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机床夹具柔性化技术研究及设计,机床,夹具,柔性,技术研究,设计
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3-1第章 支承与定位原理学习目标学习完该章后，学生应具备下述能力： 对应用于夹具中的定位与支承进行区分； 能详细说明定位与支承的用途； 分析典型零件，并针对每一种典型零件，选取最佳的定位与支承装置。参照位置为了确保在各加工工序中的精度，工件必须相对于刀具或其它工具取得一个正确的位置，我们将其称之为“参照位置”。为了确保所需要的精度，夹具设计师必须确认工件已经精确定位并具有坚固的支承。此外，定位元件与工件的位置布局应恰当。夹具设计师还必须确保夹具定位可靠，工件应正确合理地放入夹具中，装卸工件的时间也不能太长。定位要点限制工件的运动自由度并使其合理定位，需要技巧和事先谋划。工件定位装置绝不能事后想起来再进行安装。而必须事先纳入到夹具设计规划中。在设计夹具时，设计师必须牢记以下要点：（）布置工件定位装置的位置；（）工件的公差；（）工件定位可靠；（）重复定位问题。定位元件的布置只要可能，定位元件应当与工件的已加工表面相接触。只有如此，工件在夹具中的正确位置，以及夹具本身的重复精度才能得到保证。夹具的重复精度是指所有的被加工工件能始终保持在其所需的公差之内。对于任何夹具来说，正确的定位对于保证重复精度是至关重要的。定位元件在空间的相对位置距离应尽可能大。这样做的好处是所用定位元件少，且能保证其与工件定位表面之间的全面接触。此外，切屑及外部杂物容易引发事故，在布置定位元件时应切实注意加以防止。如无法防止，则定位元件应采用减摩结构。见图-。公差当设计一个夹具时，设计人员必须牢记工件的公差。按照一般规律，夹具的公差应取工件相应公差的20-50%。举例来说，如一工件上某孔的位置尺寸公差为0.010吋，则夹具相应的孔的位置尺寸公差应为0.002 吋到 0.005吋，见图3-2。这一规定对保持所需的加工精度是必要的。指定夹具公差接近工件相应公差20%，对工件质量的提高并不明显，而夹具制造成本则要显著提高。通常，夹具公差如大于工件相应公差的50%，工件的加工精度将不可能保证。因为决定工件加工精度的唯一关键Unit 3 Supporting and Locating PrincipleOBJECTIVESAfter completing this unit, the student should be able to: Identify the types of locators and supports used for jigs and fixtures. Specify the use of locators and supports. Analyze sample parts and select the locating and supporting devices best suited for each.REFERENTINGTo insure precision in any machining operation, the work must be properly positioned with respect to the cutter or other tool. This is called referencing. To insure the desired accuracy, the tool designer must make sure the part is precisely located and rigidly supported. Locators, in addition to properly positioning the part, make sure that the tool is easily loaded and unloaded. They must also make the tool foolproof. Little is gained if the part take too long to load or unload, or if they are incorrectly put into the tool.BASIC RULES FOR LOCATINTo restrict the movement of a part and have the part positioned properly requires skill and planning. Part locators should never be installed as an afterthought, but must be planned into the tool design. A tool designer must keep the following points in mind while designing the tool:Positioning the locatorsWhenever possible, locators should be contact the work on a machined surface. This permits accurate placement of the part in the tool and insures the repeatability of the jig and fixture. Repeatability is the feature of the tool that allows different parts to be machined consistently the same within their required tolerance. Accurate location is an important element in the repeatability of any tool.Locators should be spaced as far apart as possible. This permits the use of fewer locators and insures complete contact over the locating surface. Where chips or foreign matter may become a problem. The locators should be placed to avoid this interference. If this is not possible, the locators should be relieved, Figure 3-1.ToleranceWhen designing a tool, the designer must keep the part tolerance in mind. As a general rule, the tool tolerance should be between 20 and 50 percent of the part tolerance. For example, if a hole in a part must be located within 0.010 inches, then the tolerance of the hole in the jig must be between 0.002 inch and 0.005 inch, Figure 3-2. This is necessary to maintain the required precision. Specifying tool tolerances closer than 20 percent only serves to increase the cost of the tool and adds little to the quality of the part. Generally, tolerance greater than 50 percent do not guarantee the desired precision. The single factor that should determine this decision is the specified accuracy of the3-2因素并不是夹具公差，而是被加工工件的相应指定公差。设计定位元件时应确对保任一合格尺寸的工件都能安装得上。如图3-3所示工件，其允许加工到的最大与最小尺寸分别是1.260吋和1.240吋，在此尺寸范围内的工件都是合格的。如果我们将夹具上定位元件的相应尺寸设计成1.250吋，则1.250至1.260吋的工件将无法安装，尽管它们的尺寸是合格的。为防止这一情况发生，夹具上定位元件的相应尺寸应根据定位方式，设计为工件的最大或最小极限尺寸。