中英文文献翻译-小孔钻在工程塑料片材方面及其精度估算

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英文原文

Small-hole drilling in engineering plastics sheet and its accuracy estimation

Hiroki Endo and Etsuo Marui

Abstract

In 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 1 mm, 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. Introduction

Processing 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 1 mm 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 visco–elastic 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 glass–fiber-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 1 mm 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 materials

Two 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

Polyacetal 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 100 mm, width 50 mm and thickness 0.8 mm.

3. Experimental apparatus and procedure

T he 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 12,500 rpm. 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 12,500 rpm. And also the feed rate per unit time was varied between 0.405 and 1.986 mm/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 1 mm. In some extra experiments, a 0.3 mm-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 5 μm.

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 shapes

The 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 (x i,y i) are substituted. The residual in this case is v i 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 r m 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: 1 mm, spindle speed: 12,500 rpm, and feed rate: 0.405 mm/s. The least square circle is indicated by the broken line.

5. Estimation of machining accuracy and experimental results

Machining 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).

Δr i=r i?r m(8) And the position of that measuring spot on the circle is represented by the following angle θi.

(9)

The relation between Δri and θi obtained from the above method is shown in Fig. 2as a radius error curve. The drilling conditions in this figure are the same as those of Fig. 1Three 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 ±0.02 mm 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 10 μm.

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 (0.8 mm 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 2 μm when the drill rotation speed is 12,500 rpm and the feed rate is 0.405 mm/s. The rate of penetration is about 100 μm when the drill spindle speed is smallest (1250 rpm) and the feed rate is largest (1.986 mm/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 remarks

Small holes were drilled in two engineering plastics sheets POM and PEI using a drill 1 mm 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.

References

[1]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. 2317–2336.

[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. 40–60.

[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. 1097–110.

[4] E. Capello, Workpiece damping and its effect on delamination damage in drilling thin composite laminates, Journal of Materials Processing Technology 148 (2004), pp. 186–195. [5] M. Tsueda, Y. Hasegawa and H. Kimura, On walking phenomenon of drill, Transactions of the JSME 27 (1961), pp. 816–823.

[6]D.F. Galloway, Some experiments on the influence of various factors on drill performance, Transactions of the ASME 79 (1957), pp. 191–231.

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安全工程专业外语翻译

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