INTRODUCTION
It’s a system of dimensioning and tolerancing mechanical parts that is more powerful than traditional dimensioning and tolerancing which allows you to know with 100% certainty your parts will fit and function at the assembly level.
When we talk about GD and T there will be first come to our mind is Tolerance.
Basically, tolerance will divide in two parts,
1. Dimensional
2. Geometrical
Dimensional tolerance is the total amount by which a specified dimension is permitted to vary.
Means the allowable deviation from a standard. e.g., the range of variation permitted in maintaining a specified dimension in a machined item.
In GD and T, means geometrical dimensioning and tolerance.
Geometric dimension is a geometric characteristic the size of which is specified such as length, angle, location or centre of distance etc.
And Tolerance is the total permissible variation in its size, which is equal to the difference between the limits of size to allow an object to function correctly.
Symbol and its characteristics
Form -4 features
Profile -2 features
Orientation -3 features
Location -3 features
Run-out -2 features
These are the 14 geometric characteristic symbols. They are categorized with the type geometric control.
Let’s know about these symbols in details.
1st is the Form features,
1. Straightness
2. Flatness
3. Circularity
4. Cylindricity
These four form features are the individual means there will be not required any datum reference.
2nd is the Profile features
1. Profile of a line
2. Profile of surface
These two profile features are the individual or related features means there will be not compulsory any datum reference
3rd is the Orientation features
1. Angularity
2. Perpendicularity
3. Parallelism
4th is the Location features
1. Position
2. Concentricity
3. Symmetry
5th is the Run-out features
1. Circular run-out
2. Total run-out
These three form features orientation, location and run-out are the related features means there will be required datum reference.
Straightness
Surface straightness is a tolerance that controls the form of a line somewhere on the surface of the job.
Flatness
It is a very common symbol that reference how flat a surface is.
Circularity
The circularity symbol is used to describe how close an object should be to a true circle.
Cylindricity
The circularity symbol is used to describe how close an object should be to a true cylinder.
Profile of a line
Profile of a line describes a tolerance zone of a curved shape.
Profile of surface
It’s a 3D tolerance zone of advanced 3D surface.
Modifiers and its symbols
Modifiers are symbols which defines condition for the tolerance zone of a geometric tolerance.
MMC - Maximum material condition
LMC - Least material condition
Free state
Tangent plan
Projected tolerance zone
Diameter
Features control frame
Lets know about features control frame by taking an example
In this box 1st symbol indicate the geometric characteristics ,2nd is diameter symbol, 3rd is tolerance, 4th is modifier, 5th datum references A, B, C respectively primary secondary and tertiary.
In we try to understand this box in technically, then it will be read as the position of the features must be within a 0.02 diametrical tolerance zone at maximum material condition relative to datums A, B and C.
Benefits of G D and T system
GD and T is an international symbolic language, it will used in every country to define tolerance.
It comprises of symbols, rules and definitions for defining a geometric of mechanical parts.
Uniformly in drawings, better designs, no assumptions, large tolerances.
It is mostly used in manufacturing industries for dimensioning and tolerance.
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