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0819LargeAirfoil-800.jpg Exact Metrology
An airfoil is an investment casting for a turbine or propeller blade, with a cross-sectional shape like a wing. Current engine design objectives put renewed significance on the details of these shapes, in order to achieve specific airflow objectives.

Pro Tips on Scanning High-Value, High-Precision Parts

An increasing number of cast parts demand high-tech inspection and evaluation. A 3D-laser and CT scanning expert details the steps involved

Meeting and maintaining product tolerances is more than a responsibility for foundries and diecasters producing precision products: it’s a revenue driver and an image shaper. Producing high-precision parts — for example, airfoils — involves a significant investment in process technologies, materials, and talent. Certifying that those products meet the customers’ design parameters and quality certifications is as much a part of the manufacturer’s mission as melting, molding, and pouring. It’s no mystery that 3D laser and computed tomography (CT) scanning are important to product designers and product buyers — but what should the part manufacturers know about these technologies.

The scanning process
Exact Metrology, supplies 3D laser and CT scanning equipment, but also performs scanning as a service. Scanning the part can be done at the customer’s operation or at Exact Metrology in Brookfield, WI. In many cases, digitizing or scanning is done more cost-effectively at Exact’s location.

However, scanning equipment is mobile, so in instances when turnaround time is important or downtime of the scanned component is critical, onsite inspection is available.

Typically the airfoils that are provided by customers will be received by Exact Metrology for full documentation with photos, measurements, and any surface markings or serial numbers recorded prior to any high definition scanning.

Setup — The mobility of the Exact scanner allows a high degree of flexibility in performing onsite scans (Option A) at a customer facility or the more cost-efficient, in-house scan at the Exact facility (Option B).

Due to the reflective nature of airfoil blades, a thin coating of Magnaflux spray is applied. The SKD-S2 spray is a second phase of the crack checking processes. Phase 1 is a die penetrant that typically will not be used. The spray meets the requirements for AMS 2664, ASTM E165 and is ASME approved. (Health – 1, Flammability – 4, Reactivity -0, No Specific Hazard). See MSDS 0166 for more information on the SKD-S2 product.

When scanning the blade, Exact uses a fixture device. Targets also will be used to constrain data during the alignment phase. Targets will be placed using magnets on either side of the air foil. The fixture itself also will act as an alignment constraint while holding the airfoil in place and allowing Exact technicians to capture maximum data on the part without moving or touching the part. A manual or automatic turntable may be used to allow enhanced scanning and registration access to the part.

Scanning — The part is positioned on the rotating table or on a stationary marble top and scanned from multiple perspectives. This procedure is repeated multiple times to provide complete coverage of the part. Known geometric objects, cubes, and spheres often are placed in the scene for quality assurance and validation during the validation stage.

The scanner used on these projects is a Breuckmann Stereo 3D Scanning system. This device uses a novel, halogen structured-white-light projection system with dual 6.6 mega-pixel cameras. This is the highest-resolution scanner available on the market and was built in Germany especially for Exact Metrology.

Registration — The registration process can be done with or without targets. Both registration methods have different techniques with similar results.

Essentially, the targeting procedure uses an automated fitting of point clouds or scan worlds together, based upon three common positions on the part or scene. This process has been simplified with modern software technology. The software will analyze every point with overlap and run a best fit algorithm over hundreds of iterations to find the tightest and best alignment. The result of this process will yield a fully registered and organized point cloud.

Validation — It is important to verify the automated method with targeting and adding known geometric shapes to the scan. All scans are documented with notes, computer logging and digital photos accompanying the recorded scan setup within the software. A CD copy of all digital data is recorded prior to returning the part or teardown.

The modeling process
Once the data is gathered and the registration process is complete, the point cloud or data set can be worked. The first step is to triangulate or mesh the points together. As part of this early process, points are “intelligently” removed from the network by algorithms in the software. Essentially, the points of redundancy and overlap are decimated and averaged. This mesh is further manipulated by reducing the triangles and points along flatter areas and retaining more triangles in areas of curvature and complexity. This poly mesh can now be exported as a complete *.STL file or similar format.

Surfacing — The process of fitting NURBs and geometry to the poly mesh or point cloud can be a time-consuming and arduous process. A surface is basically draped over the mesh and smoothed to be reflective of the actual part. Once this task is completed and a complete watertight object is created, the model is ready to be exported to nearly any format via various methods. The end file format is entirely up to the client and the limitations of existing conversion tools available today.  A final 3D solid model can be provided as a “dumb solid” with no parametry or history tree as an imported object. The available formats include Siemens NX, Pro/E Wildfire, AutoCAD, AutoCAD Inventor, SolidWorks, as well as generic formats such as IGES, STEP, ParaSolid (X_T) and others.

Parametric modeling — The final deliverable is a parametric model. Unlike a solid model, the parametric model possesses relationships of all dimensions within the CAD. With these relationships, the CAD model can be manipulated and fine-tuned without disturbing the overall geometry of the part. For this workpiece, the specific parameters of each cross section within the model can be independently modified without disturbing neighboring cross sections. The end result is a Pro/Engineer CAD model with known geometry that is fully adjustable and possesses a design history.

Once this evaluation is complete, and meets the expectations determined, the CAD model is ready to be delivered. 
Dean Solberg is the co-president of Exact Metrology, which offers comprehensive metrology services, including 3D laser and CT scanning, reverse engineering, quality inspection, product development, 3D printing, and 2D drawings. Contact him at [email protected].

TAGS: Testing/QC
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