November 17, 2017


A few weeks ago, I surveyed on the interpretation of size dimensions based on Y14.5.  I posted the survey on several group boards on LinkedIn. This article compiles the results of the  survey.

Before I discuss the results, I would like to thank all the people that participated in the survey. Their contribution allows all of us to look at the questions in this survey from many users viewpoint.

I use these surveys as the "voice of industry" in my work on standards committees. I also believe they are helpful for the readers to know how their use of tolerancing concepts compare to others around the world. 

Purpose of this survey

The purpose of this survey was to find out if various groups in industry interpret and verify the minimum limit (or local size) of size a two point requirement or as a theoretical swept sphere requirement. . (The survey does not address the effects of Rule #1)

GD&T and Inspection Skill Levels of Survey Participants

The figure above shows the GD&T and Inspection skill levels of the survey participants.  This information is important because it shows that most of the responses came from very knowledgeable participants.

Participants Area of Work

The chart above shows that the main two areas of participation were from Engineering/design and quality/inspection. The questions were designed for the engineering community and the inspection community. 

Questions Presented to Both the Engineering /Design and Quality/Inspection Participants

The intent of the question in the figure above is to determine if the participants felt the requirement for the minimum size limit should be verified with a two-point measurement or a theoretical spherical ball.The responses show that the majority (67%) of participants feel that a two-point measurement can be used to verify the minimum limit of a size dimension. However, there is not a uniform interpretation in industry. 

A few descriptions from the "Other" responses on how the minimum size limit should be verified are below. 

  • "Both in inspection two-points and in some cases per Y14.5.1 ball check for a bended tube."
  • "If using a CMM, some points, usually 6, taken around the diameter, the best fit circle taken through the points"
  • "It is my understanding that if you take the smallest two-point measurement of the diameter, it will be the largest theoretical sphere that will pass through the diameter"

The question in the figure above is similar to the previous question about a diameter. It is interesting that when presented with a block even more people selected the response of the two point measurement.

A few descriptions from the "Other" responses on how the minimum size limit should be verified are below. 

  • "The smallest two-point measurement. The term "size dimension" should be clearly defined in Y14.5 in the definition section. It is good timing to include in the next version, it is used throughout the text".
  • "If using a CMM, we would expect measurement  points, to be taken across the surface and a best-fit plane is used - this is good enough for us."
  • "The size of the largest perfect form 2D section that can be passed through the material"

The question in the figure above asked the participants to select an answer based on the Y14.5 -2009 standard. The two point measurement still has the highest response rate. The chart shows there is variation in the interpretation of a size dimension.

Questions presented only to the Quality/Inspection Participants

The figure above shows that the amount of tolerance is the most significant factor in the choice of a measurement device for the minimum limit of a size dimension. I am a bit surprised that the standard referenced on the drawing did not score higher.

The figure above shows the two-point measurement (micrometer/caliper) is the most common method for verifying the minimum limit of a size dimension in industry. closely followed by  CMM verification. I find it interesting that the swept ball algorithm is rarely used.



These survey results are based on participant responses from 27 countries. There were 147 participants total. 61% of the responses were from the U.S. and the remainder from the international community. The highest response rates internationally were from India, Canada, and Great Britain.

Although the participation is significant, it is not high enough to make an accurate assessment of the interpretation of size dimensions in industry, but, it does provide us with information to make useful insights on the topics even though it is not a definitive answer.

The survey results indicate that the minimum limit of a size dimension is not uniformly understood by engineers, designers, quality personnel, and inspectors in industry. The variation in understanding size dimensions is partially because this topic is not covered thoroughly in the Y14.5 and Y14.5.1 standards.

Size can matter

In a number of cases, any of the methods used to verify the lower limit of a size dimension described in this survey would be adequate for the function of a part. However, in certain cases, the method of verifying a size dimension can make a difference between a functional part and a failed product. (e.g. minimum wall thickness, allowable roundness deviation, etc.)

Assumptions are effective barriers to precise communications

If you create drawings, precise communications can make the difference between a  successful product and failure. Where the interpretation of the minimum limit of a size dimension can make a difference in the function of your part, I recommend that you specify how the size dimensions should be interpreted (or verified).

Do you intend the minimum limit of size dimensions to be interpreted as a two point requirement (as implied in Y14.5) or as a  theoretical swept ball requirement (as shown in Y14.5.1)? 

