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可持续夹具及固定装置的高精度安装的方法论J. Jamshidi,P.G. maropoulos英国巴斯大学机械工程学系摘要在夹具安装阶段,夹具组件的精确测量能力决定其精确性,尤其是对大尺寸的产品和应用来说。大量定制一些在设计上有多样性的小批量产品和零部件是十分重要的。产品质量应该与迅速转换理念相互协调,对于敏感元器件及组件而言,以牺牲产品质量来提高速度是不明智的,例如在航空航天工业中所看到的那些零部件。提高精确性对于夹具的安装是很有必要的,为了要尽量少用,这可取决于夹具和夹具定位变化的耐受性预算。在航空航天工业中,灵活和可重构夹具的概念的发展及固定装置是夹具的主要开销成本。吸引他们的是重构夹具的可重用性,可持续使用是由于那些零部件也能够被重新应用到许多种产品和装配中去。一直不佳的准确性和可靠性为这类夹具的缺点,这篇论文主要是研究可持续夹具主要零部件的精确定位,影响夹具性能的因素在安装阶段会得以审查。本文介绍了一种在灵活夹具中为了最大限度地减少定位和夹紧的不确定性的方法。关键词:可持续性夹具;夹具安装;校准不确定性;夹具的监测;计量;可重复使用的夹具1引言质量和可靠性等因素早已经转换为新零件固有的特征。在他们的产品和服务范围内,近期市场趋势已经迫使制造业走向大规模定制。其次增加新陈品的多样化设计在部件和组件级别上遵循第二次高幅度的变化。有先进制造系统和技术的国家提供了更多的灵活性,能使设计师更加自由的发挥想象。例如在过去几十年发展起来的一种新的大体积测量方法,就能测量及公丈的距离。用来检测大尺寸零件的这些设备通常是由多个组装部件所制造出来的。在重组和装配期间,那些大尺寸产品的制造需要用到专用的夹具和固定装置,以便于他们的零部件能够被定位在设计基准上。对于大批量产品,在计算机辅助软件上就能估算出其主要的花费需求,否则,在某些情况下或以其他方式生产的产品成本可能会是非常高的。这个问题与客户不断用钱来寻求具有更高价值的市场需求相互矛盾。用一种典型的变化产品可以创建一个更加具有持续性的商业,因为它能够满足相对比较大地市场需求。由支持产品变化的不同形状的产品而形成的灵活和可重构夹具及固定装置则是应对上述挑战关键的解决方案。灵活夹具的概念已经在研究领域存在了几年 1。然而,它们在很大程度上没有充分利用真正的生产设施,尤其是大尺寸产品的制造,如航空航天设备。这是由于关于它们的初始安装、校准困难和可重复性等挑战往往超出公差要求。这些夹具和高品质关键部件装置的制造以及大量的集成计量系统,都可以减少上述限制。本文包括了有关安装的计量问题和灵活的夹具校准以及检测服务。2相关工作2.1大型零件的制造和装配通常把要精密制造的机械零件移动到机床工作台是必需的,先进行粗加工,然后精对准和夹紧。在这个阶段,一方面是准备用于加工高精度的关键所在,然而,这对于一些大尺寸或重的零部件并不总是可以实现的。大型产品是指那些与一些不经济的可能需要处理或在工厂周围移动的的组件一起来达到制造和装配的目的 2。制造业和这些部件的装配工艺包括所需机器和系统的运动以及这些部件的位置和方向。这些零部件通常定位在大尺寸夹具及固定装置上该定位的位置。如果这些部件以小批量生产,在航空航天工业、高开销的情况下,每个产品将会出现。在设计和制造夹具上,已经有了许多尝试,以便于能够持有许多种组件的变种。然而,这方法是不可行的,因为敏感或关键的零部件完全是由它们的高精度要求所决定的。与固定的夹具相比较,可调、可重构夹具会产生更低的重复性。固定夹具有固定的方式,可以通过焊接或铆接的接头固定在一起来实现。这些机械故障,例如夹具和固定装置由于疲劳和塑料变形就是一个终止其服务的主要原因,发送它们再进行回收。小批量制造的需求是目前常见的淘汰不合格夹具,其使用寿命取决于产品的生命周期。换句话说,在制造变种零件停止不久之后,与之有关联的夹具和固定装置将会变得多余。即使夹具有工作顺序,但它们仍然有报废并送往循环再造。这种方法会带来高负担回收的能耗。在大尺寸零件下的固定夹具和夹具的漂移以及夹具都能影响大尺寸零件组装的精确性 5。对于大尺寸的夹具已经开发出了几种分析夹具刚性的方法,可用来评估振动的影响因素。在任何情况下更加可持续生产要通过选择性方案才能得以实现。如图 2.1所示。图 2.1 典型的大型夹具元件用于制造所延误的时间是固定夹具的又一重大缺点,这些夹具应该在制造前就被订购,在生产计划和产品上市前还可以创建额外的复杂性。不管他们的类型,大型夹具有一些共同的元素,包括一个主框架、内在的框架,有可能有一个或数多个可移动的机制或更小的组件,如钳、套管、皮卡和可调螺丝 表 1。2.2柔性夹具开发的柔性夹具的概念增加了其可持续性、快速转换以及降低了成本。现在可以使用现成的模块来设计和组装夹具。根据规定,只有极少数夹具的专用零件需要设计和制造。以这种概念看,大多数散装部件、附件关节都应用的很具体。一旦产品设计变异完全制造出来,然后就可以拆卸上述组件和重组成一种新的元件来与未来的设计变异相匹配。因为组装和组件范围上的设计都相当接近,所以这种方法是最能降低夹具成本的。根据组件的变化水平和工作类型的需求,每一个不同比例的柔性夹具都需要被重新安排。为了增加使用这种类型夹具的效益,这个问题在设计阶段考虑是十分重要的。例如,在可能的情况下,皮卡车的位置和三维定位夹具上的组件的不同变种,甚至完全不同的部分应该是在靠近增加兼容性和夹具子系统间变性。夹具关键部件的收集在短期内能保证所需夹具的可用性。另外,这些夹具的存储需要很少空间,因为拆除所有模块是可能的,通常以脚手架的形式并把它们彼此放在一起。目前在一些汽车公司应用到柔性夹具,因为它们的精度等级是足够满足的。大尺寸制造的夹具设施尽管没有许多柔性夹具那样操作灵活,比如航空航天设备。准确性和定位销的不确定性、夹具的可重复性以及夹具结构的漂移都出现这样一个事实,这些夹具在发电和航空航天等工业不能满足公差要求。一旦它们的精度问题得以解决,这些夹具在上述的工业中将会有很高的利用价值。图 2.2 夹具与汽车行业中的可重构组件大量的体积计量系统和技术一直有许多新的发展。依靠计量系统和技术的现代激光技术现在就能够测量一些可接受精度的大尺寸产品甚至到几公丈的产品。在安装和初期安装调试期间,这些系统能够用来精确定位一些夹具的关键部件。夹具的安装通常开始形成机架或主要框架,然后是那些大的部件和像皮卡和夹具那样逐渐更小的零部件。计量系统能够用于柔性夹具的主要框架和其内部框架的安装,从而来保证每个零部件彼此之间的准确定位。表 1显示了这些大规模的测量系统数。跟踪系统能够测量基准点,它是实现这一目的的最合适的测量系统。一起跟踪球式反射镜在三维空间内都可以认准所要记录的位置。球式反射镜可以直接练习目标对象来提供几何位置信息或可以在机械重复的球式反射镜下应用,从这个角度用激光来跟踪目标或短期目标。像一些其他测量仪器的激光跟踪有时候不确定,在夹具的安装过程中需要做以说明。此外,激光跟踪和目标点之间的光线问题还应该考虑到。如果必要的话,多个跟踪职位都可以使用,在实际测量活动中,其结果必然是伴随着一个不确定的说明。经过测量,这些给定的分散的特征值是合理的,这个问题与一些夹具或夹具的安装及以后的审核都是一样的。