Grinders for high efficiency deep grinding (HEDG) and superabrasive high-speed grinding with plated wheels have been influenced strongly by machining center design. In many cases, the grinders are, in fact, a multipurpose machining/grinding/drilling center. The wheels are designed around sizes of 125 mm up to 300 mm running at up to 120 m/s. Numerous machine tool companies fall into this category with either a grinding background (e. g., Campbell, Edgetek, Gendron, and Huffman Magerle) or a machining background (e. g., EMAG, Makino, and Mori Seiki). The spindle requirement is, therefore, both speed and flexibility to operate over a range of speeds. Most machine builders use liquid-cooled, ceramic hybrid bearings with ac servomotors operating at speeds 5,000 rpm up to 20,000 rpm. Use of ceramic hybrid bearings allows for larger diameter shafts of 100 mm or greater and, therefore, enhanced stiffness. A similar approach has been taken by the manufacturers of high-production tool grinders that use superabrasive wheel packs in the 100- to 300-mm-diameter range.
15.13.3 Spindle Cooling for High-Speed Grinding
It is interesting to note the attention paid by a number of the machine tool builders to how the spindle is cooled. One concern with a demand for flexibility is that to achieve the necessary preload at high rotational speeds may mean a loss of preload at slower speeds. Makino [n. d.] reduces this by injecting chilled oil through the center of the shaft where centrifugal force then injects the oil from the inner to the outer bearing race. The oil is then injected through the housing setting up a temperature gradient from the inner to the outer race and between the bearing and the housing. This temperature difference, combined with the efficiency of core cooling, provides increased thermal stability at higher speeds and allows the bearings to be amply preloaded at rest.
15.13.4 Spindle Bearings for Very Small High-Precision High-Speed Wheels
It is small-diameter wheels that have pushed the limits of spindle design. The fuel injection industry, for example, especially for diesel engines, requires bores as small as 2 mm to be ground to extremely high precision. Current spindle technology is limited to about 200,000 rpm, which is only 21 m/s for a 2-mm wheel. The greatest problems at these speeds are wheel and arbor imbalance, resonance problems, and lack of stiffness of the arbor. Spindles running up to 150,000 rpm are more typical of production currently and these are dedicated to a small range of wheel diameters.
15.13.5 Active Magnetic Bearings for High-Speed Wheels
One possible solution to improve stiffness and flexibility in the use of small — and medium-sized wheels is the development of active magnetic bearing (AMB) spindles. AMB spindles have been available since the 1980s [Moritomo and Ota 1986] and a few are in use, certainly in the United States, although their acceptance has been slow.
AMB spindles use a magnetic field to support the rotor. Positional sensors and closed-loop monitoring to a controller constantly controls its position in the field. Originally developed for defense and space applications, AMB spindles have been used at speeds up to 1,000,000 rpm in, for example, centrifuges. For spindle applications, the rotor is levitated by two radial bearings and at least one thrust bearing [Wang et al. 2001]. AMBs are offered at speeds up to 200,000 rpm [IBAG 1996]. Catch bearings are provided in the event of a power loss. One of the big attractions of these spindles is the ability to provide a high power output over a very large range of spindle speeds, which in turn allows flexibility in wheel diameters when grinding a family of differentsized parts. As with any new technology there is a price barrier and learning curve to be overcome before AMB spindles become generally accepted. Certainly the avoidance of oil mist lubrication with its associated environmental issues will be an attraction.