Dimension, Arrowheads, Extension Line- Leture No-4 - Engineering Drawing

Subject: Engineering Drawing-II 

Subject Code: ME 4202

Topic: Dimension, Arrowheads, Extension Line- Lecture No 4

Teacher Name: MD. Rezaul Karim (MRK)

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Assignment Name :  Draw a regular Pentagon inscribing a circle of radius 30 mm 
Assignment Name:Orthography 
Dimension

•Engineering drawing without dimensioning is meaningless. To manufacture a part , dimensioning plays a significant role.

•Whatever may be the scale of the part, the actual size dimensions have to be always mentioned on the part.

•Dimension are indicated on the drawings by arrowheads, extension lines, dimension lines, leaders, symbols etc. in order to define the geometric characteristics such as lengths, diameters, angles etc.

•The dimensions must be clear, concise and always allow the single interpretation

 Arrowheads

•The length of the arrowhead may vary depending on the size of the drawing

•The approximate length of arrowhead may be 3 mm

•The approximate ratio of the length to width of the arrowhead is 3:1

•The arrowhead must touch the line. It must not be either away from

the line or cross the line

 

Extension line

•A gap of 1 mm has to be kept in between the extension line and  the visible line

•An extension line should extend about 3 mm from the outermost dimension line

•Extension line are usually drawn perpendicular to dimension lines 

Dimension line

Dimension line should be approximately 10 mm away from the visible line. The spacing between the consecutive parallel dimension lines may also be considered as 10 mm

Dimension lines are broken near the middle to allow space for dimension

Dimension lines should usually be placed outside the view unless it become necessary

Leaders 

•A leader should always be inclined at an angle of 60° preferably and 45° occasionally with a 3 mm horizontal bar

•A leader is either terminated by an arrowhead on a line or a small dot of about 1.5 mm diameter within the outline of the part

•All notes and dimensions in a leader have to be provided in the horizontal direction

•Leaders should not cross each other, however, they may be drawn parallel to each other 

 

Direction of Dimensions 

•Direction of dimensions is done in 2 systems: unidirectional or aligned

•Unidirectional system is often called ‘ Horizontal System’

•In unidirectional system all dimensions are  oriented to be read from the bottom of the drawing

•In the aligned system the dimensions are oriented to be read from the bottom or right side of the drawing

•The unidirectional system is preferred to aligned system