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译文题目： Drilling and Milling，Jigs and Fixtures 钻孔和铣削，钻模和夹具 Drilling and Drills Drilling involves producing through or blind holes in a workpiece by forcing a tool, which rotates around its axis, against the workpiece.Consequently, the range of cutting from that axis of rotation is equal to the radius of the required hole. In practice, two symmetrical cutting edges that rotate about the same axis are employed. Drilling operations can be carried out by using either hand drills or drilling machines. The latter differ in size and construction. Nevertheless, the tool always rotates around its axis while the workpiece is kept firmly fixed. This is contrary to drilling on a lathe.Cutting Tool for Drilling Operation In drilling operations, a cylindrical rotary-end cutting tool, called a drill, is employed. The drill can have either one or more cutting edges and corresponding flutes, which can be straight or helical.The function of the flutes is to provide outlet passages for the chips generated during the drilling operation and also to allow lubricants and coolants to reach the cutting edges and the surface being machined. Following is a survey of the commonly used drills. Twist drill. The twist drill is the most common type of drill. It has two cutting edges and two helical flutes that continue over the length of the drill body, as shown in Fig.12.1. The drill also consists of a neck and a shank that can be either straight or tapered. In the latter case, the shank is fitted by the wedge action into the tapered socket of the spindle and has a tang, which goes into a slot in the spindle socket, thus acting as a solid means for transmitting rotation. On the other hand, straight-shank drills are held in a drill chuck that is, in turn, fitted into the spindle socket in the same way as tapered shank drills. As can be seen in Fig.12.1, the two cutting edges are referred to as the lips, and are connected together by a wedge, which is a chisel-like edge. The twist drill also has two margins, which enable proper guidance and locating of the drill while it is in operation. The tool point angle (TPA) is formed by the two lips and is chosen based on the properties of the material to be cut. The usual TPA for commercial drills is 118, which is appropriate for drilling low-carbon steels and cast irons.For harder and tougher metals, such as hardened steel, brass and bronze, larger TPAs (130or 140) give better performance. The helix angle of the flutes of the commonly used twist drills ranges between 24and 30. When drilling copper or soft plastics, higher values for the helix angle are recommended (between 35and 45). Twist drills are usually made of high-speed steel, although carbide-tipped drills are also available. The sizes of twist drills used in industrial practice range from 0.01 up to 3.25 in. (i. e., 0.25 up to 80 mm). Core drills. A core drill consists of the chamfer, body, neck, and shank, as shown in Fig.12.2. This type of drill may have either three or four flutes and equal number of margins, which ensure superior guidance, thus resulting in high machining accuracy. It can also be seen in Fig.12.2 that a core drill has flat end. The chamfer can have three or four cutting edges or lips, and the lip angle may vary between 90and 120. Core drills are employed for enlarging previously made holes and not for originating holes. This type of drill is characterized by greater productivity, high machining accuracy, and superior quality of the drilled surfaces. Gun drills. Gun drills are used for drilling deep holes. All gun drills are straight-fluted, and each has a single cutting edge. A hole in the body acts as a conduit to transmit coolant under considerable pressure to the tip of the drill. There are two kinds of gun drills, namely, the center-cut gun drill used for drilling blind holes and the trepanning drill. The latter has a cylindrical groove at its center, thus generating a solid core, which guides the tool as it proceeds during the drilling operation. Spade drills. Spade drills are used for drilling large holes of 3.5 in.