这些知识阐明了一个给定的定位夹具的能力和装配任务。换句话说,它表示在其相关流程中一套夹具是否能够满足公差要求。2.3夹具理论比较为了用于多种生产装置和组装应用,在不同的公司也有大量不同形状和设计的夹具。这些夹具中的某一些的形式都是标准形式,然而另一些是经过专门设计和制造的特殊零部件。后者可以说是建立在产品的复杂性和大规模的基础上的,所以可能会是非常昂贵的 9。无论成本和目的如何,制造和组装过程都可以完成,要么不使用夹具,要么用固定帧夹具或用可重构或柔性夹具。表 2对它们的典型应用的这些方法做出了比较。固定框夹具通常用于重型应用上,它们更适合于应用在能够减轻花费开销的大量产品上。对于制造业和大型组装及复杂产品来说,柔性夹具的应用有很多好处。特别是在研究和发展工作中,以及低容量产品的情况下,制造柔性夹具和夹具可能会是非常有益的。除了在时间和金钱上使利益均衡外,柔性夹具还会有一个更加灵活的设计思路。由于在夹具的制造和装配过程中,会直接和经常改变夹具的拓扑成本。与传统夹具相比较,这种类型的柔性夹具的可重用性和可重构性是一个主要的优点。这一点尤为重要,因为它在绿色制造方面符合行业的发展方向,通过回收使用系统的组成部件,为相关项目的工装费用降低了项目成本。表 2.1 用于验证夹具大量的例子、便携式测量仪器仪器 辅助部件测量类型接触式 非接触式图像赛立信探头 是激光跟踪仪T型探头 是激光雷达 球形目标 是目标 是摄影光投影 是基于激光扫描头 是关节臂三坐标测量机接触探头 是2.4 柔性夹具的安装本节中描述的夹具在安装过程中需要考虑一些问题。夹具元件安装在合适位置可以说是一种挑战,尤其是当定位公差比较小时,柔性夹具也应当被监测以便于开发和与传统夹具相比较它们的刚性。给出了一个通用的描述阶段根据初步夹具安装在仿真软件环境和利用一些尺寸为 5m4m3m的大尺寸夹具的实际实验结果来测量指令阶段。这是不论这个夹具是否是第一次安装在夹具中还是对已经存在的夹具只做外形上的更换,其都是为了定位不同的零部件。根据其复杂性,一种典型的大尺寸夹具具有三至五个结构。通常除了有一个主框架之外,在每一个级别的基本水平,也可以由一个或多个结构。这些结构用来作为参考以便于作为夹具基准来定位。在一个自动化固定的平台上,机器系统会进行多项任务,比如定位、加工和进行装配。因此机器人的工作基准与夹具的工作框架相联系。仔细考虑夹具的综合数据,会确保后续安装达到预期的公差要求。2.4.1 测试辅助柔性夹具的安装柔性夹具的安装在第一次进行模拟仿真时有几个安装阶段。使用模拟演习,在这个工程中可以减少一些潜在的错误和返工。测量辅助安装过程是类似的跟踪对象位置的过程,是常见的大尺寸组件用到的。夹具的组件先用这种方法使其大致位置公差在 1mm以内,然后当所有的夹具组件被放到所设计的位置时,在 0.1mm至 0.15mm公差内,它们可以使用适当的扭矩收紧。典型的计量协助柔性夹具安装阶段如下:在出厂时设置初试参考结构;测量初始参照系;安装基地或其位置的主框架;安装脱机内结构;安装控股和定位;在内框和主框架位置安装钳、套管和皮卡。底座上安装内框;重点定位元件的精细调整和紧固;核查参照系和夹具;对于夹具关键位置的服务监测。这些阶段是从无到有夹具的完整安装。更不用说,在设计变化略有变化的情况下,一些下面的操作将被忽略。表 2.2 不同夹具理念之间简明的比较典型特征 固定框架 柔性夹具 夹具应用 大批量产品 小批量产品 样机唯一性 可重复性 可重构性 成本效益耐用性 非常高 高 低优点刚度 非常高 刚性不确定 低重量 重 中 低可移植性 不可移植 难以在每一个安装中定位难以进行成本 非常高 中等 低缺点生产时间 长 中等 短2.4.2 柔性夹具的安装柔性夹具安装过程中采取了以下阶段:安装夹具的主框架;移动单元和子系统组装;夹具内框架在夹具装配中的安装;皮卡和夹具的装配。主框架通常是夹具的骨干。因此,它一般不会改变外形,和内框架及较小的元素如衬套、皮卡和夹具一样具有规律性。慎重考虑制造过程可以减少夹具元件较大元素的重新排列,也会更进一步节约时间和成本。图 3显示了三组不同活动小组对于柔性夹具的安装过程。在这个过程中,假定夹具的标准件是从可供选择的现成的部件和组件中选出来的,然后提前在大量的物理安装测试和调试时以降低测量不确定度来进行一次夹具安装。一旦达到可接受水平的不确定性的物理安装才能进行。图 2.3 柔性夹具的测量辅助安装程序在夹具安装过程中,它可能需要用到多个测量系统或用到包括一套完整夹具安装关键点的测量系统。这应该包括在夹具维修期间放在工厂的地板和墙壁上的稳定性及漂移检查的目标参考点。在夹具上定位的关键点、测量仪器的不确定性应该予以考虑。作为一个经验法则,每个夹具关键位置的定位的准确性应该是在 10倍,甚至比所需的耐受性更好。换句话说,如果组件公差指定至 0.1mm,则夹具定位精度至少应为0.01mm。柔性夹具安装的最佳做法如下:一个工厂的墙壁和地板上的初始参考点的位置是首选,以减少测量的不确定性;应验证该仪器所在位置上经常使用初始参考点的基地;在每个夹具的大节上,为更好地具有跟踪和可重复性,可以附加多个球式反射跟踪系统;夹具大型组件有关于弯曲或扭曲的测量应集中在角梁的中央部分,以减少定位误差;内框架参考点应选择尽可能远的创建框的坐标系统。这可能会导致在内框坐标系时的较小的不确定性。3.不确定性由于评估的结果旨在对于所描述物体真正价值范围内的评估定位,则不确定性被定义为胶 8。这里的不确定性来自两方面的审查,首先是与测量过程中有关的,其次是与夹具定位的不确定性有关。测量不确定性的一些因素,如灰尘、重力、温度、气压、湿度、测量仪器和相关软件中的系统误差、操作人员的技能、接触探针或球式反射跟踪仪。基于胶 8的定义,测量结果应伴随着它的不确定性说明,这些错误详细来源的讨论超出了本文的范围。已经有多项研究来建立真正的不确定性的一些测量仪器 10、11 。夹具关键点的测量结果应该包括相关的置信水平。除了最初的不确定性上升的测量仪器,夹具的不确定性还有其他方面的影响。夹具框架方面的应用由于产品重量以及制造工艺过程在夹具框架中能创建一些弹性变形。夹具有了集成化、自动化和移动部分,是这个问题变得更加复杂。仔细考虑夹具设计阶段不确定性的来源可以在一个给定的情况下减少过度使用公差预算的风险。夹具的精度和夹具的成本之间有着直接的关系。然而,整体的成本也应该被考虑在内。高数额的零部件有更长的使用寿命。换句话说,质量特征接近平均值是,在使用过程中会很少失效。朝六个方向靠近在安装时是很必要的,然后是在高精度夹具和敏感部位上的可重构、柔性夹具及夹具的监测。4.讨论柔性夹具适用于组装和在加工部分有不同的设计和几何参数。尤其是当这些部件之间的设计变化小时,这种类型的夹具被证明是具有成本效益和快速的解决方案,这是因为它可能会通过夹具重新配置到一个新的产品或组件的变种上。然而,当所需的零件盒装配的设计完全不同时,可能需要不同尺寸夹具组件的完整地重排,因此,在设计一个新的零件和重组夹具的拓扑结构时应该考虑到要便于减少夹具的安装和重新配置的时间以及成本,并最大限度的重用现在的组件在当前的装配中。一个柔性夹具有许多种装配方式,是由于其灵活性。在正确的安装顺序下选择合适的组件,可以最大限度的节约成本和时间。本次选择的过程中应该对未来产品的夹具的潜在的重用予以考虑。