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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Miniature camera lens design with a freeform surface

Sasian, Jose, Yan, Yufeng 27 November 2017 (has links)
We present a miniature camera lens design method that uses a freeform surface based on the pedal curve to the ellipse in polynomial form. Two designs are presented and their benefits of optical performance and tolerance sensitivity are compared to designs with conventional aspheric surfaces. We also reverse a freeform design using even aspherical surfaces to show that the optimization solution of a freeform design cannot be reproduced by even aspherical surfaces.
2

Geometrically Adaptive Milling of Fan Blade Assembly Weld Fillets

Lin, Yu Pin 10 1900 (has links)
<p>Modern aeroengine design focuses on reducing overall weight and improving component service life. For fan blade assemblies, the blades and hub/shaft are attached by the most common dovetail (or fir tree) attachment design, which experiences fretting fatigue at the joint resulting in lower reliability and higher repair difficulty. A new joining design that connects blade /disk by welding and eliminates the attachment, has been implemented in military and commercial aeroengines. This joining design is most suitable for large diameter fan blades where single piece machining is impractical and time consuming. The joined blade requires post-process machining to remove excess weld material. However, because of varying assembly geometry, joints must be individually measured and tool paths consequently adjusted to match actual surface locations. The objective of this thesis is to develop an automated and geometrically adaptive post-process weld machining system.</p> <p>This thesis proposes a solution that integrates surface digitization, computer aided design (CAD) and computer aided manufacturing (CAM) systems, to accommodate the part-to-part variation issue. The integrated system includes precise laser digitizing, geometric modelling, tool path customizing, coordinate registration and CNC machining. The core algorithm was designed on the open and object-oriented C++ ACIS/HOOPS kernel. The customized tool paths are prepared based on the misalignment distance measured by laser digitizing, and a custom developed mathematical correction algorithm that can be implemented on a typical personal computer. At present, the machining process is designed for a three-axis machine tool. Suggested future works include implementation on a five-axis machine, and feed rate optimized tool paths.</p> / Master of Applied Science (MASc)
3

Specification and Verification of Tolerances for Parts with Free-Form Surfaces

Kale, Kishor B January 2013 (has links) (PDF)
The need for increased product variety and improved aesthetics require the manufacturing enterprise to reduce time to market and to increase use of free-form surfaces in the form of the product. These changes lead to problems in the traditional approach for specification and verification of tolerances especially for a free form surfaces. In the case of freeform surfaces, the desired performance of a product depends on its geometry and is often controlled by intrinsic parameters such as curvature. Design intent therefore requires control on variations in these parameters. Ideally therefore, tolerances have to be applied on these parameters to prescribe allowable variations in the geometry of free-form surfaces. Since only the geometry of the product is controlled in manufacturing, tolerance specification has to ensure that the tolerances specified on the part geometry will ensure that the resulting value of the parameter of interest is within the limits prescribed by the designer. Relationship between allowable range in design parameters and that in geometry is not linear. Tolerance specification therefore becomes a trial and error process requiring considerable expertise and time. This thesis provides designers with a tool to automatically derive the corresponding tolerances to be specified to the manufacturing process to realize the final shape, such that the parameters that are used to control shape of the surface are within the prescribed variations. Automation in acquiring inspection data has brought dramatic changes in procedure for tolerance verification too. Optical scanners and similar non-contact devices provide large amount of points on the surface of the part quite rapidly. The unstructured point data are then processed to determine if the part complies with the given tolerance specifications. For freeform surfaces, current methods of verification uses minimum distance criterion between the nominal surface and unstructured point data. This ignores the correspondence between the points in the two data sets and may result in the rejection of good parts and acceptance of bad parts. There are other unresolved such as the singularity at corners of polyhedral shapes and handling datum. A new approach based on the Medial Axis Transform (MAT) has been proposed. It has been shown that reasoning on the MAT of the nominal model and the measured point set respectively enables the identification of corresponding points in the two sets. Verification of the tolerance allocated is therefore free from the problem mentioned above. MAT exhibits dimensional reduction and hence reduces verification time. It also eliminates surface fitting for detected feature. Results of implementation are provided for tolerance specification and verification using MAT.

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