The following exercise exemplifies a five-step process of geometric modeling practiced in reverse engineering industries:
1. Define the scope of work. The reverse engineering process begins with defining the project scope and identifying the key require- ments. Once defined, appropriate methods will be utilized to obtain the relevant data of the part, such as the part geometry.
2. Obtain dimensional data. Step 2 utilizes dimensional metrology equipment to obtain all the relevant dimensional data necessary to create a design drawing or CAD model of the part. The use of digi- tizing or scanning may be needed. The dimensions of the part can be measured by various instruments: (a) noncontact measurement, (b) coordinate measuring machine (CMM) with contact probe, or (c) portable CMM. The 3D laser scanning is one of the most comprehen- sive, direct ways to reproduce complex geometries accurately. The capability of measuring hardware has been dramatically enhanced with advanced software. Though developed with different prin- ciples and often with specific strengths and shortcomings, most reverse engineering software packages are designed with compre- hensive application capabilities. Table 2.1 lists some commonly used TablE 2.1
Software
Parametric Modeling NURBS Modeling Analysis
Publisher Software Publisher Software Publisher Software INUS
Technology RapidformXO Innovmetric Polyworks
Modeler Innovmetric Polyworks Inspector™
Dassault
Systemes SolidWorks Raindrop Geomagic
Studio Raindrop Geomagic
Qualify Autodesk Inventor
software. RapidformXO published by INUS Technology, SolidWorks by Dassault Systemes, and Inventor by Autodesk are three widely used software packages for parametric modeling. Polyworks Modeler™ by Innovmetric, and Geomagic Studio by Raindrop are two popular software packages for NURBS modeling. Polyworks from Innovmetric and Geomagic Quality from Raindrop are used by many engineers for inspection and analysis. Nonetheless, their applications are frequently cross-referenced in both fields of model- ing and analysis.
3. Analyze data. This step formulates the nominal dimensions of the part based on the measured data. It sets the CAD analytical model with the integration of industry standards and customer specifica- tions to ensure the fit, form, and function requirements.
4. Create the CAD model. A 3D model in a suitable CAD package with the nominal dimensions is generated following a best-fit line, arc, or spline adjustment. Best practices are utilized when creating models, along with the customers’ corporate standards when applicable.
5. Verify the quality. A real-life part can be scanned to verify the analyt- ical CAD model. By comparing the point cloud data (gathered from scanning the part) with the CAD model, a comparative deviation map, usually color coded, can be generated. If any deviations are identi- fied, the CAD model can be adjusted accordingly until the part is modeled accurately. The first article can also be effectively inspected by comparing a full scan of it to the referenced CAD geometry.
In summary, precision measurement devices, advanced software, and modern reverse engineering technologies have made the reinvention of mechanical parts feasible with tight tolerance and high fidelity.
References
Cleveland, B. 2009. Prototyping process overview. Adv. Mater. Processes 167:21–23.
Chivate, P., and A. Jablokow. 1993. Solid-model generation from measured point data.
Comput. Aided Design 25:587–600.
3DScanCo/GKS Global Services. 2009a. Case studies: ARINC in the World Wide Web.
http://www.3dscanco.com/clients/case-studies/arinc.cfm (accessed October 1, 2009).
3DScanCo/GKS Global Services. 2009b. Case studies: Airbus A319 in the World Wide Web. http://www.3dscanco.com/clients/case-studies/computational- methods.cfm (accessed October 1, 2009).
3DScanCo/GKS Global Services. 2009c. Case studies: The Southern 408 in the World Wide Web. http://www.3dscanco.com/clients/case-studies/panoz-automo- tive.cfm (accessed October 1, 2009).
Weir, D., Milroy, M., Bradley, C., and Vickers, G. 1996. Reverse engineering physi- cal models employing wrap-around B-spline surfaces and quadrics. Proc. Inst.
Mech. Eng. B. 210:147–57.
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3
Material Characteristics and Analysis
Material characteristics are the cornerstone for material identification and performance evaluation of a part made using reverse engineering. One of the most frequently asked questions in reverse engineering is what material characteristics should be evaluated to ensure the equivalency of two materi- als. Theoretically speaking, we can claim two materials are “the same” only when all their characteristics have been compared and found equivalent.