在产品不断变化的情况下,计划和设计夹具框架是很重要的。使用定制的夹具、紧固件、轴套和其他夹具元件都是降低成本的关键问题。仿真软件和工具的广泛使用,可以减少返工和浪费夹具材料相关的成本。例如测量仪器,其目标点之间的视线检查就会使用这些软件来辅助。图 4显示了柔性夹具安装在测量仿真软件中的第一个阶段。由于夹具正在兴建的模拟的效益越来越明显,因此它可以突出仪器与目标测量点的水平视线的问题和不确定性。由于关于夹具操作中重量和力使夹具元件的电位漂移和变形,则可以提前在仿真中分析以及在购买夹具元件方面的财政。更重要的是,这种工具允许更好的装配和制造等操作,因为它们能提前分析出夹具的长处和弱点,揭示其物理设置。选择合适的测量仪器最重要的是要协助夹具安装的的计量。一个连贯的计量系统,其测量结果的不确定性能够决定夹具的不确定性,也暴露了夹具和是否符合一个给定的公差范围内所需任务的能力。图 4.1 测量辅助夹具安装的模拟过程由于关于夹具操作中重量和力使夹具元件的电位漂移和变形,则可以提前在仿真中分析以及在购买夹具元件方面的财政。更重要的是,这种工具允许更好的装配和制造等操作,因为它们能提前分析出夹具的长处和弱点,揭示其物理设置。选择合适的测量仪器最重要的是要协助夹具安装的的计量。一个连贯的计量系统,其测量结果的不确定性能够决定夹具的不确定性,也暴露了夹具和是否符合一个给定的公差范围内所需任务的能力。总结零件和装配体应该以尽量减少夹具成本的方式来设计,以满足使用标准组件。如果在夹具更新至下一产品时提前思考,则其速度和成本可能会增加。在夹具和固定装置的设计上有强大的总成本、碳足迹及其可持续性。它往往要求生产出只有极少数的典型设计或产品,子系统或组件,以满足产品的变化和客户的需求。对于大型产品的几何尺寸超出几米夹具的可能会造成重大的开销。此外,一旦所需的零部件倍制造出来,这些夹具则会变得多余。常规回收多余夹具是不经济的。因此,找到增加夹具的灵活性才是所需求的方法。柔性夹具和夹具的概念已存于二十余年。然而它们的潜能还没有真正发挥,是由于其精度可可重复性的不确定性。高精度计量系统现在能够用来对夹具进行初始安装和服务。然后使用该系统有可能重新设定夹具元素使其精确到一个新的拓扑结构,以便于定位不同几何位置的组件,允许几次使用夹具。本文提出了一种对于大规模柔性夹具及夹具初始定位和安装的通用的算法。计量辅助夹具安装的新概念被证明对复杂的设置和夹具的安装是有益的。当完全装配好时,夹具的稳定性可以通过慎重考虑和选择夹具几何位置上的关键点来予以保证。这种方法将应用于大尺寸元件和产品的制造以及装配件,尤其是应用于航空航天和发电工业中。G. Seliger et al. (eds.), Advances in Sustainable Manufacturing: Proceedings of the 8th Global Conference 149 on Sustainable Manufacturing, DOI 10.1007/978-3-642-20183-7_22, Springer-Verlag Berlin Heidelberg 2011 Methodology for High Accuracy Installation of Sustainable Jigs and Fixtures J. Jamshidi, P.G. Maropoulos Department of Mechanical Engineering, University of Bath, UK Abstract The ability to accurately measure the components of jigs and fixtures during their installation determines the state of their precision, especially for large size products and applications. This matter is crucial in mass customisation where small batches of products and components with high variety in design are manufactured. Product quality should be in harmony with rapid changeover philosophy as compromising quality for speed is not forgivable for sensitive components and assemblies such as those seen in the aerospace industry. It is necessary for the installation of the jigs and fixtures to be highly accurate in order to minimise the use of tolerance budget due to variations in jigs and fixture positioning. Major overhead costs for jigs and fixtures particularly in the aerospace industry led to the development of the concept of flexible and reconfigurable jigs and fixtures. Reusability of reconfigurable jigs and fixtures makes them attractive for sustainable solutions as their components can be reused for several variant of a product or assembly. The main drawbacks of this type of jigs and fixtures have been their poor accuracy and reliability. In this paper accurate positioning of the key components of sustainable jigs and fixtures is investigated. The factors affecting the performance of the jigs and fixtures are reviewed from the installation stage. The paper introduces a methodology for minimising uncertainties in positioning of the holds and clamps for flexible jigs and fixtures. Keywords: Sustainable Jig, Jig installation, Calibration