part being machined.Locators must be designed to fit the part at any size within the part limits. If the pat shown in Figure 3-3 were made at its smallest size, it would be 1.240 inches in diameter. If it were made to its largest size, the diameter would be 1.260 inches. Any parts made within these sizes would be correct. If the tool is made to fit the part at its design size of 1.250 inches, the parts between 1.250inches and 1.260inches, while correct, will not fit into the tool. To prevent this, the tool must be made to fit the parts at their largest or smallest limits of size, depending on how the part is located.POSSIBLE SIZE OF PART TOOL SIZE TO SUIT LARGEST适应工件最大可能尺寸的夹具尺寸VARIATIONS IN PART SIZES（工件尺寸变化范围）Figure 3-3 Part and tool size relationship图3-3 工件与夹具的尺寸关系Figure 3-1 Methods of relieving locators图3-1 定位元件减摩方法Figure 3-2 Tolerance relationship图3-2 公差关系3-3Figure 3-4 Foolproofing图3-4 无误定位安装REQUIRED HOLE需要的孔CORRECT LOADING工件安装正确INCORRECT LOADING工件安装错误FOOLPFOOFING PIN无误安装定位销FOOLPFOOFING PIN无误安装定位销FOOLPFOOFING PIN无误安装定位销LOCATING PIN定位销LOCATOR定位元件LOCATOR定位元件FoolproofingFoolproofing is a means by which the tool designer insure that the part will fit into the tool only in its correct position. The part in Figure 3-4A must be machined on the tapered end, so the tool designer includes a pin prevent the part from being loaded incorrectly. This pin foolproofs the tool. The part in Figure 3-4B shows a hole that must be drilled with reference to the holes in the flange. A simple pin placed in one of these holes makes it impossible to load the tool incorrectly.Other foolproofing devices are just as simple. If the foolproofing devices are not simple, they tend to complicate an otherwise easy task.Duplicate locatorsThe use of duplicate locators should always be avoided. Figure 3-5 shows examples of duplicate locators. Locator duplication not only costs more but could cause inaccuracies.For example, the flange in Figure 3-5A is located on both the underside of the flange and the bottom of the hub. Since these are parallel surfaces, only one is needed and the other should be eliminated. If the reference surface is the flange, as in Figure 3-5B, the hub locator is not necessary. If the hub is the reference surface, as in 3-5C, then the flange locator is unnecessary. To correct this, the tool designer must first determine which surface is to be referenced. Only then should the locators for that surface be specified.无误定位安装无误定位安装是一种手段，夹具设计师凭此保证工件只有在处于正确位置时，才能安装到夹具上。图3-4A所示工件需要在有锥面的一端进行加工，所以夹具设计师设置了一只销子以防止工件被误定位。这只销子使得夹具有了无误定位安装功能。图3-4所示为法兰盘上一个与其它已加工孔有相互位置要求的待加工孔，在一个已加工孔中放置一个简单的菱形销，便能保证定位无误。其它的无误定位安装装置也应尽可能简单。如果无误定位安装装置过于复杂，它可能将导致另外本来容易的问题复杂化。重复定位重复定位始终是应当避免的。图3-5表示了几个重复定位的例子。重复定位不仅造成夹具成本提高，而且会导致夹具不能使用。例如，图3-5A所示法兰盘同时以法兰下端面及底部在定位套筒上定位。由于这两个表面是平行的，因而只需要一个即可，另一个应当除去。如果定位基准是法兰，则底端定位就不须要，如图3-5B。如果定位基准是底端，则法兰定位就不须要，如图3-5C。要正确地做到这一点，夹具设计师必须首先确定工件的哪一个表面是定位基准。只有确定了其定位基准，才能为其设置定位装置。 工件定位表面与夹具定位元件的位置公差也能导致重复定位错误。如图3-6，同时以工件的侧面和孔来定位就会造成问题。首先，夹具上定位销的位置是固定的，不可能任意改变以适应任何一个工件；其次，工件上作为定位基准的孔的位置是允许在一定范围内变动的。当工件的位置尺寸处于极限状态之一时，它就可能无法装到夹具上去。要消除这一可能发生的情况，可以减小孔内定位销的尺寸，以适应工件的相应位置尺寸变化。但这样做的结果是导致定位精度最低，定位装置失去了其意义。要防止这种重复定位错误，夹具设计师必须在工件的侧面与孔之间选择其一作为定位基准，而不是二者都用。自由度问题一个自由物体具有12个运动自由度。图3-7表示了一个物体与三个坐标轴或坐标平面的关系。一个物体可以沿着任何坐标轴作任意方向旋转或平行运动。为了形象地说明这一关系，坐标平面被分别命名为“X-X” “Y-Y” 和 “Z-Z”。运动方向则从1到12进行编码。Locational inaccuracies develop because of the difference in position and location tolerances between the tool and the work, Figure 3-6. Locating the part from both its outside edge and the holes can create problems. First, the location of the pins in the tool is fixed and cannot be changed to suit each part. Second, the location of the holes in the part is variable within limits. When a part is placed in the tool which is at either extreme of the part tolerance, it may not fit. To eliminated this possibility, the hole locator can be made smaller to accommodate the variation, but if this is done, the effectiveness of the hold locator is minimized and the locator become useless. To avoid this problem, the tool designer must specify whether or not the part is to be located from its holes or edges, never both.PLANES OF MOVEMENTAn unrestricted object is free to move in any of twelve possible directions. Figure 3-7 shows an object with three axes, or planes, along which movement may occur. An object is free to revolve around or move parallel to any axis in either direction. To illustrate this, the planes have been made “X-X,” “Y-Y,” and “Z-Z.” The directions of movement are numbered from 1 to 12.Figure 3-6 Position and locational differences图3-6 工件位置与定位元件位置的差别VARIABLE POSITION OF HOLES工件定位孔的位置是变化的FIED POSITION OF LOCATORS夹具定位元件的位置是固定的DUBLICATE LOCATORS重复定位元件Figure 3-6 Duplicate locators图3-5 重复定位元件DUBLICATELOCATINGSURFACES重复定位面REFERENCESURFACE定位基准面3-4 Figure 3-7 Planes of movement图3-7 运动平面Figure 3-8 Adjusting block图3-8 调整挡块限制自由度为了使得工件能在夹具中获得并保持正确的位置，必须限制其运动自由度。