Editorial Feature

Reverse Engineering: Using 3D Scanning Technology to Create a CAD Model

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Computer-Aided Design (CAD) is one of the most important tools currently used in manufacturing processes. Its use is well established across a number of different industries, ranging from biomedical to aerospace.

Although some CAD solutions are simply 2D sketches used for the creation of blueprints, such as architectural drawings, most currently used technologies construct 3D representations, or a CAD model, of the desired product.

Prototype products can be used for a wide range of different activities such as effectively communicating the intent of the design through renders of the final part, preparing blueprints for the workshop, or even for the analysis and simulation of the performance of the component using Finite Element Analysis.

The Introduction of 3D CAD Models

Ever since their introduction to giant manufacturing companies such as Ford, GM, and Boeing during the late 1960s and early 1970s, 3D CAD models have shown a multitude of advantages compared to manual drafting.

However, this move towards digitalization has resulted in a complex issue – the transformation of blueprints into three-dimensional models. The problem becomes even more complex due to the effect of globalization as the design department of a company might be on a different continent than the manufacturing center.

The Need for Digitalisation in the Marketplace

Industries are being forced to re-engineer their design processes and products to achieve higher efficiency coupled with reduced development costs due to intense competition in the global marketplace. This increases the need to digitalize part products from blueprints in order to develop the new components.

One approach to solve this issue is using CAD specialists to construct models from the original drawings and generate the new file. However, this method may be extremely time-consuming when working with complex parts. In addition, some of the old parts might not be properly documented and some drawings can be illegible due to poor storage.

Reverse Engineering

In order to address these problems, a methodology is known as Reverse Engineering (RE) has been developed. This process consists of the generation of a reconstructed 3D model based on measurements from the original component. The methodology is quite straightforward as the basic process stages can be divided into three steps:

The first of these stages is considered the most vital of all three since the accuracy and uncertainty of the measurements will affect the rest of the procedure. In order to increase the quality of the measurements, several techniques can be seen in the modern workshop, both for RE and inspection procedures.

Coordinate Measurement Machines

The first stage is based on the recording of the X, Y, and Z coordinates of the target. The points are collected using a probe that is positioned manually by an operator, such as an Articulated Arm, or automatically using a Direct Computer Control, such as Coordinate Measurement Machines (CMMs).

Tactile probes can be mounted in machine tools, providing an onsite inspection system. The most general-purpose instruments for measurement in the industry are CMMs. This kind of equipment is very flexible and allows the measurement of points with high accuracy.

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The device is manually operated and is portable; it can be useful for the measurements of freeform shapes with accessibility difficulties on-site, such as die cavities, heavy equipment, or parts that cannot be moved. Both articulated arms and tactile probes in machine tools are less accurate than CMMs.

The main advantage of these systems is that they are traceable to the meter and can be calibrated to international standards. In addition, the accuracy of the systems has been reported to reach less than 0.5µm. To put this into perspective, the diameter of a human hair is around 30µm.

However, the main disadvantages of these systems come from the contact nature of the measurement as deformation on the probe or the surface to be measured will yield inaccuracy. In addition, the cleanliness of the probe, probing force due to deformation wear of the probe surface, and even temperature due to thermal expansion/shrinkage can cause inaccuracies.

Optical Systems for Surface Approximation

In order to address these issues, optical systems have been introduced. The main advantage of these technologies is the fast acquisition of measurement points: 48,000 points per second rather than the 200 points per second from a fully automated CMM. This generates a point cloud image rather than coordinates, which is then used for the reconstruction of the model.

Many non-contact measuring technologies are based on triangulation techniques, which rely on detecting reflected light from an external source. The main components of such systems are a collimated light source, generally a laser diode, and a detector unit consisting of an imaging lens and a position-sensitive diode (PSD).

The optical axes of the light source and the imaging lens forms a fixed angle and the object surface is brought close to the point in which both axes intersect. The diffuse reflection of the light spot on the work piece surface is projected onto the detector.

The optical characteristics of the surface of the workpiece are the main source of uncertainty, as very smooth surfaces cannot be measured due to insufficient diffusely reflected light. Typical measurement ranges of this kind of technology are between 2mm and 200mm, with an average resolution of 50 µm.

One of the main disadvantages of these technologies is the sensitivity to translucid/reflective materials which require preparation for inspection. Such preparation requires spraying parts affecting accuracy with an approximate of 3-5 μm.

Another common disadvantage of non-contact techniques, in general, is that the uncertainty contributors are often not well known or documented and the traceability with the optical methods is not well defined. In order to address these issues, an international effort has been carried out over the last 30 years for the development of calibration and measurement standards and guidelines with optical systems.

The Reconstruction of the Model

Once the measurement data has been extracted, either via tactile or optical means, the reconstruction of the model can be carried out through different mathematical approaches and the model can be used as required.

Although RE might be viewed in a negative light due to the possibility for its use in copying a competitor’s components, it has been shown to be a reliable way to recover or update documentation, carry out an inspection in the workshop and even analyze our surroundings.

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In 2008 at the University of New South Wales in Australia, researchers used measurement data from a Computerized Tomography Scanner (CT) in order to generate a computer model of the bite force of a great white shark.

This study shows the potential of RE to diminish the gap between the real world and the digitalized realm, which is used not only to improve the development cycle of products but also to enhance our understanding of the world around us.

Sources and Further Reading

 

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