Uncertainty, Jig Monitoring, Metrology, Reusable Jig 1 INTRODUCTION Factors such as quality and reliability have long converted to implicit characteristics of the new products. Recent market trends have forced manufacturing industries to move towards mass customisation in their products and service range. Increased variation in the design of new products is followed by a second wave of variation with higher amplitude at subassemblies and component level. State of the art manufacturing systems and technologies have provided more flexibility, enabling designers to think more freely. For instance new large volume measurement systems, developed in the past few years, are capable of measuring several decametre distances. Such technologies facilitate the verification of large size components that used to be manufactured from several assembled components. The manufacturing of large size products requires specialist jigs and fixtures in order for their components to be held in the desired orientation during build and assembly. This requires major overhead cost that can only be justified by mass production in some cases or otherwise the cost of finished products can be very high. This issue contradicts with the market trends where customers are constantly looking for higher value for their money. In a typical product the variation in the product creates a more sustainable business as it can fulfil the needs of a relatively larger market. Flexible and reconfigurable jigs and fixture that can be formed in different shapes to support different variation of products is a key solution for the above challenges. The concept of flexible jig existed for several years in the research domain 1. However, they are not fully utilised to a great extent in real production facilities especially for large size product manufacturers, such as aerospace. This is due to the challenges related to their initial installation, poor calibration, and repeatability that often exceed the tolerance requirement. The manufacturing of these jigs and fixtures from high quality key components as well as their integration with large volume metrology systems can reduce the above limitations. This paper covers metrology issues related to the installation and calibration of flexible jigs and fixtures as well as their monitoring during service. 2 RELATED WORK 2.1 Manufacturing and assembly of large scale parts Typically prior to precision manufacturing of mechanical parts it is essential to move the raw material to the machine bench, proceed with rough cutting then fine alignment and clamping. At this stage the part is ready for machining of its high precision key features. However, this is not always possible for large size and/or heavy components. Large scale products refer to those with components that are not economically possible to handle or move around in the factory for fabrication and assembly purposes 2. The manufacturing and assembly processes of these parts encompass movement of the machines and systems to the desired location and orientation with respect to