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    Measurement

    Measurements realities and metrology’s answers Part 2: The tools we can use

    • By MTDCNC
    • July 16, 2020
    • 8 minute read

    In my last article in the May issue, I described the difference between accuracy and precision as laid out in the ‘Vocabulary of Measurement’. I also explained the concept of the true value. If you missed that article it is free to read online and this article builds on the concepts I discussed there. One element was how measurement variation dynamically occurs all the time, defining what we mean by ‘the true value’, accuracy and precision, which are impacted by systematic and random errors. By Ian Wilcox

    Priorities

    In any manufacturing environment, there are a lot of priorities that all demand attention. As a business owner or manager, it must often feel like spinning plates, if any one of them stops spinning and crashes to the floor, the game is up, and you must walk away or start again. Metrology and its resultant measurement often gets lost down that priority list. It is seen as an indirect function of manufacturing, a cost rather than a value. Nothing could be further from the truth. Measurement is your intelligence service, your early warning system, feeding you useful data to make decisions. When applied effectively and efficiently it will give you confidence that all is running well and if not, it will give you early warning signs of when things are about to go bad; allowing you to correct it before it becomes a serious issue. Functioning poorly, it will leave you guessing and feed you misinformation, a fifth columnist subverting your attempts to build quality and productivity into your company.

    Get a grip on your measurement

    Firstly, recognise the difference between Measurement and Metrology. Measurement is a process, like any other process. It has inputs and outputs. Who does what, where, when and how – it needs to be managed. The same as any production process. In the two extremes of a process, you can either create a rigid and strict set of procedural instructions to be followed for each and every individual measurement instance (including automation). Or you can use skilled and competent technicians who can make individual and flexible decisions about where and how to achieve the objective, no matter what the measurement task. Of course, they are the extremes. It is more likely to be some combination of process, procedure and skill that is hoped, will enable good measurement practice.

    So, we can look at managing measurement in three ways, each as important as the other and dependant on each other. Three distinct things that if done well, will give you a good start in managing and stabilising your measurement.

    1. Developing a blend of measurement processes, procedures and skills to mitigate measurement precision

    2. Ensuring calibration and verification of equipment to help control accuracy

    3. Applying metrology tools to monitor capability and gain understanding of measurement performance

    Managing measurement processes, procedures and skills

    ‘Processes and Procedures’ should be common practice in manufacturing. Production is all about creating and continuously improving processes. You have full control over them, with plenty of good practice examples readily available. But often metrology as that ‘non-productive’ function maybe misses that same scrutiny over process. As many products pass through the same small group of measurement people and equipment, it can often be overlooked as requiring a process.

    Mass production with little product diversity opens measurement automation as a potential solution. Built into the production processes. Of course, there is always the option of leaving measurement processes as a creative process. Letting the technician choose which equipment to use and what strategy of measurement, as well as assessing how to best meet design intent through GD&T and its datums. That balance of skill and procedure can be a real challenge, overdo it and you unnecessarily ramp up your indirect costs, undercook it and the risk of incorrect measurement data is almost a certainty. This, in turn, will also push extra costs throughout the production process in the form of firefighting, confusing and contradictory quality issues, lowering productivity.

    As a ‘non-productive’ function maybe metrology misses that same scrutiny over process that production gets

    I will write more about specific measurement and metrology skills in my next article. In the meantime, you can get a flavour of the skills within the courses I have designed, and that myself and my colleagues have developed materials to give you a feel for those core measurement/metrology skills. They can be found at www.coventry.ac.uk/metrology and then scroll down for the CPD and Training link. 

    Calibration and verification of equipment to help control accuracy

    For a measurement, we either set or are set a unit to measure to. In engineering, to measure length we commonly measure in mm. But the mm is not the base standard unit for length, that unit is the Metre. The Metre is defined internationally, all countries who have signed up to the metre convention use this definition. In the UK, our National Measurement Institute the National Physical Laboratory (NPL) use that definition to realise the UK metre as the UK’s master length. The milli of a millimetre means a fixed ratio of one thousandth of the metre, the mm. The micro of a micrometre is a ratio of one millionth of the metre (also sometimes known as a micron).

    This master Metre held in the UK by the NPL is what all calibrations of length are traceable to. Every piece of measurement equipment in the UK cannot be compared directly to the NPL’s master Metre. So, artefacts are compared against the master metre and they then become a secondary reference available for comparison against the next stage down in the traceability chain. So, a traceability chain is established from NPL’s Metre down to your selected calibration laboratory’s standard artefacts, such as a high-quality box of gauge blocks.

