19-12-2012, 06:07 PM
GENERATION OF TOOL PATH FOR AN AXIAL FLOW TURBINE
GENERATION OF TOOL PATH FOR AN AXIAL FLOW.doc (Size: 66.5 KB / Downloads: 27)
ABSTRACT
The objective of this presentation is to create a CAD model and then to generate NC codes for machining a blade of an axial flow turbine by using reverse engineering methods. Reverse engineering has been performed on a model of the above mentioned turbine using a manual operated co-ordinate measuring machine (Spectra 6.4.4). Only one of the four blades have been considered for reverse engineering since the other blades are symmetrical. The co-ordinate points obtained from the CMM have been imported to the ProEngineer Wildfire interface. These cloud points have been used to create datum curves, which have been further used to create a solid extrusion model of the axial flow turbine. The model then obtained has been used in Master Cam package to generate an NC code for the milling sequence required to create the model of the axial flow turbine. A multi axis machining sequence has been opted for and a ball end type tool has been selected. The NC code generated which when applied to a CNC machine will generate a model.
WHAT IS REVERSE ENGINEERING?
The process of disassembling a hardware or software product from another company to find out how it works, with the intention of duplicating some or all of its functions in another product is popularly called reverse engineering. The major function of reverse engineering in industrial applications is to create CAD models based on physical models. This is also used in editing CAD models based on any alterations in the corresponding physical model. Reverse engineering is also used in creating STL files used for rapid prototyping. Reverse engineering is most useful in modern industrial processes as a tool for quick reproduction of parts, enabling manufacture of tool – die sets and in general, easy exchange of information.
ADVANTAGES OF REVERSE ENGINEERING
• Fast availability of CAD models
• Physical model is used as the starting point
• Shortened development process
• Fully developed product at the start of production
• Reduction in product and production costs
METHODOLOGY
In our attempt in reverse engineering a turbine blade model, we have undertaken the following steps:
1. Obtaining cloud points of a single turbine blade using a CMM
2. Using the obtained cloud points to create a CAD model in Pro-Engineer.
3. Importing the CAD model into MasterCAM and obtaining the NC code for the milling sequence.
The various stages in the development of the NC code have been discussed below:
OBTAINING CMM CLOUD POINTS
The CMM that has been used for obtaining the cloud points for the turbine blade is SPECTRA 6.4.4(manually operated). The specifications of the CMM used are:
1. Type: Bridge type CMM
2. Work Volume: 800 x 800 x 450 mm3
3. Probe Manufacturer: Renishaw
4. Sensitivity: 0.1 microns
5. Repeatability: 0.5 microns
6. Stylus Length: 80 mm (60 mm + 20 mm)
7. CMM – Controller Interface: RS232 Port
8. Probe Diameter: 5 mm
First, the turbine model was placed in the CMM worktable. The probe, after performing tests on its accuracy, is contacted on the diameter of the shaft hole in the hub. Then the probe is contacted on the step surface adjoining the shaft hole on the hub. Using these inputs, the CMM controller software calculates the origin and the position of the z-axis. The calculated origin coincides with the point of intersection of the axis of the hole and the horizontal plane of the step adjoining the shaft hole. The z-axis is oriented away from the body of the turbine. Now the cloud points are obtained from the CMM by manually positioning the probe at the required point.
Most of the points we obtained are along the edges of a blade of the turbine. We also obtained several points along the surface of the blade, as a mesh of points on a surface. But we found those points superfluous as we required only the edge points to create the CAD model.
The points obtained in the CMM are then obtained as a printout from the software interface of the CMM.
USING THE CLOUD POINTS TO CREATE A CAD MODEL.
1. After obtaining the cloud points, we converted the requisite points into a text file using only those points required for modeling, taking the probe diameter compensation into account. Then, we converted the text file into a “.pts” format file.
2. This file is the imported into Pro-Engineer interface using the “Offset Co-ordinate System Datum Point Tool”. The required points are obtained as datum points in the modeling window.
3. Datum curves are drawn through these points. Totally seven datum curves are obtained as shown. Using these datum curves, surfaces are generated. The intersecting surfaces form quilts.
4. These quilts are extruded to form the solid blade. Using the copy rotate option, three more blades are copied along the periphery of the hub, each at an interval of 45 degrees. After this rounding off of the blade edges are performed.