【正文】 effective damping capacity is requiredto achieve high accuracy and highefficiency cuttingoperations, since even a small vibration generated in amachine tool seriously affects machining accuracy.Some papers have reported damping capacityimprovement, including those dealing with steel–resinconcrete posite structure [1], overlaid with fiberreinforced plastics [2] which are a laminated positeof steel and high damping capacity material, and sandwich beam [3] in which a viscoelastic core is insertedbetween two steel plates. Moreover, various techniqueswere devised. In the nonobstructive particle damping(NOPD) technique [4], smalldiameter holes are drilledat appropriate locations inside the vibrating structuresand filled to the optimum level with a layer of particles,and the maximum damping capacity is attained for thedesired mode (or modes). An impact damper with granular materials [5] packed and movable in a container fixed* Corresponding author. Fax: +81565365924.Email address: (Y. Wakasawa).08906955/02/$ see front matter ? 2020 Elsevier Science Ltd. All rights reserved.PII: S08906955(01)001432to the primary vibrating system and a powder positedamper [6] packed with powder in a steel container witha high packing ratio, have also been reported. However,when packed materials were inserted in the structurecavity, the effects of impulsive excitation magnitude, thekinds of packed material, cavity shape etc. on the damping capacity were not made sufficiently clear in suchreports.In a precision machine tool, measures must be considered against the impulsive force from surroundingmachines in the light of many uncertain and unknownfactors. Many practical vibration isolation systems [7]for machinery and tools and for floor and building havebeen developed. However, when these plicated isolation systems are applied to machine tools, machiningcosts may be increased. If the damping capacity of themachine structure can be improved without anyadditional apparatus, it is more practical for machinetool operation.In the present study, the damping characteristics ofmachine structures packed with glass balls were investigated experimentally. Experiments were carried out forvarious impulsive forces, sizes of model structure andpacked ball. When balls were closely packed by excitation, the packing ratio of the balls was determined by468 Y. Wakasawa et al. / International Journal of Machine Tools amp。 Manufacture 42 (2020) 467–472When balls are packed by excitation in the cavity ofthe model structure, the maximum packing ratio and thepacked arrangement depend on the ball size. This dependency of the packing ratio affects the damping capacityetc. In this experiment, when the ball size is less than 10mm and the maximum packing ratio is high, the naturalfrequency bees low as a result that the glass ballsand square pipe are united so the equivalent massincreases. Thus, when small balls are packed, it is considered that the friction between the balls and the innersurface of the square pipe is the main factor in the damping capacity generation. On the other hand, when theball size is larger than 12 mm, the movements of ballsin a cavity have a considerable degree of freedom. Thepacked balls in vibration repeat the collisions with aninner surface of the square pipe, and are almost in asuspended state. For this reason, it is considered thatmass of balls contributes little to the equivalent mass,and the natural frequency does not bee low. Whenlarge balls are packed, the energy dissipation by the collision between balls and an inner surface of the squarepipe is considered to be the main factor in dampingcapacity improvement. Both collisions are inelastic.. Effect of repulsion coefficient on dampingcapacityFrom the previous considerations, it can be forecastthat in the damping capacity improvement by packedballs the collision characteristics between balls and theinner surface of the square pipe (., the repulsioncoefficient) also affects the damping capacity improvement. Thus, Fig. 10 shows the result of repulsion coefficient measurement which was obtained by droppingglass balls of different size on the center of squ