夾具類外文翻譯-一種自動化夾具設(shè)計(jì)方法【中文4660字】【PDF+中文WORD】
夾具類外文翻譯-一種自動化夾具設(shè)計(jì)方法【中文4660字】【PDF+中文WORD】,中文4660字,PDF+中文WORD,夾具,外文,翻譯,一種,自動化,設(shè)計(jì),方法,中文,4660,PDF,WORD
【中文4660字】
一種自動化夾具設(shè)計(jì)方法
塞西爾
美國，拉斯克魯塞斯，新墨西哥州立大學(xué)，工業(yè)工程系，虛擬企業(yè)工程實(shí)驗(yàn)室（VEEL）
本文描述了一種新的計(jì)算機(jī)輔助夾具設(shè)計(jì)方法。對于一個(gè)給定的工件，這種夾具設(shè)計(jì)方法包含了識別加緊表面和夾緊位置點(diǎn)。通過使用一種定位設(shè)計(jì)方法去夾緊和支撐工件，并且當(dāng)機(jī)器正在運(yùn)行的時(shí)候，可以根據(jù)刀具來正確定位工件。該論文還給出了自動化夾具設(shè)計(jì)的詳細(xì)步驟。幾何推理技術(shù)被用來確定可行的夾緊面和位置。要識別所完成工件和定位點(diǎn)就還需要一些輸入量包括CAD模型的技術(shù)要求、特征。
關(guān)鍵詞：夾緊；夾具設(shè)計(jì)
1.動機(jī)和目標(biāo)
夾具設(shè)計(jì)是連接設(shè)計(jì)與制造的一項(xiàng)重要任務(wù)。自動化夾具設(shè)計(jì)和計(jì)算機(jī)輔助夾具設(shè)計(jì)開發(fā)（夾具CAD）是下一代制造系統(tǒng)成功實(shí)現(xiàn)目標(biāo)的關(guān)鍵。在這篇論文里，討論了一種夾具設(shè)計(jì)的方法，這種方法有利于在目前環(huán)境下夾具設(shè)計(jì)的自動化。
夾具設(shè)計(jì)方法的研究已成為國內(nèi)多家科研工作的重點(diǎn)。作者周在[1]中對工件的穩(wěn)定和總需求約束了雙重標(biāo)準(zhǔn)，突出重點(diǎn)的工作。在夾具設(shè)計(jì)中廣泛地運(yùn)用了人工智能（AI）以及專家系統(tǒng)。部分CAD模型幾何信息也被用于夾具設(shè)計(jì)。Bidanda [4]描述了一個(gè)基于規(guī)則的專家系統(tǒng)，以確定回轉(zhuǎn)體零件的定位和夾緊。夾緊機(jī)制同時(shí)用于執(zhí)行定位和夾緊功能。其他研究者（如DeVor等，[5,6]）分析了切削力鉆井機(jī)械和建筑模型及其他金屬切削加工?？涤袨榈仍赱2]中定義了裝配約束建模的模塊化與夾具元件之間的空間關(guān)系。一些研究人員采用模塊化夾具設(shè)計(jì)原則，用以生成[2,7-11]，另一些夾具設(shè)計(jì)工作者已經(jīng)報(bào)告了[1,3,9,12-23]?？梢栽赱21,24]中找到夾具設(shè)計(jì)相關(guān)的大量的審查工作。
在第二節(jié)中，對夾具設(shè)計(jì)任務(wù)中各種步驟進(jìn)行了概述。在第三節(jié)和第四節(jié)中描述了工件的加工過程，要夾緊工件表面，否則將面臨工件的全面自動測定。第五節(jié)討論了對工件的夾緊點(diǎn)的測定。
2.夾具設(shè)計(jì)的整體方法
在本節(jié)中，描述了整體夾緊的設(shè)計(jì)方法。通常對較理想的位置的那一部分進(jìn)行夾緊，并減低切削力的影響。夾緊的位置和夾具設(shè)計(jì)中定位的位置是高度相關(guān)的。通常，夾緊和定位可以通過同樣的方法來完成。但是，不明白這兩個(gè)是夾具設(shè)計(jì)中不同的方面，則可能導(dǎo)致夾具設(shè)計(jì)的失敗。多數(shù)人的在規(guī)劃過程中首先解決定位問題，這樣可以使開發(fā)的定位與設(shè)計(jì)的定位相契合。不過，整體定位及設(shè)計(jì)方法不在本文討論范圍內(nèi)。
除了零件的設(shè)計(jì)（為此夾具設(shè)計(jì)有待開發(fā)），公差規(guī)格，過程序列，定位點(diǎn)和設(shè)計(jì)等因素外，還應(yīng)投入CAD模型到夾具設(shè)計(jì)方法中。這樣的夾具可以夾緊并支撐定位器。指導(dǎo)使用的主要內(nèi)容應(yīng)盡量不抵制切割或加工過程和其中所涉及的操作。相反，應(yīng)定位夾具，使切削力在正確的方向，這將有助于保持在一個(gè)特定的部分加工操作安全。通過引導(dǎo)對定位器的切割力量，部分（或工件）被固定，固定定位點(diǎn)，因此不能移動的定位器。
