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Achive > 2008 >
Development of a fabrication method for the orbital project “Microscope” |
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Scientific news from division 5 | ||||
| Development of a fabrication method for the orbital project “Microscope” |
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| The Scientific Instrumentation Department of the PTB is one of the partners in ESA's “Microscope” project. The goal of the project is to validate the Equivalence Principle with an accuracy of 10-15. It is the task of the Scientific Instrumentation Department to develop a course of fabrication and to actually fabricate test masses for the differential accelerometer with a precision in the micrometer range. The test masses are to be fabricated from a titanium alloy (TiAl6V4) as well as from the alloy PtRh10, for which no experience of its machining processing is available. Figures 1a and 1b show a test mass as a CAD (Figure 1a) as well as one of the first fabricated prototypes (Figure 1b) made of TiAL6V4. The (outer) test mass shown here is to be fabricated with a length of 79.83 mm (± 0.002 mm), an outer diameter of 68.017 mm (± 0.0015 mm) as well as an inner diameter of 60.8 mm (± 0.0015 mm). The deviations of the parameters parallelism, planarity, concentricity and rectangularity are also in the range of 1 µm, in part also below this value. On the surface, four flat areas are to be fabricated at the degrees of angle 0°, 90°, 180°, 270°. In addition, six counter sinks, which serve as bearings and retention of the test masses during the launch phase, are to be produced on each end face. The counter sinks have to have a cone angle of 90° (± 0°1,5'). An inserted, ideal sphere with a diameter of 3 mm has to come on a circle which has a distance of 0.539 mm (+0.003 mm, -0 mm) to the end face. |
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| Figure 1a: CAD diagram of the outer test masses for the Microscope Project to verify the Equivalent Principle. Figure 1b: First prototype fabricated from the Ti alloy TiAl6V4 at the PTB. In order to fabricate an object of this type with the precision required, the specimen may not be unmounted to determine the dimensions achieved between the processing steps. It is, thus, necessary to integrate the measuring technology (tactile measuring probes) into the precision fabrication facility. The temperature in the fabrication room is regulated by air-conditioning. While a test mass is being fabricated, the fabrication facility is not switched off. To fabricate the counter sinks on the second end face it is, however, essential to rechuck the test mass. In doing this, the angle error relating to the central axis is to be kept sufficiently low. The re-adjustment of the angle at circumference takes place by determining the relationship between the test mass and the measuring system within the machine by means of a plane-parallel mirror glued to one of the flat areas and an autocollimator. The mirror is needed as the flat area does not possess sufficient reflectivity for the collimator due to the processing technology. The angle error while re-assembling the test mass can, thus, be reduced to a few minutes. The fabrication of the test mass from the material PtRh10 is an extraordinarily great challenge. Figure 2 shows the results of several processes. PtRh10 is an extremely soft material, which tends to agglutinate during processing, Figure 2a. A series of different approaches to processing was needed in order to find suitable working parameters, allowing the required surface quality to be attained, Figure 2b. |
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| Figure 2a: First machining processing of the PtRh10 test mass by means of a tool with a mono-crystalline diamond Figure 2b: Advanced processing with a poly-crystalline diamond along with adjusted cutting speeds and inserted cooling lubricant. After completing the development of the basic course of fabrication, qualification modules will initially be fabricated. Following intensive tests by the French development partner ONERA the fabrication technology will be optimized and further measuring technologies will be integrated, before the flight and spare modules can be produced. |
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