Error budget and stiffness chain assessment in a micromilling machine equipped with tools less than 0.3 mm in diameter

The current trend towards product miniaturisation is leading to a major increase in microtechnologies, including micromilling. Although this technique is highly similar to conventional scale milling, the great reduction in dimensions (a scale of around 40/1) means that cutting phenomena and mechanisms appear that are hardly ever encountered on a conventional scale. This scale reduction can be seen in some of the usual parameters of micromilling: feed per tooth less than 1 μm, depth of cut 2–15 μm, spindle rotational speed more than 50,000 rpm and tool diameter less than 0.3 mm. The milling machine itself must also be specific to this application and designed and built to ultraprecision requirements, with positioning accuracies on the order of 0.1 μm. The first stage of the work presented here was to establish the overall error budget for a micromilling machine with tools less than 0.3 mm in diameter. Most errors were found to originate from collet angular deformation and tool deflection, so this study concentrates on these two areas. First the stiffness chain of the micromilling machine was obtained experimentally by defining the stiffness levels of the machine, the spindle, the tool-holder and the tool itself. These were measured using an experimental approach, but taking the design levels of each element in the system into account. Tests became complicated by the difficulty of handling the small sizes, forces and displacements found in micromilling in experiments. This paper sets out the results of all the tests performed. The conclusion reached is that the most flexible element is the tool. Its compliance (reciprocal of stiffness) in the face of cutting forces can be obtained, but its attachment to the tool-holder collet must also be taken into account. Finally, results of several ball-end milling tests on inclined planes (15°, 30°, 45°) are presented.