(408) 674–1451    gene@ttc-cogorno.com


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Zero Positional Tolerance at MMC Recently I received an email from a person who had red my book and asked why the use of zero positional tolerance at maximum material condition was not more widely used. Her question was, “If zero positional tolerance at MMC has the ability to increase through put of parts that would normally be rejected and not adversely affect quality, then why wouldn’t more companies at least investigate this method off tolerancing?” My response to her email is the subject of this month’s newsletter.
The Tangent Plane Applied to Perpendicularity The concept of the tangent plane was introduced in the 1994 standard. Its purpose it to maintain the orientation of the high points – that is the contact points – of a surface relative to its' mating feature. The perpendicularity, parallelism, and angularity controls of a plane flat surface not only control the orientation of a feature, but also control the flatness of the feature to the same tolerance. Specifying the circle T modifier insures the proper orientation without requiring flatness to the same tight tolerance.
Flatness of a Derived Median Plane In the 2009 Standard, the standards committee converted straightness of a derived median plane to flatness of a derived median plane. Flatness of the median plane better describes the three-dimensional aspect of this control. Open the attached file below to learn more about the application of flatness of a derived median plane.
The Datum Reference Frame Datums are the reference surfaces or the starting points for the location and orientation of features. They are essential for appropriate and complete tolerancing of a part. Datum geometries can become very complicated when they are features of size, compound datums, or features of an unusual shape. Geometric Dimensioning and Tolerancing provides the framework necessary for dealing with these complex datum features. The simpler plus or minus tolerancing system ignores these complexities which means that plus or minus toleranced drawings cannot adequately tolerance feature of sizes. As a result, many plus or minus toleranced drawings are subject to more than one interpretation.
Rule #1 I have discussed rules in a previous newsletter; however, Rule #1 is not well understood, and it is so important that it requires additional discussion. Many individuals in my classes are surprised to learn that the size tolerance of a feature not only controls its size, but also controls its form.
Imaginary GD&T Concepts Imaginary concepts are imaginary because they are considered to be perfect. Perfect geometries do not exist in this universe; therefore, they are imaginary. In the world of geometric tolerancing, measurements are thought to start from a perfect surface and end in a perfect tolerance zone. Of course measurements can’t actually be taken from imaginary surfaces, so very accurate simulated datum features are established. Imaginary datums are assumed to exist in and be simulated by these simulated datum features. Simulated datum features are also known as tooling.
Frequently Used Drawing Symbols Geometric dimensioning and tolerancing is a graphic language that consists of very specifically defined symbols, terms, and rules. These symbols, terms, and rules are the alphabet, the definitions, and the syntax of this language. Recently a client asked me if he had to memorize all of the GD&T symbols. I suggested that he memorize the most frequently used symbols and not worry about the drawing symbols that are only occasionally used. This newsletter deals with the symbols most frequently used on drawing.
A Pattern of Features Located to a Second Pattern of Features Patterns of features may be toleranced to a second pattern of features as well as to an individual feature(s). Many times there is more than one way to tolerance features on a part. The example shown in this month’s newsletter is one of those parts that can be toleranced with more than one method. The most practical tolerancing method is illustrated by feature control frame number 1. If that method is not practical or does not satisfy the tolerancing requirement, feature control frame number 2 is the next best option.
A Strategy for Tolerancing a Part When tolerancing a part, the designer must determine the attributes of each feature or pattern of features and the relationship of these features to their datum features and to each other. In other words, the designer must determine the size, the size tolerance, the form tolerance, the location dimensions, and the location and orientation tolerances of each feature. Also, the designer must decide which are the most appropriate datum features, and what material conditions apply to any features of sizes. All of these questions must be answered in order to properly tolerance a part. Some designers believe that parts designed with a solid modeling CAD program do not require tolerancing; nothing could be further from the truth. The Dimensioning and Tolerancing standard clearly indicates that, with the exceptions of reference, maximum, minimum, or stock dimensions, each dimension shall have a tolerance.
Composite Profile Tolerancing Composite profile tolerancing is very similar to composite positional tolerancing. The upper segment controls the size, form, orientation, and location just like any other profile control. The lower segment is a refinement of the upper segment.
Coplanarity What is coplanarity? Coplanarity is the condition of two or more surfaces having all elements in one plane. This month's newsletter includes four drawings that show how a profile tolerance may be used to control coplanarity.
