What is GD&T!!!!!!!!!!!!!

Drawing With GD&T Symbol

It is very difficult for an engineer to design, for a supervisor to instruct or for an operator to machine a part as required to fulfil it's functionality just by giving the verbal instructions or by giving drawing with some base dimensions. We need a globally accepted communication language in which we can associate some sort of symbols to the dimensions which defines the geometry and mating conditions of the parts. That global communication language is GD&T.


GD&T stands for Geometric Dimensioning And Tolerancing.



History of GD&T.




Geometric Dimensioning And Tolerancing(GD&T) symbols have been in use since last 100 years.It was especially important during the second world war when the European and American countries were desperate to build heavy ships, air crafts and ground vehicles. The automotive industry has also benefited from GD&T. The computer industry have have used GD&T extensively to increase their yields of high-volume and low-margin hard disc drives. 
Insipte of all these, GD&T was not effectively established and dominated until later in the twentieth century. ANSI y14.5M was the turning point which was published by American National Standards Institute in the year 1982.
Now-a-days the automotive industry is making full use of the GD&T application due the growing demand for vehicles.











GD&T TERMINOLOGY:
Before getting into GD&T, let us understand the terms used in GD&T

1.Dimension:

Basic Dimensions

                   It is a nominal value expressed in appropriate units that defines a feature of size or orientation of size.
     Types Of Dimension:

     1.Basic Dimension:- A theoretical exact value which specifies the location, orientation and  position of a feature. This dimension is generally shown in a box.

Ref Dimension

     2.Reference Dimension:- A dimension which is shown for reference only. The referense dimension is shown in paranthesis. It is given for the information only.

2.Feature:

               It is a physical entity on a part of our interest. For example, if we put a hole on a part(may be for holding purpose), the hole is called a Feature.

3.Size:

          It is nothing but a maximum dimension of a part.
       Types Of Sizes:-
       1.Actual Size: It is the size achieved after manufacturing.
       2.Limits Of Size:These are the permitted variations.
       3.Basic Size:It is the size from where the deviation starts.
       4.Nominal Size:It is the dimension given for general identification.

Feature Of  Size

4.Feature Of Size:
                           It is a feature that defines the size on a part. In feature of size, surfaces or elements must be opposed.
In this given drawing, the dimensions 25.0, 35.0 and Dia10.0 are Feature Of Size because they defines the size of a block and a hole respectively, where as the dimensions 12.5 and 17.5 defines the position of the hole.

Fundamental Rules Of GD&T: 

1.   All dimensions must have a tolerance. Every feature on every manufactured part is subject to variation, therefore the limits of allowable variation must be defined. Positive and/or negative tolerance may be applied directly to the dimensions or applied from a general tolerance table or general note. For basic dimensions, geometric tolerances are indirectly applied in a related Feature Control Frame(FCF). The only exceptions are for the dimensions marked as Minimum, Maximum, Stock or Reference.
2.   Dimensioning and Tolerancing shall completely define the nominal geometry and allowable variation. Measurement and scaling of drawing is not allowed except in certain cases.
3.  Engineering drawing define the requirements of finished parts. Every dimension and tolerance required to define the finish part must be shown on drawing. Additional dimensions would be helpful, but are not required, They may be marked as reference.
4.   Dimensions should be applied to features and arranged in such a way that the function of the feature must be represented.
5.   If certain dimensions are required during manufacturing but are not required in the final geometry(due to shrinkage), they should be marked as non-mandatory.
6.   All dimensioning and tolerancing should be arranged for maximum readability and should be applied to visible lines(edges) only in true profiles but not to the hidden lines(edges).
7.   When the geometry is normally controlled by gage sizes or by codes(Ex-Stock material), the dimensions shall be included with gage or code number in parantheses following or below the dimension.

8.   Angle of 90 degrees are assumed when lines(including center lines) are shown at right angles.

9.   Unless explicity stated, all dimensions and tolerances are only valid when the item is in free state.

10. Dimenasions and tolerances apply for the full length, width and depth of a feature including form variation.