机械外文翻译文献翻译小孔钻在工程塑料片材方面及其精度估算.doc

机械专业中英文文献翻译编号英文原文Small-hole drilling in engineering plastics sheet and its accuracy estimation Hiroki Endo and Etsuo MaruiAbstractIn recent manufacturing processes, the small diameter hole drilling process is frequently used owing to its good characteristics. The drilling process can easily be adapted to wide variations in lot size, processing accuracy, processing spot patterns where holes are made, and so on. Many machine elements, which have small diameter holes, are manufactured using engineering plastics of superior material and machining properties. However, it is not easy to drill holes with a diameter smaller than 1mm, in recent machining technology as well. In this report, 1-mm diameter holes are drilled on two engineering plastics sheets and their drilling accuracy is discussed. Keywords: Small diameter hole; Drilling; Engineering plastics; Machining accuracy 1. IntroductionProcessing of small diameter holes is done in various materials, corresponding to the trend of downsizing or high accuracy in parts incorporated into electronic equipments, medical instruments or textile machineries. Many techniques are put to practical use, including drilling, ultrasonic machining, electric discharge machining, electrolytic machining, laser beam machining, electron beam machining, fluid or abrasive jet machining, and chemical blanking. Depending on the workpiece material, the machining accuracy, and the lot size, the best process for making holes of small diameter may be appropriately selected. Within these various machining processes, the drilling process can readily deal with a wide variety of machining conditions. However, there are some difficult problems in drilling holes smaller than 1mm in diameter. For example, a large load cannot be put on small drills, owing to their low strength and rigidity. Thus, the feed rate per unit drill rotation must be set small. The removal of drilled chips is difficult owing to the small drill flute area. In many cases, engineering plastics are used in making various machine parts because they are light and have superior specific strength (that is, the ratio of tensile strength to density) compared with carbon steel. Also, the material cost of engineering plastics is competitive and their machinability is fairly good. With these points as background, the orthogonal cutting of engineering plastics was investigated 1 and it was suggested here that the viscoelastic properties of engineering plastics have some effects on the magnitude of cutting force and the surface roughness of machined surfaces. There is a review paper 2 regarding the machining of engineering plastics. In this review paper, drilling process was also treated. It was pointed out that the heating up of the workpiece due to build-up of swarf on drill flutes is an obstacle to the drilling process of engineering plastics. Recently, some experiments have been attempted on drilling glassfiber-reinforced engineering plastics sheets 3 and 4, and the thrust force and torque during drilling have been measured. In these papers, it was reported that the delamination phenomenon decreases the drilled hole integrity, when holes of about 5-mm diameter are drilled. However, the investigation on the accuracy in small hole drilling of engineering plastics is left pending. Then in this paper, small diameter holes of 1mm are drilled in two typical engineering plastics sheets, and the effect of spindle speed and feed rate on the accuracy (radius error) is estimated. 2. Workpiece materialsTwo typical engineering plastics sheets, polyacetal (POM) and polyetherimide (PEI), were drilled. The materials properties are listed in Table 1. Table 1. Material properties of workpiece engineering plastics PerformanceUnitPOMPEISpecific gravity1.411.27Rate of water%0.220.25Melting pointC165210Coefficient of linear thermal expansioncm/cm/C91055.6105Tensile strengthMPa61124Tensile extension (Yielding point)%4023Bending strengthMPa89157Bending elasticityGPa2.603.07Compressive strengthMPa103118Izote impact valueJ/m7442Rockwell hardnessM scale119127Polyacetal is a crystallized engineering plastics material. The main raw materials are acetal co-polymer and