Filling the Gap

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Repair of worn or damaged turbine blades using an adaptive machining process is an economic alternative to fitting new parts.

December 6, 2010

The first stage in blade repair is to determine the real shape of the part by probing with a Renishaw stylus so the modified CAD model can then be used to ensure accurate machining.The traditional relationship between machining and inspection is that machining is completed first and the component is then transferred to a dedicated piece of inspection equipment to be approved or rejected. However, as machining techniques become more sophisticated, and as components become larger and more complex, there are a growing number of cases where closer integration is required to give higher productivity and reduced waste. Instead of a simple linear progression from CAD to CAM to machining to inspection, a more complicated series of steps is needed, with extra data needed to fill any gaps in the information available at the various stages. These new processes can be grouped under the heading of adaptive machining.

The programming of most machining operations is based around knowing three things: the position of the workpiece on the machine, the starting shape of the material to be machined, and the final shape that needs to be achieved at the end of the operation. Adaptive machining techniques allow successful machining when at least one of those elements is unknown, by using in-process measurement to close the information gaps in the process chain.

Electronic fixturing
The most common cases when adaptive machining techniques are needed are those where the exact position of the workpiece on the machine is unknown. With larger components, such as aerospace structures, achieving the correct position and orientation of the stock on the machine is a major challenge, taking many hours of checking and adjustment. It is often easier to adjust the datum for the toolpaths to match the position of the workpiece, than it is to align the stock in exactly the desired position. This approach has been used in the machining of geometric features for some time. An equivalent solution for the manufacture of complex shapes and surfaces is now available that gives the same benefits of shorter setup times and improved accuracy.

The first stage in this approach is to create a probing sequence in the inspection software, preferably using off-line programming so there is no interruption to the machine tool’s cutting time. This sequence is used to collect a series of points from the workpiece, which can be used by the range of best-fit routines in the inspection program to determine the exact position of the stock. Any mismatch can then be calculated between the nominal position used in the CAM system to generate the toolpaths and the actual position of the workpiece on the machine-tool bed. The software can then feed the results to the machine tool control as a datum shift or rotation to compensate for the alignment differences.

On-Machine Verification
On-Machine Verification (OMV) is another technique which uses probing equipment on the machine tool. It allows initial checking of machined parts to be carried out in situ on the machine rather than having to transfer them to a coordinate measuring machine (CMM) for inspection. The main advantage is that any mistakes are discovered where they can be corrected – on the machine tool. Repeated cycles of machining and inspection, interspersed with long setup times on the respective pieces of equipment, are avoided, meaning that overall manufacturing times can be reduced.

The most obvious benefit of OMV is for those companies that do not have existing inspection capabilities to implement the process. For companies that do have specialist equipment for their final inspections, OMV can still give huge time savings by enabling the quality of the component being machined to be monitored at all stages in the manufacturing process. OBy carrying out an initial verification on the machine, errors can be detected, and corrected, that might otherwise not be found until after the component had been shipped to the inspector.

Companies already having suitable inspection equipment might think that OMV is an unnecessary operation that will decrease machining time. However, if the whole process is considered, there is considerable potential to reduce delivery times. If a part has to be transferred to a dedicated CMM, and the inspection shows any errors, the component must be returned to the machine tool and re-clamped in position before being machined again. This is time-consuming for any component but can take many hours for a large, heavy item. In addition, any mistakes during the setup back onto the machine tool could result in a new series of errors in the component, leading to a further cycle of inspection and re-machining.

With OMV, the part can be checked at each stage. The inspection on specialist measuring equipment only needs to be undertaken once at the end of the manufacturing process. This more regular verification ensures that there will be greater confidence that the component will be produced within specification.

There are also concerns about the reliability of using a machine tool to check its own work. Of course, measurements made with a machine tool on the shop floor cannot duplicate the very high accuracy made possible on a dedicated CMM in a climate-controlled environment. However, while that level of precision may be very impressive, it is rarely needed in most manufacturing operations. In addition, the quality of the results from machine tools can be checked against known artefacts in exactly the same way that the inspection accuracy of a CMM can be confirmed. Trials undertaken by Renishaw have shown the results from machine tool measurements to be both more accurate and more consistent than was expected.

