GD&T (Geometric Dimensioning and Tolerancing): A language for communicating

Presentation

ABCD of Engineering Drawing

It is not possible to build a part that exactly planed but its fitment can match it. The engineering drawing is the controlling document that ensures the manufacturability of part. It creates with GD&T and controlled precisely so that machinists and quality engineers will use, print dimensions, and drawing notes to develop a manufacturing process and inspection methodology. It will construct high-precision components and matching the designer’s original vision.

Drawing is a graphical and GD&T is a symbolic language that communicates ideas and information from one engineer to another.

Levels of Design

Three levels of design are considered in engineering design, which are as follows:

System Design: Design of a system which fulfill the specific function and purpose.

Parameters Design: Mechanical parameters, electrical parameters, thermal parameters, quantity parameters designing... of a system.

Tolerance Design: Design for tolerances for fitment of assembly.

Specification and Tolerance

• 10 ± 0.5: Specification is 10 and tolerance is 1.

• Part to Part variation is control by Size tolerance

• Within Part variation is control by Geometric Tolerance (Shape)

• Size Tolerance > Geometric (Shape) Tolerance e.g. ±1 > ± 1/32 (0.03)

Tolerance: Allowance for specific variation

Size tolerance is independent tolerance while Geometric tolerance controlled by its Feature Control Frame (FCF).

14 GD&T characteristics in 5 categories = 14.5

1. FORM = (4)

1.     Flatness, 

2.     Straightness 

3.     Circularity 

4.     Cylindricity

2. ORIENTATION = (3) 

1.     Perpendicularity 

2.     Parallelism  

3.     Angularity

3. LOCATION = (3) 

1.     Symmetry 

2.     Position

3.     Concentricity

4. RUNOUT = (2) 

1.     Circular run-out 

2.     Total run-out

5. PROFILE = (2) 

1.     Profile of a line

2.     Profile of a surface

8/4/2 Rule for Datums : 8Yes / 4No / 2Yes or No

(Orientation+Location +Runout) / Form / Profile

Symbols except for the Form tolerances (straightness, flatness, circularity and cylindricity) can use datums.

Basic Rules of Drawing

• Dimensions are measured at 68°F (degree fahrenheit) or 20°C in mechanical engineering system design.

               (68°F − 32) × 5/9 = 20°C

• Minimum, Maximum, Basic, Stroke and Reference dimensions never have any tolerances limit. These dimensions are free from tolerances.

• Dimensions shall have only one interpretation in engineering drawing. It never gives you conflict in between the interpretation and understanding of drawing.

• Reference dimensions should be kept as minimum value.

• Centerlines and featurelines are at right angle and angle is not mentioned in drawing.

• No zero allowed before decimal and digits must be equal after decimal in Inch unit system. For example; .12, .25 and .50 . It should not be 0.12, 0.25 and .5 in this case.

• Zero is must before decimal and no extra zero allowed after decimal in MM unit system. For example; 0.12, 0.25 and 0.5 It should not be .12, .25 and .50

• Primary datum control the Orientation of the feature in the drawing.

• All associates dimensions are basic dimensions (tolerance free) in profile tolerance.