Technology suitable for both serial and small quantity production
Microgrinding is a material removal technique by means of mechanical force used for machining pins and grooves with small dimensions and for obtaining flat surfaces with very fine finishing. The main reason for this is that the chip size in microgrinding is very small because cutting is realized by means of micrograins.
Due to the very small obtainable depth of cut, microgrinding is particularly advantageous for brittle materials which can be mirror finished. The tool is generally in the form of a wheel, constituted of an abrasive and a matrix. In order to accomplish smooth surfaces of less than 10 nm peak to valley depth, the grain depth of cut has to be kept to less then 100 nm [4]. Two possible approaches to achieve a very small grain depth of cut are: maintaining a small wheel depth of cut using coarse abrasives or using grinding wheels consisting of very fine abrasives. The first method requires high precision machine tools and an accurate dressing of the grinding wheel. Ultra-precision 4-axes grinding machines with a maximum resolution of 1 nm for 3-axis linear feeding and a prototype ultrastiff machine tool, Tetraform C, which produced a repeatable surface finish of less than 10 nm using 76 µm CBN grit on hardened bearing steel have been developed for this first method [5]. Special techniques for wheel dressing, as electrolytic in-process dressing (ELID), have been developed for controlling and maintaining the desired protrusion of the grains on the surface of the wheel., enabling mirror finishing of silicon wafers and other materials as ceramics, ferrite, glass, steel, etc. [6].
Concerning the second method, abrasive pellets composed of ultrafine silica particles of 10 to 20 nm have been obtained applying electrophoretic deposition [4]. A particular technique is referred to as Nanogrinding, where diamond abrasive grains are embedded in a soft tin plate, resulting in a very minimal grain protrusion. The average surface roughness obtained with this technique was Ra = 1.14 nm for Al2O3-TiC and Ra = 0.79 nm for SiC [7].
The recent development in the fabrication technology of grinding tools has led to the application of grinding in the fabrication of 2D or 3D microcavities in a system similar to mechanical or EDM/milling. In this case a tool with a microsized tip is used. Because of the considerable grinding force, the aspect ratio of the tool has to be low. Therefore, deep microholes or deep, narrow cavities are not promising targets of microgrinding. The machinable shapes are almost the same as those in milling by mechanical cutting.
Among the limitations of microgrinding is the minimum obtainable tip radius of the tool which is strongly influenced by the grit size [5]. It determines the rounding radius when machining concave shapes as V grooves. Ultrasonic vibrations have been applied to grinding in order to reduce the grinding force. This has led to the production of pins in cemented carbides of 11 µm in diameter and 160 µm in length as well as micro flat drills of 17 µm in diameter and 100 µm in length. Microgrinding is also applied to surface finish thermally sprayed hard coatings [8]. However, considerably high values of compressive biaxial subsurface stresses are generated, with a large gradient in the thickness direction. One of the technological problems is the fact that the tool must be made up of an abrasive and a matrix. When the tool size is very small, the grain size cannot be ignored; this leads to certain difficulties in forming the precise shape of the grinding tool.
[4] Ikeno J. et al., 1990, Nanometer grinding using ultrafine abrasive pellets – manufacture of pellets applying electrophoretic deposition, Annals of the CIRP, 39/1: 341-344
[5] Ohmori H. et al., 2001, Ultraprecision microgrinding of germanium immersion grating element for mild-infrared super dispersion spectrograph, Annals of the CIRP, 50/1: 221-224
[6] Ohmori H., Nakagawa T., 1990, Mirror surface grinding of silicon wafers with electrolytic inprocess dressing, Annals of the CIRP, 39/1: 329-332
[7] Gatzen H., Maetzig C., 1997, Nanogrinding Precision Engineering 21: 134-139
[8] Zhang B., Liu X. B., Brown C., 2002, Microgrinding of nanostructured material coatings, Annals of the CIRP , 51/1: 251-254