The main challenge for aerospace manufacturers today is to develop the ability to produce high-quality products on a consistent basis as quickly as possible and at the lowest possible cost. At the same time, rising material prices are making the cost of scrap higher than ever, making minimization of waste more important than ever.
Proper inspection and quality control methods are no longer a luxury; they are an essential part of every manufacturing operation that wants to grow and be successful. However, simply bolting on some quality control procedures to the existing manufacturing processes 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 and then the components are transferred to dedicated inspection equipment. However, as machining techniques become more sophisticated and as components become larger and more complex, there is a growing number of cases where closer integration is required to give the highest productivity and the biggest reductions in wastage.
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. 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. This allows much more accurate machining, often eliminating costly and tedious hand work operations. It also allows any errors to be spotted earlier in the manufacturing process and resolved more quickly and at lower cost.
The most common cases when adaptive machining is required are those where the exact position of the workpiece on the machine is unknown. With large or heavy components—such as landing gear components, molds for bigger parts like composite body structures, or forming tools for wing and fuselage 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 setup times and improved accuracy.
The first stage in this approach is to create an inspection sequence using off-line programming so there is no interruption to the machine tool’s cutting time. This sequence is run using the machine tool’s spindle probe just like a CMM and collects a series of points from the workpiece. These points are then used by the software to generate a best-fit alignment between the CNC code and the part. Any mismatch that is found is then used to define a datum shift between the nominal position of the CNC code to be used to machine the part and the actual position of the workpiece on the machine tool. The software can then transfer this data to the machine tool control to compensate for the alignment differences, eliminating the mismatch.
On-Machine Verification 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 of this approach 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 time can be reduced.
This article is based on SAE technical paper 2009-01-3130 by Brett L. Hopkins and Peter J. Dickin, both of Delcam.