On-machine Metrology for Surface Fabrication

人们常说，“如果你不能衡量它，你就不能创造它”。测量是制造技术的基础，需要专业和广泛的知识，不仅要进行关键的测量，还要适当地解释和应用结果。抛光精密镜片和镜子的老式方法是用抛光工具（后期可能是验光师的拇指！）用抛光粉的水浆以可控的方式摩擦它们。在大多数应用中，既要达到无缺陷的极佳光滑抛光，又要达到精确到百万分之几英寸的表面形状（我们称之为“表面形状”）。要做到这一点，需要进行多次测量和抛光。现代计算机控制的抛光机，如项目合作伙伴之一的Zeeko有限公司生产的，可以加快和控制这一过程，使其更加自动化和可预测。然而，由于抛光背后的微观物理和化学的潜在复杂性，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，并将其移动到测量仪器上，这增加了生产时间并引入了损坏的风险。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。这些包括“球面”（与球体的一部分不同的表面）和真正不受控制的表面，称为“自由曲面”（可能看起来像品客薯片一样随机隆起和凹陷）。今天，以可控的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。这是循环的测量部分，严重限制了可以实现的精度，并阻碍了工业利用这种表面所能赋予的优势的能力。那么，为什么工业界需要这些复杂的表面呢？考虑两个例子。在光学中，复杂的表面为设计者提供了更多的特征，当设计一个特定的透镜或反射镜时，这些特征可以在计算机中改变。一般来说，这意味着同样的工作可以用更少的玻璃片完成（更轻，更紧凑的系统），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——医学中，人工膝关节是复杂的鞍状关节，高质量的人工膝关节可以延长患者的寿命。为了理解测量和控制复杂表面的日益增长的需求，需要广泛的知识，包括测量仪器、计算机接口、数据分析、软件技术、错误来源等等。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，掌握最终应用的需求对成功制造至关重要。这就是为什么技术转让是拟议项目的本质，这样工业伙伴可以提高他们自己的技能，以解决市场问题，而且科学界也可以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，最好的方法之一就是作为一个团队来解决一个共同的问题。我们解决的核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，该项目侧重于开发一种具有挑战性的原型仪器，该仪器将以一种新颖的方式结合两种测量方法。其结果将是一个紧凑的测量模块，将适合到Zeeko机器上的刀架。这将使一个复杂的零件作为一组重叠的补丁进行测量，使用机器的7轴运动系统提供表面扫描。剩下的就是把这些分开的碎片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.