Grinding Journal - Summer 2007 - (Page 28)

Grinding glass (Ra < 2.5 μm) MÄGERLE configures its MFP machine to grind optical glass, bringing an advanced level of precision to the surface grinder W hen Xiamen University asked Mägerle if their machines could also grind optical glass, the initial reaction was one of surprise. But since Mägerle has a long tradition of coming up with engineering and technological innovations in response to new challenges, they took to the task with considerable enthusiasm. The objective was to find a way to grind plates of BK7 optical glass with sides measuring 33 x 18 inches (850 x 450 mm) and a thickness to length ratio of 1 to 20 to a roughness average of less than 100 mico-inches (2.5 μm). The reason for the required tolerance? Optical glass is lapped after grinding, but that is an extremely time-consuming procedure. The better the flatness achieved during grinding, the less time required for the subsequent lapping operation. The required tolerances could only be achieved with perfect synchronization of the machine guideways, clamping technology, wheel, coolant system and grinding process. That is one reason Mägerle selected the MFP as the base machine for the project. Its full “wrap-around” hydrostatic guideways perfectly compensates for the slightest local straightness errors, ensuring highly precise and even surfaces. Furthermore, Mägerle was able to acquire the 2 Grinding Journal consultant services of an experienced and competent specialist in the machining of optical glass, Dr. Yaolong Chen from the Aalen Zhang Wei engineering company for innovation technology. Clamping technology and tool quality Glass is much more difficult to clamp than metal. “Compared with a steel workpiece of the same thickness, glass has a rigidity of less than 40 percent,” explains Dr. Chen. That is why the glass plate to be ground is first bonded to metal and then separated again after the grinding process has been completed. Any deformation caused by the clamping force would result in a form defect in the ground part after it has been detached, so the machine must to support the glass over its whole surface to induce the least possible stress. The level of the grinding forces acting on it must be low and constant. Since the dimensions of the glass plate necessitate a long cutting path for the tool, the grinding wheel must be subject to only minimal wear. In order to achieve this, Mägerle uses a diamond grinding wheel and dresses the wheel for trueness and straightness before each grinding cycle. The programmable dressing roller is mounted firmly on the grinding table and is driven by a dedicated motor. An automatic balancing unit in the grinding

Table of Contents Feed for the Digital Edition of Grinding Journal - Summer 2007

Contents My Thoughts Exactly When Lives Are at Stake Walter Consolidating Production The Art of Grinding: Knowing the Source of Grinding Errors and How to Fix Them The How and Why of Conventional Vitrified Grinding Wheel Selection Grinding Glass Flat Problem Solver

Grinding Journal - Summer 2007

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