On-machine Metrology for Surface Fabrication

人们常说，‘如果你不能测量它，你就不能成功’。测量是制造技术的基础，需要专业而广泛的知识，不仅要进行关键的测量，而且要恰当地解释和应用结果。抛光精密镜片和镜片的老式方法是用抛光工具(在后期阶段可能是验光师的拇指)用抛光粉末的水浆以受控的方式摩擦它们。在大多数应用中，既要获得没有缺陷的极好的光滑抛光，又要有好到百万分之一英寸的精确形状的表面(我们称之为‘表面形式’)。要做到这一点，需要进行多次测量和打磨。现代计算机控制的抛光机，如项目合作伙伴之一Zeeko有限公司生产的抛光机，可以加快和控制这一过程，使其更加自动化和可预测。然而，由于抛光背后的物理和化学的微观层面的复杂性，仍然需要重复的测量周期。在实践中，这通常意味着从抛光机床上卸下透镜或反射镜，并将其移到测量仪器中，这会增加生产时间并带来损坏的风险。当试图制造具有复杂形状的精密表面时，测量的挑战变得尖锐起来。其中包括“非球面”(与球体的一部分不同的曲面)，以及真正难以驾驭的称为“自由曲面”的曲面(它们看起来可能有像品客一样的随机凸起和凹陷)。今天，以受控方式抛光这种复杂表面的技术远远领先于测量它们的能力。这是周期的测量部分，严重限制了可以达到的精度，并阻碍了工业利用这些表面所能提供的优势的能力。那么，为什么工业界想要这些复杂的曲面呢？考虑两个例子。在光学方面，复杂的表面为设计者提供了更多的功能，当设计特定的透镜或镜子时，这些功能可以在计算机中改变。一般来说，这意味着可以用更少的玻璃(更轻、更紧凑的系统)来完成相同的工作，或者可以实现更好的性能(更清晰的图像)。在一个完全不同的领域-医学-人工膝关节是复杂的鞍状形状，优质可以增加患者的关节寿命。为了理解日益增长的测量和控制复杂表面的需求，需要广博的知识，包括测量仪器、计算机接口、数据分析、软件技术、误差来源等。在一个极端，测量与制造过程的关系是至关重要的；另一方面，对最终应用的需求的把握是成功制造的关键。这就是为什么技术转让是拟议项目的本质，以便工业合作伙伴能够提高自己在应对市场方面的技能，也使科学界能够通过提高技术能力而受益。我们所说的技术转移都是关于人的，而最好的方法之一就是作为一个团队来解决共同的问题。我们解决的中心问题是如何最有效地测量在Zeeko抛光机上加工的复杂零件。考虑到技术转让，该项目专注于开发一种具有挑战性的原型仪器，它将以一种新颖的方式结合两种测量方法。结果将是一个紧凑的测量模块，它将适合Zeeko机床上的刀架。这将使复杂的零件能够被测量为一组重叠的面片，使用机器的7轴运动系统来提供表面扫描。仍然需要将这些单独的补丁用数学方法缝合在一起。

It has oft been said that, 'if you can't measure it, you can't make it'. Measurement is fundamental to manufacturing technology, and requires specialised and broad knowledge, not only to make a critical measurement, but also to interpret and apply the results appropriately. The old-fashioned way to polish precision lenses and mirrors is to rub them in a controlled manner using a water-slurry of polishing powder with a polishing tool (which might in the later stages be the optician's thumb!). In most applications it is necessary to achieve both an excellent smooth polish free of defects, and a precisely shaped surface (we call it 'surface-form') good to a few millionths of an inch. To achieve this requires many cycles of measurement and polishing. A modern computer controlled polishing machine, such as produced by one of the project-partners Zeeko Ltd, can speed up and control the process, making it more automated and predictable. Nevertheless, repeated cycles of measurement are still needed, because of the underlying complexity at the microscopic level of the physics and chemistry behind polishing. In practice, this usually means de-mounting the lens or mirror from the polishing machine-tool, and moving it to a measurement instrument, which increases production-time and introduces risk of damage. The challenge of measurement becomes acute when trying to manufacture precision surfaces which have complex forms. These include 'aspheres' (surfaces which differ from part of a sphere), and the truly unruly surfaces called 'free-forms' (which may have seemingly random humps and hollows like a Pringle). Today, the technology to polish such complex surfaces in a controlled manner is well ahead of the ability to measure them. It is the measurement part of the cycle which is severely limiting the accuracy that can be achieved, and thwarting the ability of industry to capitalise on the advantages which such surfaces can confer. So why does industry want these complex surfaces? Consider two examples. In optics, complex surfaces provide the designer with more features that can be changed in the computer, when designing a particular lens or mirror. In general, this means that the same job can be done with fewer pieces of glass (lighter, more compact systems), or better performance can be achieved (sharper images). In a completely different field - medicine - artificial knee joints are complex saddle-like forms, and superior quality can increase the joint's life in the patient. To make sense of the increasing need to measure and control complex surfaces requires breadth of knowledge, spanning measurement instrumentation, computer-interfaces, data-analysis, software techniques, sources of errors, and much more. At one extreme, the relationship of measurement to the manufacturing processes is crucial; at the other, a grasp of the demands of the final application is critical to successful manufacturing. This is why technology transfer is the very essence of the proposed project, so that the industrial partners can enhance their own skills in addressing the marketplace, but also so that the scientific community can benefit through enhanced technical capabilities. Technology Transfer as we call it is all about people, and one of the best ways to do it is to address a common problem as a team. The central problem we address is how most effectively to measure complex parts as they are processed on the Zeeko polishing machines. With technology-transfer in view, the project focuses on developing a challenging prototype instrument which will combine two measuring methods in a novel way. The result will be a compact measuring module which will fit into the tool-holder on the Zeeko machines. This will enable a complex part to be measured as a set of overlapping patches, using the machine's 7-axis motion-system to provide the surface-scanning. It remains to take these separate patches and mathematically stitch them together.