在這里討論的夾具的設(shè)計(jì)方法必須在整體夾具設(shè)計(jì)方法的范圍內(nèi)。在此之前進(jìn)行定位器/支撐和夾具設(shè)計(jì)的初步階段，涉及到的分析和識別的功能、相關(guān)的公差和其他規(guī)范是必要的。根據(jù)初步的評估和測定，定位/支撐設(shè)計(jì)與夾具設(shè)計(jì)結(jié)果的在此基礎(chǔ)上可以同時(shí)進(jìn)行。本文對所描述夾具設(shè)計(jì)的方法討論基于定位器/支撐設(shè)計(jì)與先前已經(jīng)確定的假設(shè)（包括適當(dāng)?shù)亩ㄎ缓椭С譁y定一個(gè)工件的定位，以及識別和夾具，如V元素的支持面塊，基礎(chǔ)板，定位銷等）。
（1） 夾具設(shè)計(jì)的輸入
輸入包括對特定產(chǎn)品的設(shè)計(jì)翼邊模型，公差信息，提取的特征，過程順序和部分在給定的每一個(gè)設(shè)計(jì)的相關(guān)特性的加工方向，面向的位置和定位裝置，以及加工過程中的各種工序，須出示每個(gè)相應(yīng)的功能。
（2） 夾具設(shè)計(jì)的方法
圖一是自動化夾具設(shè)計(jì)主要步驟總結(jié)圖。對這些步驟概述如下：
第一步：設(shè)置配置清單以及相關(guān)的[進(jìn)程_功能]條目。
第二步：確定方向和夾緊力。輸入必要的加工方向向量mdv1，mdv2……mdvn，面對nvs的支持力，并確定法向量。如果加工方向向下（對應(yīng)的方向向量[0，0，-1]），和面的支持向量平行于加工方向，那么，夾緊力方向平行向下加工方向[0，0，-1]。如果必需要側(cè)面夾緊并沒有可夾緊的地方，那么在其中放置一個(gè)夾具夾緊下調(diào)，然后邊鉗方向計(jì)算如下。讓sv和tv輔助常規(guī)的向量代替次要的和三級定位孔。然后，使用夾緊機(jī)構(gòu)夾緊一個(gè)方向，例如，av應(yīng)平行于這兩個(gè)法向量，即，正常向量應(yīng)分別與每塊表面的sv和tv向量平行。側(cè)面夾緊面應(yīng)該是一對分別平行于面sv和tv的平面孔。
第三步：從列表中選出最大有效加工力。這樣能夠有效的平衡各加工力。
第四步：利用計(jì)算出的最高有效加工力，才能確定用來支撐工件加工的面積的夾具尺寸（例如，一個(gè)帶夾子可以作為一個(gè)夾緊機(jī)構(gòu)使用）。
第五步：確定給定工件的夾緊面。這一步在第四步中所述過。
第六步：該夾具的夾緊面的實(shí)際位置自動在第5節(jié)中確定?？紤]接下來的步驟并返回第一步。
3.判斷夾具尺寸
在這項(xiàng)工作中所用到的夾具都來自一個(gè)系列。夾具的原理與圖二相同。在這一節(jié)里，描述了一個(gè)自動化夾具。鎖模力所需的有關(guān)螺桿的螺紋裝置大小或保存到位鉗。夾緊力平衡加工工件使工件保持恰當(dāng)?shù)奈恢?。讓鎖模力為W和螺桿直徑為D。各種螺絲夾緊力大小，可以按以下方式確定：最初，極限拉伸強(qiáng)度（抗拉強(qiáng)度）和該夾具的材料（供應(yīng)情況而定）可以從數(shù)據(jù)檢索庫檢索。各種材料有不同的拉伸強(qiáng)度。該夾具材料的選擇，也可直接采用啟發(fā)式規(guī)則進(jìn)行。例如，如果部分材料是低碳鋼，那么鉗材料可低碳鋼或機(jī)器鋼。為了確定設(shè)計(jì)應(yīng)力，抗拉強(qiáng)度值應(yīng)除以安全系數(shù)（如4或5）。根區(qū)的螺絲格A1（如一個(gè)螺絲鉗）可以被確定：[鎖模力/設(shè)計(jì)應(yīng)力]。隨后，螺栓截面全面積可以計(jì)算為等于{格A1 /（65％）}（因?yàn)槁萁z的地方可能會發(fā)生根切面積約為65％螺栓的總面積）。螺釘?shù)闹睆紻可以被確定等同于（D2的3.14 / 4）。另一項(xiàng)涉及可用于方程有關(guān)的寬度B，高度H和跨度的鉗L的螺絲直徑為D（B，H和L可以為不同的值計(jì)算D）：d2=4/3 BH2/L.
