1308-機床夾具柔性化技術(shù)研究及設(shè)計

1308-機床夾具柔性化技術(shù)研究及設(shè)計,機床,夾具,柔性,技術(shù)研究,設(shè)計 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 the第３章 支承與定位原理學(xué)習(xí)目標(biāo)學(xué)習(xí)完該章后，學(xué)生應(yīng)具備下述能力：? 對應(yīng)用于夾具中的定位與支承進行區(qū)分；? 能詳細說明定位與支承的用途；? 分析典型零件，并針對每一種典型零件，選取最佳的定位與支承裝置。參照位置為了確保在各加工工序中的精度，工件必須相對于刀具或其它工具取得一個正確的位置，我們將其稱之為“參照位置” 。為了確保所需要的精度，夾具設(shè)計師必須確認工件已經(jīng)精確定位并具有堅固的支承。此外，定位元件與工件的位置布局應(yīng)恰當(dāng)。夾具設(shè)計師還必須確保夾具定位可靠，工件應(yīng)正確合理地放入夾具中，裝卸工件的時間也不能太長。定位要點限制工件的運動自由度并使其合理定位，需要技巧和事先謀劃。工件定位裝置絕不能事后想起來再進行安裝。而必須事先納入到夾具設(shè)計規(guī)劃中。在設(shè)計夾具時，設(shè)計師必須牢記以下要點：（１）布置工件定位裝置的位置；（２）工件的公差；（３）工件定位可靠；（４）重復(fù)定位問題。定位元件的布置只要可能，定位元件應(yīng)當(dāng)與工件的已加工表面相接觸。只有如此，工件在夾具中的正確位置，以及夾具本身的重復(fù)精度才能得到保證。夾具的重復(fù)精度是指所有的被加工工件能始終保持在其所需的公差之內(nèi)。對于任何夾具來說，正確的定位對于保證重復(fù)精度是至關(guān)重要的。定位元件在空間的相對位置距離應(yīng)盡可能大。這樣做的好處是所用定位元件少，且能保證其與工件定位表面之間的全面接觸。此外，切屑及外部雜物容易引發(fā)事故，在布置定位元件時應(yīng)切實注意加以防止。如無法防止，則定位元件應(yīng)采用減摩結(jié)構(gòu)。見圖３-１。公差當(dāng)設(shè)計一個夾具時，設(shè)計人員必須牢記工件的公差。按照一般規(guī)律，夾具的公差應(yīng)取工件相應(yīng)公差的 20-50%。舉例來說，如一工件上某孔的位置尺寸公差為?0.010 吋，則夾具相應(yīng)的孔的位置尺寸公差應(yīng)為?0.002 吋到 ?0.005 吋，見圖 3-2。這一規(guī)定對保持所需的加工精度是必要的。指定夾具公差接近工件相應(yīng)公差 20%，對工件質(zhì)量的提高并不明顯，而夾具制造成本則要顯著提高。通常，夾具公差如大于工件相應(yīng)公差的 50%，工件的加工精度將不可能保證。因為決定工件加工精度的唯一關(guān)鍵 3-13-2Figure 3-1 Methods of relieving locators圖 3-1 定位元件減摩方法Figure 3-2 Tolerance relationship圖 3-2 公差關(guān)系因素并不是夾具公差，而是被加工工件的相應(yīng)指定公差。設(shè)計定位元件時應(yīng)確對保任一合格尺寸的工件都能安裝得上。如圖 3-3 所示工件，其允許加工到的最大與最小尺寸分別是 1.260 吋和1.240 吋，在此尺寸范圍內(nèi)的工件都是合格的。如果我們將夾具上定位元件的相應(yīng)尺寸設(shè)計成1.250 吋，則 1.250 至 1.260 吋的工件將無法安裝，盡管它們的尺寸是合格的。為防止這一情況發(fā)生，夾具上定位元件的相應(yīng)尺寸應(yīng)根據(jù)定位方式，設(shè)計為工件的最大或最小極限尺寸。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 TOOLSIZE TO SUIT LARGEST適應(yīng)工件最大可能尺寸的夾具尺寸VARIATIONS IN PART SIZES（工件尺寸變化范圍）Figure 3-3 Part and tool size relationship圖 3-3 工件與夾具的尺寸關(guān)系 無誤定位安裝無誤定位安裝是一種手段，夾具設(shè)計師憑此保證工件只有在處于正確位置時，才能安裝到夾具上。