Measurement Tolerances and Techniques

ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratoriesQ: I am looking for some information regarding blueprint tolerances and measurement tools used to measure certain features.

For example, can the same type of tolerance be applied over the length of 450 mm as it could be for a distance of 3 mm?  Is there additional measurement error or gage error that needs to be applied for longer distances?  If one uses a 1” micrometer for measuring a feature, does it make a difference in the measurement error by using the top end of the instrument versus using it to measure just very small features?

A: Thank you for your questions about measurement tolerances. First of all, since your questions were multi-layered, my answers will be as well. Nonetheless, I think I should be able to help you.

As for using the same type of tolerance for a dimension of 450 mm and a dimension of 3 mm, there is more than one answer. We’re talking about 17.7165 inches vs. .118 inches. The 3 F’s must first be considered.  That is Form, Fit, and Function.  In other words, where will this product be used?  If this will be for a medical product or for anything whatsoever where safety is a factor, the design engineer will most likely use a tighter tolerance. So both dimensions could be ± .001 or a more liberal ± .010.  The difference between the two sizes would just change the way they are manufactured.  For example: a 17.717 inch product with a tolerance of ± .030 could probably be rough machined or even made on a burn table.  If the size or location of the smaller dimension is critical, you would machine it with appropriate equipment and hold a tighter tolerance.  OK, enough Manufacturing 101 lingo.

With regard to measurement error, larger/longer dimensions can introduce the possibility of increased measurement error. However, if a “qualified” and experienced individual is doing the measurement, that should not be a major factor.  The same basic skills and standards would apply. The type of measurement equipment can make a difference.  In other words; if you use a Dial Caliper, you can probably rely on it to be accurate within .001-.002 inches.  If you use a 0-1 inch micrometer, you should be able to trust it’s accuracy within .0001 inch.

A qualified metrologist and/or a quality technician would know to check a micrometer at numerous points over its measuring range.  Measurement error should not increase significantly from one end to the other.  If it does, there is something wrong with the calibration or with the tool itself.

I know the above can be perceived as general answers, but I am confident you will see the specifics there as well.

Bud Salsbury
ASQ Senior Member, CQT, CQI

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ISO 17025 Certified Facility

ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratories

Q: We have a specification that states test reports shall be from an facility certified to ISO 9001:2008 Quality management systems–Requirements. Our test reports are from a facility certified to ISO/IEC 17025-2005: General requirements for the competence of testing and calibration laboratories.

Isn’t ISO 17025:2005 under the ISO 9001:2008 umbrella?

A: Your interpretation is, indeed, correct. Actually, for a testing lab, accreditation to ISO/IEC 17025 is superior to registration to ISO 9001! As you know, your accreditation agency actually observed your personnel performing tests. They had to demonstrate competency. This was in addition to the verification that you had a working management system in place (that’s why they call it accreditation and not registration. We won’t even get into the misuse of the word certification).

To make sure your customer gets the assurance they want, I recommend you contact your accreditation agency. Ask them for a letter that states this equivalency. That will probably blow your customer away – or at least amaze them! Unless you can show the text you provided to ASQ was from one of the ISO or ANSI standards-writing committees, as an official interpretation, it probably holds little weight.

Your customer is right to monitor your performance this way. Recent food safety issues, prominent in the news, have a common element to them — insufficient attention to supplier performance. Expect to see more of this as the manufacturers and distributors pay more attention to their supply chain. I expect you are or will be doing the same for your critical sub-suppliers. Remember too, there are many ways to monitor supplier performance. Registration/accreditation is one of the ways.

Dennis Arter
ASQ Fellow
The Audit Guy
Columbia Audit Resources
Kennewick, WA

For more on this topic, please visit ASQ’s website.

Coordinate Measuring Machines (CMMs) and Digital Bore Gages

Gage R&R, Torque Wrence

Q: When inspecting diameters with tolerances of .0005 and below, are there any studies relating to the accuracy of different inspection methods, such as a coordinate measuring machine (CMM) versus a digital bore gage with setting ring combination?

A: The answer to this question can often be one of opinion and/or personal preference.  What I will present are my opinions, along with some known facts.

Non-contact measurement systems such as optical and laser equipment are bulky, expensive and impractical.  With these systems, the part must be taken to the system. This is not much good in a production environment.

