外文文献及译文
本科毕业设计
外文文献及译文
文献、资料题目: Dimensioning
文献、资料来源：English in Mechatronic Engineering 文献、资料发表（出版）日期：2010.6
院（部）：机电工程学院
专业：机械工程及自动化
班级：机械092
姓名：赵常阳
学号：2009071135
指导教师：吕英波
翻译日期：2013.4.10
外文文献：
Dimensioning
The design of a machine includes many factors other than those of determining the loads and stresses and selecting the proper materials. Before construction or manufacture can begin, it is necessary to have complete assembly and detail drawings to convey all necessary information to the shop men. The designer frequently is called upon to check the drawings before they are sent to the shop. Much experience and familiarity with manufacturing processes are needed before one can become conversant with all phases of production drawings. Drawings should be carefully checked to see that the dimensioning is done in a manner that will be most convenient and understandable to the production departments. It is obvious that a drawing should be made in such a way that it has one and only one interpretation. In particular, shop personnel should not be required to make trigonometric or other involved calculations before the production machines can be set up. Dimensioning is an involved subject and long experience is required for its mastery. Tolerances must be placed on the dimensions of a drawing to limit the permissible variations in size because it is impossible to manufacture a part exactly to a given dimension. Although small tolerances give higher quality work and a better operating mechanism, the cost of manufacture increases rapidly as the tolerances are reduced, as indicated by the typical curve of Fig14.1. It is therefore important that the tolerances be specified at the largest values that the operating or functional considerations permit.
Tolerances may be either unilateral or bilateral. In unilateral dimensioning, one tolerance is zero, and all the variations are given by the other tolerance. In bilateral dimensioning, a mean dimension is used which extends to the midpoint of the tolerance zone with equal plus and minus variations extending each way from this dimension. The development of production processes for large-volume manufacture at low cost has been largely dependent upon interchangeability of component parts. Thus the designer must determine both the proper tolerances for the individual parts, and the correct amount of clearance or interference to permit assembly with the mating parts. The function of the gearbox is to provide a number of different spindle speeds (usually 6 up
to 18 speeds). Some modern lathes have headstocks with infinitely variable spindle speeds, which employ frictional ,electrical ,or hydraulic drives.
The spindle is always hollow, i. e., it has a through hole extending lengthwise. Bar stocks can be fed through that hole if continuous production is adopted. Also, that hole has a tapered surface to allow mounting a plain lathe center. The outer surface of the spindle is threaded to allow mounting of a chuck, a face plate, or the like. Tailstock. The tailstock assembly consists basically of three parts, its lower base, an intermediate part, and the quill. The lower base is a casting that can slide on the lathe bed along the guideways, and it has a clamping device to enable locking the entire tailstock at any desired location, depending upon the length of the workpiece. The intermediate part is a casting that can be moved transversely to enable alignment of the axis of the tailstock with that of the headstock. The third part, the quill, is a hardened steel tube, which can be moved longitudinally in and out of the intermediate part as required. This is achieved through the use of a handwheel and a screw, around which a nut fixed to the quill is engaged. The hole in the open side of the quill is tapered to enable mounting of lathe centers or other tools like twist drills or boring bars. The quill can be locked at any point along its travel path by means of a clamping device.