This can be prohibitively expensive, and might be technically impossible. In engineering practice, when sufficient data have demonstrated that both the materials having equivalent values of relevant characteristics will usually deem having met the requirements with acceptable risk. The determination of relevant material characteristics and their equivalency requires a compre- hensive understanding of the material and the functionality of the part that was made of this material. To convincingly argue which properties, ultimate tensile strength, fatigue strength, creep resistance, or fracture toughness, are relevant material properties that need to be evaluated in a reverse engineer- ing project, the engineer needs at least to provide the following elaboration:
1. Property criticality: Explain how critical this relevant property is to the part’s design functionality.
2. Risk assessment: Explain how this relevant property will affect the part performance, and what will be the potential consequence if this material property fails to meet the design value.
3. Performance assurance: Explain what tests are required to show the equivalency to the original material.
The primary objective of this chapter is to discuss the material characteris- tics with a focus on mechanical metallurgy applicable in reverse engineering to help readers accomplish these tasks.
The mechanical, metallurgical, and physical properties are the most rele- vant material properties to reverse engineer a mechanical part. The mechan- ical properties are associated with the elastic and plastic reactions that occur when force is applied. The primary mechanical properties include ultimate tensile strength, yield strength, ductility, fatigue endurance, creep resistance, and stress rupture strength. They usually reflect the relationship between stress and strain. Many mechanical properties are closely related to the met- allurgical and physical properties.
The metallurgical properties refer to the physical and chemical character- istics of metallic elements and alloys, such as the alloy microstructure and chemical composition. These characteristics are closely related to the ther- modynamic and kinetic processes, and chemical reactions usually occur dur- ing these processes. The principles of thermodynamics determine whether a constituent phase in an alloy will ever be formulated from two elements when they are mixed together. The kinetic process determines how quickly this constituent phase can be formulated. The principles of thermodynamics are used to establish the equilibrium phase diagram that helps engineers to design new alloys and interpret many metallurgical properties and reactions.
It takes a very long time to reach the equilibrium condition. Therefore, most grain morphologies and alloy structures depend on a kinetic process that determines reaction rate, such as grain growth rate.
Heat treatment is a process that is widely used to obtain the optimal mechan- ical properties through metallurgical reactions. It is a combination of heating and cooling operations applied to solid metallic materials to obtain proper microstructure morphology, and therefore desired properties. The most com- monly applied heat treatment processes include annealing, solution heat treatment, and aging treatment. Annealing is a process consisting of heating to and holding at a specified temperature for a period of time, and then slowly cooling down at a specific rate. It is used primarily to soften the metals to improve machinability, workability, and mechanical ductility. Proper anneal- ing will also increase the stability of part dimensions. The most frequently utilized annealing processes are full annealing, process annealing, isother- mal annealing, and spheroidizing. When the only purpose of annealing is for the relief of stress, the annealing process is usually referred to as stress relieving. It reduces the internal residual stresses in a part induced by cast- ing, quenching, normalizing, machining, cold working, or welding. Solution heat treatment only applies to alloys, but not pure metals. In this process an alloy is heated to above a specific temperature and held at this temperature for a sufficiently long period of time to allow a constituent element to dis- solve into the solid solution, followed by rapid cooling to keep the constituent element in solution. Consequently, this process produces a supersaturated, thermodynamically unstable state when the alloy is cooled down to a lower temperature because the solubility of the constituent element decreases with temperature. The solution heat treatment is often followed by a subsequent age treatment for precipitation hardening. From the heat treatment perspec- tive, aging describes a time-temperature-dependent change in the properties of certain alloys. It is a result of precipitation from a supersaturated solid solu- tion. Age hardening is one of the most important strengthening mechanisms for precipitation-hardenable aluminum alloys and nickel-base superalloys.
Physical properties usually refer to the inherent characteristics of a mate- rial. They are independent of the chemical, metallurgical, and mechanical processes, such as the density, melting temperature, heat transfer coeffi- cient, specific heat, and electrical conductivity. These properties are usually
measured without applying any mechanical force to the material. These properties are crucial in many engineering applications. For example, the specific tensile strength (strength per unit weight) directly depends on alloy density, and it is more important than the absolute tensile strength when engineers design the aircraft and automobile. However, most material char- acteristics do not stand alone. They will either affect or be affected by other properties. As a result, some material properties fall into both mechanical and physical property categories, depending on their functionality, such as Young’s modulus and shear modulus. An accurate Young’s modulus is usu- ally measured by an ultrasonic technology without applying any mechanical force to the material. However, Young’s modulus is also commonly referred as a ratio between the stress and strain, and they are the key elements in mechanical property evaluation. The interrelationships between metallurgi- cal and mechanical behaviors also cause some material properties to fall into both categories, such as hardness and stress corrosion cracking resistance can be referred to as either metallurgical or mechanical properties.
3.1 Alloy Structure Equivalency