There is a wide variation in sizes for engineering objects.
Some are very large (eg. Aero planes, rockets, etc) Some are vey small ( wrist
watch, MEMs components)
There is a need to reduce or enlarge while drawing the objects on paper. Some objects can be drawn to their actual size. The proportion by which the drawing of aan object is enlarged or reduced is called the scale of the drawing.
There is a need to reduce or enlarge while drawing the objects on paper. Some objects can be drawn to their actual size. The proportion by which the drawing of aan object is enlarged or reduced is called the scale of the drawing.
Definition
A scale is defined as the ratio of the linear dimensions of the object as represented in a drawing to the actual dimensions of the same.
A scale is defined as the ratio of the linear dimensions of the object as represented in a drawing to the actual dimensions of the same.
· Drawings
drawn with the same size as the objects are called full sized drawing.
· It is not
convenient, always, to draw drawings of the object to its actual size. e.g.
Buildings,
· Heavy
machines, Bridges, Watches, Electronic devices etc.
· Hence
scales are used to prepare drawing at
o Full size
o Reduced size
o Enlarged size
BIS Recommended Scales are shown in table 1.
Intermediate scales can be used in exceptional cases
where recommended scales can not be applied for functional reasons.
Engineers Scale : The relation between the dimension on the drawing and the actual dimension of the object is mentioned numerically (like 10 mm = 15 m).
Graphical Scale: Scale is drawn on
the drawing itself. This takes care of the shrinkage of the engineer’s scale
when the drawing becomes old.
Types of Graphical Scale :-
· Plain Scale
· Diagonal
Scale
· Vernier
Scale
· Comparative
scale
· Scale of
chords
Representative fraction (R.F.) :-
When a 1 cm long line in a drawing represents 1 meter length
of the object
Length of scale = RF x Maximum distance to be
represented
Plain scale :-
· A plain
scale is used to indicate the distance in a unit and its nest
subdivision.
· A plain
scale consists of a line divided into suitable number of equal units. The first
unit is subdivided into smaller parts.
· The zero
should be placed at the end of the 1st main unit.
· From the
zero mark, the units should be numbered to the right and the sub-divisions to
the left.
· The units
and the subdivisions should be labeled clearly.
· The R.F.
should be mentioned below the scale.
Construct a plain scale of RF = 1:4, to
show centimeters and long enough to measure up to 5 decimeters.
· R.F. = ¼
· Length of
the scale = R.F. × max. length = ¼ × 5 dm = 12.5 cm.
· Draw a line
12.5 cm long and divide it in to 5 equal divisions, each representing 1 dm.
· Mark 0 at
the end of the first division and 1, 2, 3 and 4 at the end of each subsequent
division to its right.
· Divide the
first division into 10 equal sub-divisions, each representing 1 cm.
· Mark cm to
the left of 0 as shown.
· Draw the
scale as a rectangle of small width (about 3 mm) instead of only a line.
· Draw the
division lines showing decimeters throughout the width of the scale.
· Draw thick
and dark horizontal lines in the middle of all alternate divisions and
sub-divisions.
· Below the
scale, print DECIMETERS on the right hand side, CENTIMERTERS on the left hand
side, and R.F. in the middle.