圖 3-4A 所示工件需要在有錐面的一端進行加工，所以夾具設(shè)計師設(shè)置了一只銷子以防止工件被誤定位。這只銷子使得夾具有了無誤定位安裝功能。圖 3-4Ｂ所示為法蘭盤上一個與其它已加工孔有相互位置要求的待加工孔，在一個已加工孔中放置一個簡單的菱形銷，便能保證定位無誤。其它的無誤定位安裝裝置也應(yīng)盡可能簡單。如果無誤定位安裝裝置過于復(fù)雜，它可能將導(dǎo)致另外本來容易的問題復(fù)雜化。重復(fù)定位重復(fù)定位始終是應(yīng)當(dāng)避免的。圖 3-5 表示了幾個重復(fù)定位的例子。重復(fù)定位不僅造成夾具成本提高，而且會導(dǎo)致夾具不能使用。例如，圖 3-5A 所示法蘭盤同時以法蘭下端面及底部在定位套筒上定位。由于這兩個表面是平行的，因而只需要一個即可，另一個應(yīng)當(dāng)除去。如果定位基準是法蘭，則底端定位就不須要，如圖 3-5B。如果定位基準是底端，則法蘭定位就不須要，如圖 3-5C。要正確地做到這一點，夾具設(shè)計師必須首先確定工件的哪一個表面是定位基準。只有確定了其定位基準，才能為其設(shè)置定位裝置。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.3-3Figure 3-4 Foolproofing圖 3-4 無誤定位安裝REQUIRED HOLE需要的孔CORRECT LOADING工件安裝正確INCORRECT LOADING工件安裝錯誤FOOLPFOOFING PIN無誤安裝定位銷FOOLPFOOFING PIN無誤安裝定位銷FOOLPFOOFING PIN無誤安裝定位銷LOCATING PIN定位銷LOCATOR定位元件LOCATOR定位元件 3-4工件定位表面與夾具定位元件的位置公差也能導(dǎo)致重復(fù)定位錯誤。如圖 3-6，同時以工件的側(cè)面和孔來定位就會造成問題。首先，夾具上定位銷的位置是固定的，不可能任意改變以適應(yīng)任何一個工件；其次，工件上作為定位基準的孔的位置是允許在一定范圍內(nèi)變動的。當(dāng)工件的位置尺寸處于極限狀態(tài)之一時，它就可能無法裝到夾具上去。要消除這一可能發(fā)生的情況，可以減小孔內(nèi)定位銷的尺寸，以適應(yīng)工件的相應(yīng)位置尺寸變化。但這樣做的結(jié)果是導(dǎo)致定位精度最低，定位裝置失去了其意義。要防止這種重復(fù)定位錯誤，夾具設(shè)計師必須在工件的側(cè)面與孔之間選擇其一作為定位基準，而不是二者都用。自由度問題一個自由物體具有 12 個運動自由度。圖 3-7 表示了一個物體與三個坐標(biāo)軸或坐標(biāo)平面的關(guān)系。一個物體可以沿著任何坐標(biāo)軸作任意方向旋轉(zhuǎn)或平行運動。為了形象地說明這一關(guān)系，坐標(biāo)平面被分別命名為“X-X” “Y-Y” 和 “Z-Z”。運動方向則從 1 到 12 進行編碼。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圖 3-6 工件位置與定位元件位置的差別VARIABLE POSITION OF HOLES工件定位孔的位置是變化的FIED POSITION OF LOCATORS夾具定位元件的位置是固定的DUBLICATELOCATORS重復(fù)定位元件Figure 3-6 Duplicate locators圖 3-5 重復(fù)定位元件DUBLICATELOCATINGSURFACES重復(fù)定位面REFERENCESURFACE定位基準面 限制自由度為了使得工件能在夾具中獲得并保持正確的位置，必須限制其運動自由度。這一功能是由定位元件和夾緊元件共同完成的?，F(xiàn)以圖 3-8 所示零件的夾具為例來說明限制自由度的原理。圖 3-9 所示為該工件被置于三銷組合支承面上，其 2、5、1、4 及 12 這五個自由度被限制了。采用銷式或按扣式定位元件使工件避免與切屑接觸，再通過限制定位元件與工件的接觸面積，能最大程度地減少定位差錯。平面型支承也可使用，但應(yīng)將其做成裝配式的，而不是直接加工成一個支承平面。裝配式定位元件其裝配省時且便于更換，所示從使用角度來說是經(jīng)濟的。當(dāng)采用按扣式或平面型支承元件時，最為重要的問題是使工件避免與切屑接觸，并切實保證工件與三個定位元件的支承面都可靠接觸。