这一功能是由定位元件和夹紧元件共同完成的。现以图3-8所示零件的夹具为例来说明限制自由度的原理。图3-9所示为该工件被置于三销组合支承面上，其2、5、1、4及12这五个自由度被限制了。采用销式或按扣式定位元件使工件避免与切屑接触，再通过限制定位元件与工件的接触面积，能最大程度地减少定位差错。平面型支承也可使用，但应将其做成装配式的，而不是直接加工成一个支承平面。装配式定位元件其装配省时且便于更换，所示从使用角度来说是经济的。当采用按扣式或平面型支承元件时，最为重要的问题是使工件避免与切屑接触，并切实保证工件与三个定位元件的支承面都可靠接触。为了限制工件绕”Z-Z”轴转动及沿8方向移动，设置了双销式定位元件，如图3-10。为了限制工件沿7方向移动，采用了一个销式定位元件。这种3-2-1支承点，或称 6支承点定位方法，是用于对正方或长方形零件的外表面进行定位的最为普遍的方法。如果工件上有可以利用来定位的孔，则是最好不过了。如图3-12所示，中心孔被用来作为第一定位基准，而其它孔中的一个被用来作为第二定位基准。第一定位基准采用圆柱销为定位元件，而第二定位基准则采用菱形销。可见，基础板及中心孔中的圆柱定位销，限制了工件的9个运动自由度(1,2,4,5,7,8,10,11,12)，菱形销又进一步限制了其2个自由度(6, 3)。这些定位元件合起来共限制了工件的11个自由度。这样，工件唯一能运动的方向是垂直向上。因此，夹紧装置仅需要限制工件的这一个自由度就可以了。Restricting MovementIn order to accurately locate the part in a jig or fixture, movements must be restricted. This is done with locators and clamps.The fixture for the part in Figure 3-8 illustrates the principle of restricting movement. By placing the part on a three-pin base, five directions of movement (2,5, 1, 4, and 12) are restricted., Figure 3-9. Using pin- or button-type locators minimizes the chance of error by limiting the area of contact and raising the part above the chips. Flat bases may also be used, but these should be installed rather than machined into the base. Installed locators are less expensive to use because they take less time to install and are replaceable. If button or flat locators are used, the most important consideration is keeping the part above the chips and in constant contact with all three locators.To restrict the movement of the part around the ”Z-Z”-axis and the direction 8, two more pin-type locators are positioned. Figure 3-10. To restrict direction 7, a single-pin locator is used, Figure 3-11. The remained directions, 9,10and 11, are restricted by a clamping device. This 3-2-1, or 6-point, locating method is the most common external locator for square or rectangular parts.When a workpiece having holes is located, the holes provide am excellent method of locating the complete part. As shown in Figure 3-12., the center hole is used as a primary locator and one of the other holes is used as a secondary locator. Here the primary locator is a round pin and the secondary locator is a diamond pin. As shown, the base plate with the round pin positioned in the center hole will restrict nine degrees of movement (1,2,4,5,7,8,10,11 and 12). The diamond pin, located as shown, further restricts the another two degrees of movement (6 and 3). Together, these locators restrict 11 degrees of movement. The only direction the workpiece can move in is straight up, so the clamping device is actually only holding one direction of movement.3-5 3-6给工件定位工件几乎能制造成任一种可能的形状及尺寸。不管工件是如何制造的，夹具设计人员必须有能力对任何一个工件设计正确的定位装置。要做到这一点，夹具设计人员必须充分了解各种不同类型定位元件的功能特点，以尽可能使用最少的定位元件，得到最佳的定位效果。用平面定位工件以平面作为定位基准时，主要有三种定位方法：固定支承、可调支承和自平衡支承。这些支承元件在工件定位表面的垂直方向上对工件进行支承，并防止工件在加工过程中产生变形。固定支承的应用是最为容易的。这种支承既可直接在夹具体上加工出来，也可做成装配式的。如图3-13。这一类支承通常用于工件以已加工表面作为定位基准的场合。