This information provided by Bryan R. Fischer, of TDP360 LLC.
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To answer this question, let’s start with a recap of why GD&T is so important.
GD&T is a rule-based feature-based system.
GD&T provides rules, tools, and methods to clearly, completely, and unambiguously define geometric requirements that can be reliably and repeatably recreated on actual, imperfect, manufactured parts and assemblies.
GD&T provides rules, tools, and methods to clearly, completely, and unambiguously define the allowable variation in the relationship between features on parts and assemblies, such as orientation and location.
In summary: GD&T is based on rules, it applies to features (surfaces) on parts and assemblies, and GD&T specifications define clear and unambiguous requirements for features and the relationship between features that can be recreated on actual parts and assemblies. Plus and minus tolerancing alone does not and cannot do these things.
Plus and minus dimensioning and tolerancing is a specification-based system. The focus is on dimensions and tolerances, not on surfaces on parts and assemblies.
Plus and minus dimensioning and tolerancing does not provide rules, tools, and methods to clearly, completely, and unambiguously define geometric requirements, and it does not define requirements that can be reliably and repeatably recreated on actual, imperfect, manufactured parts and assemblies.
Plus and minus dimensioning and tolerancing does not provide rules, tools, and methods to clearly, completely, and unambiguously define the allowable variation in the relationship between features on parts and assemblies, such as orientation and location. GD&T provides this capability by relating geometric tolerances to a datum reference frame. Plus and minus dimensioning and tolerancing cannot be related to a datum reference frame, thus, inspectors must guess how to relate plus and minus tolerance requirements on actual, imperfect, manufactured parts and assemblies.
Part of the problem is that there are no dimensions on an actual manufactured part. Dimensions are specifications on drawings and annotated models, consisting of lines, arrowheads, text, numbers, and symbols. There are only features (surfaces) on actual parts and assemblies. Thus, inspectors must guess how dimensions and tolerances apply to actual, imperfect, manufactured parts and assemblies. (Note that many experienced inspectors are very good at guessing, as they have had to deal with incomplete and ambiguous specifications throughout their career.)
Aside from defining the size of features of size, inspectors must guess which requirements are defined by plus and minus tolerancing and they must guess how to recreate the requirements on actual, imperfect, manufactured parts and assemblies.
To recap, these shortcomings for plus and minus dimensioning and tolerancing mean that inspectors must guess what the specifications mean, what requirements they impose on the actual product geometry, and how to determine if those requirements are met. Three levels of guessing. Informal methods. For most companies that do not properly apply GD&T to their product definition data, they find out that there are problems with their part geometry during assembly, installation, the customer use phase, and during maintenance and service. Parts don’t fit or align properly, products don’t work properly or with lower quality, replacement parts do not fit or align properly, service takes longer than it should, etc. What seems like an easy low-cost approach, using +/- instead of GD&T ends up adding significant cost and lowers the quality over the lifecycle of the product. Everybody suffers. Look up the Taguchi Loss Function (from Dr. Genichi Taguchi) for a deeper understanding of these concepts.
The following examples illustrate some of the ambiguities and guesswork required when evaluating dimensions with plus and minus tolerances used to control the relationship between features.
Drawing with Directly-Toleranced Dimensions (+/-)
Directly-Toleranced Dimensions Used to Define Distance or Location (Unclear Meaning)
Dimension and Tolerance to be Studied
As-Produced Part with Imperfect Surfaces
The part shown in this figure has imperfect geometry, as all manufactured parts are imperfect. The imperfection is exaggerated to highlight the challenges of trying to determine what the ±1 mm tolerance shown in the previous figures means. The ASME Y14.5 standard does not explain what this tolerance means. Thus, it is not clear which feature is controlled by the tolerance (e.g. does it control the distance from the hole to the edge of the part, does it control the distance from the edge of the part to the hole, do we need to use the entire surface as the origin, does it control the axis of the hole or a center point at one end of the hole…). The conformance criteria are unclear. The designer, manufacturer, and inspector must guess what the tolerance means, and it is likely that each party will guess that the tolerance means something different.
As a starting point, we will assume that the 30 ±1 dimension and tolerance control the location of the hole relative to the bottom surface, and that the tolerance controls the location of the hole’s axis. We assume that the dimension locates the hole from the bottom surface. These are all assumptions, layers of assumptions, as the ASME Y14.5 standard does not explain what this directly-toleranced dimension controls.