4.判斷夾緊表面
確定夾具經(jīng)常出現(xiàn)的相關(guān)參數(shù)包括了產(chǎn)品的CAD模型，提取的特征信息，特征尺寸，定位面和定位器的選擇?？紤]所有潛在的加緊面，如圖3。最關(guān)鍵的是夾緊表面不應(yīng)重疊或與該面相交，如圖4所示。夾緊面積是與工件表面（或PCF）接觸的是一個(gè)二維輪廓線段組成的（見圖6）。利用線段相交測試，可以測定在給定的光子晶體光纖的任何范圍內(nèi)是否可能有接觸面夾緊面重疊。
夾緊面的確定可以如下所示：
第1步：鑒別平行于二級和三級定位面（lf1和lf2）是分別到lf1和tcj最遠(yuǎn)的距離的面。如下所示：（一）鑒別面tci,tcj,使面tci和 tcj平行l(wèi)f1和tcj平行l(wèi)f2。（二）在TCF中列出面對tci的面。（三）通過檢查所有TCF中面對tci的面，確定的面對tci和tcj的面是到lf1和lf2分別最遠(yuǎn)的面，并舍棄所有其他TCF中的面。
第2步：鑒別平行面的位置，除了不相鄰的附加面。最好是選擇一個(gè)不與其他定位面垂直相鄰的面。這一步如下所示：
(a) 考慮TCF列表中的tci面，獲得與每個(gè)tci面垂直或相鄰的面然后，在FCF列表中插入每個(gè)fci面。
(b) 檢查每個(gè)FCI面，并執(zhí)行以下測試：如果FCI是相鄰、垂直于lf1或lf2，然后從列表中舍棄它并插入NTCF列表中。
第3步: 確定加緊面都在有效的加緊面上，如下所述夾緊面：
例1:如果沒有條目在列表NTCF中，就使用TCF中的面并繼續(xù)執(zhí)行步驟4。如果任何面發(fā)現(xiàn)，垂直于第二，第三位置的面孔lf1和lf2，這將要面臨的是下次選擇可行的夾具。在這種情況下，唯一剩下的選擇是重新審視在列表NTCF的面。
例2：如果列表中NTCF條目數(shù)為1時(shí)，可行夾緊面為FCI。與TCI的法向量垂直相鄰的相應(yīng)軸是夾緊軸。
例3：如果在列表NTCF項(xiàng)數(shù)大于1，確定最大的TCI加緊面再進(jìn)行步驟4。
例4：:夾緊力的方向可以是[1，0，0]或[0，1，0]，可以夾緊TCI面的中心位置。
在其他幾何位置可確定使用零件幾何形狀和拓?fù)湫畔?，這在下一節(jié)中描述。
5.判斷夾緊表面上的夾緊點(diǎn)
確定夾緊面后，必須確定實(shí)際夾緊位置。輸入夾具側(cè)面積，沿著[X，Y，Z]和潛在的夾緊面CF方向。容下使用CF幾何獲得夾具側(cè)面積：
第一步是確定一個(gè)箱體的大小，這是用來測試它是否包含在它里面的任何部分。相交測試也可以在前面介紹的方法使用。如果相交測試返回一個(gè)負(fù)的結(jié)果，那么有部分箱體與夾具相交，如圖4所示。如果相交測試返回一個(gè)正的結(jié)果，可以執(zhí)行下列步驟：
1. 劃分成更小的矩形大小條（1 W）夾框輪廓(圖5和圖6)。
2. 執(zhí)行指定與功能配置文件出現(xiàn)在CF面的零件設(shè)計(jì)的相交測試。
3. 沒有功能相交的條形區(qū)域，都是可行夾緊區(qū)域。如果有一個(gè)以上的長方形候選面，矩形配置文件，向中沿軸夾緊CF面點(diǎn)的是夾緊配置文件（夾點(diǎn)）。
如果沒有發(fā)現(xiàn)配置文件，夾具寬度可減少一半，夾具數(shù)可以增加兩個(gè)。使用這些修改過的夾具尺寸，執(zhí)行前面描述的特征相交測試。如果此測試也失敗了，那么可以用相鄰的面作為夾緊面用于執(zhí)行端夾緊。這面可以重復(fù)進(jìn)行PCF和功能相交測試。
5.1試驗(yàn)曲線的交點(diǎn)
輸入需要的二維輪廓P1、P2，使用下列方法可以自動確定該配置文件的交集。每一個(gè)輸入的資料組成一個(gè)封閉環(huán)。此配置文件測試的步驟如下：
(T1) 考慮P1線段中的L（i，1）和P2線段中的L（2，j）。
(T2) 采用L（i，1）線段和L（2，j）線段的相交段。如果邊緣相交測試返回一個(gè)正值，那么特征面和潛在面相交。如果它返回一個(gè)負(fù)值，繼續(xù)執(zhí)行步驟3。
（T3）重復(fù)與步驟（T1）相同的部分或者緩慢走過其余P1中的(Li,1)段直到P2中的[(L2, j+1) till j=n–1]段。
(T4) 其余部分邊和P1中的L12、L13到L1n段重復(fù)（T1）和（T2）步驟。
如果特征面與夾緊面重復(fù)，線相交測試將決定該事件。相交的邊可以進(jìn)行自動檢測兩個(gè)面是否相互交叉。輸入所需的邊L12{連接 (x1, y1) 和 (x2, y2)}和L34{連接 (x3, y3) 和(x4, y4)}。
L12型方程的可表示為：
F(x,y) =0 （1）
L34型方程的可表示為：
H(x,y) =0 （2）
第一步：使用等式（1）計(jì)算R3 = F(x3, y3)，用X和Y取代X3和Y3；計(jì)算R4 = F(x4, y4)，用X和Y取代X4和Y4。
第二步：如果R3和R4都與0不相等，但R3與R4結(jié)果相同（R1與R2在相同的一邊），則邊L12與L34不相交。如果這樣不滿足條件，那么進(jìn)行第三步。
第三步：使用等式（2）計(jì)算R1 = H(x1, y1)。接著，計(jì)算R2 = G(x2, y2)再進(jìn)行第四步。
第四步：如果R1與R2都不等于0，且R1與R2的結(jié)果相同，那么把R1與R2放在相同的一邊并輸入不相交。如果，這個(gè)也不滿足條件，那么進(jìn)行第五步。