為了限制工件繞”Z-Z” 軸轉(zhuǎn)動及沿 8 方向移動，設(shè)置了雙銷式定位元件，如圖 3-10。為了限制工件沿 7 方向移動，采用了一個銷式定位元件。這種 3-2-1 支承點，或稱 6 支承點定位方法，是用于對正方或長方形零件的外表面進行定位的最為普遍的方法。如果工件上有可以利用來定位的孔，則是最好不過了。如圖 3-12 所示，中心孔被用來作為第一定位基準，而其它孔中的一個被用來作為第二定位基準。第一定位基準采用圓柱銷為定位元件，而第二定位基準則采用菱形銷?？梢?，基礎(chǔ)板及中心孔中的圓柱定位銷，限制了工件的 9 個運動自由度(1,2,4,5,7,8,10,11,12)，菱形銷又進一步限制了其 2 個自由度(6, 3)。這些定位元件合起來共限制了工件的 11 個自由度。這樣，工件唯一能運動的方向是垂直向上。因此，夾緊裝置僅需要限制工件的這一個自由度就可以了。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.3-5Figure 3-7 Planes of movement圖 3-7 運動平面Figure 3-8 Adjusting block圖 3-8 調(diào)整擋塊Figure 3-9 Three-pin base restricts five directions of movement圖 3-9 三銷支承限制工件五個方向的運動Figure 3-10 Five-pin base restricts eight directions of movement圖 3-10 五銷支承限制工件八個方向的運動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.給工件定位工件幾乎能制造成任一種可能的形狀及尺寸。不管工件是如何制造的，夾具設(shè)計人員必須有能力對任何一個工件設(shè)計正確的定位裝置。要做到這一點，夾具設(shè)計人員必須充分了解各種不同類型定位元件的功能特點，以盡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為定位基準時，主要有三種定位方法：固定支承、可調(diào)支承和自平衡支承。這些支承元件在工件定位表面的垂直方向上對工件進行支承，并防止工件在加工過程中產(chǎn)生變形。固定支承的應(yīng)用是最為容易的。這種支承既可直接在夾具體上加工出來，也可做成裝配式的。如圖 3-13。這一類支承通常用于工件以已加工表面作為定位基準的場合。3-6Figure 3-11 Six-pin base restricts eight directions of movement圖 3-11 六銷支承限制工件九個方向的運動Figure 3-12 Primary and secondary locators圖 3-12 主要定位元件與第二定位元件SECONDARY LOCATOR(Diamond pin locator)第二定位元件(菱形銷)PRIMARY LOCATOR(Round pin locator)主要定位元件(圓柱銷)Figure 3-11 Six-pin base restricts nine directions of movement圖 3-11 六銷支承限制工件九個方向的運動3-7Figure 3-13 Solid supports圖 3-13 固定支承Figure 3-14A Adjustable supports, threaded type.圖 3-14A 可調(diào)支承(螺紋式)STATIONARY MOVABLE LOCK NUT LOCKADJUSTMENT LOCK NUT SCREW固定調(diào)節(jié) 活動調(diào)節(jié) 鎖緊螺母 鎖緊螺紋CARBIDE INSERT滲碳淬硬塊（插入）OPTIONAL BASE CAN BEPRESSED INTO TOOL BODY可換基礎(chǔ)支座能壓入夾具體中AVAILABLE WITH OR WITHOUT ADJUSTING SCREW調(diào)節(jié)螺釘（可用亦可不用）LOCKSCREW鎖緊螺紋Figure 3-14B Adjustable supports, spring type.圖 3-14B 可調(diào)支承(彈簧式)Figure 3-14C Adjustable supports, push type.