(90mm) or more. Their design results in a marked saving in cost of the tool as well as a tangible reduction in its weight, which facilitates its handling. Moreover, this type of drill is easy to grind. Milling and Milling Cutter Milling is a machining process that is carried out by means of a multiedge rotating tool known as a milling cutter.In this process, metal removal is achieved through combining the rotary motion of the milling cutter and linear motions of the workpiece simultaneously. Milling operations are employed in producing flat, contoured and helical surfaces as well as for thread- and gear-cutting operation. Each of the cutting edges of a milling cutter acts as an individual single-point cutter when it engages with the workpiece metal. Therefore, each of those cutting edges has appropriate rake and relief angles. Since only a few of the cutting edges are engaged with the workpiece at a time, heavy cuts can be taken without adversely affecting the tool life. In fact, the permissible cutting speeds and feeds for milling are three to four times higher than those for turning or drilling. Moreover, the quality of the surfaces machined by milling is generally superior to the quality of surfaces machined by turning, shaping, or drilling. A wide variety of milling cutters is available in industry. This, together with the fact that a milling machine is a very versatile machine tool, makes the milling machine the backbone of a machining workshop. As far as the direction of cutter rotation and workpiece feed are concerned, milling is performed by either of the following two methods. Up milling (conventional milling). In up milling the workpiece is fed against the direction of cutter rotation, as shown in Fig.12.3a. As we can see in that figure, the depth of cut (and consequently the load) gradually increases on the successively engaged cutting edges. Therefore, the machining process involves no impact loading, thus ensuring smoother operation of the machine tool and longer tool life. The quality of the machined surface obtained by up milling is not very high. Nevertheless, up milling is commonly used in industry, especially for rough cuts. Down milling (climb milling). As can be seen in Fig.12.3b, in down milling the cutter rotation coincides with the direction of feed at the contact point between the tool and the workpiece. It can also be seen that the maximum depth of cut is achieved directly as the cutter engages with the workpiece. This results in a kind of impact, or sudden loading. Therefore, this method cannot be used unless the milling machine is equipped with a backlash eliminator on the feed screw. The advantages of this method include higher quality of the machined surface and easier clamping of workpieces, since the cutting forces act downward.Jigs and FixturesIntroduction It has already been stated that the workpiece must be located relative to the cutting tool, and be secured in that position. After the workpiece has been marked out, it is still necessary to position it with respect to the machine movements, and to clamp it in that position before machining is started. When several identical workpieces are to be produced the need to mark out each part is eliminated by the use of jigs and fixtures, but if a casting or forging is involved, a trial workpiece is marked out, to ensure that the workpiece can be produced from it, and to ensure that ribs, cores, etc. have not become misplaced. Jigs and fixtures are alike in that they both incorporate devices to ensure that the workpiece is correctly located and