第五步：給定相交線段。這樣就完成了測試?？紤]如圖7所示的一部分樣品。將要生產(chǎn)一個(gè)盲孔。起初，完成定位設(shè)計(jì)。定位器的（或主要定位器）是一個(gè)基盤（放在F4面）和二級和三級定位器面臨F6和F5（對應(yīng)到定位面lf1和lf2在第4節(jié)中討論）。一個(gè)輔助定位器也被使用，這是一個(gè)V型塊（對F3和F5面輔助定位），如圖8所示。在前面討論的夾具設(shè)計(jì)方法中所述的步驟的基礎(chǔ)上，候選面孔（這是平行的，并在從lf1和lf2最遙遠(yuǎn)的距離）是面對F3和F5面。沒有面孔，這是平行到定位面，但他們不相鄰。在這種情況下使用的優(yōu)先權(quán)規(guī)則（如步驟3第4步討論），剩余的候選面面對的是F2面。夾具方向向下的V型塊徑向定位器和其他與對工件夾緊底面提供所需位置。
根據(jù)第五步選擇夾具的位置。如果沒有功能發(fā)生在面F2上，那么也沒有必要進(jìn)行相交測試確定夾具優(yōu)美加緊。夾具位置應(yīng)遠(yuǎn)離V型定位器（這是輔助定位位置）的夾緊面毗鄰輔助定位面（這確保了更好的快速夾緊）。最終位置和夾具的設(shè)計(jì)如圖8所示。
本文討論的方法，毫不遜色于其他夾具設(shè)計(jì)文獻(xiàn)中討論的方法。本文所討論的方法的獨(dú)特性是零件的夾緊面的幾何形狀，拓?fù)浜凸δ馨l(fā)生了被加工為基礎(chǔ)的系統(tǒng)鑒定。其他方法都沒有利用了定位器的位置，該方法使用定位器在對持有一級，二級和三級定位器加工的工件。這種方法的另一個(gè)好處是在可行的候選面上確定在面上用夾具面交點(diǎn)測試（如前所述），并迅速和有效地確定潛在的下游過程中可能出現(xiàn)問題，夾緊和加工的功能檢測。
6.總結(jié)
本文在一個(gè)夾具設(shè)計(jì)方法的總體框架之下進(jìn)行了夾具設(shè)計(jì)方面的討論。設(shè)計(jì)定位器，規(guī)范零件設(shè)計(jì)，和其他相關(guān)被用來確定夾緊面和夾緊方向。各種自動化步驟也有涉及。
8
Int J Adv Manuf Technol (2001) 18:784–789
ó 2001 Springer-Verlag London Limited
A Clamping Design Approach for Automated Fixture Design
J. Cecil
Virtual Enterprise Engineering Lab (VEEL), Industrial Engineering Department, New Mexico State University, Las Cruces, USA
In this paper, an innovative clamping design approach is described in the context of computer-aided fixture design activi- ties. The clamping design approach involves identification of clamping surfaces and clamp points on a given workpiece. This approach can be applied in conjunction with a locator design approach to hold and support the workpiece during machining and to position the workpiece correctly with respect to the cutting tool. Detailed steps are given for automated clamp design. Geometric reasoning techniques are used to determine feasible clamp faces and positions. The required inputs include CAD model specifications, features identified on the finished workpiece, locator points and elements.
Keywords: Clamping; Fixture design
1. Motivation and Objectives
Fixture design is an important task, which is an integration link between design and manufacturing activities. The automation of fixture design activities and the development of computer-aided fixture design (CAFD) methodologies are key objectives to be addressed for the successful realisation of next generation manufacturing systems. In this paper, a clamp design approach is discussed, which facilitates automation in the context of an integrated fixture design methodology.