LOCATING THE WORKParts are made in almost every possible shape and size. The tool designer must be able to accurately locate each part regardless of how it is made. To do this, the tool designer must know the various types of locators and how each should be used to get the best part placement with the least number of locators.Locating from a Flat SurfaceThere are three primary methods of locating work from a flat surface: solid supports, adjustable supports, and equalizing supports. These locators set the vertical position of the part, support the part, and prevent distortion during the machining operation.Solid supports are the easiest to use. They can be either machined into the tool base or installed, Figure 3-13. This type of supports is normally used when a machined surface acts as a locating point.Figure 3-9 Three-pin base restricts five directions of movement图3-9 三销支承限制工件五个方向的运动Figure 3-10 Five-pin base restricts eight directions of movement图3-10 五销支承限制工件八个方向的运动3-7Figure 3-11 Six-pin base restricts eight directions of movement图3-11 六销支承限制工件九个方向的运动Figure 3-12 Primary and secondary locators图3-12 主要定位元件与第二定位元件SECONDARY LOCATOR(Diamond pin locator)第二定位元件(菱形销)PRIMARY LOCATOR(Round pin locator)主要定位元件(圆柱销)Figure 3-11 Six-pin base restricts nine directions of movement图3-11 六销支承限制工件九个方向的运动Figure 3-13 Solid supports图3-13 固定支承Figure 3-14A Adjustable supports, threaded type.图3-14A 可调支承(螺纹式)STATIONARY MOVABLE LOCK NUT LOCKADJUSTMENT LOCK NUT SCREW固定调节 活动调节 锁紧螺母 锁紧螺纹CARBIDE INSERT渗碳淬硬块（插入）3-8OPTIONAL BASE CAN BEPRESSED INTO TOOL BODY可换基础支座能压入夹具体中AVAILABLE WITH OR WITHOUT ADJUSTING SCREW调节螺钉（可用亦可不用）LOCKSCREW锁紧螺纹Figure 3-14B Adjustable supports, spring type.图3-14B 可调支承(弹簧式)Figure 3-14C Adjustable supports, push type.图3-14C 可调支承(推入式)SPRING弹簧Adjustable supports are used when the surface is rough or uneven, such as in cast parts. There many styles of adjustable supports. A few of the more common are the threaded, Figure 3-14A, spring, Figure 3-14B, and push types, Figure 3-14C. The threaded style is the easiest and most economical, and has a larger adjustment range than the others. Adjustable locators are normally used with one or more solid locators to allow any adjustment needed to level the work.CONTACT POINTS接触点CONTACT POINTS接触点PIVOT POINT支点Figure 3-15 Equalizing supports图3-15 自平衡支承SCREWED AND DOWELED PRESS FIT THREADED PRESS FIT AND THREADED螺钉与销固定 压入式装配 螺纹定心紧固 压入及螺纹紧固HEXAGONLOCK NUT六角锁紧螺母Figure 3-16 Internal locators图3-16 内孔定位元件Figure 3-17 Pin locators and bushing图3-17 销式定位元件与衬套ROUND BULLET PLAIN CONICAL DOWEL圆头 子弹头 平头 圆锥头 沉孔销 REGULAR CONICAL 常规型 锥孔型Figure 3-18 Round and tapered locators图3-18 圆头与圆锥头定位元件 CANTACT AREA接触面 CANTACT AREA接触面ROUND TAPERED OR BULLET圆头 圆锥头或子弹头可调支承用于粗糙不平的表面，如铸造表面。这类支承有多种类型，图3-14A、3-14B和 3-14C分别表示的是螺纹式、弹簧式和推入式。其中，螺纹式结构最为简单和经济，而且比其它类型的调整范围要大得多。通常，可调支承与一或多个固定支承配合使用，以使工件调整至需要的位置。3-9Equalizing supports, Figure 3-15 are also a form of adjustable supports. They provide equal support through two connected contact points. As one point is depressed, the other raises and maintains contact with the part. This feature is especially necessary on uneven cast surfaces.The terms locator and support are used interchangeably when the devices used under a workpiece are discussed. The locating devices used to reference the edges of a part are called locators or stops.Before choosing a support, the tool designer must consider the shape and surface of the part and the type of clamping device to be used. The support selected must be strong enough to resist both the clamping pressure and the cutting forces. The clamps should be positional directly over the supports to avoid distorting or bending the part.Locating from an Internal DiameterLocating a part from a hole or pattern is the most effective way to accurately position work. Nine of the 12 directions of movement are restricted by using a single pin, and 11 are restricted with 2pins. When possible, it is logical to use holes as primary part locators.Several types of locators are used for locating work from holes. Figure 3-16 shows a few used for large holes. When large holes locate the work, fasten the internal locator with both screws and dowels. Under normal conditions, two dowels and two screws are needed to hold the locator. With more force, it is better to use larger dowels and screws rather than to increase their number.With shank-type locators, it is a good practice to use the press-fit rather than the threaded locator for accuracy. Threaded locators are useful in areas where the construction of the tool will not permit the other type to be pressed out. Another type has the advantages of the press