these parts. Such parts are normally held in their positions using large size jigs and fixtures. If these parts are produced in small batch sizes that is the case for aerospace industries, high overhead cost per product will occur. There have been many attempts to design and manufacture jigs and fixtures so that they can hold a number of variants of components 3, 4. However, this approach is not feasible for parts with sensitive or key features due to their high accuracy requirements. J. Jamshidi, P.G. Maropoulos 150 Adjustable, reconfigurable jigs and fixtures produce lower repeatability over time compared to fixed ones. Fixed jigs have permanent topology achieved through their permanent joints that are welded or riveted. Mechanical failure of these jigs and fixtures for example due to fatigue and plastic deformation is a main cause of terminating their service and sending them for recycling. With small batch manufacturing requirements it is now common to retire a conforming jig as their service life depends on the life of products. In other words soon after the cease of manufacturing a parts variant, the associated jigs and fixtures become redundant. Even if the jigs are still in working order, they have to be scrapped and sent for recycling. This method brings the burden of high energy consumption for recycling. Even for the fixed jigs and fixtures the drift in the large size parts and jig can affect the accuracy of a large size assembly 5. Several methods for analysing jig rigidity have been developed 6 to evaluate the impact of vibration on large size jigs. In any case a more sustainable manufacturing can only be achieved by alternative solutions. Figure 1: Typical components of large scale jig (image courtesy of Electroimpact /G150TFIX/gallery.asp) Extensive lead time to manufacture is another major drawback for fixed jigs and fixtures. These jigs should be ordered well in advance of any manufacturing processes. This can create additional complexity in production planning and product time to market. Regardless of their type, large scale jigs have a number of common elements including one main frame, one or a number of inner frames, potentially one or a number of moving mechanisms, and smaller components such as clamps, bushings, pickups and adjustable screws (Figure 1). 2.2 Flexible jigs and fixtures The concept of flexible jigs and fixtures is developed for increased sustainability, rapid changeover as well as low cost. It is now possible to use off the shelf modules and clamps for jigs and fixtures design and assembly. Depending on the requirement only a handful of specialised components for the jigs and fixtures might be needed to be custom designed and manufactured. In this concept the majority of bulk components, joints at the attachments are used in for a specific application. Once the product design variant is fully manufactured it is then possible to disassemble the above components and reassemble them in a new topology to suite the next design variant. This cycle can be repeated over a large number of