A popular board game in the 1980s famously took, "a minute to learn... a lifetime to master". Portable arms are a deceptively simple metrology system for which the same catchphrase applies. Arms are designed to be highly flexible and easy to use for a wide variety of measurement tasks. However, understanding measurement datasets and how to use them for verification, reverse engineering or any other application can be far more involved than it first appears.
Here are five steps to help provide a sound basis for adopting more robust and effective methods when using portable arms.
1. Understand Your Measurement Requirements
A portable arm is a form of coordinate measuring machine (CMM) that records points in 3-dimensional space. This makes it a powerful tool for verifying not just dimensional tolerances, but also geometrical tolerances such as form, orientation and location of features. To do this accurately and correctly, a clear understanding of tolerancing is essential. NPL offers formal training in measurement fundamentals which includes a strong focus on geometrical dimensioning and tolerancing (GD&T), and this can help enormously to ensure components are well specified and correctly inspected.
2. Understand the Sources of Variation
Einstein defined insanity as doing the same thing over and over and expecting to get different results. Sadly, when assessing the capability of measurement systems, that is precisely what we must do! We measure the same things many times over and we look at how much the results vary. Understanding the influences of operators, environmental stability, datum alignments and so on, is extremely important in arm measurement as they are typically not used in highly-controlled laboratory environments. Following a formal “gauge repeatability and reproducibility” (GR&R) study is a powerful way to understand if an arm is truly capable of verifying a feature against its tolerance.
3. Document Your Processes
When developing a measurement process, it is vital that it is well documented to ensure that everyone will follow the same method and therefore reduce variation and possibilities for making mistakes. Standard Operating Instructions (SOIs) do not need to resemble War and Peace (if they do, no one will read them), but they do need to eliminate variation as far as possible. For example, how to set up the inspection, what pre-checks to perform, what CAD file to use, what probes are needed or what analysis process to follow, should all be clearly stated. Generally, many of the basic set up procedures for an arm can be covered in an over-arching SOI for arm measurement, and the specifics of each individual job can then be kept relatively brief.
4. Make the Most of Workplace Training
Typically arm vendors deliver a brief familiarisation session when an arm is purchased to cover its basic operation. Coordinate metrology software vendors provide more in-depth training to cover the functionality of their measurement software. Unfortunately, neither of these routes tends to provide adequate guidance focused on the real-world requirements for portable arms. More application-focused training covering good-practice considerations and how to ensure the arm is providing robust usable data can be very valuable in ensuring that arms are used appropriately.
5. Get Creative!
Often when discussing arms, too much negative focus is given to the challenges arising from their flexibility and diverse applications, as these can make it hard to ensure that the equipment is used in a reliable and repeatable way. Of course, flexibility is one of the greatest strengths of a portable arm – it can be used quickly and easily for a myriad of applications. If an organisation has developed robust methods to testing capability and documenting good practice processes, then the arm should not only be used for the one or two operations for which it was bought – it can be used in many different processes. The more you use an arm, the better you will use it, and the more useful it will become.
INSPHERE have used portable arms to gather robust and effective data in a vast variety of ways, from calibrating robots to checking foot pedal positions in a Formula One car cockpit. As measurement experts, we have an implicit understanding of the factors that can influence portable arm metrology and regularly deliver training courses to help ensure good practice principles can be adopted by organisations and their operators. Visit our training page for more information or contact us to discuss how we can help.