Mixed-Light Inspection for UV Inspection of Forgings
The ML 2000 mixed-light system uses a specialized camera, patented driver electronics and a self-contained UV light source. It is suitable for inline inspection of forgings on a conveyor and also permits the inspection of forgings too big or too heavy to be moved into a darkened inspection room. The system’s software can be adapted to automatically detect and report any class of surface or process defect that falls both inside and outside of the range of user-defined acceptance criteria.
The ability to automatically scan the entire surface of forgings for tiny cracks, pores or other glitches is of great benefit to the forging industry. Fluorescent-penetrant testing is a well-established inspection technique used in the metalworking industries.
Though used most often on castings, the technology is also useful with forged products. The technique can be applied on both ferrous and nonferrous materials, but it usually requires darkened rooms or the use of tents and awnings because the effectiveness of this technique requires that the ambient lighting conditions are very low. This limits certain NDT inspection operations and makes others impossible. Furthermore, for inline inspection in a continuous production environment, the extra time and resources necessary for operating under low-light conditions can cause serious production bottlenecks and is expensive in terms of manpower.
“Mixed-light” inspection is the term given to a new industrial technique that enables the detection of the very small levels of light emitted from fluorescent-penetrant dyes against a background of strong daylight or other brightly lit environments.
A U.K. company, Inspection Technologies Ltd., has developed a mixed-light system that eliminates these production bottlenecks and significantly increases throughput rates at inline inspection points. Furthermore, the system comes with software that completely automates the inspection process, removing the need for expensive, highly qualified inspectors. After the automated inspection process is complete, visual photographic records of all inspections – pass or fail – are permanently stored for compliance and 100% quality-audit purposes.
The ML 2000 mixed-light system (Figure 1) is comprised of a specialized camera, patented driver electronics and a self-contained UV light source.
Not only suitable for inline inspection of forgings on a conveyor belt, the ML 2000 also makes it possible to inspect forgings or other components that are too big or too heavy to be moved into a darkened inspection room in the forge or other workshop. Moreover, for both inline inspection of parts on a conveyor belt and inspection of fixed plant in-situ, the system’s software can be adapted to automatically detect and report any class of surface or process defect that falls both inside and outside of the range of user-defined acceptance criteria. The system also employs digital signal processing-assisted noise reduction, filtering and contrast enhancement methods to provide more contrast and a sharper, clearer image in which any detected fluorescence can be more easily seen.
The final image, which eliminates all background light and shows only the part, is shown in real-time, thus allowing the user to visually identify an area in which only the fluorescent material is present as a continuous-streaming video feed. Such an arrangement is advantageous in an NDT setting where the fluorescence corresponding to a crack or other defect can be detected as part of a dynamic or moving inspection process, such as in-line inspection of objects on a moving conveyor belt in a production setting.
The system’s detection software also makes it possible to represent surface defects, deviations and imperfections with overlain color graphics on top of traditional photographic images of the inspected part. Optionally, as desired, the software can stream real-time visual overlays of fluorescent regions and identified defects on top of ordinary visual images of the object under inspection. Moreover, all captured images, regardless of whether they contain detected defects or other regions of interest, are permanently stored. This has several advantages in terms of compliance and insurance issues in that it effectively provides a truly 100% quality audit.
Apart from the cost savings due to the reduction of production bottlenecks by speeding up inspection and increasing overall production throughput, other advantages are:
- No limitations on metal composition or heat-treated condition
- No limitations on the size or shape of forgings that can be inspected
- Liquid-penetrant inspection can be performed at any stage of manufacture, regardless of background lighting conditions.
The system also overcomes the ever-increasing, industry-wide shortage of qualified inspectors emerging from the training system. Since the ML 2000 is an expert system in its own right, this means that technician-grade staff could be used in lieu of more fully trained inspectors. Alternatively, the productivity of a qualified inspector could be increased dramatically if they were able to inspect remotely via telemetry by supervising or reviewing the work of technicians on the ground in multiple geographically separated sites.
If an inspected part falls within the acceptable limits as defined by the user, the part is accepted and proceeds to packing and shipping. However, if the inspected part is defective in any way, it is either rejected and/or appropriate alerts are delivered to operators and management. The entire test cycle takes just seconds.
Depending on the size of the parts and, if necessary, the availability of appropriate mechanical-handling equipment such as robot arms, etc., the throughput is typically hundreds of parts per hour. Throughput also depends on the size of each part, the complexity of its shape and the relative size of any defect found.
The ML 2000 system has been specifically designed to eliminate the false acceptance of defective parts due to human error. Field trials have shown that, in some deployments, the end-of-line forgings are delivered with no significant defects for the first time ever. This addresses a very real customer need because it is a generally accepted estimate that a typical magnetic-particle operator detects only about 80% of the defective parts.
Collateral benefits are that plant scrap rate would also be reduced because parts with superficial marks, scratches, thin flaps or overlaps of metal that do not extend structurally into the forging, and other surface blemishes are saved because these defects do not alter the structural integrity of the part.
With forgings, the cost of a single part may often be relatively small and quality control in the factory can discard fair numbers of forged parts without any significant financial consequences. If a forged part fails while in service, however, the damage and associated cost will be considerably larger. For example, if a piston rod in an engine breaks, it means that the vehicle will have to be taken out of service, the engine unmounted and the rod replaced. Such a failure will almost certainly cause damage in other parts of the engine too.
The conclusion is that quality control during the manufacture of all classes of forged parts is of paramount importance and can potentially save considerable cost to the customer. Consequently, a system such as the ML 2000, apart from saving production costs with increased throughput and reduced labor overhead, also offers added value to the customer by ensuring a higher level of quality assurance.
Some illustrative examples of the system in action are shown in Figure 3. In the first instance, test pieces provided by Magnaflux® EMEAR are shown. These were of a type that are normally used for quality-assurance purposes on fluorescent magnetic-particle dyes (used on ferrous metals) and fluorescent-penetrant dyes (commonly used for the detection of defects associated with nonferrous metals). Figures 3 and 4 show the performance of the mixed-light apparatus on these samples under laboratory conditions.
Figure 4 shows one configuration of the mixed-light system in which the scanning head is attached to a robot arm, which then performs either spot inspections at preset positions or a continuous sweep of the surface. Visual records of all inspections are permenantly stored for compliance and quality-assurance purposes.
Because the precise positions of the defects are recorded, the software is also useful for process control if, for example, faults occur more frequently in some locations as opposed to others.