times resulting in reduced overhead cost for jigs and fixtures. Needless to mention the other factors such as disassembly time, resetting time, operators time should be considered for evaluating the real cost benefit of using this type of jigs and fixtures. This approach best reduces the cost of jigs and fixtures for the assembly and component ranges that are fairly close in design. Depending on the level of variations in the components and the type of work required on each a different percentage of the flexible jigs need to be rearranged. This matter is crucial to be considered at design stage in order to increase the benefit of using this type of jigs and fixtures. For example, when possible, the location and 3D positioning of pickups and clamps on different variants of the component or even totally different parts should be in close proximity to increase compatibility and inter-changeability of sub-systems of jigs and fixtures. Having a collection of the key components of the jigs can guarantee the availability of the desired jigs in a short time. In addition to this the storage of the jigs required less space as it is possible to dismantle all the modules that are typically in the form of scaffolding and place them next to each other. Flexible jigs are currently utilised in some of the automotive companies (Figure 2) as their accuracy level is sufficient for this sector. Despite the above benefits there are Clamp Main frame Inner frame Component Adjustable holds Moving mechanisms Methodology for High Accuracy Installation of Sustainable Jigs and Fixtures 151not many flexible jigs in operation in large size manufacturing facilities such as aerospace factories. Accuracy and uncertainty of positioning pins, repeatability of the clamps and drift of the jig structure are all contributing to the fact that these jigs cannot meet the tolerance requirements of the power generation and aerospace industries. These jigs have high potentials for utilisation in the above industries once their accuracy problems are resolved. Figure 2: Fixture with reconfigurable components for automotive industry (image courtesy of Witte http:/www.horst-witte.de/en/) There has been a number of new developments in large volume metrology systems and technologies. Modern laser based metrology systems and technologies are now capable of measuring large size products up to several decametres with acceptable accuracy. These systems can be used to accurately position mountings of the key components of the jig during its installation and initial setup. The installation of jigs and fixtures typically starts form its base or main frame then large components and gradually to the smaller components such as pickups and clamps. Metrology systems can be used for the installation of flexible jig main frame and its inner frames to guarantee the correct positioning of each and every component. Table 1 shows a few of these large scale measurement systems. For technological review of these systems see 7. Laser tracker systems capable of measuring reference points, are