GD&T

DEFINITION:

An International language which is used in Engineering drawings to precisely portray a given part is called as Geometric Dimensioning and Tolerancing (GD&T).
It consist of a set of symbols, rules, definitions and conventions.
GD&T is a mathematical language which helps us in understanding the size, form, orientation and location of part features.

DESIGN PHILOSOPHY OF GD&T

A dimensioning philosophy which is supported by Geometric Tolerancing is called as “functional dimensioning”.
Functional dimensioning is a philosophy which defines a part based on its function in the final product.
If you go through the ASME Y14.5 you’ll notice that functional dimensioning philosophy is used a lot of times.
When designing a product, the designer should take into consideration other factors as well and not only the functional dimensioning philosophy.
Simultaneous Engineering is a process which is widely adopted by most of the companies when marketing, engineering, manufacturing, inspection, assembly, and service are included for better design of a product.
This process helps to form better products at a much cheaper rate.

GD&T BENEFITS

Communication Improvement

GD&T helps to communicate the intent of the part to the user and eliminate any guesswork or assumptions.

Provides better product design

GD&T usage helps the designer to convey what they want to say by abiding to functional dimensioning philosophy.

Increases production tolerance

Two ways in which the tolerances are enhanced due to GD&T are:

  1. It provides bonus or extra tolerance which helps in reducing the rejection parts and thereby reduces the production costs.
  2. Tolerances are given to the parts based on their functional requirements by implementing functional dimensioning philosophy. At times designers copy the existing tolerances which may be tighter or provide more freedom for manufacturing process that increases the chances of rejection.

PRINCIPLES

A part cannot be manufactured without any deviation from its nominal shape.
When these deviations are too large the function of the part gets hampered.
The production gets too expensive when reduction in deviations are to be done.
The main reason of reducing these deviations is the competition of companies associated with it.

Incomplete implementation of tolerance in a drawing results in:

  1. Questions from production planning engineer.
  2. Questions from manufacturing engineer.
  3. Questions from Inspection engineer.
  4. Reworking.
  5. Defects and damages.

Complete implementation of tolerance in drawings helps to create accurate and precise parts.
Mostly general tolerances should be applied as mentioning tolerances for all the features overcrowds the drawing which makes it hard to read.
When there is no ISO standard or National standard available a company standard should be used.

COMPARISON BETWEEN GD&T AND COORDINATED TOLERANCING

Cylindrical vs Square Tolerance Zone
Cylindrical vs Square Tolerance Zone
  1. The first major problem with coordinate tolerancing is that it has “square tolerance zones”.
  2. The second major problem of coordinate tolerancing is ” fixed size tolerance zone”.
  3. The third major problem of coordinate tolerancing is that it has “ambiguous instructions for inspection”.

GREAT MYTH OF GD&T

GD&T is been widely adopted by many companies but there is still a great myth associated with it.
The great myth of GD&T is related to the greater cost associated with it.
The myth is created mainly by 2 factors:
The first one is due to the fear of not knowing the methodology associated with it.
When a drawing containing GD&T is sent out for cost estimation (quotation) manufacturers tend to give more cost as they feel that it is difficult to manufacture the part considering the constraints the designer has applied.
GD&T gets the blame that the cost is increased due to it but in reality this methodology has provided more tolerance to the parts which helps in reducing the rejection parts.
The incapability of the drawing user to read the drawing led to the formation of this issue.
The second factor which fuels this myth is related to the poor design practices implemented.
At times due to lack of knowledge, the designer tends to provide unnecessary tolerances which is not achievable during manufacturing.
The designer should take care that good design practices should be followed.
But in this case also GD&T gets all the blame.

GD&T can save more money is the actual fact.
This can only be achieved if the drawing creating and reading capability of the designer and user is improved.

Before jumping onto the symbols of GD&T we must understand different geometrical deviations associated with a surface.

Size deviation

Size deviation is associated with the difference between actual and nominal size.
They are assessed over the complete geometrical part.
They are produced by the imperfections developed through the manufacturing process.

Form deviation

The deviation of a feature (line, surface or geometric element) from its nominal form is called as form deviation.
They are mainly formed by the errors in machine, fixtures and alignment of tool and workpiece.

Orientation deviation

This deviation is associated with a feature which deviates from its nominal form and orientation. This includes form deviation as well.

Locational deviation

When there is a deviation of a feature from its nominal location then that deviation is said to be locational deviation. Locational deviation includes form as well as orientational deviation.

Waviness

It is the periodic irregularities of a workpiece surface with more spacings as compared to roughness spacings.

Roughness

It consists of periodic or non periodic irregularities of a workpiece with small spacings as compared to waviness.

Surface discontinuity

It is mainly related to the cracks, pores or laps developed in the surface.

Edge deviations

Deviations in the edge zone of the workpiece is the edge deviation. The best example is the formation of the burr on the edges instead of producing sharp edges which was intended.

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