fit and the locking properties of a thread.Pin-type locators are used for smaller hole and for aligning members of the tool, Figure 3-17. When the pins are used for alignment, special bushings should also be used so they can be replaced when they wear. Pins used for part location are made with either tapered or rounded ends, Figure 3-18, allowing the parts to be installed and removed easily.The main difference between the pins used for location and the pins used for alignment is the amount of bearing surface. Alignment pins usually have a longer area of contact. Locating pin usually have a contact area of one-eighth to one half of the part thickness. More than this makes placement and removal operations difficult.Another style of pin common to jigs and fixtures is the diamond or relieved pin, which is normally used along with the round pin to reduce the time it takes to load and unload the tool. It is easier to locate a part on one round and one diamond pin than to locate it on two pins.图3-15所示的自平衡支承，也是一种可调支承。其特点是一个支承，却有二个互动的接触点。当一个接触点下降时，另一个就上升，以保持两点对工件同时接触。对于粗糙不平的铸造表面进行支承时，这一技术特点是极为必要的。当所讨论的装置是用于工件时，定位元件与支承元件这两个术语，可交替使用。用来确定工件边缘位置的定位装置，称为定位元件或挡块。在选择支承元件之前，夹具设计人员必须充分考虑工件表面的形状，以及所要采用的夹紧装置的类型。所选取的支承元件必须具有足够的支承刚性，以抵抗夹紧力与切削力。夹紧元件应直接位于支承元件之上，以防止夹紧力引起工件颠覆或产生弯曲变形。以内孔定位要精确地对工件进行定位，以孔或内部成形面作为定位基准是一个最为有效的方法。用1只销就能限制工件的9个自由度，用2只销则能限制工件的11个自由度。只要可能，应尽量选择内孔作为主要定位基准。给工件内孔定位的元件有多种形式。图3-16表示的是几种用于大直径孔定位的定位元件。当工件以大直径孔为定位基准时，要使用螺钉和连接销来固定内孔定位元件。在通常情况下，需要采用二只螺钉和二只连接销。定位元件受力较大时，最好是采用大规格的螺钉和连接销，而不是增加他们的数量。采用台阶轴式定位元件时，实践经验表明，压入装配式定位元件的精度要高于螺纹定心式的。只有在夹具结构不允许压出定位元件时，才使用螺纹定心式定位元件。另外一种定位元件则同时兼具压入装配及螺纹锁紧之双重优点。销式定位元件如图3-17所示，它们用来对工件的小直径孔进行定位，以及用来确定夹具零件之间的相对位置。当其用途为后者时，应当使用特殊的衬套，以便磨损后进行更换。用来对工件定位的定位销，其头部制成球帽状或锥帽状，以便于工件的装卸。见图3-18。定位用的销与连结用的销之间的主要区别，是摩擦面的大小。连结销的接触长度一般较长；而定位销的接触长度一般为工件厚度的1/8到1/2。接触长度过大将使工件的装卸变得困难。另一种普遍用于夹具上的定位销是菱形销，或称减摩销。这种销通常与圆柱销以其使用，以减少夹具的装卸时间。将一个零件装到一只圆柱销和一只菱形销上，要比装到二只圆3-103-11柱销上要容易得多。从作用上看，圆柱销对工件进行定位，而菱形销则防止工件绕圆柱销转动。如图3-19。请注意，工件运动的方向是绕圆柱销转动。通过设置上如图所示的菱形销，这个运动自由度被限制了。为使定位有效，菱形销必须布置在限制这个自由度的位置上。图3-20表示的是两个菱形销怎样对一个工件进行定位的情况。要注意二者是怎样相互配合来限制工件自由度的。当工件具有适当的位置公差时，应当使用两个菱形销对其进行定位。除了菱形销以外，某些工件的定位也采用其它形式的减摩式定位元件。图3-21表示的是一些典型的减摩式定位元件。是否采用特殊的减摩式定位元件，取决于工件的结构及其定位的需要。减摩式定位元件能够减少工件与定位元件之间的接触面积。增加接触面积对夹具总的定位精度的提高，影响甚微，或没有影响。而减少接触面积却能便于夹具的装卸，并能降低灰尘、切屑及毛刺引起的麻烦。图3-22所示的分段接触定位元件，是减摩式定位元件的一种，用来对厚的工件进行定位。在工件厚度较大时，不应当使工件的全部厚度都用以定位。而应将定位元件中部设计成与工件不接触，仅以头部和底部与工件接触。这种设计能可靠地减少定位元件与工件定位孔之间的咬合现象，避免了二者之间全面接触带来的弊病。图3-23所示的高位接触定位元件，是减少摩擦以达优越定位功能之范例。这种定位元件头部与底部的接触区域均被切除掉了。这一设计不仅减少了接触面积，而且提高了定位元件与工件之间的接触点。使接触点离开基准面，移到工件中部，有利于减少灰尘、切屑与毛刺的影响。高位接触的定位设计，利用了全部的定位表面，但减少了定位元件在工件孔中的接触几率。为了提高定位功效，人们设计了大量不同类型的定位元件。然而，只有一种定位元件在任何定位孔中都不会产生咬合现象。这就是球形定位元件。通过切除与工件接触区域的全部材料，球形定位元件极大地减少了接触面积。球形定位元件不会产生咬合现象的原因，在于其接触区两侧之间的距离总是相等的，而圆柱形定位元件则非如此。如图3-24A，若工件在一圆柱销上定位，除非工件孔的中心线与定位元件的中心线正好重合，否则二者之间就要发生咬合。产生这一现象的原因，是由于二条中心线不重合而形成的直角三角形的斜边效应。d所示的销直径要小于D所示的椭圆式销的直径。在直径方面的这一差别，正是圆柱销会产生咬合现象的原因所在。pins. In use, the round pin locates the part and the diamond pin prevents the movement around the pin, Figure 3-19. Notice the direction of movement the part has around the round pin. By installing the diamond pin as shown, this movement is restricted.To be effective, the diamond pin must always be placed to resist this movement. Figure 3-20 shows how two diamond pins could be used to locate a part. Notice how each restricts the direction of movement of the other. Two diamond pins should be used to locate a part when the part has adequate locational tolerance.In addition to the diamond-pin type of relieve locator, other types are used for some workpiece. A few examples of relived locators are shown in Figure 3-21. The specific design of any relieved locator is determined by the workpiece and the type of location required. Relieved locators reduce the area of contact between the workpiece and the locator. Decreasing the contact area has little or no effect on the overall locational accuracy; however, reducing the contact area helps make the jig or fixture easier to load and unload and lessens the problems caused by dirt, chips, and burrs.The split contact locator, shown in