among the most suitable measurement systems for this purpose. The instrument tracks a Spherically Mounted Retroreflector (SMR) target the position of which can be registered in three dimensional space. SMR can be contacted directly with the target object to provide geometrical positional information or can be used within a mechanically repeatable SMR nest known as Laser tracker target or in short target from this point on. Laser trackers like any other measurement instrument have a level of uncertainty that need to be accounted for during the jig installation process. Also the line of sight issues between the laser tracker and its target point should be considered and if necessary multiple tracker positions should be used. In real measurement activity the result must be accompanied with a statement of uncertainty. Such statement characterises the dispersion of the values that are reasonably attributed to the measurand 8. This issue is the same for the installation and later verification of any jig or fixture. This knowledge clarifies the jig capability of a given positioning and assembly task. In other word it indicates if a jig can meet the tolerance requirements for its related processes. 2.3 Comparison of jig philosophies There are a large number of different shape and design jigs and fixtures in different companies for various manufacturing and assembly applications. Some of these jigs and fixtures are readily available in standard forms, while some are designed and manufactured specific to particular parts and tasks. The latter can be very expensive based on the complexity and scale of the products 9. Regardless of cost and purpose a manufacturing or assembly process can be performed using with no jig, with fixed frame jigs, or with reconfigurable or flexible jigs. Table 2 provides a comparison of these methods with their typical applications. Fixed frame jigs are typically for heavy duty applications. They are more suitable for applications with a large number of products that can relax the overhead cost of the jig. There are several advantages in the application of flexible jigs and fixtures for the manufacturing and assembly of large and complex products. In particular for research and development work, as well as for cases where low volume products are manufactured flexible jig and fixture can be very beneficial. In addition to time and money saving benefits the possibility of having a flexible jig gives more freedom to the design, manufacturing and assembly processes due to the low direct and recurrent cost of changing the overall topology of the jig. Reconfigurability and reusability of flexible jig is a main Clamp Main frame Inner frame Component J. Jamshidi, P.G. Maropoulos 152 advantage for this type of jig compared to conventional jigs. This is particularly important as it is in line with the industry direction in terms of green manufacturing by recycling components from a used system, reducing project costs with regards to expenses for tooling of associated items. Table 1: Examples of large volume/portable