Papers by Author: Mitsuyoshi Nomura

Abstract: Micro end mills, for example, smaller than 0.5 mm in diameter have low strength and stiffness. They are rather difficult to be re-sharpened by grinding. Therefore they are usually used until their breakage or are exchanged for a new one when the machining results lose quality. In the previous study [, tool life up to breakage was experimentally investigated under various feed rates and some useful information was obtained to predict tool life considering a sort of bending fatigue. For each experiment, a new tool was used to machine slots till it broke due to fatigue and/or wear. In this study, in order to measure tool life based upon another point of reference, the machining accuracies of the above slots were investigated. The main results obtained are as follows: (1) Slot depth first increased due to thermal deformation of the spindle and then decreased due to tool wear, (2) Slot width decreased as the tool wear increased, (3) Slot bottom corner radius increased as the tool wear increased, (4) Burr size increased as the tool wear increased, (5) Surface roughness of the slot bottom seemed to be influenced by feed rate, tool wear and chatter.

Abstract: This paper describes on the influence of thermal behavior of spindle on machining accuracy in micro-endmilling. In order to make clear the reasons why the depth of slot varies during slotting repeated, temperatures around the spindle head and the thermal expansion of spindle axis were investigated. The machine tool used for experiment was a small type of vertical machining center. The spindle used for experiment was a ball bearing spindle which rotated at 10000-40000 min-1. The spindle head made of aluminum alloy had various cooling systems for spindle motor and ball bearings, and the temperature of spindle head changed only a few degrees C. Therefore it seems difficult to assume that the primary cause was thermal deformation of spindle head structure, even though the coefficient of thermal expansion of aluminum alloy is approximately twice of that of steel. Finally the thermal expansion of spindle axis was partly measured and relation between the thermal expansion and the variation of depth of slots was investigated. The main results obtained are as follows, (1) Slotting after a long period warming-up of spindle resulted in a small change of depth of slot, but slotting without any warming-up caused an initial rapid increase and then saturation of depth of slot, (2) Temperature rise of spindle head and adjacent structure seemed out of relation to the variation of depth of slot, (3) The partially measured thermal expansion of spindle as well as temperature of lower flange of spindle showed qualitatively similar change corresponding to depth of slot.

Abstract: To obtain some knowledge to predict the tool life caused by breakage, a series of slotting experiments was performed at various feed rates for high carbon steel by using coated carbide square end mills of 0.5mm in diameter. The main results obtained are as follows, (1) There found a possibility to obtain an optimum feed rate for the maximum slotting distance. (2) Lower feed rate caused chatter but higher feed rate rarely caused chatter. (3) A newly defined cumulative damage seems to be useful in some extent to predict tool life by monitoring cutting force.

Abstract: This study aims to develop an ultrasonically assisted grinding technology for precision internal grinding of a small hole measuring several millimeters in diameter, such as those formed in a fuel injector for an automotive engine. In a previous work, an experimental apparatus mainly composed of an ultrasonic vibration spindle was designed and constructed, and grinding experiments were carried out. The previous investigation found that applying ultrasonic vibration to the wheel decreased the normal and tangential grinding forces, respectively, and improved the surface roughness in surface grinding. The purpose of this paper is to examine the effect of ultrasonic vibration on grinding force and surface roughness in internal grinding when the grain sizes of small cBN grinding wheel are changed. The experimental results indicate that applying ultrasonic vibration to the wheel decreases the normal and tangential grinding forces by more than 83 % and 80 %, respectively, and improves the surface roughness by as much as 62 % while the wheel grain size is changed. In addition, over the range of grinding conditions employed in this paper, the grain size as small as 3 µm can be used in ultrasonically assisted internal grinding.

Abstract: In micro endmilling, because of small uncut chip thickness comparable to the tool edge radius and low rigidity of tool, the cutting process must frequently transit between rubbing/ploughing and cutting, and it may deteriorate the machining stability, surface finish and tool wear. In this report, such unique cutting phenomena are investigated by modeling a mechanism, computer simulations and experiments. As a result, a possibility of the unique cutting phenomena proposed has been certified.

Abstract: In micro drilling, the run-out of drill due to rotational motion error of spindle or eccentric chucking of drill causes machining error and larger radial forces which may break the drill. Furthermore, because the spindle speed is very high, rotating bending fatigue of drill may dominate the tool life. In the previous work, relationship between bending deflection and radial cutting force, and fatigue fracture of drill were investigated theoretically and experimentally. The objective of this paper is to investigate more in detail the relationship between the radial cutting force/deflection of drill and its fatigue life. A series of pseudo rotating bending fatigue tests was performed by using three types of ultrafine grain carbide drill blank with 0.1mm diameter. The main results obtained are as follows, (1) Rotating bending fatigue curve did not depend on cobalt content and tungsten carbide grain size. This differs from the results of conventional cemented carbides. (2) The fatigue fracture surface was dominantly occupied by fast fracture regions.

Abstract: This study aims to develop an ultrasonically assisted grinding technology for precision internal grinding of a small hole measuring several millimeters in diameter, such as those formed in a fuel injector for an automotive engine. In a previous work, an experimental apparatus mainly composed of an ultrasonic vibration spindle was designed and constructed, and grinding experiments were carried out. The purpose of this paper is to examine the effect of ultrasonic vibration on grinding force and surface roughness when the grain size and concentration of small cBN grinding wheel are changed. The experimental results indicate that applying ultrasonic vibration to the wheel decreases the normal and tangential grinding forces by more than 50% and 78%, respectively, and improves the surface roughness by as much as 10% when the wheel grain size and concentration are changed. In addition, over the range of grinding conditions employed in this paper, the grain size as small as 5μm can be used in ultrasonically assisted grinding.

Abstract: This paper describes an experimental investigation of the effects of ultrasonic vibration in the truing and dressing of the small CBN grinding wheel used for the internal ultrasonic grinding of small holes. In the precision machining of small holes measuring several millimeters in diameter, improvement in the wheel truing accuracy is significantly levels off when using a single diamond dresser or a rotary GC wheel dresser. In the present work, a new truing and dressing technique was proposed, by which the grinding wheel is ultrasonically vibrated in its axial direction during the truing operation using a rotary GC cup dresser. In order to validate the proposed new technique, experiments were carried out. During experimental operations, the GC cup wheel was traversed along the vitrified CBN grinding wheel axis with an in-feed motion toward the grinding wheel in the wheel radial direction. The influences of the truing parameters on the truing force, the run-out of grinding wheel and the grinding wheel surface properties were investigated. As a result, it was found that applying ultrasonic vibration to the grinding wheel decreased the truing force by more than 22%, and the run-out of grinding wheel decreased from an original value of 150µm to a final one of less than 0.8µm, while that obtained without ultrasonic vibration was more than 1.1µm. As well, better surface properties of the grinding wheel were obtained by the application of ultrasonic vibration.

Abstract: This paper discusses the mechanism behind the grinding force decrease associated with ultrasonication of the grinding wheel in constant-depth-of-cut ultrasonically assisted grinding (UAG). By introducing a grinding model describing the cutting trace of an abrasive grain, an equation relating the grinding force decrease to such process parameters as the amplitude and frequency of vibration and the grinding wheel speed, is established. Experiments are conducted to confirm the theoretical prediction. Theoretical and empirical results both indicate that the decrease in grinding force is due to the grinding chips becoming smaller and fracturing more easily under ultrasonication. The results also suggest that the grinding force decrease is greater at higher vibration amplitudes and at lower grinding wheel speeds.