Tolerance of basic knowledge of CNC machining: GD&T

Tolerance of basic knowledge of CNC machining: GD&T

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As we all know, the process of R&D design, process designation, production and processing of parts involves various fields. In each field, each engineer has his own understanding of product requirements and standards, and in order to communicate more smoothly in the product development process, a unique and authoritative (with legal effect) standard is needed to constrain everyone. GD&T Thus was born.

So what is GD&T? How should we use it? Should tighter tolerances always be pursued? We will answer you in the following content!

What is GD&T?

GD&T is a symbolic engineering language widely used in the world, which clearly and precisely defines the size, shape, direction, and position of the geometric product function, so that the product can obtain a looser control as much as possible on the premise of meeting its functional needs. Tolerance requirements can effectively reduce manufacturing costs; the continuous improvement of GD&T popularization and application can effectively reduce or eliminate the understanding of drawings of the same product in different countries, or the same country but different departments (R&D, process, manufacturing, quality inspection) disagreement;
GD&T originated in the 1940s, and has been widely used in many industries such as national defense, aerospace, automobile, construction machinery, electronic appliances, medical equipment, etc., and has a decisive influence on a global scale.

Application of GD&T

In order to understand the qualitative application of GD&T in a production environment, it is necessary to understand the design model. A design model consists of a set of geometric shapes called features. In the finished design, the geometry of the features must be defined, and deviations from their geometric properties (such as position, orientation, size, and shape) must be constrained. There are two common methods to constrain the deviation of the geometry of features in the design model. GD&T can be applied to each feature depending on the function of the part or how the part is manufactured. It is generally considered a best practice for GD&T applications based on part functionality.

Step 1: Apply GD&T Using an Established Baseline Reference

The first step in applying GD&T to a design model is to establish a datum reference frame (DRF). The first datum coordinate system established is called the dominant datum coordinate system. In GD&T’s functional approach, the surfaces that orient and position a part in its assembly are often used as datum features for establishing the primary DRF. There are additional considerations for datum feature selection:
Manufacturing considerations: Orient and position the surface of the part in the machining fixture as a datum feature.
Inspection Considerations: Select the surfaces that orient and position the part as datum features during inspection.

Step 2: Apply GD&T Constraint Feature Attributes

Once the master DRF has been established, GD&T can be used to constrain the geometric properties of each feature in the model relative to the function-based DRF. In complex parts, multiple DRFs may need to be established, but all features in the final part must be directly or indirectly constrained to the main DRF. There are tools to define features on a part, apply GD&T to those features, and then evaluate each feature and the conditional GD&T applied on the feature to determine whether all geometric properties of the feature are fully constrained.

Geometric feature symbols are combined in a feature control frame along with tolerance values, modification symbols, and datum reference frames. A feature control framework, sometimes referred to as a GD&T callout, defines allowable feature deviations based on feature function and design intent.

Should tighter tolerances be pursued?

Often, too tight tolerances can require a part to undergo a secondary grinding or EDM operation to complete, unnecessarily increasing cost and lead time. But tolerances that are “too loose” or inconsistent with the tolerances of the mating parts can prevent assembly, require rework, and at worst render the finished product unusable.
The best way to apply tolerances is to apply tight or geometric tolerances only to critical areas when design criteria need to be met to minimize cost.

Hope the above content is helpful to you!

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