The move of the checking process from the CMM room to the factory floor means that the results need to be both quick and easy to produce and understand. Since there will no longer be specialist metrologists to interpret the data produced, making the best use of OMV requires software that is not only simple enough for machine-tool operators to use, but that also gives both quick and easy comparison of tooling and sample components against CAD data. The output must be clear, comprehensive reports that can be understood by everyone involved in the product development process, not just inspection specialists.

For this reason, Delcam’s PowerINSPECT software has proven particularly successful for OMV. The system offers a Wizard-based approach to developing the inspection sequence, making it easy to generate the required probe path. An extra level of security is provided by the process simulations that can be carried out on the computer, allowing any potential collisions to be detected before the routine is run on the shop floor. The software is also very flexible, allowing extra measurements to be added in any areas that may be causing concern.

Machining Near-Net Shapes
Most examples where the exact starting shape is unknown result from near-net-shape manufacturing processes, like casting and forging, or from imprecise repair techniques, such as welding. The main requirement in these cases is to allow an even distribution of material to be removed around the component to avoid over-machining in some areas and under-machining in others. Other benefits include the ability to give a smooth transition between machined and un-machined areas, a reduction in air cutting, and improved control over the feed rate as the cutter enters and leaves the material.

Depending on the degree of uncertainty of the shape, a probing solution or a reverse engineering solution can be used. Typically, machining of near net shape preforms uses a probing path to determine the form of the starting stock. This is generated and executed in the same way as the probing paths used to determine the part position in the electronic fixturing process described above. The final shape to be achieved can then be orientated within the envelope representing the starting shape to give an even thickness of material on the surfaces to be machined.

When there is greater uncertainty over the starting shape, which can result from component or tooling repair, reverse engineering software can be used to create a complete model of the areas to be machined. This can then be used within the CAM system to create toolpaths specific to that component. Many CAM systems can now produce toolpaths from the triangle models generated by reverse engineering programs, so eliminating the need to create a fully-surfaced CAD model.

Machining Unknown Shapes
The most challenging adaptive machining operations are those where the final shape of the component is unknown. This usually is needed when undertaking repairs to components that have been changed from their nominal CAD shape during service, for example, turbine blades that have been distorted by the high temperatures in aircraft engines. A similar problem can arise when repairing tools that have been modified after their initial manufacture, such as press tools that may have been adjusted to compensate for spring back, so that the original CAD data no longer matches the actual component.

The initial stage in these cases is to probe the component to determine the extent of its deviation from the nominal CAD data. Then, the morphing functionality in PowerSHAPE can be used to bring the CAD model into line with the actual geometry. Finally, toolpaths can be generated for the required areas with PowerMILL.

Another application area is the trimming and drilling of large composite components, such as hulls and super-structures for yachts, and aerospace panels. These parts are relatively flexible and their manufacturing methods do not have the consistency of metal panels. These factors mean that automated finishing methods are difficult to apply. Manual methods are too slow to meet the increased demands in both the aerospace and marine industries, and do not give the required level of accuracy.

The solution developed by Delcam, in conjunction with router manufacturer CMS Advanced Materials, allows a 5-axis machining center of any size or configuration to be used as a production trimming station. The software is used to measure the real position of the features on the component and then the trimming paths are adapted so that the cutting is performed exactly where it is needed, rather than where the CAD model thinks it should be.

The solution can also identify the real positions for holes to be drilled to give consistent, accurate holes normal to the surface.

Companies wanting to use adaptive machining processes must understand that they tend to be much more complex and process-specific than conventional CAM programming. There is a core of common functionality at the heart of all the solutions described above, and Delcam is utilizing and extending its product range to standardize this core functionality. Nevertheless, most adaptive machining projects will require some specific consultancy and customization work as part of their implementation.

Despite this added complexity, anything that can reduce waste or improve efficiency must be worthy of further investigation. Adaptive machining processes have the potential to achieve both these goals, making them something that progressive manufacturers cannot afford to ignore. 

Delcam plc
Salt Lake City, UT