Saturday, October 18, 2008

GD & T - Power of Feature Control Frames using SolidWorks

Feature Control Frame forms the heart of the Geometric Dimensioning and Tolerancing ( GD & T ) practice for engineers involved in creating, manufacturing and inspecting designs. In fact it is the greatest invention of engineering expressions in symbolic language that it is finding unilateral acceptance by the engineering community as a whole.

For the un-initiated, let us considere a hole dimensioned as follows:
Read from left to right, the Feature Control Frame states that "Position of the Axis of a pattern of 8 holes when produced within stated size limits, can be off-centre within a diametral tolerance zone of 0.50 when produced at Maximum Material Condition, when the part is located on Datum A as Primary, Datum Feature of Size B when produced at Maximum Material Condition as Secondary and Datum C as Tertiary references."

Feature Control Frames have a characteristic symbol in the first cell, a tolerance value with a zone descriptor and material modifier (if any) as the second cell followed by cells having Datum references ranging from 1 to 3 depending on the design specification that the tolerance definition is intended to convey. Number of datums depend and possible material modifiers (on datum features of sizes) depends on the intention and feature that is controlled.

Many a times, design intent and design specifications are wrongly used interchangeably while producing drawings. GD & T Drawing is intended to convey design specifications in an unambiguous manner and not the design intent. Sounds strange? Read again !

GD & T symbols, when used with care and purpose, have been proven to reduce costs and greatly improve quality while ensuring part interchangeability and protecting the parts' intended fit, form and functions.

SolidWorks helps designers achieve a great level of definition control with GD & T using DimXpert. Even Imported geometries can have the complete Dimensional schematic completed in no time with accurate representation of the design specifications.
Feature Control Frame Definition using DimXpert

User specified Datums with selected order of precedence alongside features of sizes can be defined for various levels of control based on size, form, orientation and location. Features defined using feature control frame are ascertained for completeness of definition. Features shown as green are complete in dimensional definition. If the feature selected is a part of a pattern of features, say a hole pattern, then a common reference frame with necessary number of features are provided automatically.

Using SolidWorks, this is possible even in the case of imported geometries as shown. This approach enhances accuracy and adequacy, when dimensioning a part with a huge history of features requiring dimensioning.

When a circular feature of size, such as Datum B shown in the Figure, is selected as a datum for another feature, the feature dimensioning is automatically defined in all aspects with appropriate Feature control frame (in this case Perpendicularity) in context to the datum feature used (in this case Datum A) in precedence. This approach, not only saves time, but also ensures dimensional completeness thereby preventing ambiguity in manufacturing and inspection downstream during part manufacture.

Hole depth is also indicated in the Feature Control Frame while defining size limits. Tolerances specified as based on default values specified by the user that can be modified based on cost and practical manufacturing considerations (including machine Cp and Cpk).

When multiple features refer to the same Datum, this is automatically recognized by SolidWorks and common Datum sequence is followed in all relevant Feature Control Frames, signifying a single setting during manufacture and inspection.

In summary, DimXpert inside SolidWorks reduces effort required to develop GD & T drawings while improving accuracy of representation and completeness in definition. This eliminates ambiguity, resulting in overall cost and time savings.

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