The carriage. The main function of the carriage is mounting of the cutting tools and generating longitudinal and/or cross feeds. It is actually an H-shaped block that slides on the lathe bed between the headstock and tailstock while being guided by the V-shaped guideways of the bed.The carriage can be moved either manually or mechanically by means of the apron and either the feed rod or the lead screw. When cutting screw threads, power is provided to the gearbox of the apron by the lead screw. In all other turning operations, it is the feed rod that drives the carriage. The lead screw goes through a pair of half nuts, which are fixed to the rear of the apron. When actuating a certain lever, the half nuts are clamped together and engage with the rotating lead screw as a single nut, which is fed, together with the carriage, along the bed. When the lever is disengaged, the half nuts are released and the carriage stops. On the other hand, when the feed rod is used, it supplies power to the apron through a worm gear. The latter is keyed to the feed rod and travels with the apron along the feed rod, which has a keyway extending to cover its whole length. The manner of placing tolerances on drawings depends somewhat on the kind of product or type of manufacturing process. If the tolerance on a dimension is not specifically
stated, the drawing should contain a blanket note which gives the value of the tolerance for such dimensions. However, some companies do not use blanket notes on the supposition that if each dimension is considered individually, wider tolerances than those called for in the note could probably be specified. In any event it is very important that a drawing be free from ambiguities and be subject only to a single interpretation. ? Dimension and Tolerance In dimensioning a drawing, the numbers placed in the dimension lines represent dimension that are only approximate and do not represent any degree of accuracy unless so stated by the designer.
To specify a degree of accuracy, it is necessary to add tolerance figures to the dimension. Tolerance is the amount of variation permitted in the part or the total variation allowed in a given dimension. A shaft might have a nominal size of 2.5in.(63.5mm), but for practical reasons this figure could not be maintained in manufacturing without great cost. Hence, a certain tolerance would be added and, if a variation of±0.003in.(±0.08mm) could be permitted, the dimension would be stated 2.500±0.003(63.5±0.08mm). Dimensions given close tolerances mean that the part must fit properly with some other part. Both must be given tolerances in keeping with the allowance desired, the manufacturing processes available, and the minimum cost of production and assembly that will maximize profit Generally speaking, the cost of a part goes up as the tolerance is decreased. If a part has several or more surfaces to be machined, the cost can be excessive when little deviation is allowed from the nominal size. Allowance, which is sometimes confused with tolerance, has an altogether different meaning. It is the minimum clearance space intended between mating parts and represents the condition of tightest permissible fit. If a shaft, size 1.498-0.003, is to fit a hole of size 1.500+0.003, the minimum size hole is 1.500 and the maximum size shaft is 1.498. Thus the allowance is 0.002 and the maximum clearance is 0.008 as based on the minimum shaft size and maximum hole dimension. Tolerances may be either unilateral or bilateral. Unilateral tolerance means that any variation is made in only one direction from the nominal or basic dimension. Referring to the previous example, the hole is dimensioned 1.500+0.003, which represents a unilateral tolerance. If the dimensions were given as 1.500±0.003, the tolerance would be bilateral; that is, it would vary both over and under the nominal dimension. The unilateral system permits changing the tolerance while still retaining the same allowance or type of fit. With the bilateral system, this is not possible without also changing the nominal size dimension of one or both of the two mating parts.
In mass production, where mating parts must be interchangeable, unilateral tolerances are customary. To have an interference or force fit between mating parts, the tolerances must be such as to create a zero or negative allowance.
The drawing must be a true and complete statement of the designer’s requirements expressed in such a way that the part is convenient to manufacture. Every dimension necessary to define the product must be stated once only and not repeated in different views. Dimensions relating to one particular feature, such as the position and size of a hole, should, where possible, appear on the same view. There should be no more dimensions than are absolutely necessary, and no feature should be located by more than one dimension in any direction. It may be necessary occasionally to give an auxiliary dimension for reference, possibly for inspection. When this is so, the dimension should be enclosed in a bracket and marked for reference. Such dimensions are not governed by general tolerances. Dimensions that affect the function of the part should always be specified and not left as the sum or difference of other dimensions. If this is not done, the total permissible variation on that dimension will form the sum or difference of the other dimensions and their tolerances, and this will result in these tolerances having to be made unnecessarily tight. The overall dimension should always appear. All dimensions must be governed by the general tolerance on the drawing unless otherwise stated. Usually, such a tolerance will be governed by the magnitude of the dimension. Specific tolerances must always be stated on dimensions affecting function or interchangeability. A system of tolerances is necessary to allow for the variations in accuracy that are bound to occur during manufacture, and still provide for interchangeability and correct function of the part. A tolerance is the difference in a dimension in order to allow for unavoidable imperfections in workmanship. The tolerance range will depend on the accuracy of the manufacturing organisation, the machining process and the magnitude of the dimension.
