New adaptive machining methods for the foundry industry

Transcription

1 New adaptive machining methods for the foundry industry Peter Dickin Delcam, UK. Abstract Computer-based methods for machining and inspection are well established in the foundry industry. More recently, these two technologies have been brought closer together in a new development adaptive machining. This approach uses a combination of software, such as Delcam s PowerMILL CAM system and PowerINSPECT software, to give new solutions to a range of challenging manufacturing problems. 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. Key words (5 maximum). Machining; inspection; in-process measurement 238/1

2 Introduction Computer-based methods for machining and inspection are well established in the foundry industry. While a small number of companies still rely on programming on the machine tool control or still use copy milling, the great majority now use a CAM system to produce CNC code for their machining operations. Similarly, inspection against CAD data is increasingly used for quality control instead of inspection against drawings. With component designs always becoming more and more complex, these computer-based techniques are often the only practical way to undertake their manufacture. Despite the many benefits these systems bring, there are a range of operations that still remain very challenging, even for companies using the latest technology. The problems usually come from a lack of information at some stage in the process from CAD model to approved component. For successful CAM programming, the user needs to know 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. By bringing machining and inspection technology closer together, in-process measurement can be used to close the information gaps in the process chain and allow successful machining when one or more of those elements is unknown. This approach is known as adaptive machining. It uses a combination of machining and inspection software to give new solutions to a range of challenging manufacturing problems. Uncertain position The most common applications of adaptive machining are those where the exact position of the workpiece is unknown. When manufacturing large patterns or when finish machining heavier castings, such as the bodies of large pumps, achieving the correct position and orientation of the component 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 part in exactly the desired position. This approach has been used for the machining of geometric features for some time. Adaptive machining technology now offers an equivalent solution for the manufacture of complex surfaces that gives the same benefits of shorter set-up times and improved accuracy. The Delcam process in these cases uses PowerINSPECT, together with a new program, PS-Fixture. First, a probing sequence is created for the inspection software using its off-line programming capabilities. This sequence is used to collect a series of points from the workpiece, which can be used by a range of best-fit routines to determine its exact position. Any miss-match between the nominal position used in the CAM system to generate the toolpaths and the actual position of the workpiece can be calculated in PS-Fixture. The software can then feed the results to the 238/2

3 machine tool control as a datum shift or rotation to compensate for the alignment differences. Figure 1. Software can be used to adjust toolpaths to the actual position of the part, rather than aligning the part to a specified position A similar approach can be used when the relationship between different design elements is more important than the absolute position of each feature based on a single datum for the complete part. For example, it might be necessary to drill a series of holes at a constant spacing around a central bored hole. By basing the drilling operations on the actual position of the central hole, rather than relying on its nominal position within a CAD model, the toolpaths can be adjusted to ensure that the secondary holes are produced to the highest possible accuracy. As well as giving greater accuracy and reducing set-up times, the use of this approach can significantly cut the cost of fixtures. Since there is no longer the need to locate the component in an exact position, considerably less accuracy is demanded of the fixtures. As long as the component is in approximately the correct position and orientation, the software can compensate for any deviation. 238/3

4 Uncertain starting shape The second application of adaptive machining comes when there is some uncertainty over the exact shape of the component to be machined, a common problem given the inherent inaccuracy of the majority of casting processes. The main requirement for finish machining is to allow an even distribution of material to be removed around the stock to avoid overmachining in some areas and under-machining in others. This can be achieved by first creating a probing path within the inspection software to determine the form of the casting. The final shape to be reached can then be orientated within this envelope to give an even thickness of material on the surfaces to be machined. Other benefits of knowing where material exists, and where it doesn t, 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. Figure 2. Ensuring a good alignment of the finished shape within the stock can save material and machining time There is also the potential for material savings. Most castings are designed with worst-case tolerances to ensure that there will always be sufficient material in all critical areas. The ability to tweak the position of the finished shape within the casting means that these tolerances can be reduced, which in turn means that there is less need for excess material. Furthermore, having less material to be removed from the casting reduces the time needed for finish machining. More comprehensive reverse engineering can be needed for cases where no nominal CAD data exists, for example when repairing older tooling. The surface of the tool must first be scanned to give the data required to create CAD surfaces in the reverse engineering software. Data can also be collected in a similar way to generate a model of the excess weld that needs to be machined away from the repair. The difference between the shape of the weld and the desired surface of the tool can then be used to calculate the necessary machining program. This approach is both faster and more accurate than trying to remove the excess weld by hand grinding. 238/4