homo-polymer. POM has good fatigue properties and machinability. Many cams, guides and liners are made of POM. Very high accuracy is needed in these machined parts. PEI is an amorphous engineering plastic having superior thermal resistance characteristics. Special electrical parts, for example, electric insulators, connectors, are made of PEI, which is superior in mechanical strength but inferior in machinability to POM. The workpiece size was: length 100mm, width 50mm and thickness 0.8mm. 3. Experimental apparatus and procedureThe drilling machine used is for small diameter holes, and is equipped with an automatic feed mechanism. A high-frequency induction motor positioned at the uppermost position of the main spindle drives the spindle. Maximum spindle speed is 12,500rpm. The net spindle speed of the spindle during the drilling is measured by a tachometer, which counts number of the laser beam reflected from a reflective tape pasted on the scroll chuck. A servomotor for drill feed drives the feed motion of the spindle. The feed is stepless, and a dial gauge equipped at the spindle head measures the length of the drill motion in the spindle axis direction. A stopwatch was used to measure the time needed for this length. The ratio of the moved length to the time is the substantial feed rate per unit time. The spindle speed was varied between 1250 and 12,500rpm. And also the feed rate per unit time was varied between 0.405 and 1.986mm/s. Spindle speed was varied in keeping with the feed rate per unit time. Hence, the feed rate per unit drill rotation became small with the increase in the spindle speed of the drill. The drill spindle end is attached to the scroll chuck. The drill used here is a conventional twist drill made of high-speed steel with a diameter of 1mm. In some extra experiments, a 0.3mm-diameter drill was also used. Such drills have no surface treatment. A dial gauge estimates deflection accuracy of the drill on the scroll chuck during rotation. Extreme care was taken so that the drill deflection was smaller than 5m. The same drill made five holes under the cutting condition of the same spindle speed and the same feed rate. Another drill was used in the drilling under another cutting condition. Of course, the size accuracy of these drills exists within the above-mentioned size scattering. Any evidence of the wear of drills and the build-up of swarf on drill flutes were not recognized after five holes drilling. Formerly mentioned workpiece of engineering plastics were set on the base of the drilling machine by clamping bolts. Dry cutting without fluid was performed. 4. Calculation of drilled hole shapesThe 1-mm diameter holes drilled on engineering plastics sheets by the process described above are not geometrically true circles, but have a small radial deviation. Shape accuracy of the drilled holes is estimated by the following process. An optical microscope equipped with digital measuring device measures the shape of the drilled hole. The cross wire of the microscope is set at the circumference of the hole. Then, the coordinates (x,y) of the hole circumference are read. Dividing the circumference into 18 equal parts, the same measurements are then repeated on each spot on the circumference. Using these 18 sets of measured qualities, the equation of the circle that fits closely to the drilled hole is calculated. This is called a least square circle, and in the calculation, the least squares method is applied. The equation of the least square circle is assumed as follows:x2+y2+Ax+By+C=0 (1)Owing to the shape error of the hole, the right hand side of Eq. (1) does not become zero when the above-mentioned measured qualities (xi,yi) are substituted. The residual in this case is vi and the following equation is obtained: (2)Here, the coefficients A, B, C in Eq. (1) are determined as the sum of the squared values of the residual vi becomes minimum. Values of these coefficients are obtained by solving the following simultaneous linear equations. In the calculation, N=18.