Clamp design approaches have been the focus of several research efforts. The work of Chou [1] focused on the twin criteria of workpiece stability and total restraint requirement. The use of artificial intelligence (AI) approaches as well as expert system applications in fixture design has been widely reported [2,3]. Part geometry information from a CAD model has also been used to drive the fixture design task. Bidanda [4] described a rule-based expert system to identify the locating and clamping faces for rotational parts. The clamping mech- anism is used to perform both the locating and clamping
Correspondence and offprint requests to: Dr J. Cecil, Virtual Enterprise Engineering Lab (VEEL), Industrial Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA. E-mail: jcecil@nmsu.edu
functions. Other researchers (e.g. DeVor et al. [5,6]) have analysed the cutting forces and built mechanistic models for drilling, and other metal cutting processes. Kang et al. [2] defined assembly constraints to model spatial relationships between modular fixture elements. Several researchers have employed modular fixturing principles to generate fixture designs [2,7–11]. Other fixture design efforts have been reported in [1,3,9,12–23]. An extensive review of fixture design related work can be found in [21,24].
In Section 2, the various steps in the overall approach to automate the clamping design task are outlined. Section 3 describes the determination of the clamp size to hold a work- piece during machining and in Section 4, the automatic determi- nation of the clamping surface or face region on a workpiece is detailed. Section 5 discusses the determination of the clamp- ing points on a workpiece.
2. Overall Approach to Clamp Design
In this section, the overall clamping design approach is described. Clamping is usually carried out to hold the part in a desired position and to resist the effects of cutting forces. Clamping and locating problems in fixture design are highly related. Often, the clamping and locating can be accomplished by the same mechanism. However, failure to understand that these two tasks are separate aspects of fixture design may lead to infeasible fixture designs. Human process planners generally resolve the locating problem first. The approach developed can work in conjunction with a locator design strategy. However, the overall locator and support design approach is beyond the scope of this paper.
