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Performing Accurate GD&T Measurements

A SCANtek 5 multi-sensor CMM pictured on the LK Metrology stand at last year’s Control exhibition

It’s been more than 80 years since the first publication of Y14.5, back then under ASA, and Y14.5 and GD&T has evolved to become the primary means of communicating product requirements for mechanical components and assemblies of virtually all types of products. While there are several different versions of GD&T published by various nations. There are two standards that are dominant among the global industries and throughout global supply chains. These are the ISO and ASME systems. There are several notable differences in the ISO and ASME specifications of GD&T.

Nations with Documented ASME/ISO GD&T Presence

While there are businesses that elect to use the ISO standard within the United States, the U.S. founded system of GD&T (Y14.5), today published under the American Society of Mechanical Engineers (ASME), is found in more commonly around the globe. Even in western Europe, where ISO GD&T is more common, ASME Y14.5 is often the GD&T training and applications professionals are using. Social media, ASME’s list of certified geometric dimensioning and tolerancing professionals (GDTP), and personal experience from a decade of GD&T training were combined to show where these two GD&T standards are dominant.

  • Concepts – ASME maintains that there are two types of size to be reported for most features of size; local size (or opposing points) and the mating envelope. ISO, by comparison only reports local size. The associated feature (fit cylinder, parallel planes, etc.) is used for certain geometric tolerances, but not the reported size.
  • Applications – The position tolerance is highly common in both versions, but ASME reserves the use of position tolerances for locating features of size only. ISO employees position tolerances to locate not only to features of size, but to nominally planar features as well.
  • Mathematics – ASME defines actual mating envelopes in terms of smallest size that can be contracted about an external feature or largest size that can be expanded within an internal feature so that it coincides with the surface(s) at the highest points. This means a maximum inscribed and minimum circumscribed fitting algorithm must be applied to the data collected during inspection. ISO defaults to a least squares fitting, although where specification operators are applied the maximum inscribed, minimum circumscribed or other fitting algorithms can be applied.

There are many reasons for the ASME Y14.5 system of GD&T to be more dominant standard globally:

  • The mass proliferation of ASME Y14.5 based GD&T training material means even when many companies have sought to train employees to use the ISO system of GD&T the reality is that employees have been taught to apply the ASME system of symbols and rules.
  • ASME’s certification system (GDTP or Geometric Dimensioning and Tolerancing Professional) has employees and employers looking to ASME’s professional certification to demonstrate proficiency with GD&T. In a world flooded with “I know GD&T” resume statements, actual proven proficiency matters.
  • Using the U.S. standards facilitates receiving outsourced engineering and manufacturing work for Non-U.S. nations
  • The United States has been both an economic power player and a leader in technology and mechanical innovations for most of the past century.
  • Supporting standards like the DMIS CMM operating language have been based on ASME’s GD&T practices.
Statement from DMIS 5.2 Standard Section 3

In all industries, failure to meet engineering specifications can result in serious consequences. Warranty actions and product recalls can be multi-million-dollar expenses. Serious injuries or a customer death attributed to defective products have damage brands and rendered brilliant marketing campaigns useless. Even something as mundane as late shipments and mal-fitting parts have sunk the stock price of public corporations. Manufacturing variations will happen, so the ability to evaluate specifications, with exactness to the standards, ensures that the critical limits of a tolerance are upheld. Where they are not, we have witnessed medical implants that cause patient chronic aches and pains, ground vehicles that maim drivers, and aircraft that can be hazardous to fly.

“We have learned to live in a world of mistakes and defective products as if they were necessary to life. It is time to adopt a new philosophy…”  Dr. Edward Deming

Historically there have been massive deficiencies in what computer aided inspection (CAI), such as a CMM operating system, do for the evaluations of the engineering drawing specification in the effort to generate a reported value.

Chief among these issues has been the fact that many CMM operating systems use the alignment coordinate system, also known as a part coordinate system (PCS), as a proxy for a datum reference frame. This was a poor idea from its inception, but it was intuitive for the customer, and easy to program so the idea took root. A simple explanation of the differences involves pointing out the deficiencies of the PCS as a proxy (or substitute) for an actual datum reference frame. The axis of the alignment’s coordinate system can only point in one direction at a time with the intersection of these axis defining only one possible origin for the CMM. GD&T datum reference frames however, may require more than one fixed axis direction and the origin can often shift of float about feature like a cylinder or parallel plane width. The results are that these discrepancies in the software may report false passing or false failure values.

CAMIO’s standard selection dialog

LK Metrology’s CAMIO software utilizes a truly independent datum reference frame. The datum reference frame and the alignment coordinate system will often have different constraints on the degrees of freedom, different origins, and even different directions for the X, Y, and Z axis. The ability to select between major standards is also provided under the software’s preference settings. ISO and ASME standards selection permit users of both global standards to determine how geometric tolerances should be applied. CAMIO’s preference selection dialog allows users to fulfill the engineer’s specification in ways legacy CAI software simply cannot.

It should be vital to the interest of quality professionals to have a solid understanding that measurement involves far more than probing parts or taking readings from metrology instruments. One definition of “measure” is: “to ascertain the extent, dimensions, quantity, capacity, etc., of, especially by comparison with a standard.” In the case if mechanical engineering the ISO and ASME dimensioning and tolerancing standards play a vital role in applying proper measurement.

Measuring Position

In the past many CMM programmers have attempted to work around software deficiencies with different alignment and reporting methods. ASME Y14.5-2009 figure 7-5 can be used to discuss some of the gimmicks that have been applied to work past the deficiencies of the PCS.

In this first example the PCS was used as a proxy for a datum reference frame.