Assuming the Dimension and Tolerance Control the Hole’s Location – Step 1
In this figure, the 30 ±1 dimension and tolerance are represented in the context of the manufactured part. The first guess here is that the dimension locates the hole from the bottom surface. The second guess is that we should use a tangent plane as the origin for the dimension. The third guess is that the 1 mm tolerance defines a tolerance zone. The fourth guess is that the ± 1 mm tolerance controls the hole’s axis, as represented by the 2 mm wide tolerance zone. The fifth guess is that the part should be oriented relative to the origin plane as shown. Note that the shape of the bottom surface is convex, which means the setup may be unstable.
In this setup, the axis of the hole, which is represented by the dot in the center of the hole, is within the tolerance zone, which could mean it conforms to the ±1 mm tolerance. Again, this is a guess.
Assuming the Dimension and Tolerance Control the Hole’s Location – Step 2
In this figure, the part was rotated counterclockwise, so the origin plane mainly contacts the left end of the bottom surface. Perhaps the center of mass for this part is such that the part naturally rotates counterclockwise and is at rest as shown against a surface representing the origin plane, such as a surface plate. The relationship between the bottom surface and the plane are one of many possible relationships. Thus, this setup is another possibility, another guess.
In this setup, the axis of the hole is not within the tolerance zone, which could mean it does not conform to the ±1 mm tolerance. This is yet another guess.
Assuming the Dimension and Tolerance Control the Hole’s Location – Step 3
In this figure, the part is rotated clockwise, so the origin plane mainly contacts the right end of the bottom surface. Perhaps the part is also stable in this setup, as there may be more than one stable setup between a surface and a plane. Notice that in this figure the axis of the hole is within the 2mm wide tolerance zone.
In this setup, the axis of the hole is within the tolerance zone, which could mean it conforms to the ±1 mm tolerance. This is yet another guess.
Assuming the Dimension and Tolerance Control the Hole’s Location – Step 4
In this figure, we show another possible interpretation of the dimension and tolerance, that it represents a point-to-point distance rather than a distance from an origin plane. This is yet another guess. Trying to measure the distance from a surface to a hole using hand tools or a measuring tape is tricky and fraught with errors. For example, looking at the figure, the distance is ~ 28 mm from where the measuring tape contacts the bottom surface to the center of the hole.
In this interpretation, it looks like the axis of the hole (more likely an approximated center at one end of the hole) does not conform to the ±1 mm tolerance. This is yet another guess.
Assuming the Dimension and Tolerance Control the Hole’s Location – Step 5
In this figure, we show another possible point-to-point distance between the bottom surface and the hole. In this example, the measuring tape contacts a different point on the bottom surface, which yields a different distance between the bottom surface and the hole. Looking at the figure, the distance is ~ 30 mm from where the measuring tape contacts the bottom surface to the center of the hole.
In this interpretation, it looks like the axis of the hole (more likely an approximated center at one end of the hole) conforms to the ±1 mm tolerance. This is yet another guess.
Problem Solved – Hole is Clearly Related to Datum Reference Frame A|B|C
In this figure, the ambiguous 30 ±1 directly-toleranced dimensions used to locate the hole are replaced by GD&T. The location of the hole is defined by two 30mm basic dimensions. These basic dimensions locate the true position of the hole relative to datum reference frame A|B|C, which is referenced in the positional tolerance feature control frame. The hole’s location tolerance is defined by the positional tolerance, which clearly indicates that the hole’s tolerance zone is cylindrical, it is 2mm in diameter, and it is located relative to datum reference frame A|B|C. The meaning of these specifications, the requirements they impose upon the product geometry, and their conformance criteria are completely, clearly, and unambiguously defined in the ASME Y14.5 standard.
Often, using what seems to be a simpler approach, using directly-toleranced dimensions to control the relationships between features instead of GD&T, ends up causing a lot of undue confusion, iteration, wasted time, argument, scrap and rework. Whereas using directly-toleranced dimensions may seem like a less expensive alternative to GD&T, defining the relationships between features using directly-toleranced dimensions increases the lifecycle cost of the product. Using ambiguous specifications and the conflict that comes with them often has a negative effect on morale.
Use GD&T. Everyone will be better off.