Thursday, 7 November 2019

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

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
  • MinimumMaximumBasicStroke 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; 0.120.25 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.

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