QMT Features: September 2015
Best practice approach to preparing to take a measurement
The third in 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)

Our last article (published in June’s QMT) looked at the measurement considerations that will allow you to make better measurements in the workplace, outlining the advice NPL gives to measurement professionals at the preliminary stages of the measurement process. In this article we examine the next step of the process, when professionals are preparing to take a measurement.

In your preliminary assessments you will have determined what needs measuring, the next stage of planning is to ensure you have selected the right measurement instrument for the task and have considered the environmental requirements in your planning. We look at both of these issues in this article, as well as advising on what to do with the measurement results.

Choosing your measurement instrument
The choice of tool you need will depend on whether you are working to a procedure or if planning a measurement for the first time. With a procedure you will know which instrument to use, as it will be clearly stated within the procedure. However, not everybody will have that luxury, so let’s continue as if you do not have access to a procedure.

The first step is to refine the choices available to you. The drawing tolerance allows you to put a limit on the uncertainty of measurement of the equipment you can use. This, in turn will limit the number of potential tools you can use.

From those tools available do not make the mistake of believing the most accurate is the best option. Doing so could increase costs, waste time and take a valuable resource away from being used where it is genuinely needed.  For example, do you use a general-purpose measuring instrument such as a CMM or would a more specific instrument like a height gauge - whose purchase price can be up to 10 times less than a CMM - perform the task equally well?

Understanding the capabilities of any equipment will inform you what is appropriate for a given measurement. However, this knowledge very much depends on you understanding the manufacturer’s specification and the calibration certificate for the instrument. Armed with this information, international standards such as ISO 14253 and the drawing tolerances you should be able to make an informed decision.
However, manufacturers’ specifications can be hard to interpret as different manufacturers may have different test methods and articulate their test results in different ways. They will quote terms such as accuracy, resolution and error. This can be confusing, and mixing up terms like these and others such as tolerance, uncertainty and precision can mislead your measurement preparation as a result.

To help you make an informed decision NPL has developed a series of good practice guides that aim to improve measurement understanding and technical abilities. NPL Good Practice Guide #80, Fundamental Good Practice in Dimensional Metrology, has a section dedicated to such terms, and can be downloaded free of charge on the NPL website.

Environmental considerations
Once the right tool has been selected, measurement professionals need to consider environmental effects in their planning. Adequate lighting in the area where the measurement will be made is needed, as are controlled levels of dust, moisture and vibration.

Perhaps the largest environmental issue to consider when planning a measurement task is temperature, as this can influence measurement results (both the part being measured and the measurement tool itself).  
Data from the Coordinate Metrology Society (CMS) Measurement study reports show that temperature is one of numerous considerations often overlooked. The CMS invited delegates at its annual conferences from 2010-2014 to participate in a series of measurement tasks, using either a laser tracker or an Articulated Arm CMM. These tasks were observed and monitored to provide data and knowledge for on-going metrology education.  The results showed that temperature considerations were often overlooked, with up to 85% of delegates failing to inquire about temperature during the test.

There are several considerations on temperature to remember when planning a measurement; from ensuring the work-piece is at ambient temperature to having suitable thermal stablisation procedures. Also, as defined in ISO 1, all dimensional measurements should be reported at a standard reference temperature of 20 °C.  More detailed advice on temperature compensation can be found in NPL’s Good Practice Guide #80.

What to do with the measurement results
Measurement planning should also consider how the results are recorded, tracked and stored. As part of this plan you should ensure there is a clear way for the data gathered to be related to the job. For example, is there a task reference number that should be used for consistency?

If you are using software to collect or store the data you need to check you are using the right version – both of the CMM software and of the part programme. It is simple to go online and update most software. If you are using a CMM to take the measurement a programme will be written to move the machine, contact the relevant surfaces, compute Gaussian associated features and then make calculations based on those features. A mistake in any of those steps can lead to errors in the final output. There is a certain amount of good practice that needs to be followed to ensure the correctness of any program created and this can be accessed in Good Practice Guide #41 CMM Measurement Strategies.

Once the data is collected you also need to know how to process and analyse it. For example, is there a script you need to follow?  Also, bear in mind what the final output of the measurement result will be, and how the measurement will be reported as part of this, such as visually or through a written report. Finally, when you share the results make sure you know and account for any data security issues, such as commercially confidential information.

Any problems?

Remember that help is always available should you encounter a problem at any stage of the measurement process. This help can be from someone senior within your company, from the original manufacturer of the equipment, especially if using a CMM or through a recognised external body such as NPL.  As well as Good Practice Guides, NPL provides a range of training programmes, available through the classroom and via e-learning, that can give measurement professionals the confidence they need to make better measurements in the workplace.
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