GY01-154@机床夹具柔性化技术研究及设计
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GY01-154@机床夹具柔性化技术研究及设计,机械毕业设计全套
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Unit 3 Supporting and Locating Principle OBJECTIVES After 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. REFERENTING To 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 LOCATIN To 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 locators Whenever 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. Tolerance When 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 the 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 3-1 nts3-2 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 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 nts 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。 Foolproofing Foolproofing 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 locators The 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. 3-3 Figure 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 nts 3-4 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 MOVEMENT An 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 DUBLICATE LOCATING SURFACES 重复定位面 REFERENCE SURFACE 定位基准面 nts 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 Movement In 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 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 ntsFigure 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 LOCATING THE WORK Parts 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 Surface There 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. 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。 3-6 ntsFigure 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 3-7 nts 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 LOCK ADJUSTMENT 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） OPTIONAL 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） LOCK SCREW 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 3-8 ntsAdjustable 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 HEXAGON LOCK 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。 3-9 ntsEqualizing 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 Diameter Locating 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，
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