The greater the tolerance range, the cheaper the manufacturing process. A bilateral tolerance is one where the tolerance range is disposed on both sides of the nominal dimension. A unilateral tolerance is one where the tolerance zone is on one side only of the nominal dimension, in which case the nominal dimension may form one of the limits. Limits are the extreme dimensions of the tolerance zone. For example, nominal dimension 30mm tolerance limits Fits depend on the relationship between the tolerance zones of two mating parts, and may be
broadly classified into a clearance fit with positive allowance, a transition fit where the allowance may be either positive or negative (clearance or interference), an interference fit where the allowance is always negative. Type of Limits and Fits The ISO System of Limits and Fits, widely used in a number of leading metric countries, is considerably more complex than the ANSI system. In this system, each part has a basic size. Each limit of size of a part, high and low, is defined by its deviation from the basic size, the magnitude and sign being obtained by subtracting the basic size from the limit in question. The difference between the two limits of size of a part is called the tolerance, an absolute amount without sign. There are three classes of fits: 1) clearance fits, 2) transition fits (the assembly may have either clearance or interference), and 3) interference fits. Either a shaft-basis system or a hole-basis system may be used. For any given basic size, a range of tolerances and deviations may be specified with respect to the line of zero deviation, called the zero line. The tolerance is a function of the basic size and is designated by a number symbol, called the grade—thus the tolerance grade. The position of the tolerance with respect to the zero line also a function of the basic size—is indicated by a letter symbol (or two letters), a capital letter for holes and a lowercase letter for shafts. Thus the specification for a hole and shaft having a basic size of 45 mm