Locating Coaxial Features at MMC A client asked me to review a drawing that is similar to the one shown in this month's newsletter. This part consists of a cylinder with two coaxial holes drilled through it. Neither the holes nor the inside and outside diameters of the cylinder are controlled for coaxiality. Not controlling coaxiality is the most common error that I find on drawings. Many designers believe that the title block tolerance controls coaxiality. It does not. Some designers don't think it is necessary to control coaxiality at all. I have been told that specifying a tolerance for coaxiality is "overkill." Yet, when parts won't assemble because coaxiality has not been properly controlled, both time and money are wasted. This newsletter illustrates several ways to control coaxiality.
Specifying the Clocking of Patterns of Features One of the most common tolerancing errors found on drawings is the incorrect clocking of features. Clocking is the rotation of a feature or patterns of features around a datum axis in a manner similar to the movement of the hands of a clock. The first drawing in the attached article shows patterns of features incorrectly clocked. The three drawings are examples of correctly clocked features and patterns of features.
Multiple Patterns of Features A client asked me to review a drawing similar to the one shown in the attachment. This drawing is the perfect example to explain the concept of multiple patterns of features. When two or more patterns of features are located with basic dimensions, to the same datum features, in the same order of precedence, and with the same material condition modifiers, they are considered to be one composite pattern of features. This drawing also includes composite tolerancing and the concept of simultaneous requirements.
Composite Positional Tolerancing Composite positional tolerancing can be used where the tolerance from feature-to-feature must be small and the tolerance between the pattern and its datum features can be larger and consequently less expensive. Open the attachment to see a detailed explanation of Composite Positional Tolerancing.
Tolerancing Fixed Fasteners Last month's article was about floating fasteners. This article is about fixed fasteners. Because of habit, lack of training, or both, clearance holes and threaded holes are often toleranced too tightly. Open the enclosed attachment to see a detailed explanation that will explain how to make sound tolerancing decisions for fixed fasteners.
Tolerancing Floating Fasteners One of the most frequent tolerancing acactivities that engineers perform is tolerancing clearance holes for fasteners. Often, because of habit, lack of training, or both, clearance holes are toleranced too tightly. This section on floating fasteners provides information that will allow engineers to make sound tolerancing decisions for floating fasteners assemblies.
Zero Positional Tolerance at Maximum Material Condition Zero positional tolerance at MMC is just what it says–no tolerance at MMC. However, bonus tolerance is available. As the size of the feature departs from MMC toward LMC, the bonus tolerance increases; consequently, the location tolerance is directly proportional to the size of the feature as it departs from MMC toward LMC. Zero positional tolerance at MMC provides the manufacturing staff with maximum flexibility and the greatest possibility of producing good parts. Open the enclosed attachment to see a more detailed explanation of the benefits of zero positional tolerance at MMC.
Datum Features of Size Specified with a Material Condition Modifier If, in a feature control frame, there is no material condition symbol, such as a circle M or circle L, following a datum feature of size, the regardless of material boundary (RMB) modifier automatically applies. The RMB modifier requires the datum feature to make physical contact with the processing equipment. Consequently, no shift tolerance is allowed. If, in a feature control frame, the maximum material boundary (MMB) modifier, a circle M, is specified following a datum feature of size, it allows the datum feature to shift within its virtual condition boundary. The MMB modifier allows the acceptance of more parts. Open the enclosed attachment to see a more detailed explanation of how the RMB and MMB modifiers affect the total tolerance of a part.
Specifying the Position Tolerance at MMC The maximum material condition (MMC) modifier, a circle M specified in a feature control frame following the geometric tolerance of a feature of size, modifies the tolerance to allow all of the tolerance available for that feature. This modifier indicates that the specified tolerance applies at the maximum material condition size of the feature and that a bonus tolerance is available as the size of the feature departs from its maximum material condition toward its least material condition.
Specifying the Position Control The position control is often, incorrectly, referred to as "true position." True position is the theoretically exact location of a feature. The theoretically perfect axis of a cylindrical tolerance zone oriented and located by basic dimensions is an example of true position. Tolerance zones are located at true position. The position control is used to locate and orient features of size and is represented by the symbol that looks like the cross hairs on a surveyer's transit.
The Tolerance of Position Position is a composite tolerance that controls both the location and the orientation of features of sizes at the same time. It is the most frequently used of the fourteen geometric controls. The position tolerance significantly contributes to part function, part interchangeability, the optimization of tolerance, and the communication of design requirements.
Establishing a Datum Reference Frame

