooker one of the most startling aspects of computer numerically controlled machining is the rapid metal-removal rates used. That there are cutting tools capable of withstanding such treatment can seem quite incredible. Add to this indexing times of less than one second and automatic tool changing providing a “chip-to-chip” time of around five seconds and it is easy to understand why many production engineers consider tooling to be the most fascinating aspect of computer numerically controlled machining.
Materials for Cutting Tools
Although high-speed steel (HSS) is used for small-diameter drills, taps, reamers, end mills, and spot drills, the bulk of tooling for computer numerically controlled machining involves the use of cemented carbide.
The physical properties necessary in a cutting tool are hardness at the metal-cutting temperature, which can be as high as 600°C, and toughness. High-speed steel is tougher than cemented carbide but not as hard and, therefore, cannot be used at such high rates of metal removal. On the other hand, the lack of toughness of cemented carbide presents problems, and this has meant that a tremendous amount of research has gone into developing carbide grades that, when adequately supported, are able to meet the requirements of modem machining techniques. It is only necessary to observe a computer numerically controlled machine in action to see how successful this research has been.
The hardness of cemented carbide is almost equal to that of diamond. It derives this hardness from its main constituent, tungsten carbide. In its pure form tungsten carbide is too brittle to be used as a cutting tool, so it is pulverised and mixed with cobalt.
The mixture of tungsten carbide and cobalt powder is pressed into the required shape and then sintered. The cobalt melts and binds the tungsten carbide grains into a dense, nonporous structure.
In addition to tungsten carbide, other hard materials such as titanium and tantalum carbides are used, and by providing tungsten carbide tools with a thin layer of titanium carbide, resistance to wear and useful life are increased by up to five times.