Figure 3-22, is a type of relieved locator used for thick workpieces. Here, rather than using the complete thickness of the part for location, the locator is relieved in the middle and only the top and bottom areas of the locator contact the workpiece. This design provides full location and also makes the locator less likely to bind in the workpiece. The raised contact locator, shown in Figure 3-23, is an example of relieving a locator for better function. Here the top and bottom contact areas of the locator have been removed. This design reduces the contact area as well as raises the point where the locator and workpiece touch. Moving this contact point off the baseplate, to the middle of the workpiece, helps reduce the effects of dirt, chips, or burrs. The raised-contact design supplies a complete locating surface and reduces the chance of the locator binding in the hole.To increase the overall locational efficiency, a great number of locator designs have been examined. However, there is only one style that will not bind in any locating hole. This is the spherical locator. A spherical locator greatly reduces the contact area by removing all the material not directly in contact with the workpiece.Spherical locators are impossible to bind because, unlike with cylindrical locators, the distance between the opposite sides of the contact area is always the same. As shown in Figure 3-24A, if a part is located on a cylindrical pin, it can and will bind unless the center line of the hole is precisely aligned with the center line of the locator. This phenomenon is caused by the effect of the hypotenuse of the triangle formed by the center lines if they are not perfectly aligned. The diameter of the pin shown at d is smaller than the elliptical form of the pin shown at D. This difference in diameters is what causes the cylindrical pin to bind. 3-12SPHERICALLOCATOR球式定位元件CYLINDRICALLOCATOR圆柱定位元件Figure 3-22 Split contact relieved locator图3-22 分段接触减摩定位元件SPLITCONTACTAREA分段接触区域THICKERWORKPIECE较厚工件RAISEDCONTACTAREA高位接触区域CONTACT AREA RAISED OFF THE BASEPLATE离开基准面的高位接触区域Figure 3-23 Raised contact relieved locator图3-23 高位接触减摩定位元件Figure 3-24 Spherical locators图3-24 球式定位元件WORKPIECE工件PART MOVEMENT ARRESTED WITH RELIEVED PIN采用减摩定位销限制工件自由度RELIEVEDLOCATOR减摩定位元件ROUNDLOCATOR圆柱定位元件Figure 3-19 Locating with one relieved locator图3-19 采用一个减摩定位元件的定位方式DIAMOND OR RELIEVED LOCATOR菱形或减摩定位元件Figure 3-20 Locating with two relieved locator图3-20 采用二个减摩定位元件的定位方式Figure 3-21 Relieved locators图3-21 减摩定位元件SPLIT RAISED SPHERICALCONTACT CONTACT CONTACT分段接触 高位接触 球式接触CONTACTAREA接触区域CONTACTAREA接触区域SPHERICALCONTACT球式接触RELIEF GROOVE减摩槽Figure 3-25 Alternative relieved locator designs图3-25 改良型减摩定位元件THIN BAND CONTACT细窄接触带However, as shown in Figure 3-24B, a spherical locator always has the same diameter regardless of how the center lines of the part and the locator vary. This can be seen by the three positions of the diameter lines. Regardless of where the diameter, d, is measured it is always the same size. This design results in a locator that only contacts the workpiece with a thin, single line of contact around the locator. While such locators cannot bind in the hole, they also have their own problems. Since a spherical locator only contacts the workpiece with a single-line contact, locator wear can become a real problem. Should any wear occur on this locator, the locational accuracy is immediately affected. To minimized this problem, these locators should only be used where sufficient tolerance for wear is permitted. To reduce the effects of wear, the spherical surface may also be hardened.To achieve both the nonjamming characteristics of a spherical locator and the extended service life of a relieved locator, a modified form of locator can be used. Two examples of this type are shown in Figure 3-25. The locator shown at the left is a form of raised contact relieved locator that has a very thin contact band. This design offers more resistance to wear, while the 45-degree relief angles make the locator less likely to bind or jam during loading and