Geometric tolerance and dimensioning, or GD&T symbols, is the method of allocating dimensions and tolerances to the part’s components to make it viable to manufacture, monitor, and assemble the pieces.
This allows to attach specific dimensions and tolerances and via a sequence of letters and symbols describing different relationships and characteristics of the part.
When specifying geometric tolerances, such as in Solidworks, these symbols are used to GD&T tolerance specifications.
This system is commonly used in industries such as manufacturing to produce parts and components with a high degree of precision.
In most situations, including in SOLIDWORKS, GD&T symbols follow the American Society of Mechanical Engineers (ASME).
The ASME changes the GD&T guidelines on an annual basis, cited for specifically identifying sections and assemblies, but how does this function in practice?
If you’ve ever been in a situation where GD&T is part of your daily workflow, you’ve probably come across how complicated and time-consuming this can be.
Below, we offer an intro about how to optimize this method using SOLIDWORKS and glance at the available resources to help us achieve this.
How does SolidWorks handle GD&T?
SolidWorks has several methods to tackle geometric dimensioning and tolerance. There is a complete range of “manual” drawing resources that will enable you to incorporate any GD&T symbols or standards you want. Concerning these, SolidWorks has a DimXpert that simplifies a lot of our GD&T.
SolidWorks is a professionally configured software for modeling and detailing, and therefore has excellent toolsets for sketches with the correct implementation of GD&T rules.
How GD&T Works
Engineering drawings would display the dimensions of all the aspects of a product.
A tolerance value must be defined with a min and max reasonable limit in addition to the measurements. Tolerance is the variation between the minimal and the highest limits. E.g., if we had a table that we would consider with a height between 750 mm and 780 mm, the tolerance will be 30 mm.
However, the table resistance assumes that we can consider a table with a height of 750 mm on one side and a height of 780 mm on the other, or a wave surface with a difference of 30 mm. To better tolerate the product, we need to have a symbol that expresses a plain top surface’s design purpose. In addition to the overall height resistance, we need to have an extra flatness tolerance.
Similarly, if the cylinder is slightly bent during the production process, a cylinder with a toleranced diameter would not fit into the opening. It also requires a straightness monitor, which will be impossible to communicate with conventional plus-minus tolerances. Or a tube must be seamlessly matched to a complicated surface that is welded to require surface profile power.
GD&T set up a library of symbols to showcase such design purposes that we’ll discuss below.
Tolerance involves determining only the right combinations for all particular design elements to optimize the acceptance of the component within the production process’s constraints and based on the item’s graphic and functional intent.
There are many International Tolerance (IT) grades in the metric system that can also define tolerances through symbols. For reference, the 40H11 symbol means a 40 mm diameter hole with a loose running fit. The maker would just need to look up the simple table for the gap features to decide the precise tolerance value.
Standards refer to engineers and designers, and quality inspectors by telling them how to determine dimensions and tolerances. The use of specialized instruments such as digital height gauges, surface plates, micrometers and calipers, dial indicators, and a coordinate measuring machine (CMM) is essential for tolerancing practice.
GD&T Tolerancing Guidelines
The engineering drawing must express the product clearly without the inclusion of excessive ambiguity or constraints. It is useful to consider the established principles:
- The transparency of the drawing is the most crucial, perhaps more important than its precision and completeness. To increase visibility, draw measurements and tolerances beyond the boundary of the part, add them to visible lines in genuine profiles, use a one-way reading path, and express the function of the portion, group, and stagger measurements, then use white space.
- Often design with the least tolerable tolerance possible to hold costs down.
- Use the available tolerance array at the base of the drawing for all dimensions of the component. The general tolerance would then replace the unique closer or looser tolerances.
- Tolerance functionalities and their interrelationships first, then pass on to the rest of the element.
- Whenever practicable, leave GD&T to the manufacturing expertise and not identify the production processes in the engineering drawing.
- As it is implied, do not assign a 90-degree angle.
- Dimensions and tolerances are accurate at 20 °C/101.3 kPa until otherwise specified.
Solidworks Geometric Tolerancing Symbols
GD&T is a function-based system, with each feature defined by separate controls. These tolerating symbols fell into five groups:
- Form controls determine the shape of a feature, including:
- Straightness is split into straightness aspect line and straightness axis.
- Flatness means straightness in various dimensions, measured between the highest and the lowest points of the surface.
- Circularity or roundness can be defined as straightness bent into a circle.
- Cylindricity is simply flatness twisted into a barrel. It requires straightness, roundness, and taper, making it expensive to check.
- Profile controls define the three-dimensional tolerance region across the surface:
- Line Profile contrasts a two-dimensional cross-section with an optimal shape. The tolerance zone shall be identified by two offset curves unless otherwise specified.
- Surface Profile produces two offset surfaces between which the surface of the feature would slip. This is a dynamic regulation generally calculated with a CMM.
- Orientation controls concern measurements that range from angle to angle, including:
- Angularity is the flatness at the angle to the date and is also calculated by two reference planes which separate the tolerance value.
- Perpendicularity means flatness at 90 degrees to the date. It defines two ideal planes that must lie between the features of the plane.
- Parallelism stands for straightness at a distance. Axis parallelism can be defined by defining a cylindrical tolerance zone by putting the diameter symbol in front of the tolerance value.
- Location controls describe function positions using linear dimensions:
- Position is the place of the functions relative to each other or the dates and is the most widely used control.
- Concentricity matches the position of a function axis to a data axis.
- Symmetry means that non-cylindrical sections are identical around the data plane. This is a dynamic regulation generally calculated with a CMM.
- Runout controls describe the sum by which a specific function can differ for the dates:
- Circular Runout is used when several separate defects, such as ball-bearing mounted sections, need to be compensated for. During the inspection, the component is rotated on the spindle to test the difference or ‘wobble’ along the rotation axis.
- Total runout is calculated at several surface locations, representing the runout of a circular feature and the entire surface. This regulates straightness, profile, angularity, and all other difference.
Indeed, Geometric Dimensioning and Tolerance (GD&T) provides enormous advantages for engineers and designers working on complicated systems where proportions need to be closely controlled. GD&T symbols convey linear dimensions and design intentions, which help to express engineering design more directly to project stakeholders.
With just over a dozen icons, the control frame feature, and the datum feature, it is possible to enrich production drawings to guarantee that engineering fits stay compatible across component assemblies.
GD&T also encourages developers to learn about how their components can be optimally tolerated in the desired manufacturing process, as different processing methods contribute to different characteristic variations.
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