For some of the components used on ships and submarines, the US Navy does not always have sufficient documentation from the Original Equipment Manufacturer (OEM) to competitively procure replacement components.
In the past, this meant it needed to purchase replacements from the original manufacturer, which was often very expensive.
Recently, the US Naval Undersea Warfare Center (NUWC) has begun using laser scanning to reverse engineer components with complex geometries in order to help out.
A laser scanner captures points at a much faster rate than a coordinate measurement machine (CMM) and also operates without the need for an operator.
‘The time needed to reverse engineer a typical component, including both measurement and modelling time, has been reduced from 100 hours with a CMM to 42 hours with a laser scanner,’ said Patrick Bergan of the NUWC.
‘The Navy saved $250,000 by purchasing the first part produced with laser scanning through competitive bidding.’
As US Navy systems and platforms age, the demand created for spare parts increases significantly. In cases where the Navy does not have detailed drawings and documentation for these parts it has been in the past forced to work with the original suppliers. Purchasing the parts from a single bidder without the benefit of competition can be very expensive.
In other cases, the companies that originally built the parts have left the business and drawings and documents do not exist. Or, partial documentation may be available which is not sufficient to manufacture the part.
To reduce costs, the Navy wants the capability to quickly and accurately reverse engineer components so that it will possess complete documentation and can from that point on purchase the parts at a much lower cost through the competitive bidding process.
Typically, Navy components can be particularly challenging to reverse engineer because of their complex geometries including pump impellers, pump housings, compressor impellers, compressor housings, hinges and brackets.
The challenge for the Navy is that traditional reverse engineering methods are very time consuming, labour intensive and capture only a limited number of points so they require a considerable amount of manual modelling effort in order to complete the model.
Another concern with traditional reverse engineering methods is that they capture such a limited number of points that the accuracy of the finished solid model is open to question.
A major limitation of a CMM is that an operator must manually move the steering system to track each point to be measured one at a time. The amount of time required to measure each point means that it is only practical usually to capture a few thousand points. This is not nearly enough to uniquely define the surface geometry of a complex part. So the CMM operator captures cross-sections of the part and relies up a computer aided design (CAD) operator to manually develop smooth surfaces to tie the cross-sections together. This process adds considerably to the amount of time required for reverse engineering and makes the accuracy of the finished model dependent upon the skill of both the CMM operator and the CAD operator.
Recently, NUWC Keyport evaluated new reverse engineering methods based on the emerging technology of laser scanning.
Laser scanning systems work by projecting a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, enabling the object to be accurately replicated.
The laser probe computer translates the video image of the line into 3D coordinates, providing real-time 3D coordinate data that gives the operator immediate feedback on areas that might have been missed.
Laser scanners are able to quickly measure large parts while generating far greater numbers of data points than probes without the need for templates or fixtures. Since there is no contact tip on a laser scanner that must physically touch the object, the problems of depressing soft objects, measuring small details, and capturing complex free form surfaces are eliminated.
Instead of collecting points one by one, the laser scanner picks up tens of thousands of points every second. This means that reverse engineering of the most complicated parts can often be accomplished in significantly less time with greater accuracy.
Laser scanning can reverse-engineer parts that are so complex that they would be practically impossible one point at a time. Finally, the software provided with the scanner greatly simplifies the process of moving from point cloud to CAD model, making it possible in minimal time to generate a CAD model of the scanned part that faithfully duplicates the original part.
Special software can be used to compare the original design geometry to the actual physical part, generating an overall graduated colour error plot that shows at a glance where and by how much surfaces deviate from the original design. This goes far beyond the dimensional checks that can be performed with touch probes on CMM.
NUWC engineers surveyed available laser scanning technology and through a competitive bidding process purchased the DS-3060 Surveyor from Laser Design, Minneapolis, Minnesota.
They originally purchased several Laser Design RPS series probes and the most advanced SLP-330 laser scanning probe. The DS-3060 includes a 30 inch by 60 inch by 24 inch gantry table, offering the capability to automatically scan objects up to that size without operator intervention.
The SLP-330 laser probe captures up to 50,000 points per second and weighs less than a pound. Another advantage of the new laser probe is that dual detectors view the laser line from two different angles, reducing the number of scanning passes required to capture steep sidewalls and deep geometries. NUWC Keyport purchased the probe with the Renishaw PH10 indexing system which provides two additional axes of motion and a Newport rotary stage for a total of 6 axes.
‘With the Laser Design DS-3060, the operator can simply map out the area to be scanned and walk away while the laser scanner operates,’ Bergan said.
‘The scanner will capture millions of points that completely define the surface of the part. So laser scanning dramatically reduces the amount of time required for reverse engineering. In the past, it typically took 20 hours to perform measurements with a CMM and 80 hours to convert the measurements into a coherent solid model.
With the laser scanner we can scan the part in only two hours. The laser scanner captures far more points than the CMM which reduces the amount of modelling time to only 40 hours. The overall time for reverse engineering of a typical component is thus reduced by more than half from 100 to 42 hours. In addition, engineers have more confidence in the accuracy of the resulting solid model since it is based nearly entire on scanned data and relies far less on the CAD operator’s judgment.’
NUWC Keyport also uses the laser scanner to inspect parts, typically first articles, produced by its suppliers. In this situation, NUWC engineers scan the part provided by the supplier and convert the resulting point cloud to a solid model.
They import the solid model along with the original CAD model that defines the design intent into software that displays differences between the two in a range of colours. These colours indicate the magnitude of the variation from the design intent.
CAD Model, scanned model and the 3D comparison
‘The colour-coded error map shows us visually exactly how the first article varies from the CAD model,’ Bergan said.
‘This chart saves a considerable amount of time by helping us understand the manufacturing variation. We can see at a glance if the variation is in a critical area or one that has no impact on the part’s performance. In many cases the error map helps diagnose the cause of the manufacturing variation. The graphic, for example, makes it easy to see how the part has twisted out of shape while cooling after a forging operation. This would be much harder or even impossible to spot on a printout of dimensions captured by a CMM.’
‘The time and cost savings that can be achieved with laser scanning are enormous,’ Bergan concluded.
‘Laser scanning easily reduces the time required for reverse engineering by more than 50%. The models that are produced with laser scanning are also more accurate because laser scanning generates so many more measurements. The real value comes in the savings that the Navy achieves when it reverse engineers a component and then procures it through the competitive bidding process. On the very first part that we scanned the savings amounted to $250,000 which was nearly enough to pay for the scanner. Since then we can have scanned 25 additional parts and we expect the savings on each to be significant. Clearly laser scanning offers major benefits to the Navy and other organizations that need to reverse engineer complex components.’