14.8in Cement Mortar Trowel Wall Concrete Tool Hard

In 1895, the German overtone made the world's first DC drill. The housing is made of cast iron and can drill a 4mm hole in the steel plate. Then a three-phase power frequency (50Hz) electric drill appeared, but the motor speed failed to exceed 3000r/min. In 1914, electric tools driven by single-phase series excitation motors appeared, and the motor speed reached more than 10000r/min. In 1927, an intermediate frequency power tool with a power supply frequency of 150-200Hz appeared. It not only has the advantages of high speed of a single-phase series motor, but also has the advantages of a simple and reliable structure of a three-phase power frequency motor, but it needs to be powered by an intermediate frequency current. , use is restricted. In the 1960s, with the development of battery manufacturing technology, battery-type power tools without power cords using nickel-cadmium batteries as power sources appeared. But at that time, the development was slow due to the high price. By the mid-to-late 1970s, due to lower battery prices and shorter charging times, this type of power tool was widely used in Europe, America and Japan. The power tools were initially made of cast iron, and later changed to aluminum alloys. In the 1960s, thermoplastic engineering plastics were used in power tools, and double insulation of power tools was realized to ensure the safety of power tools. Due to the development of electronic technology, electronic speed-regulated power tools also appeared in the 1960s. This kind of power tool uses thyristor and other components to form electronic circuits, and adjusts the speed by the depth of the switch button, so that the power tool can be used according to the different objects to be processed (such as different materials, hole diameter, etc. ), select a different speed. [1]