    When you have your equipment calibrated and it is compared to their traceable and quantified equivalent of the master metre (or ratio of the metre). The estimated uncertainty is assessed by that laboratory for accuracy and precision and a stated uncertainty is ascribed to the result given. Records are kept so that the process can be externally audited and the uncertainty on your digital callipers certificate forms part of a traceability chain right back to the UK’s master metre held by NPL.

    Definition of calibration as given in VIM “operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication”.   

    With this infrastructure and sets of standards in place, we can tie our result to the defined Metre. This enables us to have confidence that our equipment is measuring ‘accurately’.

    But this is only one part of potential systematic accuracy errors. Earlier in the article we discussed another systematic error, for example a 0-25mm Micrometer being incorrectly set when zeroing. A calibration is like an M.O.T. it says it passed the test and you are given a period of time, where this test should remain valid in normal circumstances, but it is not a guarantee. What if someone drops the Calliper and fails to report it, it may have changed. It is recommended that a standard operating procedure is developed and put in place. These procedures, if followed should give confidence that at the point of use, all reasonable precautions have been taken to stop accuracy errors occurring that may invalidate the calibration or misuse of equipment. This interim check is often called a verification and can be a simple as checking the zero or a set size against a calibrated gauge block.   

    Both calibration and verification are needed to help lock in your control of accuracy.

    Metrology tools to monitor capability and understand performance

    We have looked at linking your equipment to the master Metre via calibration and confirming any change from that state by in house verification, thus managing accuracy. This is your best chance of getting close to that unknown ‘true value’.

    We have also discussed that this alone is not enough, you also need to control your own consistency when applying measurement, with a balanced combination of process procedures and skilled measurement technicians. This enables you to manage your measurement result variation, also known as precision.

    The last piece of my tripartite of ways to manage measurement is to monitor and test your capability and deepen your knowledge of what you do, enabling continuous improvement. It is only through the continuous improvement process that is unlocked by knowledge, that we can drive down costs, improving quality and productivity. 

    https://cdn.mtdcnc.global/cnc/wp-content/uploads/2020/07/16173841/54-56-image-1-960x500.jpg

    Measurements realities and metrology’s answers Part 2: The tools we can use

    In my last article in the May issue, I described the difference between accuracy and precision as laid out in the ‘Vocabulary of Measurement’. I also explained the concept of the true value. If you missed that article it is free to read online and this article builds on the concepts I discussed there. One element was how measurement variation dynamically occurs all the time, defining what we mean by ‘the true value’, accuracy and precision, which are impacted by systematic and random errors. By Ian Wilcox

    Priorities

    In any manufacturing environment, there are a lot of priorities that all demand attention. As a business owner or manager, it must often feel like spinning plates, if any one of them stops spinning and crashes to the floor, the game is up, and you must walk away or start again. Metrology and its resultant measurement often gets lost down that priority list. It is seen as an indirect function of manufacturing, a cost rather than a value. Nothing could be further from the truth. Measurement is your intelligence service, your early warning system, feeding you useful data to make decisions. When applied effectively and efficiently it will give you confidence that all is running well and if not, it will give you early warning signs of when things are about to go bad; allowing you to correct it before it becomes a serious issue. Functioning poorly, it will leave you guessing and feed you misinformation, a fifth columnist subverting your attempts to build quality and productivity into your company.

    Get a grip on your measurement

    Firstly, recognise the difference between Measurement and Metrology. Measurement is a process, like any other process. It has inputs and outputs. Who does what, where, when and how – it needs to be managed. The same as any production process. In the two extremes of a process, you can either create a rigid and strict set of procedural instructions to be followed for each and every individual measurement instance (including automation). Or you can use skilled and competent technicians who can make individual and flexible decisions about where and how to achieve the objective, no matter what the measurement task. Of course, they are the extremes. It is more likely to be some combination of process, procedure and skill that is hoped, will enable good measurement practice.

    So, we can look at managing measurement in three ways, each as important as the other and dependant on each other. Three distinct things that if done well, will give you a good start in managing and stabilising your measurement.