(3)Moreover, the coordinates (x0,y0) of the center of the least square circle and its radius rm are obtained as follows:Corresponding to the above process, the least square circles are described. An example is shown in Fig. 1, where the workpiece material is PEI, drill diameter: 1mm, spindle speed: 12,500rpm, and feed rate: 0.405mm/s. The least square circle is indicated by the broken line. 5. Estimation of machining accuracy and experimental resultsMachining accuracy of the drilled holes is estimated by the radius error obtainable from the least square circles. The calculation process of the radius error is given here. Radius ri at the each measuring spot (xi,yi) is obtained from the coordinate of the least square circle center (x0,y0) of Eqs. (4) and (5) as follows: (7) Then, the radius error is calculated by the following equation. The parameter rm in the equation is the radius of the least square circle given by Eq. (6).ri=rirm (8) And the position of that measuring spot on the circle is represented by the following angle i. (9)The relation between ri and i obtained from the above method is shown in Fig. 2 as a radius error curve. The drilling conditions in this figure are the same as those of Fig. 1 Three concavities and convexities are recognized on the circumference. Then, the drilled hole shape is approximately triangular. Similar results were obtained in other workpiece materials for other drilling conditions. Furthermore, it is seen that the circumference of the drilled hole exists in the vicinity within 0.02mm from the least square circle. This drilled hole shape is similar to that produced by the so-called drill walking phenomenon 5. The radius of the least square circle is slightly larger than that of the drill. The difference between them is about 10m. Result of Fig. 2 is obtained in the measurement at the drill entrance into workpiece. Small burr was formed at the drill exist and the accuracy measurement could not carry out as it is. Then, the burr was forcibly removed and the accuracy was measured. Almost the same accuracy was confirmed, because the workpiece is thin (0.8mm thickness). These radius errors are rearranged as functions of the spindle speed or the feed rate for every workpiece material. The results are given in Fig. 3, Fig. 4, Fig. 5 and Fig. 6. Error bars indicate the distribution range of the experimental data. The radius error becomes small hyperbolically with the increase in the feed rate and becomes large linearly with the spindle speed. Small diameter drills were used in this experiment and their bending rigidity is low. Rotational cutting speed is almost zero near the chisel point. At that point, the drill has only a small axial velocity corresponding to the drill feed motion. Accordingly, the rate of penetration 6 is extremely small when the feed rate is small. As mentioned above, the walking phenomenon occurs owing to small errors in drill size. This phenomenon is compounded with the effect of small rate of penetration when small feed rate and large spindle speed are applied. Hence, the positioning accuracy of the drill point against the workpiece is not very good at small feed rate and large spindle speed. As a result, it is supposed that the radius error becomes large. For example, the rate of penetration, that is the feed rate per unit drill rotation, is about 2m when the drill rotation speed is 12,500rpm and the feed rate is 0.405mm/s. The rate of penetration is about 100m when the drill spindle speed is smallest (1250rpm) and the feed rate is largest (1.986mm/s). One reason for the radius error worsening when the rotation speed becomes high is that chatter 6 related to the drill dynamic characteristics is possible. However, the small drill size errors and the relative drop in the feed rate per unit drill rotation corresponding to the spindle speed increase have a large effect on the radius error. In conclusion, it is important to drill a small hole in the drilling condition so as to maintain a sufficiently high feed rate per unit drill rotation. An example of superposition of the results of POM and PEI is given in Fig. 7. It is recognized in this figure that the radius accuracy in the drilling of PEI is slightly inferior to that of POM. PEI is a kind of supper engineering plastics. PEI is superior to POM in tensile strength, compressive strength, bending strength, bending elasticity and Rockwell hardness, as seen in Table 1. Owing to this, the machinability of PEI may be worse than that of POM and the result of Fig. 7 regarding the radius error was be obtained. 