GD&T Applications and Interpretations Needed for Measurement

Production processes change because manufactured items often deviate from the original CAD model in size and dimensions. Engineers and manufacturers use a symbolic language known as GD&T, or Geometric Dimensioning and Tolerancing, to best regulate and convey these variances.

The permissible variation within the product assembly is communicated to production engineers and quality inspectors by GD and T, who also standardise the variation's measurement.

This article explains the GD&T method for streamlining conventional and digital manufacturing design-related communications.

How did GD&T Tolerance help to Overcome the Limitations?

Prior to the application of GD & T symbols, X-Y regions were used to specify manufacturing features. For instance, a mounting hole has to be drilled inside a specific X-Y area. However, the X-Y tolerancing wasn’t accurate and produced some failure components. 

Currently, the American Society of Mechanical Engineers (ASME Y14.5-2018) and ISO 1101-2017 are responsible for defining the GD&T standard. While other specifications address unique traits like surface roughness, texture, and screw threads, it primarily concerns the device's general shape.

How to Measure GD&T?

Various GD & T controls are used to regulate form tolerance. Let us see how GD&T is measured practically in the shop floor. The sorts of form tolerance symbols are as follows.

 

• Straightness

 

The degree of straightness tolerance determines how each feature is shaped. It is employed to regulate the degree of straightness of a line or curve element on a surface. On that surface, the straightness tolerance should be the same for each cross-section. There is no need for a datum for straightness tolerance.

It might be employed to ensure that the part is homogeneous over the surface. Also applicable to circular objects, straightness tolerance regulates the axis' straightness.

 

• Flatness

 

The tolerance range beyond which a surface shall not deviate is defined by flatness. It makes it clear how level the surface ought to be. 

A surface can frequently be controlled by its flatness without changing its dimension. Surface tolerance is always less than the surface's designated dimension.

Two parallel planes form the surface tolerance zone between which each component of the real surface should lie.

 

• Circularity

 

The circle's divergence from the ideal circle in any cross-section of a circular object is known as the circularity tolerance. Roundness is another name for circularity.

Any cross-section of a circular surface should have the same amount of circularity tolerance if indicated on the surface unless additional types of tolerances are added on top of the circularity tolerance.

 

• Cylindricity

 

The term cylindricity refers to how closely a cylinder's surface resembles an actual cylinder. It refers to the maximum deflection a cylindrical surface can have. 

Because the tolerance is for a 3D surface, cylindricity applies to the entire surface rather than just a single circular element, as in the case of circularity.

How to Integrate GD&T tolerancing in Solidworks?

After considering the requirement, execute: In DimXpert > Auto Dimension Scheme, indicate the datums and choose the Geometric option rather than Plus/Minus tolerancing.

Finally, the software automatically generates dimensions for features-of-size (FOS), such as bosses and holes. Choose "bilateral" or "limit" as the tolerance type for features where the plus and minus limits are not equal.

Before importing these tolerances into an engineering drawing, check the FeatureManager to see which planes are used in the "Annotations" folder. 

When importing the views from these planes into a drawing, ensure "Import annotations" and "DimXpert annotations" are checked. The drawing will be significantly more precise if a suitable section view is added.

GD&T Case Study 

A specific project aims to use injection molding to make 50,000 bottle caps. It needs an excellent tolerance specification to regulate how the caps feel and press against the bottles. To maintain a regular running fit, the bottle’s outer diameters should march the dimensions of the cap’s inner diameter.

The outer diameter of the thread on the bottle is 36.95 +/- 0.010 mm. Accordingly, the inner diameter of the cap ranges between 36.985 and 37.065 mm, with a mean value of 37.0 mm.

The cap also connects through a specified hole to an axle positioned below a flat surface. This enables the bottle to dangle underneath the surface of a storage cabinet and be opened with one hand. 

The axle is a typical OEM stainless steel part with a 4mm diameter and a tolerance of 0.13mm (0.005"). To force fit with a tolerance of between -0.0375 and 0.0125 mm for a secure connection. Here, the diameter of the hole is between 3.99 and 4.01 mm, resulting in a force fit for all axle diameters. 

As the range is so small, the dimension of the hole is specified as 3.85 mm and then drilled precisely to 4.00 mm, which regulates the concentricity of the two holes.

A datum was deployed to manage the dimensions correctly. Since matching the cap and bottleneck is crucial in this situation, the inner cylindrical surface of the cap was chosen as a datum.

Conclusion

With the use of GD&T, production engineers to machinists can enhance the production process. Enabling engineers to validate the design accuracy maximises production and saves money.

Enroll in our GD&T course to learn more about it. To learn the basics for free, log in to Skill-Lync’s Grow platform to unlock the GD&T course.