【正文】 val of time known as the period. The motion pleted in any one period of time is called a cycle. The number of cycles per unit of time is called the frequency. The maximum displacement of any part of the system from its staticequilibrium position is the amplitude of the vibration of that partthe total travel being twice the amplitude. Thus, “amplitude” is not synonymous with “displacement” but is the maximum value of the displacement from the staticequilibrium position. Natural and Forced Vibration A natural vibration occurs without any external force except gravity, and normally arises when an elastic system is displaced from a position of stable equilibrium and released, i. e. natural vibration occurs under the action of restoring forces inherent in an elastic system, and natural frequency is a property of he system. A forced vibration takes place under the excitation of an external force (or externally applied oscillatory disturbance) which is usually a function of time, e. g. in unbalanced rotating parts, imperfections in manufacture of gears and drives. The frequency of forced vibration is that of the exciting or impressed force, i. e. the forcing frequency is an arbitrary quantity independent of the natural frequency of the system. Resonance Resonance describes the condition of maximum amplitude. It occurs when the frequency of an impressed force coincides with, or is near to a natural frequency of the system. In this critical condition, dangerously large amplitudes and stresses may occur in mechanical systems but, electrically, radio and television receivers are designed to respond to resonant frequencies. The calculation or estimation of natural frequencies is, therefore, of great importance in all types of vibrating and oscillating systems. When resonance occurs in rotating shafts and spindles, the speed of rotation is known as the critical speed. Hence, the prediction and correction or avoidance3 of a resonant condition in mechanisms is of vital importance since, in the absence of damping or other amplitudelimiting devices, resonance is the condition at which a system gives an infinite response to a finite excitation. Damping Damping is the dissipation of energy from a vibrating system, and thus prevents excessive response. It is observed that a natural vibration diminishes in amplitude with time and, hence, eventually ceases owing to some restraining or damping influence. Thus if a vibration is to be sustained, the energy dissipated by damping must be replaced from an external source. The dissipation is related in some way to the relative motion between the ponents or elements of the system, and is caused by frictional resistance of some sort, . in structures, internal friction in material, and external friction caused by air or fluid resistance called “viscous” damping if the drag force is assumed proportional to the relative velocity between moving parts. One device assumed to give viscous damping is the “dashpot” which is a loosely fitting piston in a cylinder so that fluid can flow from one side of the piston to the other through the annular clearance space. A dashpot cannot store energy but can only dissipate it. Basic Machining Operations and Machine Tools Basic Machining Operations Machine tools have evolved from the early footpowered lathes of the Egyptians and John Wilkinson’s boring mill. They are designed to provide rigid support for both the workpiece and the cutting tool and can precisely control their relative positions and the velocity of the tool with respect to the workpiece. Basically, in metal cutting, a sharpened wedgeshaped tool removes a rather narrow strip of metal from the surface of a ductile workpiece in the form of a severely deformed chip. The chip is a waste product that is siderably shorter than the workpiece from which it came but woth a corresponding increase in thickness of the uncut chip. The geometrical shape of the machine surface depedns on the shape of the tool and its path during the machinig operation. Most machining operations produce parts of differing geometry. If a rough cylindrical workpiece revolves about a central axis and the tool perates beneath its surface and travels parallel to the center of rotation, a surface of revolution is producedand the operation is called turning. If a hollow tube is machined on the inside in a similar manner, the operation is called boring. Producing an external conical surface of uniformly varying diameter is called taper turning. If the tool point travels in a path of varying radius,a contoured surface like that of a bowling pin a can be produced。翻譯： 英文原文 Definitions and Terminology of Vibration vibration All mattersolid, liquid and gaseo