1、 英文资料 .Sawing Sawing is the parting of material by using metal disks, blades, bands, or abrasive disks as the cutting tools. Sawing a piece from stock for further machining is called cutoff sawing, while shaping of forming a piece is referred to as contour sawing. Machine sawing of metal is performe
2、d by five types of saws or processes: hacksawing, babdsawing, cold sawing, friction sawing, and abrasive sawing. Hacksaws are used principally as cutoff tools. The toothed blade, held in tension, is reciprocated across the workpiece. A vise holds the stock in position. The blade is fed into the work
3、 by gravity or spring. Sometimes a mechanical or hydraulic feed is used. Automatic machines, handling bar-length stock, are used for continuous production. Band saws cut rapidly and are suited for either cutoff or contour sawing. The plane in which the blade operates classifies the machine as being
4、either vertical or horizontal. Band saws are basically a flexible endless band of steel running over pulleys or wheels. The band has teeth on one side and is operated under tension. Guides keep it running true. The frame of the horizontal type is pivoted to allow positioning of the workpiece in the
5、vise. Horizontal machines are used for either straight or angular cuts. A table that supports the workpiece and the wide throat between the upright portions of the blade makes the vertical band saw ideal for contour work. Band saws operating at high speed are frequently used as friction saws. Cold s
6、awing is principally a cutoff operation. The blade is a circular disk with cutting teeth on its periphery. Blades range in size from a few inches to several feet in diameter. The cutting teeth may be cut into the periphery of the disk or they may be inserts of a harder material. The blade moves into
7、 the stock with a positive feed. Stock is positioned manually in some cold-sawing machines, while other models are equipped for automatic cycle sawing. Friction sawing is a rapid process used to cut steel as well as certain plastics. This process is not satisfactory for cast iron and nonferrous meta
8、ls. Cutting is done as the high-speed blade wipes the metal from the kerf after softening it with frictional heat. Circular alloy-steel blades perform cutoff work,thile frictional band saws do both cutoff and contour sawing. Circular blades are frequently cooled by water or air. Circular blades are
9、adcanced into the work,thile thick work-pieces require power-table feed then friction-cut on a band saw. Abrasive sawing is a cutoff process using thin rubber or bakelite bonded abrasive disks. In addition to steel, other materials such as nonferrous metals, ceramics, glass, certain plastics, and ha
10、rd rubber are cut by this method. Cutting is done by the abrasive action of the grit in the disk. Abrasive disks are operated either wet or dry. For heavy cutting a cooling agent is generally used. The workpiece is firmly held while the wheel traverses through it. Machines are made in manually opera
11、ted and automatic models. Drilling Holes are one of the most common features in products manufactured today. There-fore, drilling and other related processes and tools are extremely important. Holes as small as 0.005in.may be drilled using special techniques. On the other hand , holes larger than 2
12、to 221in.in diameter are seldom drilled, because other processes and techniques are less expensive. The twist drill (shown in Fig.12-3) is the most common type of drill. The shank of the drill is held by the machine tool, which in turn imparts an rotary motion. This shank of the drill is held by the
13、 machine tool. Which in turn imparts a rotary motion. This shank may be straight or tapered. The body of the drill is typically made up of two spiral grooves known as flutes, which are defined by a helix angle that is generally about 30but can vary depending on the material properties of the workpie
14、ce. The point of the drill (see Fig.12-3) generally form a 118angle and includes a 10 clearance angle and chisel edge. The chisel edge is flat with a web thickness of approximately 0.015 * drill diameter. This edge can cause problems in hole location owing to its ability to “walk” on a surface befor
15、e engaging the workpiece. In the case of brittle materials, drill point angles of less than 118 are used, while ductile materials use larger points angles and smaller clearance angles. Complex hole configurations may often be called for； these include multiple diameters, chamfers, countersinks, and
16、combinations of these, as illustrated in Fig.12-4. In each of these cases in is possible to make special combination drills that can produce the configurations shown in a single operation. Although expensive, they can be economically justified for sufficient volume. The flat chisel edge, which can “
17、walk” on the surface of the workpiece, and the long , slender shaft and body of the twist drill, which can deflect, make it difficult to machine holes to tight tolerances. A combination center drill and countersink can be used to accurately start a hole, owing to its small web thickness and its tend
18、ency to deflect only very small amounts (because of a relatively large diameter-to-length ratio) . Truing of the hole to make it straight is accomplished by boring. Reaming the hole provides a better finish as well as more accurate sizing. The feed rate of a drill is normally proportional to its dia
19、meter, because it depends on the volume of chips the flutes can handle. However the feed is independent of the cutting speed, which is a function of the tool-work combination. A rule of thumb would give a feed rate as approximately d/65,so that a 3/4-in.-diameter drill would have a feed rate of abou
20、t 0.012 in. /rev. Although the hole wall tends to support the drill when the hole depth exceeds three times the drill diameter, there is a tendency for buckling to occur and the feed rate should be reduced. Most drills are made from high speed steel because of its relatively low cost and ease of man
21、ufacture. Some types of carbide drills are now available commercially. The demands of numerically controlled machine tools have led to the development of drills that will produce pore precise holes and that will originate a hole in line with the centerline of the drill-press spindle. Drills that hav
22、e heavier webs, less stickout, double margins, and are ground with a spiral point help meet these new demands. Reaming Reaming is a machining process for enlarging, smoothing and/ or accurately sizing existing holes by means of means of multiedge fluted cutting tools (reamers) . As the reamers and /
23、 or workpiece is rotated and advanced relative to each other, chips are produced to remove relatively small amounts of material from the hole wall. Reaming may be performed on the same type of machines used for drilling. Accuracy of the hole and quality of finish produced by reaming depends primaril
24、y upon the condition of the starting bole, rigidity of the machine and fixture, correct speeds and feeds, a suitable and properly applied cutting fluid, and precise resharpening of dull tools. Since stock removal is small and must be uniform in reaming , the starting holes (drilled or otherwise prod
25、uced) must have relatively good roundness, straightness, and finish. Reamers tend to follow the existing centerline of the hole being reamed, and in limited instances it may be necessary to bore the holes prior to reaming to maintain required tolerances. With the proper conditions and operating parameters,