Because the drawing only has a one datum reference there are too few degrees of freedom constrained for the programmer to use only datum B as a PCS. The programmer chose the lower right-hand side hole (1 of 6) to align the workpiece. Minimal positional deviation is reported giving everyone a false sense of confidence in the part. This erroneous reporting will bleed into a false sense of confidence in the manufacturing process which may lead to lower rates of inspection as full production ramps up.

For a second example, the programmer again realizes that datum B alone offers too little constraint to define a PCS. In this scenario a plane, line, cylinder alignment was used with the cylinder of datum B being used only to define the X & Y origin in the PCS. In short, the axis of this PCS, compared to those in the first example, all point in slightly different directions. This also differs from the design requirements as the primary datum B sets axis direction and origins for the 2 other axis of the datum reference frame. This gimmick produces a higher deviation value for the position tolerance, however it is also incorrect. Both the first and second examples were programmed differently from the actual datum reference frame and thus report an incorrect value for the position tolerance. The design engineer, or customer, should be highly disappointed to find out that the design specifications are not being adhered to. The “solution”, using the PCS as a proxy for a datum just isn’t, and cannot be made to be,  equivalent to using an actual datum reference frame.

In a final example the alignment is irrelevant. The programmer allows CAMIO to make use of the datum reference frame as the engineer intended – only datum axis B. Additionally, the design shows the position tolerance applied to 6X holes. From a reading of the Y14.5 definitions and paragraph 4.19[vi] we find that this is a simultaneous requirement and the position of all 6 holes with relationship to this single datum axis must be looked at in the context of the collective pattern of 6 only. The report now shows that this hole is badly out of position. The part is clearly unfit for use and the process needs immediate correction. Compared to the previous examples where a line between this hole and the datum feature B had been used as part of the alignment and reporting for this hole (thus hiding deviations), this report using only datum axis B shows that a large part of this hole’s deviation is in the rotation about Z – that is to say holding the 60° angle dimension from the drawing. The two gimmick attempts were unable to detect this because they rotated the alignment based on where this feature was found to be, rather than a deviation from the 5 other holes in the pattern as the engineering specification required.

It is worth noting that the use of datum reference frames can show “better” or “worse” results than the traditional (gimmick) alternatives. In these examples it was elected to show how the true measurement could be eluding your quality department if your CMM software does not utilize datum reference frames properly – many CMM software to this day do not.

Further Understanding Location

There has been a long-standing failure in the mechanical engineering community to understand that directly toleranced linear dimensions cannot effectively define the locations of features. Often called “coordinate dimensioning”, these linear dimensions do not effectively define and limit the direction in which the dimension is to be measured.

Examples of direct tolerancing methods
Dimensions in red are location dimensions

Directly toleranced dimensions, or “coordinate dimensions”, are great for defining the characteristic of size because universally size is the linear distance between directly opposite points, lines, or planes of a part. Thus, the only direction needed is inherent in measuring to an opposite element. For this reason, calipers and micrometers remain popular, and viable, options for these measurements. However; where a linear dimension is required for locations more is required to define the direction for measurement. For this reason, ASME Y14.5 states that location dimensions need to be based in a datum reference frame. This inherently requires the dimensions shall be basic dimensions. Basic dimensions are most commonly expressed with a frame or “box” about the dimension value. Only basic dimensions are founded in a datum reference frame.

Basic dimensions from ASME Y14.5-2009

The need to move beyond these extremely limited usefulness of directly toleranced dimensions was an original catalyst for much of the development of Y14.5 and GD&T

It has been the practice of many imaginative product designer and inspector that the direction of the dimension line indicated the direction of measurement for a directly toleranced dimension. But this is simply not stated in any standard, nor would there be any real means of controlling these directions because a direction can only be understood in the mathematical theory of the drawing. However; divorced from this pure theory the physical reality of the part transforms the direction into an elusive and ambiguous entity.

It has been the practice of many CMM programmers to program the report of directly tolerance location dimensions as distances along the alignment coordinate system (or PCS).

Anytime a “measurement” must be programmed to evaluate using an axis of an alignment, the entire team should offer serious scrutiny, both to the specification and the method of evaluation, as these methods are generally saturated with opportunities for errors.

Linear directions affected by surface variations

As illustrated the direction of the axis can have a significant influence on where the hole is perceived as belonging in it’s nominal condition. The hole shown in black was the drawing location, but the alignment to the imperfect surfaces of the part illustrate that the software could easily identify a different location as nominally correct. This, of course, would be quite detrimental to assembly and function.

Commonly these will be reported as “Distance along __ axis”, “Distance between features along __ axis”, or expressed on drawings. Avoid specifying locations with directly toleranced dimensions at all times, and avoid using distance reporting involving axis directions whenever possible. The use of geometric tolerances and datum reference frames is a very effective means of avoiding the problems of directly toleranced dimensions for location for the designer and inspector. In LK Metrology’s CAMIO 8 series CMM operating system datum features receive additional processing to set the mutually perpendicular planes, and corresponding axis, of the datum reference frame in conformance with ASME and ISO standards as selected in the preferences.

LK Metrology has multiple GDTP certified professionals on staff to support customers with accurate CMM inspection program creation, proper drawing interpretation, and ensuring LK’s CMM operating system, CAMIO, performs measurements as intended by the design engineer and in conformance with the ASME Y14.5 standard.


Failure to meet engineering specifications can result in serious and expensive consequences. LK Metrology’s CAMIO software is a sophisticated and powerful means to evaluate your products for compliance to engineering specifications. The software is capable of evaluating datum reference frames from a wide range of datum feature types – planar, cylinder, parallel planes, and several combinations in patterns. CAMIO’s tolerance library supports all 14 geometric tolerances (Y14.5 1994 & 2009) and, when required, evaluates then properly to a datum reference frame rather than the PCS axis. The ability to change standards makes CAMIO a global solution.

Source: Metrology News