unloading operations. The locator shown at the right has a similar design, but uses a relief groove in place of the lower relief angle.Locating from an External ProfileLocating work from an external profile, or outside edge, is the most common method of locating work in the early stages of machining. Profile locators position the work in relation to an outside edge or the outside of a detail, such as a hub or boss. The following are examples of the most common ways a part can be located from its profile. Nesting locators position a part by enclosing it in a depression, or recess, of the same shape as the part. Nesting is the most accurate locating device for profile location. Since the nest must conform to the shape of 但是，无论工件孔的中心线和定位元件的中心线如何变化，一个球式定位元件的直径却总是保持不变的，如图3-24B。这一特点可以通过三个位置的直径线看出来。无论在何位置，被测量的直径d,，总是同一尺寸。这一结构设计所导致的结果是，定位元件与工件之间的接触，是一条环绕定位元件的极窄的线接触。因此，尽管球式定位元件不会与工件的孔发生咬合，它们仍然有自身的问题。由于球式定位元件与工件之间是单纯的线接触，其磨损便成了一个实际问题。只要定位元件发生了任何磨损，定位精度就会马上收到影响。基于这一原因，这类定位元件仅适于在允许定位元件有足够磨损量的场合应用。为了减少磨损影响，球式定位元件一般也应淬硬。为了使减摩定位元件既能具有球式元件不易卡滞的优点，又能延长使用寿命，可以采用改良型的定位元件。图3-25表示的是改良型的定位元件的两种范例。左边表示的是一种具有极窄接触带的高位接触式定位元件。其耐磨性要显著高于球式元件，而45度的减摩角也在一定程度上减轻了装卸工件时发生咬合及卡滞的可能性。右边所示的定位元件结构与左边类似，特点是用一个减摩槽代替了左边结构中较低的减摩角。以外部轮廓定位在早期的制造业中，以工件的外部轮廓或外缘定位是最为普遍的定位方法。成型定位元件的形状与工件外缘或具体的外部轮廓相适应，使工件获得定位，就像轴套或轮毂一样下面是一些工件以其外部轮廓定位的最为普通的方法。嵌套式定位元件，是将工件装入一个与工件外形一样的凹形套中，来对其进行定位。嵌套是成形定位中最精确的定位装置。由于嵌3-13套的形状必须与工件一致，所以复杂形状的嵌套从设计到制造都是很昂贵的。图3-26所示的环状嵌套是最为普通的一种嵌套，它一般用来对圆柱形轮廓的工件进行定位。图3-27所示为全包式嵌套，用于非圆柱形轮廓工件的定位。局部式嵌套是全包式嵌套的变形，它只使工件的局部得以嵌入，如图3-28.V形定位元件主要用于圆形外廓工件。它们可以对圆形外廓、角状端部及圆盘形平板工件进行定位，如图3-29。V形块通常用于轴类及其它具有柱形截面零件的定位。见图3-30。V形定位元件相对于其它定位元件的优点之一是其对中性能。当使用V形定位元件时，要确保工件的尺寸误差所引起的定位误差在许可范围之内。见图3-31。当一个工件既不能用嵌套、也不能用V形块定位时，可使用固定止挡定位元件进行定位。固定止挡可能加工在夹具体上，也可能是装配式的，见图3-32与3-33。使用装配式定位元件一般来说是较为经济的，因为其制造周期相对于机械加工成形的固定止挡定位元件要短。由于装配式定位元件在磨损后便于更换，所以不必要重新制造整个夹具体。最普通形式的固定止挡定位元件是联接销。联接销也用于其它定位装置的联接。例如图3-34所示的挡块。为了降低成本，应尽可能使用开口式或槽式定位销来代替联接销；它们的保持功能是一样的。由于开口式或槽式定位销的安装孔不需要铰制，其定位精度相对要低一些。为了节省时间与成本，当工件公差允许时，应当使用开口式或槽式定位销。使用图3-36所示的可调止挡定位元件也能使夹具成本降至最低。由于这类止挡装置是可调节的，所以它们在夹具体上的位置没有必要严格控制。一种通常的做法是同时使用固定与可调止挡定位元件。图3-37所示夹具中，工件的一边采用固定止挡定位，另一边则采用可调止挡定位；可调止挡能使工件处于正确位置。当因磨损或工件位置不正确需要进行调节时，是很方便的。可调止挡定位元件的另一优点是可作为夹紧元件使用。其方法是将可调螺钉换成滚花头螺钉。如图3-38。低精度零件的加工可采用目测定位，如图3-39。采用目测定位装置来确定工件位置的方法有两种：一种是利用蚀刻在夹具体上的直线，如图3-39A；另一种是利用加工在夹具体上的槽，如图3-39B。在这两种情况下，依照标记放置工件直至其处于大致的中心位置，而后将其夹紧并进行加工。3-14the part, nests are very expensive to design for complicated shapes. The most common type is the ring nest., which is normally used for cylindrical profile, Figure 3-26. The full nest completely encloses shapes other than cylindrical, Figure 3-27. The partial nest is a variation of the full nest and only encloses a part of the workpiece, Figure 3-28.Vee locators are used mainly for round work. They can locate flat work with rounded or angular ends and flat discs, Figure 3- 29. The vee-block locator is normally used to locate round shafts or other workpieces with cylindrical sections, Figure 3-30.One advantage Vee locators have over other locators is their centralizing feature. When using a vee locator, be sure it is positioned to allow for the differences in part sizes, Figure 3-31.Fixed-stop locators are used for parts that cannot be placed in either a nest or a vee locator. They are either machined into the tool body, Figure 3-32, or installed, figure 3-33.Installed locators are normally more economical to use because of the time it takes to make the machined locators. Since installed locators can be replaced when worn, the entire tool body does not have to be made again.The most common type of fixed locator is the dowel pin. Dowel pin also attach other locational devices, such as the block in Figure 3-34.When possible, split pins or groove pins should be used in place of dowel pins to reduce the cost. Split pins and groove pins hold as well as dowel pins, Figure 3-35. However, they do not require a reamed hole, so they are not as accurate. When the tolerance permits, to reduce the time and cost, these pins should be used.Adjustable-stop locators can also be used to keep the cost of a tool to a minimum., Figure 