Until the Y14.5 and Y15.1 standards adequately cover how to interpret size dimensions, consider one of the following actions:

  • check
    Add a general note to your drawings to establish a default for how size dimensions are to be interpreted.
  • check
    If your company uses an addendum to the Y14.5 standard,  add a section that documents how size dimensions are to be interpreted/verified. Click here to the read addendum article.
  • check
    Create a measurement plan that covers the method for verifying size dimensions and reference it in your drawings..

Final thoughts

In hindsight, I can think of several additional questions that would have been useful to ask in this survey. It is always a trade-off between the depth and length of these studies. If you have an opinion on how many survey questions would not discourage participants, leave a comment or send me an email. Thanks.

I hope you found the survey results informative. If you like the article, please share it with your friends on on social media. Feel free to leave a comment about your experiences with size dimensions or on any aspect of this article.

Thanks again to all the survey participants.

About the author 

Alex Krulikowski

​Author of 22 books on GD&T, teacher, mentor, instructional designer, ASME and ISO standards member, product designer, dimensional engineering manager, speaker, and successful entrepreneur.

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  1. Alex,

    WRT…”the method of verifying a size dimension can make a difference between a functional part and a failed product. (e.g. minimum wall thickness, allowable roundness deviation, etc.)”

    Alex, Do not dismiss 2 point measurement using roundness as the reason. Diameter (size) is not the same as roundness.

    Although roundness and size both play a significant role in the way things fit together, roundness is evaluated independent of size and must therefore be measured in a different manner.

    Definition of roundness
    A component is described as round if all points of a cross section are equidistant to a common center. Therefore, to measure roundness, rotation of the component is necessary coupled with the ability to measure change in radius.

    Vee-block (3 point) method
    Rotational datum method
    Radial variations output from the gauge as a polar profile or graph from a manual template overlay or computer generated “perfect circle.”

    With Respect Always,

    Charles Roscoe
    Senior Designer ~ Retired

  2. From your assumption section. In the following text, we shall read Y14.5.1 instead of Y15.1.
    “Until the Y14.5 and Y15.1 standards adequately cover how to interpret size dimensions, consider one of the following actions:”
    My interrogation
    Where is the definition of “Size dimension” in ASME Y14.5?
    How could we explain the difference between “limits of size” and “size dimension”?
    Does “size dimension” is reserved to limit the size of a regular feature of size?
    Does size dimension is only express using limits of size?
    Does a size dimension of a feature of size could be express as a basic dimension and its variation is express by using a profile tolerance?
    Why ASME Y14.5 maintain Rule No 1 as a default? Does an industry survey could be performed which will demonstrate if the default Rule No 1 is functionally required on most regular features of size defined with a limits of size?
    If we go back in the industry standard (mil-std-8), rule No 1 was applied only when the only controlled we have is a limit of size.

  3. I am having some trouble understanding how the inspection method can be the difference between design success and failure. A Diameter, for example, must have all points on the surface comply with the controls shown (They must lie within the boundary set by the diameter tolerance, and any additional form tolerance). When I inspect a part, I recognize that using a micrometer to check this diameter can be very risky, allowing a section of surface I did not check wander outside of the tolerance zone. Lastly, as a design engineer, I have always been taught not to place manufacturing or inspection methods on the drawing. I can force a more detailed inspection of critical surfaces by using a 3D geometric tolerance in place of a 2D control (Cylindricity in place of circularity or total runout vs. circular), but I cannot force the QA inspector to measure the part in a particular way. How do you propose I force the inspector to utilize a specific method like the theoretical swept ball?

    1. Hi James,

      Thanks for you comment. I apologize for my late response.

      I think the method of inspection can make a difference between the success and failure of a part. The survey results showed that a significant number or respondents believe that a two point measurement is all that is needed to verify the size of a diameter. At the same time, any roundness deviation (on a three lobed part) will not be detected with a two -point size verification. However, if another method was used to verify the size of the diameter, roundness deviation could be detected. The undetected roundness deviation may impact the function of the part.

      By the way, I am not a proponent of making the swept ball the default for verifying a diameter. But, it is being debated in the standards community.

  4. I think this highlights the issues when the standard runs into practical considerations.

    Technically (as per the survey, ignoring the effects of rule 1), per the standard, you need to determine the axis from the unrelated actual mating envelope. You can then sweep the surface, and determine that the distance from this axis of all points is within the limits of size.

    With a CMM, this might be relatively easily achieved, if your machine supports it.
    Otherwise, you could perhaps set up something clever to lock the part’s axis along the axis of its unrelated actual mating envelope, allow it to rotate on this axis, and then set up an indicator that can sweep along the direction of the axis.

    …But how often is this actually going to happen?

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