圖 3-14C 可調(diào)支承(推入式)SPRING彈簧3-8Adjustable 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接觸點CONTACT POINTS接觸點PIVOT POINT支點Figure 3-15 Equalizing supports圖 3-15 自平衡支承SCREWED AND DOWELED PRESS FIT THREADED PRESS FIT AND THREADED螺釘與銷固定 壓入式裝配 螺紋定心緊固 壓入及螺紋緊固HEXAGONLOCK NUT六角鎖緊螺母Figure 3-16 Internal locators圖 3-16 內(nèi)孔定位元件Figure 3-17 Pin locators and bushing圖 3-17 銷式定位元件與襯套ROUND BULLET PLAIN CONICAL DOWEL圓頭 子彈頭 平頭 圓錐頭 沉孔銷 REGULAR CONICAL常規(guī)型 錐孔型Figure 3-18 Round and tapered locators圖 3-18 圓頭與圓錐頭定位元件CANTACT AREA接觸面CANTACT AREA接觸面ROUND TAPERED OR BULLET圓頭 圓錐頭或子彈頭可調(diào)支承用于粗糙不平的表面，如鑄造表面。這類支承有多種類型，圖 3-14A、3-14B 和 3-14C 分別表示的是螺紋式、彈簧式和推入式。其中，螺紋式結(jié)構(gòu)最為簡單和經(jīng)濟，而且比其它類型的調(diào)整范圍要大得多。通常，可調(diào)支承與一或多個固定支承配合使用，以使工件調(diào)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.圖 3-15 所示的自平衡支承，也是一種可調(diào)支承。其特點是一個支承，卻有二個互動的接觸點。當(dāng)一個接觸點下降時，另一個就上升，以保持兩點對工件同時接觸。對于粗糙不平的鑄造表面進行支承時，這一技術(shù)特點是極為必要的。當(dāng)所討論的裝置是用于工件時，定位元件與支承元件這兩個術(shù)語，可交替使用。用來確定工件邊緣位置的定位裝置，稱為定位元件或擋塊。在選擇支承元件之前，夾具設(shè)計人員必須充分考慮工件表面的形狀，以及所要采用的夾緊裝置的類型。所選取的支承元件必須具有足夠的支承剛性，以抵抗夾緊力與切削力。夾緊元件應(yīng)直接位于支承元件之上，以防止夾緊力引起工件顛覆或產(chǎn)生彎曲變形。以內(nèi)孔定位要精確地對工件進行定位，以孔或內(nèi)部成形面作為定位基準是一個最為有效的方法。用1 只銷就能限制工件的 9 個自由度，用 2 只銷則能限制工件的 11 個自由度。只要可能，應(yīng)盡量選擇內(nèi)孔作為主要定位基準。給工件內(nèi)孔定位的元件有多種形式。圖 3-16表示的是幾種用于大直徑孔定位的定位元件。當(dāng)工件以大直徑孔為定位基準時，要使用螺釘和連接銷來固定內(nèi)孔定位元件。在通常情況下，需要采用二只螺釘和二只連接銷。定位元件受力較大時，最好是采用大規(guī)格的螺釘和連接銷，而不是增加他們的數(shù)量。采用臺階軸式定位元件時，實踐經(jīng)驗表明，壓入裝配式定位元件的精度要高于螺紋定心式的。只有在夾具結(jié)構(gòu)不允許壓出定位元件時，才使用螺紋定心式定位元件。另外一種定位元件則同時兼具壓入裝配及螺紋鎖緊之雙重優(yōu)點。銷式定位元件如圖 3-17 所示，它們用來對工件的小直徑孔進行定位，以及用來確定夾具零件之間的相對位置。當(dāng)其用途為后者時，應(yīng)當(dāng)使用特殊的襯套，以便磨損后進行更換。用來對工件定位的定位銷，其頭部制成球帽狀或錐帽狀，以便于工件的裝卸。見圖 3-18。定位用的銷與連結(jié)用的銷之間的主要區(qū)別，是摩擦面的大小。連結(jié)銷的接觸長度一般較長；而定位銷的接觸長度一般為工件厚度的1/8 到 1/2。接觸長度過大將使工件的裝卸變得困難。另一種普遍用于夾具上的定位銷是菱形銷，或稱減摩銷。這種銷通常與圓柱銷以其使用，以減少夾具的裝卸時間。將一個零件裝到一只圓柱銷和一只菱形銷上，要比裝到二只圓3-10 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