Scaling In engineering Drawing

Scales

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.

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.

·        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.

Types of Scale :-
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.

Diagonal Scale :-

·        Through Diagonal scale, measurements can be up to second decimal places (e.g.  4.35).

·        Are used to measure distances in a unit and its immediate two subdivisions; e.g. dm, cm & mm, or yard, foot & inch.

·        Diagonal scale can measure more accurately than the plain scale.

Diagonal scale…..Concept

·        At end B of line AB, draw a perpendicular.

·        Step-off ten equal divisions of any length along the perpendicular starting from B and ending at C.

·        Number the division points 9,8,7,…..1.

·        Join A with C.

·        Through the points 1, 2, 3, etc., draw lines parallel to AB and cutting AC at 1΄, 2΄, 3΄, etc.

·        Since the triangles are similar; 1΄1 = 0.1 AB, 2΄2 = 0.2AB, …. 9΄9 = 0.9AB.

·        Gives divisions of a given short line AB in multiples of 1/10 its length, e.g. 0.1AB, 0.2AB, 0.3AB, etc.

Construct a Diagonal scale of RF = 3:200  showing meters, decimeters and centimeters. The scale should measure up to 6 meters. Show a distance of 4.56 meters

·        Length of the scale  =  (3/200) x 6 m  = 9 cm

·        Draw a line AB = 9 cm . Divide it in to 6 equal parts.

·        Divide the first part A0 into 10 equal divisions.

·        At A draw a perpendicular and step-off along it 10 equal divisions, ending at D.

Diagonal Scale

·        Complete the rectangle ABCD.

·        Draw perpendiculars at meter-divisions i.e. 1, 2, 3, and 4.

·        Draw horizontal lines through the division points on AD. Join D with the end of the first division along A0 (i.e. 9).

·        Through the remaining points i.e. 8, 7, 6, … draw lines // to D9.

PQ = 4.56 meters