CAD models of the part design (for which the clamp design has to be developed), the tolerance specifications, process sequence, locator points and design, among other factors, are the inputs to the clamp design approach. The purpose of clamping is to hold the parts against locators and supports. The guiding theme used is to try not to resist the cutting or machining forces involved during a machining operation. Rather, the clamps should be positioned such that the cutting forces are in the direction that will assist in holding the part securely during a specific machining operation. By directing
A Clamping Design Approach
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the cutting forces towards the locators, the part (or workpiece) is forced against solid, fixed locating points and so cannot move away from the locators.
The clamp design approach discussed here must be viewed in the context of the overall fixture design approach. Prior to performing locator/support and clamp design, a prelimi- nary phase involving analysis and identification of features, associated tolerances and other specifications is necessary. Based on the outcome of this preliminary evaluation and determination, the locator/support design and clamp design can be carried out. The clamp design approach described in this paper is discussed based on the assumption that locator/support design attributes have been determined earlier (this includes determination of appropriate locator and support faces on a workpiece as well as identification of locator and support fixturing elements such as V-blocks, base plates, locating pins, etc).
2.1 Inputs to Clamp Design
The inputs include the winged-edge model of the given product design, the tolerance information, the extracted features, the process sequence and the machining directions for each of the associated features in the given part design, the location faces and locator devices, and the machining forces for the various processes required to produce each corresponding feature.
2.2 Clamp Design Strategy
The main steps in the automation of the clamping design task are summarised in Fig. 1. An overview of these steps is as follows:
Step 1. Consider the set-up SUi in the set-up configuration list along with the associated [process I feature] entries.
Step 2. Identify the direction and type of clamping. The inputs required are the machining direction vectors mdv1,mdv2,
. . .,mdvn and identified normal vectors of support face nvs. If the machining directions are downward (which correspond to the direction vector [0, 0, -1]), and the normal vector of the support face is parallel to the machining direction, then the direction of clamping is parallel to the downward machining direction [0, 0, -1]. If sideways clamping is required, and if there are no feasible regions at which to position a clamp for downward clamping, then a side-clamp direction is obtained as follows. Let sv and tv be the normal vectors of the secondary (sv) and tertiary (tv) locating faces. Then, the direction of clamping used by a side-clamping mechanism such as a v- block should be parallel to both these normal vectors, i.e. the normal vectors of the each of the v-surfaces in the v-block will be parallel to sv and tv, respectively. The side clamping face should be a pair of faces parallel to the faces sv and tv, respectively.
Step 3. Determine the highest machining force from the mach- ining forces list (for each feature) MFi (i = 1, . . .,n). This will be the effective force FE that must be balanced while designing the clamp for this set-up SUi.
Step 4. Using the value of the calculated highest machining force FE, the dimensions of the clamp to be used to hold the
Fig. 1. The clamp design activities.
workpiece can be determined (for example, a strap clamp can be used as a clamping mechanism). The approach for this task is explained in Section 3.
Step 5. Determine the clamping face on a given workpiece. This step can be automated as described in Section 4.
Step 6. The actual position of the clamp on the clamping face is determined in an automated manner as explained in Section 5.
Consider next set-up SU(i + 1) and proceed to step 1.
3. Determination of the Clamp Size
In this work, the clamps used belong to the family of clamps referred to as strap clamps. A strap clamp is based on the same principle as that of the lever (see Fig. 2). In this section, the automated design of a strap clamp is described. The clamping force required is related to the size of the screw or a threaded device that holds the clamp in place. The clamping force should balance the machining force to hold the workpiece in position. Let the clamping force be W and the screw diameter be d. The dimensions of the various screw sizes for various clamping forces can be determined in the following manner. Initially, the ultimate tensile strength (UTS) of the material of the clamp (depending on availability) can be retrieved from a data library. Various materials have different tensile strengths. The selection of the clamp material can also be performed directly using heuristic rules. For example, if the part material is mild steel, then the clamp material can be low
Fig. 2. The strap clamp.
carbon steel or machine steel. To determine the design stress, the UTS value can be divided by a safety factor (such as 4 or 5). The root area A1 of the screw (for a clamp such as a screw clamp) can then be determined: [Clamping force required/Design Stress DS]. Subsequently, the full area FA of the bolt cross-section can be computed as equal to {A1/(65%)} (since the root area of the screw where shearing can occur is approximately 65% of the total area of the bolt). The diameter of the screw d can then be determined by equating FA to (3.14 d2/4). Another equation which can be used involves relating the width B, height H and span L of the clamp to the screw diameter d (B, H, and L can be computed for various values of d): d2 = 4/3 BH2/L.