might be 45H8/g7. Twenty standard grades of tolerances are provided, called IT01, IT0, IT1~18, providing numerical values for each nominal diameter, in arbitrary steps up to 500mm (for example 0~3, 3~6,6~10, ......, 400~500 mm). The value of the tolerance unit, i, for grades 5~16 isWhere i is in microns and D in millimeters. Standard shaft and hole deviations similarly are provided by sets of formulas, however, for practical application, both tolerances and deviations are provided in three sets of rather complex tables. Additional tables give the values for basic sizes above 500 mm and for “Commonly Used Shafts and Holes” in two categories—“General Purpose” and “Fine Mechanisms and Horology”.
标注尺寸
机械设计除了计算载荷和应力、选择合适的材料外 还包括许多其它因素。在建造或制造开始前 完成装配图和零件图以把必要信息传达给车间工人是必须的。在送往车间前设计者常常被召集来检查图纸。而在精通生产图纸的所有情况之前 需要有许多经验并熟悉制造工艺。图纸必须仔细检查其尺寸是否按生产部门最方便易懂的方式标注。很明显图
纸应该只有唯一的解释。尤其是不能要求车间工人在生产机械安排前进行三角或其它复杂的计算尺寸标注是一项复杂的工作 要掌握它需要有丰富的经验。由于要把零件加工到正好为给定尺寸是不可能的 因此图纸的尺寸必须加上公差以限制其可允许的变化。虽然较小公差能得到较高加工质量和较好操作机构 但随着公差的减小制造成本会迅速增加 如图14.1的典型曲线所示。因此公差被定为从操作或功能考虑允许的最大值是重要的。
公差既可以是单向的也可以是双向的。单向标注有一公差为零 所有变化都由另一公差给定。而双向标注则采用一平均尺寸 它将公差带中点从该尺寸双向扩展为相等的正负变化范围。主轴箱 主轴箱固定在车床床身的左侧 它包括轴线平行于导轨的主轴。主轴通过装在主轴箱内的齿轮箱驱动。齿轮箱的功能是给主轴提供若干不同的速度(通常是6到18 速)。有些现代车床具有采用摩擦、电力或液压驱动的无级调速主轴箱。
主轴往往是中空的 即纵向有一通孔。如果采取连续生产 棒料能通过此孔进给。同时 此孔为锥形表面可以安装普通车床顶尖。主轴外表面是螺纹可以安装卡盘、花盘或类似的装置。尾架 尾架总成基本包括三部分 底座、尾架体和套筒轴。底座是能在车床床身上沿导轨滑动的铸件 它有一定位装置能让整个尾架根据工件长度锁定在任何需要位置。尾架体为一能横向运动的铸件 它可以调整尾架轴线与主轴箱轴线成一直线。第三部分 套筒轴是一淬硬钢管 它能根据需要在尾架体中纵向进出移动。这通过使用手轮和螺杆来达到 与螺杆啮合的是一固接在套筒轴上的螺母。套筒轴开口端的孔是锥形的 能安装车床顶尖或诸如麻花钻和镗杆之类的工具。套筒轴通过定位装置能沿着它的移动路径被锁点。
大拖板 大拖板的主要功能是安装刀具和产生纵向和/或横向进给。它实际上是一由车床床身V形导轨引导的、能在车床床身主轴箱和尾架之间滑动的H形滑块。大拖板能手动或者通过溜板箱和光杆(进给杆)或丝杆(引导螺杆)机动。在切削螺旋时 动力通过丝杆提供给溜板箱上的齿轮箱。在其余车削作业中 都由光杆驱动大拖板。丝杆穿过一对固定在溜板箱后部的剖分螺母。当开动特定操作杆时 剖分螺母夹在一起作为单个螺母与旋转的丝杆啮合 并带动拖板沿着床身提供进给。当操作杆脱离时 剖分螺母释放同时大拖板停止运动。另一方面 当使用光杆时则通过蜗轮给溜板箱提供动力。蜗轮用键连接在光杆上 并与溜板箱一起沿光杆运动 光杆全长范围开有键槽。一根轴可能的名义尺寸为2.5in.(63.5mm) 但由于实际原因不用大成本是不能在制造中保持这个数字的 因此要增加确定的公差。如果允许有±0.003in.(±0.08mm)的变化 则此尺寸可表达为 2.500±0.003(63.5±0.08mm)。具有紧密公差的尺寸表示该零件必须恰当地与某些其它零件配合。
