机械专业英语董建国第11章.ppt

Unit 11 TEXT READING MATERIAL Unit 11 TEXT Drills and Drilling Machines The twist drill is a very efficient tool. It is generally formed by forging and twisting grooves in a flat strip of steel or by milling a cylindrical piece of steel, high-speed steel being commonly used. High-speed steel costs more but tools made of it withstand heat much better than those made of ordinary tool steel. The twist drill may be divided into three principal parts: body, shank, and point. The flutes are the spiral grooves that are formed on the side of a drill, drills being made with two, three, or four flutes. Those having three or four flutes are used for following smaller drills or for enlarging holes already drilled, and are not suited for drilling into solid stock. Fig.11-1 The twist drill Spiral flutes have four main advantages: (1) ?They give the correct rake to the lip of a drill. (2) ?They cause the chip to curl so tightly that it occupies the minimum amount of space. (3) ?They form channels through which chips escape from the hole. (4) ?They allow the lubricant to flow easily down to the cutting edge. The margin is the narrow strip on the cutting edge of the flute. It is practically the full diameter of the drill and extends the entire length of the flute, its surface being a part of a cylinder. The portion of the body next to the margin is of less diameter than the margin. This lessened diameter, called body clearance, reduces the friction between the drill and the walls of the hole, while the margin insures the hole being of accurate size. The shank is the end of the drill which fits into the socket, spindle, or chuck of the drill press. The tang is usually found only on tapered shank tools. The drilling machine is the second oldest machine tools, having been invented shortly after the lathe, and is one of the most common and useful machines. The drilling machines may be classified into three general types: vertical