Design engineers strive to create clear, complete and unambiguous component specifications that accurately communicate design intent. Manufacturing engineers use engineering drawings and annotated CAD models to produce parts that meet these requirements, while metrologists and quality inspectors verify that the finished components conform to specification.
Without a common language, the same component specification can be interpreted differently by design, manufacturing and inspection teams. Miscommunication and inconsistent interpretation can lead to increased manufacturing costs, longer lead times, higher scrap rates, quality issues and components that are not fit for purpose.
This is where Geometric Dimensioning and Tolerancing (GD&T) earns its place in everyone’s toolbox.
Geometrical dimensioning and tolerancing is a symbolic language that defines how far manufactured parts may deviate from perfect geometry, while still meeting functional requirements. It replaces vague notes and overly tight linear tolerances with clear, standardised controls that every stakeholder can read the same way.
Consider a machined pump housing. In CAD, every planar face is perfectly flat, every bore perfectly formed, aligned and located, etc. On the shop floor, tool wear, poor fixturing, thermal effects and programming errors, amongst others, result in no manufactured product being truly perfect. GD&T is crucial as it allows the Design Engineer to specify, very precisely, how much of this inherent geometrical deviation is tolerable. How far from flat a sealing face must be, how much a bolt pattern may wander from true location, and in short, which geometrical aspects of which features matter most to function. A component should be defined according to its functional requirements.
GD&T is applicable to a wide variety of design and manufacture industry sectors across which standards like ASME Y14.5 and BS8888 are used to employ a shared language within global supply chains. This promotes more than just tidier drawings; it encourages consistency in approach and supports modern practices such as Model-Based Definition and automated inspection.
The true strength of GD&T lies in its ability to communicate design intent clearly and unambiguously throughout the product design and manufacture lifecycle. It provides a common language that preserves functional requirements from the 3D CAD model through manufacturing and ultimately to the CMM (co-ordinate measuring machine) inspection report.
GD&T primarily focusses on the geometrical deviation of the functional aspects of the features of a component.
Consider a shaft running in a pair of bearings. The designer chooses the bearing seats as primary functional features and specifies them as datums. Functionally related features can then be controlled with respect to the datum features, thus ensuring that the finished parts, if manufactured to drawing, will be functional.
During manufacturing the component can be fixtured from those same surfaces when turning and grinding associated features. The finished part is then set up on the same datums during the inspection process when, for example, checking runout of the referenced features. It is clear to everybody involved in the process, what is functionally important and how the shaft will behave in the assembly. Functional success is not always about satisfying size and location dimensions alone. There is so much more to geometrical deviation than size and location.
Organizations that adopt GD&T often report fewer concessions and less time spent in conflict over the interpretation of engineering drawings. These difficulties don’t disappear completely, but issues can be resolved efficiently using the precise, yet flexible and mutually understood standardised language of GD&T.
GD&T on an engineering drawing can look a bit daunting to the uninitiated, but once understood, the benefits of the approach become clear, and a drawing begins to ‘speak’ of the component’s function.
Understanding these five categories is essential to the effective and application and interpretation of GD&T.
1. Form tolerancing defines how feature can deviate from being perfect in shape. These controls don’t control the orientation of a feature, nor do they control the size of a feature. Form is used when the function of the feature is purely reliant on its shape rather that it being in any particular orientation or location.
Straightness, flatness, circularity and cylindricity all fall within this category.
As an example, flatness may be used to control deviation of a planar sealing face and cylindricity to control the deviation of a bearing bore. Form controls are often used to define datum features.
2. Orientation tolerancing defines how far a feature can deviate in angularity from a referenced datum feature. These controls don’t control the location of features. Orientation tolerancing is used when the function of the feature is reliant on its relationship between itself and one or more associated functional features, rather than on it being in a particular location.
Parallelism, perpendicularity and angularity all fall within this category.
As an example, perpendicularity may be used to control the orientation of the axis of an input shaft bearing bore to a mounting face of a gearbox. By identifying the mounting face as a datum, and applying a perpendicularity tolerance for the bore, to that datum and then referencing both datums, a clear and measurable requirement for assembly alignment can be specified.
3. Location tolerancing defines how a feature, or pattern of features, can deviate from their true position relative to datums, or to each other, as functional requirements dictate. There is often an inherent orientational aspect to this control which is defined by the referenced datums. The location of features is extremely useful and therefore widely used. When the location of features is functionally important with respect to datums or associated features, controlling how far features can deviate from being in true position is crucial. This can be applied to all manner of features such as bores, bosses, fixing holes, keyway slots and bolt patterns.
Position, Concentricity* and Symmetry* (*Not available in ASME Y14.5 2018) all fall within this category.