Tolerancing a part consists of two major steps:

  • First, establishing a datum reference frame
  • Second, tolerancing the other features to that datum reference frame
Establishing a datum reference frame consists of selecting appropriate datum features and controlling the proper relationship between them. The file illustrates how to establish a datum reference frame from a plane surface and a pattern of two features.
Perpendicularity Refinement

When locating and orienting an individual feature, locate the feature with the position control and refine the orientation tolerance with the appropriate orientation control. The file shows how to properly tolerance a drawing with the perpendicularly control.

Free-State Variation

Where a form or location tolerance is specified for a feature in the free state, the free state symbol is placed inside the feature control frame following the tolerance and any modifiers. A size dimension and tolerance is specified, followed by the abbreviation AVG indicating that the tolerance applies to the average of measurements.


This month's article is about flatness ? the definition of flatness, how to specify flatness, and how to in interpret the flatness specification. Also included is the flatness of a median plane, a new concept in the most recent edition of the standard.

Irregular Datum Features of Size

Irregular datum features of size is a new concept illustrated and explained in the latest revision of the Geometric Dimension and Tolerancing standard.

Datum Features of Size

Where a datum feature symbol is placed in line with a dimension line or on a feature control frame associated with a feature of size, the entire feature of size is the datum feature.

Datum Sequencing

Consider this sentence. ''The dog walked I.'' We know something is wrong: either the order is incorrect or the personal pronoun is the incorrect part of speech. Depending on your assumption, you might interpret this sentence in one of two ways, ''I walked the dog'' or ''The dog walked me.''

In the same way, when datum features are incorrectly sequenced, the tolerance is ambiguous. I frequently see drawings that have datum features specified in feature control frames in an order of precedence that makes no sense. Proper datum sequencing is extremely important to insure that only one, correct interpretation is possible. The attached excerpt from my book will help clarify this subject.

Datum Feature Identification

All datum features must be specified in order of precedence. Each datum feature must be identified with a datum feature symbol. Datum feature symbols must not be applied to centerlines, center planes, or axes.

I often see drawings with datum feature symbols attached to axes or center planes, and I sometimes have difficulty convincing engineers that it is impossible for inspectors to orient or locate features relative to imaginary lines and planes. The discussion on the attached file will help clarify this issue.

Three Rules Included are the 3 dimensioning and tolerancing rules. These three rules apply to drawings in general, and to GD&T in particular. They govern specific characteristics of features on a drawing.
Sixteen Critical Terms

Symbols, terms, and rules are the basics of geometric dimensioning and tolerancing. They are the alphabet, the definitions, and the syntax of this language. The GD&T practitioner needs to be very familiar with these concepts and how to use them. This month's article includes the 16 GD&T terms that I consider to be essential to understand the current geometric tolerancing standard.

New Symbols Included are the new symbols that appear in the latest revision of the geometric dimensioning and tolerancing standard.