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調(diào)研報(bào)告
我的畢業(yè)設(shè)計(jì)是組合機(jī)床雙面鉆孔，加工的零件是變速箱體，它是用HT200材料制造成的。我要鉆的是M10和M14孔，加工量是年加工10萬(wàn)件，是大批量的生產(chǎn)。
接到任務(wù)的第二天就帶上筆記本到圖書(shū)館查資料，我通過(guò)《機(jī)械加工工藝設(shè)計(jì)手冊(cè)》查到刀具的一系列參數(shù)，通過(guò)計(jì)算得到刀具的耐用度，切削功率等。我還確定了主軸的一系列參數(shù)。
通過(guò)這次調(diào)研，使我知道組合機(jī)床有組合鉆床、組合鏜床、鉆擴(kuò)組合機(jī)床、鉆擴(kuò)鉸組合機(jī)床等，
組合機(jī)床是以通用部件為基礎(chǔ)，配以少量專(zhuān)用部件，對(duì)一種或若干種工件按預(yù)先確定的工序進(jìn)行加工的機(jī)床。它能夠?qū)ぜM(jìn)行多刀、多軸、多面、多工位同時(shí)加工。在組合機(jī)床上可以完成鉆孔、擴(kuò)孔、鉸孔、鏜孔、攻絲、車(chē)削、銑削、磨削及滾壓等工序。隨著組合機(jī)床技術(shù)的發(fā)展，它能完成的工藝范圍日益擴(kuò)大。在組合機(jī)床自動(dòng)線上可以完成一些非切削工序，例如：打印、清洗、熱處理、簡(jiǎn)單的裝配、試驗(yàn)和在線自動(dòng)檢查等工序。
組合機(jī)床及其自動(dòng)線所使用的通用部件是具有特定功能，按標(biāo)準(zhǔn)化、系列化、通用化原則設(shè)計(jì)、制造的組合機(jī)床基礎(chǔ)部件。每種通用部件有合理的規(guī)格尺寸系列，有適用的技術(shù)參數(shù)和完善的配套關(guān)系。
1990年前后的幾年中,躍進(jìn)汽車(chē)集團(tuán)從大連組合機(jī)床研究所、大連機(jī)床廠、常州機(jī)床廠、保定機(jī)床廠、豫西機(jī)床廠等十余個(gè)生產(chǎn)組合機(jī)床的廠家訂購(gòu)了200多臺(tái)組合機(jī)床及自動(dòng)線, 其中使用量最大的第二發(fā)動(dòng)機(jī)廠用于索菲姆缸體、缸蓋、連桿等零件生產(chǎn)的組合機(jī)床120多臺(tái), 包括12條自動(dòng)線。投產(chǎn)幾年來(lái), 依維柯汽車(chē)的生產(chǎn)起到了重要的保證作用。這批設(shè)備普遍采用了引進(jìn)德國(guó)Hubller - H ille公司的通用部件制造技術(shù), 使組合機(jī)床的產(chǎn)品技術(shù)提高到了一個(gè)新水平。在機(jī)床控制系統(tǒng)方面, 改變了傳統(tǒng)常規(guī)繼電器、接觸器控制系統(tǒng), 普遍應(yīng)用了微機(jī)控制, 大大提高了機(jī)床的先進(jìn)性和使用的可靠性。從總體上看, 組合機(jī)床行業(yè)的總體水平, 經(jīng)過(guò)幾十年的發(fā)展有了很大的提高, 特別是自動(dòng)線的技術(shù)水平比“六五”期間又大大前進(jìn)了一步。從用戶的角度看, 這些設(shè)備與引進(jìn)的組合機(jī)床的水平差距還較大。
為了對(duì)組合機(jī)床有一個(gè)感性的認(rèn)識(shí)，這次為期一天的生產(chǎn)見(jiàn)習(xí)是我們理論聯(lián)系實(shí)際很重要的一部分，在范真老師的帶領(lǐng)下我們見(jiàn)習(xí)了揚(yáng)州柴油機(jī)廠?？梢哉f(shuō)我們?cè)谶@一天的實(shí)習(xí)中學(xué)到了很多在課堂沒(méi)學(xué)到的知識(shí),受益匪淺。內(nèi)容是：通過(guò)實(shí)際觀察和操作對(duì)專(zhuān)用組合鉆床有一個(gè)全方位的了解和認(rèn)識(shí)，從基本的組成到各個(gè)部件的功能有一個(gè)理論聯(lián)系實(shí)際的升華，為即將到來(lái)的畢業(yè)設(shè)計(jì)做準(zhǔn)備。
這是是我們專(zhuān)業(yè)知識(shí)結(jié)構(gòu)中不可缺少的組成部分，并作為一個(gè)獨(dú)立的項(xiàng)目列入專(zhuān)業(yè)教學(xué)計(jì)劃中的。其目的在于通過(guò)實(shí)習(xí)使學(xué)生獲得基本生產(chǎn)的感性知識(shí)，理論聯(lián)系實(shí)際，擴(kuò)大知識(shí)面；同時(shí)又是鍛煉和培養(yǎng)學(xué)生業(yè)務(wù)能力及素質(zhì)的重要渠道，培養(yǎng)當(dāng)代大學(xué)生具有吃苦耐勞的精神，也是學(xué)生接觸社會(huì)、了解產(chǎn)業(yè)狀況、了解國(guó)情的一個(gè)重要途徑，逐步實(shí)現(xiàn)由學(xué)生到社會(huì)的轉(zhuǎn)變，培養(yǎng)我們初步擔(dān)任技術(shù)工作的能力、初步了解企業(yè)管理的基本方法和技能；體驗(yàn)企業(yè)工作的內(nèi)容和方法。這些實(shí)際知識(shí)，對(duì)我們學(xué)習(xí)后面的課程乃至以后的工作，都是十分必要的基礎(chǔ)。
