QMT Features: June 2015
How to meet your measurement needs in 2015
The second of a series of articles designed to help QMT readers in their everyday measurement needs from Keith Bevan, the Delivery Manager for Training at the National Physical Laboratory (NPL).  This series aims to help readers understand the value of metrology, and particularly dimensional metrology in the workplace


In our introductory article (published in April’s QMT) we looked at the broad principles of metrology good practice. In this article I want to look at the measurement considerations that will allow you to make better measurements in the workplace. Issues need to be addressed throughout the measurement process but I want to look just at the preliminary stages, and the advice we give to any professional looking to make a measurement.

Before starting any measurement there are two critical questions you should ask yourself, to ensure you are ready to plan your measurement task.

1. Have I understood all the design requirements?
2. Have I identified all the measurement requirements?

Taking an example from dimensional metrology, when dealing with the first question there are many things to consider... Are you sure you are using the correct technical drawing to inspect the part, or has there been an update to a tolerance or dimension? Also, what standards do you need to refer to (ISO, ASME, BS, etc.)? For example, drawings will make reference to the standards, tolerances, material specifications, and surface texture. In addition, are you interpreting the projection and the units correctly on the technical drawing? Are there any specific statements or standards that require further investigation? Understanding this information is critical to ensure you are able to develop an accurate measurement strategy. The next step will be to associate the dimensional and if appropriate, the geometrical tolerances with the available measuring equipment. For example, should you measure a length with a calliper or micrometer; or a depth with a height gauge or depth micrometer and so on?

In some instances the measurements and associated equipment can be defined by the use of a documented procedure or plan. Once you have, read through the procedure in detail. Make sure you are comfortable with the content and fully understand how to implement it. If not, make sure you seek assistance from either the plan’s author or a colleague who has worked to it before. If you haven’t been advised on an existing procedure, are you in a position to design one yourself? If not, seek assistance from someone who can help you.
Now you have confidence in the approach to the task, check everything been identified to move to the next step of the measurement process.

Let’s look at co-ordinate measuring machines (CMMs) as an example. Hundreds of thousands of measurements are made every year to check that components meet their designated tolerances, and many of these measurements are made using CMMs. It is important that the measurement uncertainty of the measurements made on the CMM is known so that you have a better understanding of how this relates to conformance/non-conformance decisions. This is where NPL can help. We work with a range of businesses across the UK to provide information that enables people making measurements to interpret the requirements of the international standard relating to CMM measurement uncertainties. This includes supporting documentation available from the NPL website, such as the Good Practice Guide to co-ordinate measuring machine task-specific measurement uncertainties.

What NPL has found from the businesses we engage with is that operators can have a tendency to rely on manufacturers specifications. These specifications are based on ISO 10360, only relate to measurements made of the distance between two points on an artefact and are only a sample of the machine’s performance. The error associated with a real world measurement could be larger because of factors such as the choice of stylus configuration, probing strategy and environmental effects.

Equipment like CMMs should periodically undergo a performance verification exercise such as that described in ISO 10360 to ascertain the performance of the machine to manufacturer’s specifications. Performing weekly interim checks is also a recommended way to check that your CMM has not deteriorated between periodic performance verification tests. In addition, checking and validating your part programmes is good practice to support your measurement strategy and programming methodology. This will give you confidence in your part programme to provide sound measurements.

So what else can people making measurements do to ensure they have confidence in the measuring equipment they are using? NPL recommend developing a process to check measurement consistency. There are many tools and techniques available to do this such as measurement systems analysis (MSA) where the capability of the equipment to repeat and reproduce results can be calculated and monitored. The consistency of the measurement is crucial to making informed decisions about the capability of the equipment. You need to know the capabilities of your equipment to be able to take an evidential approach to the jobs you can confidently take on and also be confident in your measurement capability to support audit requirements.

Businesses working within the automotive, aerospace and medical sectors already have to supply MSA data including gauge reproducibility and repeatability, to prove to companies at the top of the supply chain that their measurement systems can measure their components to their requirements. NPL recommends the use of consistency charts to help identify potential issues in the equipment.

Once the measuring systems are proven to be capable we can have confidence in the capability of our equipment to move to the next stage of the measurement strategy.

Ensuring a standardised approach to measurement across a business is critical. Companies need to be confident that their staff work in a consistent manner, have a strategy for measuring a datum or features for example, and make sure staff are fully aware of the risks of a lack of consistency. NPL is helping companies to think about this methodology, and how to document this methodology to ensure quality and consistency across their operation. For example, NPL’s product verification programme enabled Suffolk-based Brafe Engineering to develop an action plan focused on a number of key improvement areas. This included improved CMM verification arrangements and a system to demonstrate capability to Brafe’s customers and third parties.
To help companies improve their systems and drive their profitability NPL can provide training, advice and consultancy. For example, we have recently published and updated a number of CMM good practice guides including the Good Practice Guide to CMM Verification, and developed an accompanying training course, CMM Verification to ISO 10360.

At NPL we work with a variety of companies, from SMEs to multinationals, to find ways to improve their productivity and reduce waste through the implementation of better measurement processes, practices and techniques. In the next issue I will look at the next stage of the metrology process – how you prepare to make a measurement.
www.npl.co.uk
  
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