3-36. Since these stops are adjustable, their position on the tool body does not have to be as closely controlled.One common way to locate parts is to use both fixed and adjustable stops. The tool in Figure 3-37 shows how the fixed locator is used to reference the end of the part while the adjustable locators are used on both sides. Using adjustable locators for this jig allows the part to be positioned exactly. If adjustment is necessary because of wear or misalignment, it can be easily corrected.Another advantage of the adjustable-type locator is its ability to double as a clamp. This is done by replaceing the adjustment screw with a knurled-head sarew, Figure 3-38.Sight locators align rough parts in a tool for approximate machining, Figure 3-39. There are two methods of referencing a part by sight location: by lines etched on the tool, as in Figure 3-39A, or by slots, also shown in Figure 3-39B. In both cases, the part is aligned with the marks until it is in the approximate center. It is then clamped and machined.3-15顶出器顶出器用来从封闭式定位元件中取出工件，如全包嵌套或环形嵌套。这些装置能使工件快速从夹具上卸下，因而减少了占用夹具的时间，并由此提高了生产率。图3-40表示的是夹具中常用的两种顶出器。Ejector Ejectors are used to remove work from closed-fitting locators, such as full nests or ring nests. These devices speed up the unloading of the part from the tool, which reduces the in-tool time and increases the production rate. Figure 3-40 shows two styles of ejectors common to both jigs and fixtures.3-163-17弹性止挡钮与弹性定位销图3-41所示为弹性止挡钮与弹性定位销。他们不是直接定位装置，而是适当地对工件定位加以辅助。这些装置用来将工件推向定位元件，以确保夹紧过程中工件与定位元件之间有适当的接触。三种最普通的弹性止挡钮是球头钮、平面钮和扁尾平面钮，如图A；其应用如图B，三只弹性止挡钮使工件保持在与定位销接触的位置。Spring-Stop Buttons and Spring-Locating PinsSpring-Stop Buttons and Spring-Locating Pins, Figure 3-41,while not locating devices, do aid in properly locating a part. These devices are used to push the part against the locators to insure proper contact during the clamping operation. The three styles of spring-stop buttons most commonly available are the contact button, flat button, and flat with tang, shown in view A. The application of these units is shown in view B. Here the part is held against the locating pins with three spring-stop buttons. 3-18使用止挡钮不仅能帮助工件定位,而且在夹紧工件时不再需要其它辅助装置。图C所示的弹性定位销是基于上述理念的一个变种。这种弹性定位销对小型零件及结构空间受限制的场合，是极为有用的。这种销可安装在孔中，或偏心衬套中。偏心衬套能使销的位置成为可调的，所以能适应较宽松的工件公差。图D表示的是弹性定位销的应用情况。首先将工件置于夹具上方，然后将其靠紧固定定位元件，并顶着弹性定位销压下。完全就位后，弹性定位销能将工件推向固定定位元件。要点本章介绍了以下重要概念： 要获得正确的定位，定位元件必须使工件获得正确的相对位置，并确保在整个生产周期中具有可靠的重复定位精度。- 相对位置是指工件与刀具或其它工具的相对位置。- 重复定位精度是指在规定公差内的待加工工件，在生产过程中均不会因定位问题而产生废品。 夹具设计人员需要牢记的几个主要概念是定位元件的位置、定位公差、无误安装定位及防止重复定位。 任一工件都有12个运动自由度。夹具设计人员在谋划定位时，应选择能限制自由度数最多的定位方法。- 3-2-1, 或6点定位方法能限制工件的9个自由度。- 仅用一个定位元件来对一个具有（中心）孔的零件进行定位，就能限制9个自由度。再加上第二个定位元件对工件上的另一个孔进行定位，总起来能限制12个自由度。- 无论何种定位方式，工件剩余的未限制的自由度，均由夹紧装置来进行限制。 位于工件下方的定位装置称为支承，以其边缘来对工件进行定位的装置称为定位元件或止挡。- 三种主要形式的支承是固定、可调与自平衡支承。- 内孔定位元件的主要形式是定位台阶轴和定位销。定位销也可用于对工件外廓进行定位。- 其它定位装置包括嵌套、V形块、固定止挡、可调止挡与目测定位装置等。 顶出器是用于将工件从封闭式定位元件中取出的装置。顶出器形式繁多，机械杠杆式与弹簧式仅是其中的两种范例。Using these buttons not only helps position the part but also eliminates the need for a third hand when clamping some parts.The spring-locating pins, shown in view C are another variation of this idea that is very useful for smaller parts, or in a confined space. These pins may be installed in a hole or mounted in an eccentric liner. This liner permits the position of the spring pin to be adjustable to hold parts with looser tolerances. The application of these spring pin is shown in view D. The first step is to position the part over the workholder. The part is then butted against the solid locator and push down against the spring pin. When fully seated, the spring-locating pin pushes the workpiece against the solid locator, as shown.SUMMARYThe following important concepts were presented in this unit: To achieve proper location, the locators must properly reference the part and insure the repeatability of location throughout the production cycle.- Referencing is the process of properly positioning the part with respect to the cutter or other tool.- Repeatabil
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