By Patrick Beauchemin, P Eng. PhD
Pointe-Claire, Quebec, Canada
Today, the demands being made on manufacturers are greater than ever. Accuracies are tighter, turnaround times are shorter, and costs are monitored closely. In these conditions, it’s important to evaluate different approaches that can be applied in non-contact measuring to ensure that parts meet or exceed their functional requirements and throughput is maximized. There are two approaches that warrant careful consideration. One approach is to measure various critical dimensions on the parts and verify that they are within their allowed tolerance range. This can be accomplished with measurement instruments such as optical comparators, scopes, and video CMMs.
This approach has a number of benefits. It allows for the collection of numerical data (i.e. the measured dimensions), which can then be used to continuously monitor the manufacturing process and spot trends early on before they start to cause problems. The data are also useful for creating reports and producing whatever regulatory documentation that might be required for industries such as medical, aerospace, and automotive. However, this approach also has some drawbacks. When parts are being measured manually, operator-to-operator variation is virtually impossible to avoid. Working to minimize this variation, which is often the single largest source of measurement error, is a constant battle. It generally involves putting standardized procedures in place, periodically carrying out training to ensure that these procedures are properly applied, and providing ongoing monitoring. Automated measurement also has its disadvantage—the need for part-specific programming. Even in the case of simple parts, creating and validating an automated measurement program can require hours of work by a skilled programmer.
For all these reasons, measuring critical dimensions to check for part accuracy is more suitable for high-volume production where there are
longer runs of parts being produced with relatively infrequent part change-overs. In cases of lower-volume production with frequent changeovers, comparing the part to its CAD data is a prudent approach. Historically, this has been done using optical comparators. There are a number of benefits to this approach. The principal benefit is speed. These inspections can be done in a minute or two, directly on the shop floor and by the same oper-ator that is making the parts. And because optical comparators are especially well-suited for inspecting profile and form tolerances, this approach is also very well adapted to checking parts with complex geometries.
But the increasing demands made on today’s manufacturers are bringing various limitations of conventional optical comparators to light. One of the most obvious is the need for overlays (also called templates or Mylars) to allow operators to verify that parts are in tolerance. Overlays present a number of problems: they need to be physically stored and managed, they can’t be used simultaneously by multiple operators, they are subjective, they can be damaged, and they need to be re-calibrated periodically. The accuracy of optical comparators often can’t meet today’s ever-tighter tolerances or provide any record of the inspection/measurement operation. In addition, conventional optical comparator results are operator-dependent.
Recent technology developments, such as the digital optical comparator, offer an alternative to the limitations of traditional optical comparators. A digital optical comparator works directly with the part’s CAD data, and features an auto-align tool that automatically matches up the CAD file to the part, as well as an auto pass/fail tool, so the results are highly accurate and operator-independent. It can also automatically collect complete device history and documentation, a high-resolution image of a part with its CAD overlay, computed deviations from nominal, the auto pass/fail result, as well as measurements of critical dimensions and annotations. This information can be time/date-stamped and easily linked to a job and lot number. Critical part dimensions can also be measured manually, and the information collected can be exported to other applications. The digital optical comparator method is quickly gaining traction as a faster, more reliable option to conventional comparative measurement approaches.
Choosing the right approach—or the right combination of approaches—for your application will help ensure that the optimal solution is in place to meet and exceed both quality and production objectives. ME
This article was first published in the November 2011 edition of Manufacturing Engineering magazine. Click here for PDF.