While a CMM is without a doubt very accurate, they are also slow.  Like the optical or laser equipment, the parts must be taken to the system.  In many production situations it is more practical to check the part in the machine.  Also, even though CMMs come with reticulated heads, measuring at abstract angles or various depths is not always an option.  It is also wise to keep in mind that deeper bores would require longer stylus probes.  This is a situation that can introduce concerns of error and rapid movement can generate false contact readings with longer styli simply due to the motion.

A final thing to keep in mind is the high initial price of a CMM, as well as the maintenance costs.

Two and three point contact measurement is readily available.  Popular digital bore gages are calibrated to a master ring.  The rings themselves can be verified with a CMM or sent out for certification traceable to national standards. Most digital bore gages can be set up to interface with a statistical process control system. This is important when process control is vital.

Cylinder bore gages (generally two point contact) can sometimes have problems with linear accuracy. Analog versions can be more prone to operator error.

While two point systems will more readily detect ovality, where this is not a major concern, three point digital systems are, in this quality technician’s opinion, the best all-around option.

When I am inspecting parts in which ovality could be an issue, if the parts are readily portable, I will check a percentage with a CMM to verify their roundness.  However, for speed, accuracy, practicality, and price, a three point digital bore gage would be the way I would go to verify product with tight tolerances.

A final note: If parts are relatively small and can be in contact with other materials, robotics is often used with air gage instruments.  This is another expense but can be introduced in high volume manufacturing.

I hope this will help.

Bud Salsbury
ASQ Senior Member, CQT,CQI

For more on this topic, please visit ASQ’s website.

ISO Documentation Practices; Difference Between Record and Document


ISO documentation practices, requirements

Q: Is there a published ISO standard for good documentation practices (e.g., crossing out an error with a single line and initialing and dating; striking through a blank space)?

Thank you.

A: Your question has two parts:

1) Is there a standard?

2) Does it cover the specific practice you cited?

The answers are “yes” and “no.”   🙂

About a decade ago, the ISO Technical Committee (TC) 176 on Quality Management and Quality Assurance started work on a documentation standard. There was (and still is) much confusion in the world about what kind of documents were expected and what should go into them. Of course, most didn’t want to take the time and energy to understand the purpose of documents, much less describe their practices in a site-specific manual. How sad. The output of the ISO/TC 176 work was a Technical Report: ISO/TR 10013:2001 – Guidelines for quality management system documentation. Frankly, however, I do not think it will address your question.

First of all, documents and records are often confused. Even though the ISO terms and definitions standard (ANSI/ISO/ASQ 9000:2005 Quality management systems — Fundamentals and vocabulary) parks them both under the word document, it is good practice to always think document=before, and record=after.

In other words, a document tells us what to do. A record tells us what was done. Many people, not understanding this principle, have actually tried to place records under configuration control!

The record-keeping practices you cited — crossing out an error and marking in a blank space — have their origin in the early military practices of the 1950s! Back then, there were no computers, internet or even ISO standards. There was also much more falsification of information back then, as we treated the workers with little or no respect.

The practices you cite were attempts to make sure that the data entered on a record wasn’t changed. Those practices just kind of hung on for half a century. In my 40 years in the quality profession, I have never seen these “rules” written down in an external document, like a regulation or standard or policy. Sure, individual organizations have required these practices through their local Standard Operating Procedures, but I am pretty sure they are not published in higher-level documents.

With automation and networking, records are becoming much more virtual. Paper records are becoming a thing of the past. Security and protection of those electronic records is a much bigger problem than when they were all on dead trees.

Follow-up from expert: Doing some further research (for an upcoming class), I discovered that ISO/IEC 17025:2005 General requirements for the competence of testing and calibration laboratories, contains a clause about records correction, In general, the clause says all alterations must be visible (not erased, blacked out, or deleted), and all changes must be signed or initialed by the person making the change. Equivalent measures should be taken in the case of electronic records.

I don’t know why I didn’t think of this standard earlier, however, my earlier remarks about this coming from the 1950s practices B.C. (before computers) still stand.

Dennis Arter
ASQ Fellow
The Audit Guy
Columbia Audit Resources
Kennewick, WA

For more about this topic, please visit ASQ’s website.