The Adaptive Approach

Capable of providing a real boost to manufacturing productivity, adaptive machining techniques use in-process measurement to close information gaps in the process chain.

The main challenge for manufacturers today is developing the ability to produce high-quality products on a consistent basis. Both business and domestic customers are demanding better appearance and higher performance from the goods they buy. At the same time, they are increasingly likely to turn to litigation if they don’t receive the quality they believe they deserve. Rising material prices are making the cost of scrap higher than ever so making it moreimportant to minimise waste.

Proper inspection and quality control methods are no longer a luxury; they are an essential part of any manufacturing operation that wants to grow and be successful. However, simply bolting on some quality control procedures is not enough. Inspection must be fully-integrated with manufacturing for the investment to really produce significant improvements.

The traditional relationship between manufacturing and inspection is that machining is completed first on the company’s machine tools and the components are then transferred to dedicated 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 the highest productivity and the biggest reductionsin wastage.

Instead of a simple linear progression from CAD (computer-aided design) to CAM (computeraided manufacturing) 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. It also allows any errors to be spotted earlier in the manufacturing process, so helping the problems to beresolved more quickly and at lower cost.

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, moulds for bigger parts like automotive dashboards, or press tools for car body panels, 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 set-up 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 positionof 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 thealignment 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 coordinate measuring machines 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 set-up 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 manufacturers that do not have existing inspection capabilities, for example those companies making parts so large that dedicated measuring equipment would be prohibitively expensive. However, companies that do have specialist equipment for their final inspections can also benefit through errors being detected earlier and corrected more quickly, which translates to lower cost.

For example, it will be possible to check that the correct amount of stock has been left on the component after a roughing operation, rather than having to wait until all machining operations have been completed before discovering that an error has been made.

Similarly, the extent of any damage caused, for example, by a tool breakage, can be assessed and a decision made immediately to determine whether the part can still be completed within tolerance or whether it will have to be scrapped.

Of course, there are already a variety of manual methods for undertaking such checks between machining operations. However, like all manual operations, these are time-consuming and prone to human errors and inconsistencies. Furthermore, they are based on inspection against drawings, when most design data is now issued as CAD files. OMV is a more automated and more consistent process than manual measurement, and is based on checkingagainst CAD data.

On-Machine Verification also benefits companies with customers that insist on independent inspection of their work. By 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 increases machining times. However, if the whole process is considered, there is considerable potential to reduce delivery times because, conventionally, a part has to be transferred to a dedicated CMM (coordinate measuring machine) and the inspection shows any errors, returned to the machine tool, are-clamped in position, and machined again.

This is time-consuming for any component but can take many hours for any large, heavy item, such as a press tool for an automotive body panel. In addition, any mistakes during the set-up back onto the machine tool could result in a new series of errors in the component, and so lead to a further cycle of inspection and re-machining.

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 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 accuracyof a CMM can be confirmed. Trials undertaken by metrology company Renishaw have shown the results from machine toolmeasurements to be both more accurate and more consistent thanwas 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.

Machining of 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 overmachining 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 afully-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 CAD model can be adjusted to bring it into line with the actual geometry. Finally, toolpaths can be generated for the required areas with a CAM system.

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 and struggle to meet the increased demands for quality and consistency in both theaerospace and marine industries.

Potential pay-off
Companies wanting to use adaptive machining processes must understand that they tend to be much more complex and process-specific than conventional CAM programming. Most adaptive machining projects will require some specific consultancy and customisation work by the software supplier as part of their implementation.

Despite this added complexity, with productivity now a key issue for all manufacturing companies, 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.

Peter Dickin is Marketing Manager, Delcam (www.delcam.com), a supplier of advanced CADCAM solutions for manufacturing industry.