所采用的制造工艺和使利润最大化的最小生产及装配成本都要求给定公差以保持所需允差。大规模低成本制造生产工艺的发展很大程度取决于组成零件的互换性。因此设计者必须确定单个零件的合适公差以及配合零件装配允许的正确间隙或过盈量。在图纸上标注公差的方法相当程度上依赖于产品的性质或制造工艺的类型。如果尺寸公差没有特别注明 图纸应该包含一个给出这些尺寸公差值的普遍适用注释。然而有些公司不采用普遍适用注释 假定每个尺寸是单独被考虑的 可能会规定出比注释中要求的更宽的公差。在任何情况下图纸不模棱两可并只服从于单一的解释是十分重要的。在图纸标注尺寸时 除非设计者有意标明 注在尺寸线上的数字代表的尺寸仅仅是近似的 并不代表任何精度等级。
为了详细标明精度等级 有必要在尺寸上增加公差数字。公差是零件允许的变动量或给定尺寸允许的总变动。一般而言 零件的成本随着公差的减小而上升。如果一个零件有若干或较多表面要机加工 且几乎不允许偏离名义尺寸 则成本会超过正常合理的界限。允差 有时会跟公差混淆 但其具有完全不同的含义。它是配合零件之间最小的预期间隙空间 代表着允许的最紧配合条件。如果一根尺寸为 1.498-0.003的轴与尺寸为1.500+0.003的孔配合 孔的最小尺寸为1.500而轴的最大尺寸为1.498。这样允差就是0.002 而由最小轴尺寸和最大孔尺寸形成的最大间隙为0.008。公差可以是单向的也可以是双向的。单向公差意味着任何变动都是只从名义或基本尺寸出发向一个方向变动的。引用前例 孔的尺寸标注为1.500+0.003 它表示了一个单向公差。如果尺寸标为1.500±0.003 就是双向公差 即它可以在名义尺寸之上或之下变化。单向体系允许在依然保留相同允差或配合类型的情况下改变公差。而双向体系在不同时改变一个或两个配合零件名义尺寸的情况下 这是不可能做到的。大规模生产中配合零件必须能互换 单向公差是经常遇到的。为了使配合零件之间具有过盈或强制配合 公差必须产生零或负允差。公差、极限和配合图纸必须按方便制造零件的方式将设计者的要求真实和完整地表达出来。对每一描述产品所需的尺寸都只须标注一次而不必在不同的视图中重复。有关同一特性的尺寸 诸如孔的位置和大小 如果可能应出现在同一视图上除绝对需要的尺寸外 不应该有更多的尺寸 而在任意方向上 只能在一个尺寸上标注特性要求。偶尔也可能为了检查而必须给出供参考的辅助尺寸。在这种情况下 尺寸应该用括号括起来 以便参考。这样的尺寸不受通用公差控制。影响零件功能的尺寸总是应该标注的而不要留作其它尺寸的和或差。如果不是这样 那尺寸允许的总的变化将形成其它尺寸及它们的公差的和或差 这会导致这些公差不得不定得过紧。总尺寸一般应该标注。除非另行说明 所有尺寸都必须受图上的通用公差控制。一般这样的公差受到尺寸量值的控制。在影响功能或互换性的尺寸上必
须标注专门的公差。为了允许在制造过程中必然会发生的精度变化 并提供零件的互换性和正确功能 一个公差系统是必需的。公差是为了允许工艺上不可避免缺陷而存在的尺寸上的不同。公差范围取决于制造机构的精度、机加工过程和尺寸的量值。
公差范围越大 则制造过程的成本就越低。双向公差是在公称尺寸两侧都有公差带的公差。单向公差是仅在公称尺寸一侧有公差带的公差 在这种情况下公称尺寸成了两个极限中的一个。极限是公差带的极限尺寸。例如公称尺寸30毫米公差极限配合取决于两配合零件公差带之间的关系 并且可以概括地分为具有正允差的间隙配合 允差可以是正或负的过渡配合和总是负允差的过盈配合。极限和配合的类型在一些最主要采用公制的国家中广泛使用的ISO的极限和配合系统 比ANSI的极限和配合系统要复杂得多。在这个系统中 每个零件都有基本尺寸。零件尺寸的每一极限 不管大小 都通过对基本尺寸的偏差来定义 其量值和符号由正被讨论的极限减去基本尺寸得到。零件尺寸的两个极限之差称为公差 这是一个没有符号的绝对量值。存在三种配合 1)间隙配合 2)过渡配合(装配后可以有间隙或过盈) 和3)过盈配合。基轴制或基孔制均可采用。对任何给定的基本尺寸 公差范围和偏差可以相对于被称为零线的零偏差线来确定公差是基本尺寸的函数并通过一个被称为等级的数字符号标明—即公差等级。公差相对于零线的位置同样为基本尺寸的函数通过一个或两个字母符号表达 大写字母表示孔而小写字母表示轴。这样基本尺寸为45毫米的一个孔和轴配合规格可能是45H8/g7。ISO规定了二十种标准的公差等级 称之为IT01 IT0 IT1~18 给在直至500毫米强行分段(例如0~3 3~6 6~10, ......, 400~500毫米)中的公称直径提供具体数值。对5 16级而言 公差单位i的值可用下式计算这里i的单位是微米 而D的单位是毫米。标准的轴和孔偏差同样都由若干公式提供 然而对实际应用 公差和偏差都在三张相当复杂的表格中规定了。对基本尺寸大于500毫米和在“一般用途”和“精密机械和钟表”两个类别中的“常用的轴和孔”而言 由附加的表格给出数值。