clamped, but they differ in that jigs incorporate means of tool guiding during the actual cutting operation, and fixtures do not. In practice, the only cutting tools that can be guided while actually cutting are drills, reamers, and similar cutters; and so jigs are associated with drilling operations, and fixtures with all other operations. Fixtures may incorporate means of setting the cutting tools relative to the location system. The advantages of jigs and fixtures can be summarised as follows: 1)Marking out and other measuring and setting out methods are eliminated; 2)Unskilled workers may proceed confidently and quickly in knowledge that the workpiece can be positioned correctly, and the tools guided or set; 3)the assembly of parts is facilitated, since all components will be identical within small limits, and “trying” and filing of work is eliminated; 4)The parts will be interchangeable, and if the product sold over a wide area, the problem of spare parts will be simplified. Bolt holes often have 1.5mm or even 3.0mm clearance for the bolt, and the reader may doubt the necessity of making precision jigs for such work. It must be remembered that the jigs, once made, will be used on many components, and the extra cost of an accurately made jig is spread over a large output.Furthermore, it is surprising how small errors accumulate in a mechanism during its assembly. When a clearance is specified, it is better to ensure its observance, rather than to allow careless marking out and machining to encroach upon it 1) The location of workpiece. Fig.13.1 represents a body that is completely free in space. In this condition it has six degrees of freedom. Consider these freedoms with respect to the three mutually perpendicular axes XX, YY, and ZZ. The body can move along any of these axes; it therefore has three freedoms of translation. It can also rotate about any of the three axes; it therefore has three freedoms of rotation. The total number of freedoms is six. When work is located, as many of these freedoms as possible must be eliminated, to ensure that the operation is performed with the required accuracy. Accuracy is ensured by machining suitable location features as early as possible, and using them for all location, unless other considerations mean that other location features must be used. If it is necessary, the new location features must be machined as a result of location from the former location features. 2) The clamping of the workpiece. The clamping system must be such that the workpiece is held against the cutting forces, and the clamping forces must not be so great as to cause the workpiece to become distorted or damaged. The workpiece must be supported beneath the point of clamping, to ensure that the forces are taken by the main frame of the jig or fixture, and on to the machine table and bed. When jigs and fixtures are designed, the clamping system is designed to ensure that the correct clamping force is applied, and that the clamps can be operated quickly but with safety. Definition of a Drill Jig A drill jig is a device for ensuring that a hole to be drilled, tapped, or reamed in a workpiece will be machined in the proper place.Basically it consists of a clamping device to hold the part in position under hardened-steel bushings through which the drill passes during the drilling operation. The drill is guided by the bushings. If the workpiece is of simple construction, the jig may be clamped on the workpiece. In most cases, however, the workpiece is held by the jig, and the jig is arranged so that the workpiece can be quickly inserted and as quickly removed after the machining operation is performed.Jigs make it possible to drill, ream, and tap holes at much greater speeds and with greater accuracy than when the holes are produced by conventional hand methods. Another advantage is that