As an example, position control may be used to control the tolerance of location of an equi-spaced pattern of holes arranged on a pitch circle diameter and centred on a bore through a flange mounted coupling. By identifying the mounting face and the bore as datum features, defining the pattern with theoretically exact dimensions, applying a positional tolerance for the holes in the pattern and referencing the datums, the functional requirements of the features can be specified.
4. Runout tolerancing is a combination tolerance which controls the allowable deviation of two distinct types of geometrical deviation, within one measured value. Primarily, it controls the combination of form AND location of revolute features with respect to a datum axis of rotation.
Circular Runout and Total Runout both fall within this category.
Circular Runout affords a 2D control, whereas Total Runout affords a 3D control. In both cases the control can be considered to be ‘form about a referenced datum axis’.
As an example, circular runout could be used to control the form an o-ring groove to ensure an even stretch on installation and at the same time control the surface of the groove’s deviation from a referenced datum axis, thereby ensuring that the groove stays concentric within functional limits, to a referenced datum axis.
5. Profile tolerancing is a very versatile aspect of GD&T and allows simple or complex forms to be controlled. The theoretically perfect geometry is defined by the Design Engineer and profile tolerances applied to specify the maximum deviation allowable from the perfect CAD geometry. Profile can be used to control form, orientation and location, or combinations of these characteristics to different values according to functional requirements. It can also be used to control the size of a feature. Both types of profile tolerancing always refer to the actual feature’s surface rather that of a derived axis or derived mid-plane of a feature.
Profile of a Line and Profile of a Surface both fall within this category.
Profile of a line affords a 2D line control whereas profile of a surface affords a 3D surface control. In both cases the tolerance zones can be manipulated using Theoretically Exact Dimensions, TEDs, and by referencing datums, or not.
As an example of this a complex profile could be fully located with respect to a datum reference frame quite generously whilst the orientation could be to a tighter value and the form to a tighter value still, according to functional requirements.
This is a good example of the flexibility that GD&T affords the designer. This flexibility is passed on to the manufacturer enabling the part to be as easy to produce as possible.
Rather than looking at GD&T as a quality requirement, it should be viewed with its real purpose and value in mind. If appropriately applied and correctly interpreted, it has a positive impact on costs and lead times. GD&T primarily controls closely the functional aspects of features so that the manufactured components do what they are intended to do, but secondly and very beneficially, allows larger deviations where the geometry is less important.
Consider a machined bracket with a functional bearing bore. Applying overly tight size and location limits to that feature may result in the scrapping of components, that are functionally acceptable. By applying maximum material condition, MMC, to the position tolerance, it allows more positional deviation as the hole size moves away from its maximum material condition. As the measured size of the hole increases, the tolerance on location relaxes. In practice, that allows the manufacturer to ‘open up’ a hole within its limit of size and legitimately bring the part back into specification.
Publications such as CMM Quarterly describe how better geometric tolerancing reduces rework loops and clarifies acceptance criteria at the CMM stage. The key focus should always be to set tolerance values to reflect true functional need.
This approach affords manufacturing more flexibility and salvage opportunities throughout the manufacturing process.
Nothing can be perfect, nor does it need to be; the designer defines exactly how far from perfection the manufactured component can be, whilst retaining functionality.
To maximize the benefit from the application of GD&T, it must be applied thoughtfully, effectively and appropriately by the Design Engineer and should satisfy the requirements as laid out in whichever GD&T Standard is being utilised, be it ASME, BS or another.
Embracing GD&T is best achieved by a gradual implementation, increasing in complexity and therefore benefit, over time as the understanding develops through experience. Implementation is most successful in teams that treat GD&T as a crucial cross-functional skill which spans the three key stakeholders, design, manufacturing and metrology (inspection).
A practical starting point is to pick one product or product family which poses clear functional challenges. For example, a gearbox or pump assembly with recurring assembly issues. Working as a joint team, the component drawings could be reviewed and updated considering input from the three stakeholders ,with a view to up-issuing a set of drawings with explicit functional datums, geometric controls and, where appropriate, modifiers such as MMC or LMC. The aim is for the components to be functionally acceptable, manufacturable and measurable.
In parallel (no pun intended!) to that pilot, targeted training for each role will almost certainly pay dividends.
For an organisation which adopts this structured, realistic approach, GD&T becomes more than a compliance exercise; it becomes a shared engineering language that assists organisations to reduce costs and lead-times and produce components that fit first time and function according to design intent, and also reduce the time that components spend ‘locked-up’ within in the time consuming concession process.
Whether you're introducing GD&T for the first time or looking to improve consistency across design, manufacturing, and inspection teams, targeted GD&T training will establish a common understanding across all departments and reduce costly misapplication and interpretation errors.
To learn more about our GD&T training courses, please get in touch to discuss your team's requirements.