    1. Developing a blend of measurement processes, procedures and skills to mitigate measurement precision

    2. Ensuring calibration and verification of equipment to help control accuracy

    3. Applying metrology tools to monitor capability and gain understanding of measurement performance

    Managing measurement processes, procedures and skills

    ‘Processes and Procedures’ should be common practice in manufacturing. Production is all about creating and continuously improving processes. You have full control over them, with plenty of good practice examples readily available. But often metrology as that ‘non-productive’ function maybe misses that same scrutiny over process. As many products pass through the same small group of measurement people and equipment, it can often be overlooked as requiring a process.

    Mass production with little product diversity opens measurement automation as a potential solution. Built into the production processes. Of course, there is always the option of leaving measurement processes as a creative process. Letting the technician choose which equipment to use and what strategy of measurement, as well as assessing how to best meet design intent through GD&T and its datums. That balance of skill and procedure can be a real challenge, overdo it and you unnecessarily ramp up your indirect costs, undercook it and the risk of incorrect measurement data is almost a certainty. This, in turn, will also push extra costs throughout the production process in the form of firefighting, confusing and contradictory quality issues, lowering productivity.

    As a ‘non-productive’ function maybe metrology misses that same scrutiny over process that production gets

    I will write more about specific measurement and metrology skills in my next article. In the meantime, you can get a flavour of the skills within the courses I have designed, and that myself and my colleagues have developed materials to give you a feel for those core measurement/metrology skills. They can be found at www.coventry.ac.uk/metrology and then scroll down for the CPD and Training link. 

    Calibration and verification of equipment to help control accuracy

    For a measurement, we either set or are set a unit to measure to. In engineering, to measure length we commonly measure in mm. But the mm is not the base standard unit for length, that unit is the Metre. The Metre is defined internationally, all countries who have signed up to the metre convention use this definition. In the UK, our National Measurement Institute the National Physical Laboratory (NPL) use that definition to realise the UK metre as the UK’s master length. The milli of a millimetre means a fixed ratio of one thousandth of the metre, the mm. The micro of a micrometre is a ratio of one millionth of the metre (also sometimes known as a micron).

    This master Metre held in the UK by the NPL is what all calibrations of length are traceable to. Every piece of measurement equipment in the UK cannot be compared directly to the NPL’s master Metre. So, artefacts are compared against the master metre and they then become a secondary reference available for comparison against the next stage down in the traceability chain. So, a traceability chain is established from NPL’s Metre down to your selected calibration laboratory’s standard artefacts, such as a high-quality box of gauge blocks.

    When you have your equipment calibrated and it is compared to their traceable and quantified equivalent of the master metre (or ratio of the metre). The estimated uncertainty is assessed by that laboratory for accuracy and precision and a stated uncertainty is ascribed to the result given. Records are kept so that the process can be externally audited and the uncertainty on your digital callipers certificate forms part of a traceability chain right back to the UK’s master metre held by NPL.

    Definition of calibration as given in VIM “operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication”.   

    With this infrastructure and sets of standards in place, we can tie our result to the defined Metre. This enables us to have confidence that our equipment is measuring ‘accurately’.

    But this is only one part of potential systematic accuracy errors. Earlier in the article we discussed another systematic error, for example a 0-25mm Micrometer being incorrectly set when zeroing. A calibration is like an M.O.T. it says it passed the test and you are given a period of time, where this test should remain valid in normal circumstances, but it is not a guarantee. What if someone drops the Calliper and fails to report it, it may have changed. It is recommended that a standard operating procedure is developed and put in place. These procedures, if followed should give confidence that at the point of use, all reasonable precautions have been taken to stop accuracy errors occurring that may invalidate the calibration or misuse of equipment. This interim check is often called a verification and can be a simple as checking the zero or a set size against a calibrated gauge block.   

    Both calibration and verification are needed to help lock in your control of accuracy.

    Metrology tools to monitor capability and understand performance

    We have looked at linking your equipment to the master Metre via calibration and confirming any change from that state by in house verification, thus managing accuracy. This is your best chance of getting close to that unknown ‘true value’.

    We have also discussed that this alone is not enough, you also need to control your own consistency when applying measurement, with a balanced combination of process procedures and skilled measurement technicians. This enables you to manage your measurement result variation, also known as precision.

    The last piece of my tripartite of ways to manage measurement is to monitor and test your capability and deepen your knowledge of what you do, enabling continuous improvement. It is only through the continuous improvement process that is unlocked by knowledge, that we can drive down costs, improving quality and productivity.