6. Concluding remarksSmall holes were drilled in two engineering plastics sheets POM and PEI using a drill 1mm in diameter. Drilling can be done on both workpiece materials. Reading the drilled hole shape by optical micrometer, and calculating the least square circle, the drilling accuracy (radius error) can be estimated. The radius error becomes worse when the drill feed rate is small and the spindle speed is large. The feed rate per unit drill rotation is relatively small when the spindle speed is large. Hence, it is supposed that the positioning accuracy of the drill against the workpiece is not good, and that the radius error becomes worse under the drilling conditions in which the feed rate per unit drill rotation is small. From this fact, it is desirable that small diameter holes be drilled in the condition in which the feed rate does not become low.References1 K.Q. Xiao and L.C. Zhang, The role of viscous deformation in the machining of polymers, International Journal of Mechanical Science 44 (2002), pp. 23172336. 2 M. Alauddin, I.A. El Baradie and M.S.J. Hashmi, Plastics and their machining: a review, Journal of Materials Processing Technology 54 (1995), pp. 4060. 3 W.-C. Chen, Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates, International Journal of Machine Tools and Manufacture 37 (1997), pp. 1097110.4 E. Capello, Workpiece damping and its effect on delamination damage in drilling thin composite laminates, Journal of Materials Processing Technology 148 (2004), pp. 186195. 5 M. Tsueda, Y. Hasegawa and H. Kimura, On walking phenomenon of drill, Transactions of the JSME 27 (1961), pp. 816823. 6 D.F. Galloway, Some experiments on the influence of various factors on drill performance, Transactions of the ASME 79 (1957), pp. 191231. 中文译文小孔钻在工程塑料片材方面及其精度估算摘要在最近的制造流程中，小直径钻孔由于其良好的特性经常被使用。钻孔工艺能够轻易的适用于大部分尺寸，加工精度，加工点孔，等等。许多有小直径孔的机械零件，是使用优质金属材料和机器特性制造出来的。然而，在最近的加工技术中，钻一个直径小于1 毫米的孔也是不容易的。在这份报告中，直径为1毫米的孔在两个工程塑料板材的钻孔和钻孔的精度进行了讨论。关键词：小直径孔；钻孔；工程塑料；加工精度1.简介 在各种材料中都存在小直径孔的加工，根据数据趋势或从电子设备中反映零件部分的高精度，医疗器械、纺织机械。许多技术投入实际使用，包括钻孔，超声加工，电火花加工，电解加工，激光束加工，电子束加工，液体或磨料喷射加工，化学消隐。根据工件材料，加工精度，和尺寸的大小，可以选择小直径钻孔的最佳工艺。在这些不同的加工工艺中，钻孔工艺可以很容易地处理大部分的加工情况。 但是，钻孔直径小于1毫米时有一些困难的问题。例如，一个大的负载不能放在小钻头上，由于其较低的强度和刚度。因此，每单位钻孔旋转进给速度必须设置小。由于其小的钻孔区域，钻屑的去除是一个困难的问题。 在许多情况下，与碳钢相比工程塑料被使用用于制造各种机械零件（即，拉伸强度与密度的比值）是因为他们轻和具有较高的强度。同时，工程塑料材料成本拥有竞争性，其加工性能较好。 以这些问题为背景，研究了工程塑料的正交切削 1 和建议在工程塑料的弹性性质切削力的大小和加工表面的表面粗糙度的影响。有一份关于工程塑料加工的审查报告2。在这篇综述报告中，钻孔过程也被关注。指出出对于工程塑料钻孔由于钻孔过程中的切屑的累计而导致的材料过热。最近，一些实验已经尝试在钻孔中使用玻璃纤维来增强塑料片材 3 ，钻孔过程中的轴向力和扭矩进行了测量。在这些论文中，当孔直径约5毫米，分层现象会减少钻孔的完整性。但是，在工程塑料小钻孔精度调查尚未解决。 在这个报告中，1毫米的小直径孔在两个典型的工程塑料板材的钻孔，主轴转速和进给速度对精度的影响（半径误差）估计。2.工程材料在两个典型的工程塑料聚甲醛（POM）和聚醚酰亚胺（PEI）。材料特性列于表1。表1工程塑料加工件的材料特性性能单位POMPEI比重1.411.27水率%0.220.25 熔点C165210 线性热膨胀cm/cm/C91055.6105拉伸强度MPa61124拉伸（屈服点）%4023弯曲强度MPa89157弯曲弹性模量GPa2.603.07抗压强度MPa103118丝兰冲击值J/m7442 罗克韦尔硬度M scale119127 聚甲醛是一种明确的工程塑料材料。主要原料是缩醛共聚物和均聚物。聚甲醛具有良好的疲劳性能和加工性能。许多凸轮，导板和线路是聚甲醛制成的。非常高的精度是在这些加工零件所需要的。PEI是具有优良的热特性的无定形工程塑料。特殊的电气部件，例如，绝缘子，连接器，是由PEI制成的，它在较好的机械强度而机械加工性能不如POM优越。工件尺寸为：长100毫米，宽50毫米，厚0.8毫米。3.试验设备和程序 钻孔机用于小直径的孔，并配有自动送料机构。位于主轴的最高位置，驱动主轴高频感应电动机。最大主轴转速为12500 RPM。通过转速表测量钻孔时主轴的钻速，计算数量的激光束反射胶带粘贴在三爪卡盘。 伺服电机通过钻头进给驱动主轴进给运动。进给是无级的，百分表安装在主轴头上计算在主轴轴线方向的运动长度。秒表是用来测量这个长度所需的时间。长时间的移动率是每单位时间的实质性的进料速率。 主轴转速变化在1250和12500转之间。而且每单位时间的进料速率变化0.405和1.986毫米/秒。主轴转速保持在每单位时间的进料速率之间变化。因此，每单位钻旋转进给率变小，在钻主轴速度增加。 钻床主轴一端连接到三爪卡盘。这里使用的钻机是由高速钢制成的直径为1毫米常规麻花钻。在一些额外的实验中，0.3毫米直径的钻头也会被使用。这类钻孔没有表面处理。在旋转中用百分表估计在三爪卡盘钻偏转精度。特别注意使钻挠度小于5M。 同样的钻五孔的主轴转速和进给速度相同的切削条件下。另一个钻用于钻一个切削条件下。当然，这些演习的尺寸精度存在于上述尺寸的散射。任何证据表明钻头的磨损和切屑在五孔钻后无法辨认。 以前提到工程塑料工件通过夹紧螺栓的安装在钻机底座的。没有切削液。4.钻孔形状的计算 1毫米直径的钻孔工程塑料片由上述过程不是几何真界，但有一个小的径向偏差。钻出的孔的形状精度是通过以下过程估计。 光学显微镜配备数字测量装置测量钻孔的形状。显微镜的十字线设置在孔的圆周上。然后，孔的坐标（x，y）在圆周上读。把圆周分成18等份，然后重复相同的测量是在每个点在圆周上。用这18组实测的坐标，圆周的方程，紧密地配合到钻孔的计算中。这就是所谓的最小二乘圆，并在计算中，应用最小二乘法。最小二乘圆方程如下： x2+y2+Ax+By+C=0 (1) 由于孔的形状误差，方程的右手边（1）不为零时，由上述测量坐标（xi，yi）取代。在这种情况下，剩余的VI方程如下： (2) 在这里，系数A，B，C在式（1）确定为的平方值的总和为最小值VI。这些系数的值是通过求解线性方程组得到。在计算中，n = 18。(3)此外，坐标（x0，y0）的最小二乘圆，圆心和半径rm如下： 对应于上述过程，介绍了最小二乘圆。一个例子是图1所示，在工件材料PEI，钻头直径：1毫米，主轴转速：12500转速，进给速度：0.405毫米