measurement instruments for jig verification Measurement type Instrument Auxiliary components Contact Non-contact Image SMR probe 9 Laser Tracker T-probe 9 Laser Radar Spherical targets 9 Targets 9 Photogrammetry Light projection 9 Laser based scanning head 9 Articulated Arm CMM Contact probe 9 3 FLEXIBLE JIG INSTALLATION The issues and concerns that need to be considered in the jig installation procedure are described in this section. The installation of the jig components in the right position can be a challenging especially when the positioning tolerances are tights. Flexible jigs should also be monitored in order to exploit and compare their rigidity with that of the conventional ones. Stage by stage measurement instruction for the jig installation based on the results of an initial jig setup in the simulation software environment and practical experiment of a large size jig with dimensions of 5m x 4m x3m is given in a generic description. This is regardless of whether the jig is in first time installation or it is a change of an existing jig topology into a new shape, for holding a different component. Depending on the complexity a typical large size jig has between three to five levels of frames. Apart from the base level with normally one main frame, at each level there can be one or several frames. These frames are interrelated with reference to the jig datum in order to facilitate the positioning and functionality required. In an automated, fixed platform, robotic systems carry out several tasks such as part positioning, machining and assembly. The robot working datum therefore is linked with the working frame of the jig. Careful consideration of jig datuming strategy and its subsequent installation can secure achieving the desired tolerance. 3.1 Measurement assisted flexible jig installation There are several stages for the installation of flexible jigs that can be carried out in first simulation and then real world. The use of simulation exercise can reduce the number of potential errors and rework during this process. The process of measurement assisted installation is similar to tracking objects to position that is common for large size assemblies. In this approach the components of the jig are roughly positioned, within 1mm tolerance from the target position, at first. Then when all of the jig components are attached into their designated positions, within 0.1mm to 0.15mm tolerance, they are tightened using the appropriate torque. The typical stages of metrology assisted flexible jig installation are given below: 1. setting initial reference frames in the factory 2. measurement of initial reference frame 3. installation of base or main frame in its position 4. installation of inner frames offline 5. installation of holding and positioning brackets 6. installation of clamps, bushings and pickups in their rough position on inner frames and main frame 7.
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本文标题:11ZA-1.58型空气压缩机曲轴铣键槽夹具设计(含全套CAD及三维图纸)
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