5 Figure 3 Adaptive machining can be used for the finishing and repair of cast press tools This approach is used by a leading automotive manufacturer in Sunderland to replace complete sections in draw tools, especially where laser welding is used to join sheets of different grades of metal together. These areas wear more quickly because of the effects of the laser weld so there is a need to replace them with harder material to extend the overall lifetime of the tool. Surface data from the affected area is collected with a Renishaw probe fitted to the company s machine tool. The replacement section is modelled in the CAD system and then machined on the machine tool. Unknown final shape The most challenging adaptive machining operations are those where the final shape of the component is not known precisely. This is most often the case 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. 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 system can be used to bring the nominal model into line with the actual geometry. As with the tooling repair examples mentioned above, the position of any 238/5

6 excess weld can also be determined. It is then possible to generate a toolpath to remove any surplus material and create a smooth surface on the part. Figure 4 The repair of components that have distorted during service requires machining from an unknown starting shape to an unknown final shape In all cases, the component can be probed after machining to give a record of the final shape of the part. This data can be used to check the component against a CAD model of the part if one exists, or to create a CAD model for future reference if needed. Verifying the result With all of the processes described, and, indeed, with many other machining operations, it is important to check the results against a required specification. That can be undertaken on a co-ordinate measuring machine but an increasing number of companies now carry out an initial inspection on the machine tool using on-machine verification. The most obvious advantages of this technology are for those companies that do not have existing inspection capabilities. However, it is also beneficial for sub-contractors that need to machine bigger components, such as those used in the aerospace or oil and gas industry, and for the production of large patterns. Most modern machine tools either come with, or can be retrofitted with, probing capabilities to assist in the set-up of the job. With on-machine verification, this same equipment can now be used for initial quality checks at little extra cost. Data for the process can be collected using various spindle probes, such as the established MP 700 or the soon to be released compact OMP 400. Neither of these probes need to be calibrated in all the vectors in which they are to be used, so reducing the number of points required to measure a given part and, therefore, giving shorter verification cycle times. The inspection software can then use the data both to check surface accuracy or to measure features, such as circles, cylinders, cones, 238/6

7 spheres and planes, to a high level of precision. The ability to program complete verification sequences off-line means that there can be minimal interruption of the machining operations. Figure 5 On-machine verification can give significant time savings when manufacturing large components On-machine verification can give huge timesavings by enabling any errors to be detected earlier, and so corrected more quickly and at lower cost. Similarly, the extent of any damage caused, for example, by a tool breakage, can be assessed accurately and a decision made immediately to determine whether the part can still be completed within tolerance or whether it will have to be scrapped. On-machine verification will also benefit 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 equipment might think that onmachine inspection is an unnecessary operation that can loose 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 timeconsuming for any component but can take many hours for any heavy 238/7

8 item, such as a large aerostructure or 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. With on-machine verification, the part can be checked before it is moved. Any significant errors can then be detected and corrected before the component is transferred to the CMM for its final inspection. The ability to check that the part is reaching its specifications at the various stages of the manufacturing process will save time, reduce the amount of scrap and increase confidence that time is not being wasted working on components that are already too far out of tolerance. Conclusions This paper has described a number of adaptive machining methods that can be used to overcome problems with more challenging manufacturing operations, and the use of on-machine verification to confirm that the desired result has been achieved. Recent software developments allow users to undertake these complex operations more accurately, more reliably and with considerable savings in time and costs. However, companies wanting to use adaptive machining processes must understand that they tend to be much more complex than conventional CAM programming. They may often be specific to a particular process or even to a single component. There is a core of common functionality at the heart of all the solutions described above, and software developers like Delcam are extending their product ranges to standardise this core functionality. Nevertheless, most adaptive machining projects will still require some specific consultancy and customisation work as part of their implementation. It now seems inevitable that most standard production operations to produce large numbers of identical parts will move to countries with lower labour costs. There will still remain a large demand for shorter runs of more complex, more individual products and it is this area that the more traditional manufacturing centres can still be competitive, providing that they make the best use of modern manufacturing methods. Of course, the choice of the best approach will be different for each company. However, it is clear that doing nothing is not an option for companies that want to grow their turnover and profitability. Failure to invest in any of these new techniques will see companies losing business to those that do use the latest technology. For further information, Peter Dickin can be contacted by to pjd@delcam.com 238/8