Delcam UK, Birmingham, UK
Edited by Leslie Gordon
Many shops use machine tools equipped with in-process probing systems that ease job setup by capturing data which locates a part and establishes a work-coordinate system. Some manufacturers hesitate to use the systems to monitor machining quality, fearing it will drive up cycle times. But this is far from true. In fact, savvy shops can save money and boost productivity by checking parts such as large components, patterns, and molds, on the machine tool.
We use the phrase " onmachine verification" to emphasise that verification, rather than inspection is key. Inspection tends to be an independent measurement, while verification implies a somewhat less-precise check because it's not, for example, carried out in a temperature-controlled environment. Even so, catching mistakes on the machine tool eliminates the need to remove parts for inspection on CMMs, except for the final inspection. Mid-process part-removal entails repeated cycles of machining and long equipment setup times. This is time-consuming for any component but can take many hours for large, heavy parts, such as press tools for automotive body panels. In addition, any mistakes during the set-up back onto the machine tool could result in a new series of errors in the component.
VERIFYING ON THE MACHINE
On-machine verification technology comprises in-process probes and inspection software that lets manufacturers automatically monitor, correct, and document in-process operations. One such system combines MP 700 or soon-to-be-released OMP 400 spindle probes from Renishaw Inc., Hoffman Estates, Ill., and PowerInspect software from Delcam USA, Salt Lake City, Utah.
The probes, touch-trigger sensors that mount in a machinetool spindle or turret, feed data gaged relative to the machine's coordinate system back to the controller for compensation in the machining program. They are used just like any other tool in the machine. However, instead of cutting, the probe touches partsurface gage-reference points (typically defined in CAD/CAM systems) to monitor the size and position of critical points at certain stages of machining. Renishaw probes don't require calibration in all the vectors in which they are to be used. This reduces the number of points required to measure a given part and speeds verification.
The inspection software allows CAD-to-part comparison, with GD&T capabilities such as perpendicularity, angularity, parallelism, and concentricity. Wizards help users develop the inspection sequence, making it easy to generate required probe paths. All measurements are initially displayed as green, red, or blue dots, depending on whether the point taken is within, above, or below tolerance. At the end of the process, customizable reports can be generated that can quickly show whether the part has been made accurately and highlight areas that might give cause for concern. The program also allows offline programming of the inspection sequence and includes simulation and collision checking. This minimizes time required for verification operations.
The system can monitor component quality at all stages of the manufacturing process. For example, it can check that the correct amount of stock has been left after a roughing operation, or assess the extent of damage due to tool breakage. This permits immediate decisons as to whether a part can be completed within tolerance, or must be scrapped.
Of course, measurements made with a machine tool on the shop floor cannot duplicate the accuracy of a dedicated CMM in a climate-controlled environment. Such a level of precision may be impressive, but it's rarely needed in most manufacturing operations. In addition, the quality of the on-machine results can be checked against known artifacts in a technique invented by Renishaw.
An artifact is a master part made from the same material by the same manufacturing operations as parts being machined. It need not have all the features of the finished part, but it does have the critical gage points.
Technicians first measure the artifact in a lab on a CMM calibrated to traceable international standards such as those from the National Institute of Standards in the U.S. or the National Physics Laboratory in the U.K. The machine tool is qualified to an industry standard, such as ISO 230 or ASME B5.54, which both specify ballbar and lasercalibration checks. The standards determine the accuracy a machine can meet or it's process capability.
Next, the artifact is located in the machining envelope, usually as part of the fixture. At various points in the machining cycle, a spindle mounted probe measures the size of a feature on the artifact for comparison with the program part dimensions. Any difference can then be compensated in machining that feature on the workpiece. According to the company, manufacturers can use this technique on almost any machining center, holding tolerances near the machine's repeatability specification as well as compensating for thermal effects.
Probing for productivity
On-machine verification reduces waste and improves efficiency, letting companies check parts at almost all stages of machining. The system can handle: Job identification. Probes determines if correct parts have been loaded and call up the right NC program.
Setup. Eliminates complex and expensive locating fixtures. Probes "find" the parts, so fixtures need merely hold it.
Toolsetting. Probes sets tool length and diameter and provide an economical way to verify tool geometry and condition.
Final inspection. Shops can "buy-off" parts while they are still fixtured.
First-part inspection First-part inspection is seamless with Renishaw's master artifactcomparison technique.