通過(guò)這次調(diào)研，我對(duì)組合機(jī)床有了一定的了解，對(duì)我的畢業(yè)設(shè)計(jì)會(huì)有很大的幫助。
A CNC machine tool interpolator for surfaces of cross-sectional design
Sotiris L. Omiroua,_, Andreas C. Nearchou
Abstract
A machining strategy for milling a particular set of surfaces, obtained by the technique of cross-sectional design is proposed. The
surfaces considered are formed by sliding a Bezier curve (profile curve) along another Bezier curve (trajectory curve). The curves are
located in perpendicular planes. The method employs a three-axis CNC milling machine equipped with suitable ball-end cutter and is
based on the locus-tracing concept.
1. Introduction
In the automobile, aerospace and appliances industry, a variety of functional or even aesthetic free-form surfaces are engaged by engineers and designers to achieve the desired performance of a product. The machining of such complex geometries is a basic problem in computer-aided
manufacturing since the available NC machines are constrained, by their software, to linear and circular motions. In this paper we deal with a set of surfaces obtained with this design technique. More particularly we use Bezier curves to define the shapes of both the profile and the trajectory. Bezier curves as free-form curves are a powerful designing tool. They need only a few points to define a large number of shapes, hence their wide use in CAD systems. The principle for generating the considered surfaces is shown in Fig. 1. The curves are located in perpendicular planes. The upper end of the profile curve lies on the trajectory curve which is a plane contour. Fig. 2 shows a sample surface obtained by the above-mentioned technique. This paper, following the present intention of research engineers to take advantage of the hardware capabilities of modern CNC systems, proposes a real-time surface interpolator for machining the specified surfaces on
Fig. 1. Surface is generated by sliding the profile curve along the
trajectory curve.