4. The Determination of the Clamping Face
The required inputs to determine the clamping region include the CAD model of the product, the extracted features infor- mation, the feature dimensions and faces on which they occur, the locating faces and locators selected. Consider a potential clamping face PCF as shown in Fig. 3. The crucial criterion to be satisfied is that the clamping surface should not overlap or intersect with the features on that face, as shown in Fig. 4. The clamping surface area, which is in contact with the workpiece surface (or PCF) is a 2D profile consisting of line segments (see Fig. 6). By using line segment intersection tests, it can be determined whether the potential clamping area of contact overlaps any of the features on the given PCF.
The determination of clamping faces can be automated as fol- lows:
Fig. 3. Potential clamping face and feature profiles.
Fig. 4. Potential clamping face and clamp box profile.
Step 1. Identify faces that are parallel to the secondary and tertiary locator faces (lf1 and lf2) and at the farthest distance from lf1 and tcj, respectively. This is performed as shown below:
(a) Identify faces tci, tcj such that tci is parallel to lf1 and
tcj is parallel to lf2.
(b) Insert candidate faces tci in list TCF.
(c) By examining all faces tci listed in TCF, determine faces tci and tcj that are farthest from face lf1 and lf2, respect- ively, and discard all other faces from list TCF.
Step 2. Identify the face that is parallel to the location faces but not adjacent to the additional locator faces. It is preferable to select a clamp face that does not have to share the adjacent perpendicular face with a locator. This step can be automated as shown below:
(a) Consider each face tci in list TCF and obtain correspond- ing faces fci that are adjacent and perpendicular to each tci. Then, insert each face fci in list FCF.
(b) Examine each fci and perform the following test: If fci is adjacent, perpendicular to lf1 or lf2,
then discard it from list FCF and insert it in list NTCF. Step 3. Determine the clamping faces, based on the availability of potential clamping faces, as described below.
Case (a). If there are no entries in list NTCF, then use the faces in list TCF and proceed to step 4. If any faces were found that were perpendicular to the secondary and tertiary location faces lf1 and lf2, such faces are the next feasible choices to be used for clamping.
In this case, the only remaining choice is to re-examine the faces in list NTCF.
Case (b). If the number of entries in list NTCF is 1, the feasible clamping face is fci. The normal vector of the corresponding adjacent, perpendicular face tci is the axis of clamping.
Case (c). If number of entries in list NTCF is greater than 1, determine the face tci with larger area and proceed to step 4.
Step 4. Depending on the direction of clamping which is either [(+ or -)1, 0, 0] or [(+ or -) 0, 1, 0], the clamp can be positioned along the centre of the face tci. The candidate geometrical positions of the clamp can be determined using part geometry and topological information, which is described in the next section.
Fig. 5. Determination of the clamp profile dimensions.
5. Determination of the Clamping Points on a Clamping Face
After the clamp face has been determined, the actual clamping positions on that face must be determined. The inputs are the clamp profile dimensions, clamp directions [x, y, z], and poten- tial clamping face CF. The clamp profile dimensions are obtained (as in case (g)) using CF geometry as follows.
The first step is to determine a box size, which is tested to determine whether it contains any features inside it. Profile intersection tests can also be performed using the method described earlier. If the intersection test returns a negative result, then no feature intersects with the clamp box profile, as shown in Fig. 4. If the intersection test returns a positive result, the following steps can be performed:
1. Divide the clamp box profile into smaller rectangular strips of size (1 ′ w) (Figs 5 and 6).
2. Perform the intersection tests with the feature profiles of features that occur on the face CF for the given part design.
Fig. 6. Profiles intersection test of feature and clamp regions.
3. The rectangular strips, where no feature intersection occurs, are feasible clamping regions. If there is more than one candidate rectangle for clamping, the rectangle profile that is toward the mid-point of the CF face along the clamping axis is the clamp profile (and clamp points).
