tailieunhanh - Metal Machining Episode 6

Tham khảo tài liệu 'metal machining episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Forces in machining 143 Fig. Octagonal ring and parallel beam dynamometer designs a Octagonal ring type tool dynanometer b parallel beam type tool dynanometer - a A Ebt2 b B Ebt2 . The manufacture of the ring outer surface as an octagon rather than a cylinder is just a practical matter. The need to generate detectable strain imposes a maximum allowable stiffness on a dynamometer. This in turn with the mass of the dynamometer depending on its size or on the mass supported on it imposes a maximum natural frequency. Simple beam 144 Experimental methods Fig. The loading of a ring by radial and tangential forces dynamometers suitable for measuring forces in turning from 10 N to 10 kN can be designed with natural frequencies of a few kHz. The ring and the strut types of dynamometer tend to have lower values of several hundred Hz Shaw 1984 Chapter 7 . These frequencies can be increased tenfold if semiconductor strain gauges Ks from 100 to 200 are used instead of wire gauges. However semiconductor gauges have much larger drift problems than wire gauges. They are used only in very special cases an example will be given in Section . An alternative is to use piezoelectric force sensors. Piezoelectric dynamometers For certain materials such as single crystals of quartz Rochelle salt and barium titanate a separation of charge takes place when they are subjected to mechanical force. This is the piezoelectric effect. Figure shows the principle of how it is used to create a three-axis force dynamometer. Each force component is detected by a separate crystal oriented relative to the force in its piezoelectric sensitive direction. Quartz is usually chosen as the piezoelectric material because of its good dynamic low loss mechanical properties. Its piezoelectric constant is only 2 X 10-12 coulombs per Newton. A charge amplifier is therefore necessary to create a useful output. Because the electrical impedance of quartz is high the amplifier must .