Fig. 2. Sample surface obtained by cross-sectional design
vertical three-axis CNC milling machine. However we keep in mind that whenever feasible, three-axis milling procedures are often preferred due to considerations of cost. For the considered surfaces, inaccessibility issues are directly dependent upon the form of the profile curve. So by controlling the form of theaccuracy are the main advantages of this manufacturing method.Finally, accuracy is obtained by applying the locus-tracing concept for driving the tool along the Bezier’s offset. The concept is generally applicable in motion generation. In this paper, its application is illustrated in the context of motion generation along Bezier’s offset. Compared to the customary offset-modeling schemes, an additional advantage besides accuracy, is the fact that we avoid the complexity of using an exact analytic expression or a piecewise-analytic approximation for the offset.
2. Cross-sectional design with Bezier curves
Many commonly seen and useful surfaces are surfaces of cross-sectional design. For example a surface of revolution is produced under this technique. The surface is generated by revolving a given curve about an axis. The given curve is a profile curve while the axis is the axis of revolution. This paper deals with a more complex type of surface which is an extension to the surfaces of revolution. We still need a
profile curve that rotates about the axis of revolution, but the rotation is controlled by a trajectory curve. Now, the profile curve swings about the axis of revolution, guided by the trajectory curve. Both curves, profile and trajectory, are Bezier curves located in perpendicular planes. A Bezier curve of degree n is a polynomial interpolation curve defined by en t 1T points defining the Bezier control polygon. The interpolation basis functions used in Bezier interpolation are the Bernstein polynomials defined for degree n as
where the binomial coefficients are given by
The parameter t is in the range [0,1] and there are n t 1 polynomials defined for each i from 0 to n. The Bezier
curve is therefore defined over the interval [0,1] as
where bi are the control points defining the Bezier polygon. A recursive algorithm defined by de-Casteljau [3,5,12], calculates for a given control polygon the point that lies on the Bezier curve for any value of t, and can be used to evaluate and draw the Bezier curve simply, without using the Bernstein polynomials. The algorithm advances by creating in each step a polygon of degree one less than the one created in the previous step until there is only one point left, which is the point on the curve. The polygon vertices for each step are defined by linear interpolation of two consecutive vertices of the polygon from the previous step
with a value of t (the parameter):
An interactive drawing tool based on the de-Casteljau algorithm, capable to design and manipulate Bezier curves supports the method proposed in this paper. Since the design process is very often iterative, the designer first lets the computer draw the Bezier curve defined by a given polygon. Next, checks whether the shape is acceptable (or optimal) based on various criteria, and, if necessary, adjusts the location and the number of the polygon vertices. The edit, add, move and delete operations of this drawing tool, presented in Figs. 3(a)–(d), respectively, were used to achieve the desired form for a profile curve. Once the forms of the profile and the trajectory curve are definitively accepted, the coordinates of their control points are advanced to the input of the CNC surface interpolator, constituting part of the geometric information required.
3. Offset tracing for a Bezier curve
An accurate machining of the considered surfaces requires accurate offset cutter paths along the trajectory and the profile curves. Since both of them are implemented in terms of Bezier curves our interest is focused on the motion generation along Bezier’s offset. The generation of an accurate motion along Bezier’s offset is treated as a
locus-tracing problem. The formulation of the interpolation algorithm demonstrates the versatility and effectiveness of the locus-tracing concept in this practical case of machining. The algorithm guides the tool-center through repeated application of two analytically implemented
construction operations, maintaining exact contact (within 1BLU1) along the entire path. In each iteration, the set of candidate steps is represented by the vector expression
assuming a unit of length equal to the step size. The number of possible steps in each point is 8 (Fig. 4). The last inequality excludes the combination of zero values for both dX, dY, which does not constitute a step. The optimal step is one, which maximizes the advance TidP (Fig. 5) along the local tangent Ti while, at the same time, it satisfies a criterion of proximity to the offset. Implementation of the proximity criterion requires the use of a proximity function which, in the neighborhood of Pi, provides a measure of closeness to the offset.
A suitable proximity function is derived from the fixed
distance property of the offset
where d is the radius of the cutting tool.
Notice that for P lying on the offset p