If no profile Pi can be found that does not intersect with the feature profiles, clamp width can be reduced by half and the number of clamps increased to two on that face. Using these modified clamp dimensions, perform the feature intersection test described earlier. If this test also fails, then the side face adjacent to the PCF can be used as the clamping surface to perform side clamping. The side face then becomes the PCF and the feature intersection test can be repeated.
5.1 The Intersection of Profiles Test
The required inputs include the 2D profile P1 another 2D profile P2. The intersection of profiles can be determined in an automated manner using the following approach. Each input profile Pi consists of a closed loop of line segments Lij. The steps in this profile test are as follows:
(T1) Consider a line segment L(i,1) in P1 and another line segment L(2, j) in P2.
(T2) For inputs L(i,1) and L(2, j), the intersection of edges can be employed. If the edge intersection test returns a positive value, then the feature profile intersects with the candidate or potential clamp profile under evaluation. If it returns a negative value, proceed to step 3.
(T3) Repeat step (T1) for the same segment or edge (Li,1) in
P1 with all remaining segments [(L2, j+1) till j = n–1] in P2. (T4) Repeat steps (T1) and (T2) for the remaining edges or segments L12, L13,. . .,L1n in profile P1.
If the feature profiles overlap the clamping profiles, the line intersection tests will determine that occurrence. The inter- section of edges test can be performed automatically to detect whether two edges intersect with each other. The inputs required for this test are the line segments L12 {connecting (x1, y1) and (x2, y2)} and L34 {connecting (x3, y3) and (x4, y4)}.
Let the equation of L12 be represented by:
F(x,y) = 0 (1)
and that of L34 by:
H(x,y) = 0 (2)
Step 1. Using Eq. (1) compute r3 = F(x3, y3) by substituting x3 and y3 for x and y and compute r4 = F(x4, y4) by substitut- ing x4 and y4 for x and y.
Step 2. If r3 is not equal to 0, r4 is not equal to 0, and the signs of r3 and r4 are the same, (which indicate r1 and r2 lie on same side), then the edges L12 and L34 do not intersect. If this is not satisfied, then step (3) is performed.
Step 3. Using Eq. (2), compute r1 = H(x1, y1). Then, compute
r2 = G(x2, y2) and proceed to step 4.
Step 4. If r1 is not equal to zero, r2 is not equal to zero, and the signs of both r1 and r2 are the same }, then r1, r2 lie on
Fig. 7. Sample part to illustrate the clamping design approach.
the same side and the input line segments do not intersect. Else, if this condition is not satisfied, proceed to step 5.
Step 5. The given line segments do intersect. This completes the test.
Consider the same sample part shown in Fig. 7. The features to be produced are a step and hole. Initially, the locator design is completed. The support locator (or primary locator) is a base plate (placed against face f4) and the secondary and tertiary locators are placed against faces f6 and f5 (which correspond to the locator faces lf1 and lf2 discussed in Section 4). An ancillary locator is also used, which is a v-block (positioned against the ancillary faces f3 and f5), shown in Fig. 8. Based on the steps outlined in the clamp design
Fig. 8. Fixture design for the sample part in Fig. 7.
approach discussed earlier, the candidate faces (which are parallel and at the farthest distance from lf1 and lf2) are face f3 and f5. There are no faces which are parallel to the locator faces but not adjacent to them. Using the priority rules in such cases (as discussed in step 3 of Section 4), the remaining candidate face is face f2. The clamp direction is downward; the v-block radial locator and other locators provide the required location with the clamp holding the workpiece down- ward against the baseplate.
The position of the clamp is determined based on the steps described in Section 5. As there are no feaures occurring on face f2, there is no need for feature intersection tests to determine collision-free clamping. The position of the clamp should be away from the v-locator (which is positioned along the ancillary location faces) as the clamping face is adjacent to the ancillary location faces (this ensures better access for quick clamping). The final location and clamping design is shown in Fig. 8.
The method discussed in this paper compares favourably with the other clamp design methods discussed in the literature. The uniqueness of the discussed approach is the systematic identification of the clamping faces based on part geometry, topology, and the occurrence of features to be machined. While other approaches have not exploited the position of the locators adequately, the proposed method uses the locators to hold the workpiece during machining against the primary, secondary, and tertiary locators. An
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