Normally used microtool types for micromilling operations are end-mill, ball-nose, drills and engraving tools. These tools are usually made of tungsten carbide, and also of diamond material when not machining ferrous workpieces.

One of the main limiting factors for the miniaturization of the cutting process is the tool due to the reduced stiffness of the tiny sizes. Common tool wear in conventional milling cannot be used as an end tool life criterion in micromilling as premature tool failures occur frequently. Various authors [46-47]state that commonly observed tool breakages may be due to the following mechanisms:

·         Excessive stress-related breakage: It will occur very quickly if the cutting force increases beyond the strength of the tool. The cutting force might increase for the following two reasons: first, the cutting edge might lose its sharpness or the cutting edge is partially damaged. And second, deposition of chips fills the microtools tiny grooves producing chip clogging. High Speed Steel (HSS) tools tolerate such chip clogging better than carbide tools since they are less rigid. However, it is almost impossible to predict chip clogging ahead of time. The workpiece starts to push the shaft of the tool and it deflects, increasing the static component of the feed direction force [46];

·         Fatigue-related breakage: It may happen if the cutting force and the stress increase as a result of tool wear, and then stay at that level for an extended period of time. The stress on the shaft will change repeatedly while it is rotating [46];

·         Increase in the specific energy: Tool failures occur due to the substantial increase in the specific energy required as the chip thickness decreases. This means that in the case of micromachining, as the chip gets thinner with smaller depths of cut, the microtool tip will be subject to greater resistance when compared with conventional machining [47].

Furthermore, tool run-out is another remarkable problem since it creates drastic changes in the cutting force profile. Tool run-out is caused by a misalignment of the axis of symmetry between the tool and the tool holder or spindle [48]. Due to that, it is quite common to see that only one cutting edge of a two-flute micro end mill performs the machining operations involving an increase in the force variation, and as a consequence, the tool wears out faster raising the tool failure probability [49] (see Fig. 3.10).

This run-out may create drastic force variations in the cutting forces since some cutting edges of the microtool perform more than the others during the machining operations.

Various partially conflictive requirements need to be met with in order to produce precision components. Depending on the required surface properties and workpiece material cemented carbide tools (CW) or diamond cutting tools are used (scarcely high-speed steel tools). Traditionally, diamond cutting tools have been used for ultra-precision machining operations, but because of the high demand for machining ferrous materials [51], carbide tools are considered better suited to achieve these ends.

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