Going in reverse

By Michael Raphael
Wednesday, 13 August, 2003



Michelangelo used the principles of reverse engineering to create great sculptures. Today, manufacturers use it to create images of products to speed up prototyping, manufacturing and testing processes.

As a technique for making products, reverse engineering used to be viewed as a not-so-ethical process of copying successful designs by OEMs but with less design capability than the originators.

Indeed, 30 to 40 years ago, a number of noteworthy products were copied and produced, from complete aircraft to motorcycles. But since then, reverse engineering has come to be viewed in a more positive light because it speeds up and simplifies many developmental procedures for OEMs. Today, it is in almost universal use because it enables OEMs to be more competitive across several disciplines.

Ergonomics and dimensional control

Reverse engineering enables OEMs to evaluate competitors' products - or their own early products - and compare them to their latest designs. From an ergonomic standpoint, reverse engineering brings greater understanding of human factors.

Using a measuring-arm co-ordinate measuring machine (CMM) as a reverse engineering tool enables engineers to precisely define the physical capabilities of the human physique with respect to tasks that must be performed. Available tables provide the limits of some human factors, but they do not define the comfort range of human motion and effort. An engineer studying ergonomic factors must do so on a case-by-case basis, and the arm makes the task easy.

From a dimensional control standpoint, OEMs gain knowledge of how well manufacturing processes can hold tolerances. Reverse engineering is used to digitally recreate a product and compare the recreation to design dimensions. This is common in prototyping circumstances where development engineers monitor pre-production runs of a product to see if the forming tools can endure the rigors of production. The beneficiary of this type of investigation is the consumer, who eventually is offered products that are easier, better balanced, quieter or simpler to use.

Time-to-market

However, the most used application of reverse engineering is in rapid prototyping where it compresses the time to get designs from the model stage to the prototype stage - or even to jump the prototype stage altogether and use images gathered via reverse engineering to develop manufacturing tools and techniques.

In either case, the basics of reverse engineering are fairly straightforward. Models made in wood, plaster, clay, wax or other solid media are duplicated digitally as computer-aided design (CAD) files.

Existing products can be substituted for the model, for dimensional comparison or where no CAD file for the product exists. These files are then transferred to machining programs, such as computer-aided manufacturing (CAM), mould-cutting or robotic welding systems, whereby the model is duplicated in the material from which the product will be produced.

Sometimes, reverse engineering is applied to only a small part of this development process, such as when a change is made in an overall design. In this way, reverse engineering becomes the feedback loop for designers to evaluate changes, such as in radii, longer shaft, new bumper-bar contour or other incremental design modification.

Digital testing

Because reverse engineering can be adapted to testing procedures that would normally be conducted on a prototype, it is sometimes used to skip the physical prototype stage, thus saving more time. The digital image created in CAD can be tested via analytical programs without having to produce a prototype.

For instance, the image of valves, propellers, airfoils, impellers and ducts can be evaluated for drag, flow lines, cavitation, tip vortices and other flow conditions via computerised fluid dynamics (CFD) programs. Structural or load-bearing objects such as gears, frames or suspensions can be analysed via stress analysis or non-destructive testing (NDT) programs. If ergonomics is the issue, images of cockpits, vehicle interiors or boat interiors can be tested using human factors software to assess reach, line-of-sight, seating comfort and other considerations.

And let's not forget kinematics, the study of how things move and the forces associated with that motion. Applied to vehicles, the process can be used to find roll centres; applied to structures, it can be used to determine stability by locating centres of gravity.

Interfacing changes

In the early days of reverse engineering, measurements were taken via calipers, rules and dial indicators, but this was time-consuming and inaccurate. Later, when portable-style CMMs were developed, accuracy improved, but they still had to be programmed to measure a specific part, a time-consuming task.

A tool that makes reverse engineering easy and fast is the new generation of measuring-arm CMM. These devices are the articulating arms, the joints of which are optical encoders. Measurements taken from a mechanical stylus or a laser scanner attached to the end of the arm are encoded for X-Y-Z location and I-J-K positioning, the accuracy of which is as close as ±0.01 mm for the mechanical type and ±0.02 mm for the laser tracker.

Using an arm, a designer can take measurements of a part or model as single points or as streams, curved or straight. This data can be converted into many forms in software, depending on the end use of the image.

Dividing line

The degree of detail required to reproduce a faithful recreation of the original will determine the measuring system - mechanical or laser - to use. The mechanical probe is preferred for parts that are mostly composed of recognisable geometric shapes such as rectangles, cylinders and flat plates.

Most products that are manufactured by welding, staking, drawing, punching, water-jet cutting or turning fall into this category. After taking point or line measurements, a designer can construct a wire-frame image of the object in a program such as AutoCAD, then transfer this data into surfacing software where it is converted into a likeness of the original.

This technique can be adequate for most types of analytical use, including ergonomic, CFD, stress and strain and NDT evaluations. The number of points gathered in a typical project will vary from 100 to a few thousand. To gather 1000 points it typically takes five to six minutes. Greater point complexity is unnecessary, and, for most current software analytical packages, becomes unwieldy.

This reverse engineering technique is suitable for most tool and mould making, as well as machining (CAD-to-CAM transfer) where parts are composed primarily of geometric shapes. A digitised image is transferred as a .dxf file to a post processor and converted into a numerical control cutting program. Man-days are often reduced to man-minutes.

However, when an item is made up of complex curves, detailed decorative work, grip patterns or intricate designs, the mechanical system of gathering data is often too slow or the data too sparse to produce an adequate image of the item. In these cases, the choice for capturing the image is usually a laser scanner. The difference is in image density.

Lasers are ideal where a point cloud of hundreds of thousands of points needs to be created. Examples include statuary, prosthesis parts, animated characters, etchings and other items where surfaces change almost continuously and cannot be described by geometric shapes.

Lasers measure large parts

In addition to projects that require greater detail, a laser-based arm can more readily scan large objects than one based on a mechanical probe. Even though mechanical systems can be 'leap-frogged' to measure beyond the reach of the arm, a laser can still scan large objects much faster than the mechanical touch-and-click system of measuring.

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