Geometric Dimensioning and Tolerancing (GD&T) - Introduction

History Of GD & T -

The GD & T has firstly introduced the concept of 'Position" or "True Position" who's named is Mr. Stanley Parker, located in Great Britain in 1938 during world war-2. All you know in the wartime deadlines are critical and Mr. Stanley Parker in the situation where some torpedo parts to be inspected according to the traditional tolerance was rejected. 

He found the similarity to the fact that traditional X-Y tolerances result in the square zone, but part outside the square may actually be good, on this condition they are within a circle hold within the square's corners

Tolerance zone form by X-Y method

As you see in the above fig, we got the logic here, 

The X-Y method clearly allows only if the four corners of the square zone were functional. In most cases, a circle area would be just functional. And you think that how many parts will be rejected unnecessarily instead of it is avoidable. 

During that time the GD & T is growing till now. After that, the idea of Mr. Parker 'Position', some other GD & T concepts introduce such as straightness, flatness, circularity, cylindricity, parallelism, perpendicularly, angularity, concentricity, symmetry, circular runout, total runout, profile of a circle and profile of a line. After that, GD & T became standardized by the military and gradually more popular in the commercial industries. Many companies use GD & T  effectively. 

What is GD & T?

It's full form is Geometrical Dimensioning and Tolerancing.

The design model is ideal for the part to be manufacture. The physical part never matches exactly to the design model due to the imperfection during manufacturing or error during an inspection. The important factor of the design model is to specify the amount of the part feature deviates from their theoretically exact geometry.

GD & T is the symbolic language used in the engineering drawings to define the allowable deviation of the feature geometry.

The production can use this symbolic language for understanding the design and what the designer wants to say what they mean. In this symbolic language consist of symbols, dimensions, rules, tolerance which is used for describing the drawing in very few words or symbols.

Why is GD & T Important?

The designer designs the model according to the manufacturing approach and when the part is actually in manufacturing it is difficult to maintain the exact match to the ideal design model, at this point the question arises how could we manufacture the part which functionally not failed to the end-use and how the part functionally effective.

One point also important that because of the GD & T, we get the idea that part should be manufacture in that that tolerance and accordingly the quality engineers check the dimensions and quality. Because of this company expense and rejection rate will be reduced.

In all, fourteen geometric characteristics arc required to be discussed for their indication and interpretation, inspection, and verification as well as ag for their relevance. However, out of these fourteen characteristics, one (Position) has been discussed separately in the chapter "Coordinate vis positional dimensioning and tolerancing". Remaining thirteen are listed as follows: 

A: These are concerned regarding the Basic form of some feature of an object. They are individual characteristics and do not require any datum for their definition.

1. Straightness 

2. Flatness

3. Circularity. 

4. Cylindricity 

B: These are concerned regarding the Orientation of some feature of an object These characteristics require one datum for their definition.  

1. Parallelism 

2. Perpendicularity 

3 Angularity 

C: These concerned regarding the Location of some feature and require one or more datums for their definitions. 

1. Concentricity

2. Symmetry

D: There is concerned regarding the functional effect due to a lack of concentricity. One or more datums are required for their definition.

1. Circular runout

2. Total runout

E: There are concerned regarding the Profile of some features and may/may not require a datum for their definition.

1. Profile of a surface

2. Profile of a line

          The characteristic "Position" (which has been discussed separately), if listed here, would have been in group C, since it is concerned regarding the location of a feature. Now, these thirteen characteristics are discussed here,

To Read More ... Straightness