skilled workers are not required when jigs are used. Responsibility for the accuracy of hole location is taken from the operator and given to the jig. The term jig should be used only for devices employed while drilling, reaming, or tapping holes. It is not fastened to the machine on which it is used and may be moved around on the table of the drilling machine to bring each bushing directly under the drill. Jigs physically limit and control the path of the cutting tool. If the operation includes machining operations like milling, planing, shaping, turning, etc., the term fixture should be used. A fixture holds the work during machining operations but does not contain special arrangements for guiding the cutting tool ,as drill jigs do. Typical Jigs and Fixtures Typical drill jig. Figure 13.2 illustrates a drilling jig for drilling four holes in the flange of a workpiece that has been completed except for the four holes. The workpiece has an accurately machined bore, and is located from the bore and the end face, from a cylindrical post. There is no need to control the rotational position about the axis of the bore, because up to the time when the holes are drilled, it is symmetrical about that axis. The four bushes used to control the drill are held in the drill plate, which, with the hand nut, is used to clamp the workpiece against the base of the fixture. Typical milling fixture. Figure 13.3 illustrates a simple milling fixture for milling the slot in the otherwise completed workpiece shown. The workpiece is located from two of the four holes in its base, and from the underside of the base. The workpiece is clamped in position, and cutter is located against the setting block, which provides setting or cutter position and depth of cut. The fixture must be positioned relative to the machine table, this is done by engaging the two tenons at the bottom of the fixture in the slot in the machine table. The fixture is secured to the machine table with T-bolts, also engaging in the slots in the table (Fig.13.3).钻削和钻头钻削就是通过迫使绕自身轴线旋转的切削刀具进入工件而在其上生成通孔或盲孔。因此，从旋转轴线开始的切削范围等于所需孔的半径。实际上，使用的是两条围绕相同轴线旋转的对称切削刃。 钻削作业既能采用手钻也能采用钻床来实现。钻床在尺寸和结构上虽有差别，然而始终都是切削刀具围绕自身轴线旋转、工件稳固定位的形式。这正好与在车床上钻孔相反。用于钻削作业的切削刀具在钻削作业中，要用到被称为钻头的圆柱形回转端切削刀具。钻头可以有一条或多条直的或是螺旋状的切削刃以及相应的出屑槽。出屑槽的功能是给钻削作业中产生的切屑提供排出通道，并允许润滑剂和冷却液到达切削刃和正在被加工的表面。下面是常用钻头的概述。麻花钻：麻花钻是最常用的钻头类型。它有两条切削刃和两条沿钻头体全长连续的螺旋状出屑槽，如图12.1所示。麻花钻还包括钻颈和钻柄，钻柄可以是直的也可以是锥形的。锥形钻柄通过楔入动作安装在主轴的锥形轴孔中，钻柄上还有柄舌插入主轴轴孔中的插槽，从而作为传递转动的可靠方法。另一方面，直柄钻头用钻头卡盘夹住，接下来钻头卡盘则象锥形钻柄钻头一样安装在主轴轴孔内。如图12.1所示，两条切削刃就是钻唇，通过凿子状边缘的楔形体连在一起。麻花钻还有两条导向边，用于作业中钻头的正确导向和定位。两条钻唇形成钻顶角，并根据被钻削材料的性能来选取其大小。商品化钻头的钻顶角一般为118，这适用于钻削低碳钢和铸铁。对于更硬更韧的金属，诸如淬火钢、黄铜和青铜，更大的钻顶角(130或140)才能有更好的效果。麻花钻常用的出屑槽螺旋角范围为24到 30。钻削紫铜或软塑料时，推荐采用更大的螺旋角(35到45)。 虽然也有硬质合金刀尖的钻头，麻花钻一般用高速钢制成。工业实际中使用的麻花钻尺寸范围为0.01到3.25英寸(即0.25到80毫米)。空心钻：空心钻包括斜面、钻头体、钻颈和钻柄，如图12.2所示。这类钻头可以有三条或四条出屑槽及相同数量的保证良好导向的导向边，这样使得加工有高精度。在图12.2中同样能看到，空心钻具有平坦的端部。斜面可以有三或四条切削刃或钻唇，并且钻唇角可以在90到120之间变化。空心钻用于扩大已有的孔而不是打孔。这类钻头具有较大生产率、高加工精度和优良钻削表面质量的特性。深孔钻：深孔钻用于钻深孔。所有深孔钻都是直出屑槽的，并且均为单切削刃。钻头体中有个孔作为导管在相当大的压力下将冷却液传送到钻头顶端。深孔钻有两种类型，即用于钻盲孔的中心切削深孔钻和套孔钻。后者在其中心有一圆柱形沟槽，这样能生成整体芯在钻孔作业过程中引导钻头。扁平钻：扁平钻用于钻削3.5英寸(90毫米)或更大的大孔。其设计使得钻头成本明显节省、重量切实减轻，重量轻又使操作更方便。此外这种钻头容易磨利。铣削和铣刀铣削是采用被称为铣刀的多刃旋转刀具完成的机加工作业。在此工艺中，金属去除是通过铣刀的旋转运动和工件的直线运动的组合实现的。铣削作业既可用于生成平面、轮廓面和螺旋面，也可用于切削螺纹和齿轮。在铣刀切削工件金属时，铣刀的每条切削刃都象一单独的单刃刀具一样作用。所以每条切削刃都适当的前后角。由于同一时间只有部分切削刃切削工件，因此可以在对刀具寿命没有不利影响的情况下承担重型切削。事实上，铣削允许的切削速度和进给比车削或钻削高三到四倍。此外，由铣削加工的表面质量通常优于车削、刨削或钻削加工的表面质量。工业上可采用的铣刀类型众多。连同铣床是极通用机床的事实，使得铣床成为机加工车间的支柱。至于涉及到铣刀转动的方向和工件的进给，铣削可以通过下列两种方法之一进行。逆铣(传统铣削)：在逆铣中，工件逆着铣刀转动的方向进给，如图12.3a所示。就像在此图中能看到的那样，切削深度(及作为结果的载荷)随着切削刃持续进入切削而逐渐增加。所以，这种工艺没有冲击载荷，从而保证了机床的较平稳运行和较长寿命。通过逆铣所得机加工表面质量不是很高。然而逆铣仍经常被用在工业上，尤其是粗切削时。这会导致一种冲击，或突然加载。因此，这种方法只有当铣床在进给螺栓上配备间隙消除器时才采用。这种方法的优点包括机加工表面质量较高和工件由于切削力向下作用而较容易夹紧。 顺铣(同向铣削)：如同在图12.3b中看到的那样，在顺铣时刀具与工件之间接触点上铣刀旋转与进给方向一致。还可以看到当刀具进入工件切削时直接达到最大切削深度。钻模和夹具导言已经说明了工件必须相对于切削刀具定位并确保到位。工件划线后，仍有必要将