Engineers Must Forward Engineer to Reverse Engineer

The goal of a restoration project involving a vintage de Havilland Vampire jet fighter was to return it to flying serviceability for display at an air show. A number of components were badly corroded, including the lower bracket for the ejection seat main gun tube (see Figure 1), which determined that the bracket was originally made of a very low-density magnesium alloy. The mass of any item within the airplane originally would have been an important design criterion because maximum lifting ability should be devoted to carrying payload, i.e., weapons and fuel. Since the aircraft's original military role was no longer relevant, it was decided that a heavier material should be used - an aircraft-grade aluminium alloy that would be less susceptible to corrosion and with the original dimensions, it also would give a much stiffer structure.

The original bracket was digitized using the FARO arm, a portable coordinate measuring machine (see Figure 2) from FARO Technologies (Lake Mary, FL) - a provider of computer-aided manufacturing measurement equipment - which can digitize points to a resolution within 76 microns (about three-thousandths of an inch) within a four-meter (12-foot) sphere around its origin. The arm can create a database of points, which must then be interpreted by a skilled operator. The probe tip is a small sphere, although other probe types can be used. With a 6-mm sphere tip, allowance must be made for the fact that the center of the sphere does not contact the body being digitized.

Software supplied by FARO allows the direct construction of CAD models from the digitized points; however, this was not done because a direct copy was not being made due to the different manufacturing method. Some reverse engineering experts have given the impression that replicating an original shape with a CAD model is the way to do reverse engineering. There is much more to it - engineers must be able to forward engineer before they can reverse engineer.

Occasionally, the design criteria are changed, and the methods and processes are different, which results in a replica that performs the same function, but may have a considerably different form.

Figure 3 shows the digitizing of the original and the production a new CAD model. While performing this task, it was noted that some dimensions are functional and some are more at the discretion of the designer. For example, the large circular holes in the vertical sidewalls need to be sized so that the pin that fits through them is a firm push fit. They were measured at 12.81 mm. This was assumed to be a nominal 1/2-inch hole and that some wear and corrosion had enlarged the original hole.

Other holes in the base plate are merely to hold the plate in position. Remembering that British imperial units were used by the original designer, a measurement of 38.252 mm was interpreted as 1.506 inches - the original design choice for the separation of the fitting holes. A more accurate measurement would have been 38.1 mm (1.5 inches), but it is important to be aware of manufacturing accuracy limitations in the original. This could be significant for designers used to English units in reverse engineering parts designed in metric units; arbitrary spacing would more likely be to the nearest 5 or 10 mm.

The new CAD solid model was built up without draw angles using MicroStation Modeler software from Bentley Systems, Inc. (Exton, PA) - a company that offers products and services for creating, managing and publishing project and facility information. A further model was developed to reflect how various stages of machining would develop the shape from an original block to the final artifact (see Figure 4).

The different colors used in the model show how the material is removed at various stages with different size ball end-milling cutters. In fact, several models were used to program the CNC milling machine since the machine available had only 2 1/2- axis capability and there was limited experience available in 3-D machining. Thus, a series of programs were developed for each cutter in a 2-D mode.

In order to test the models and programs, a physical prototype was produced in wood before more expensive materials were used.

Figure 5 shows the prototype alongside the original. The noticeable difference in form between the two illustrates well that reverse engineering is not just duplication of form. It is a design process that starts by capturing the original form, but uses the data as a starting point to replicate the required function using contemporary methods and tools.