Lines and Dimensioning
Lecture-03
Lines
Drafting is a graphic
language that uses lines, symbols, and text to describe how to manufacture or
construct a product. Line conventions provide a way to classify the content of
a drawing to enhance readability.
Engineering drawings are
prepared with the help of symbolic lines. In technical drawing and art, a line
is a fundamental element that connects two points and represents the simplest
form of graphical representation. It can define the shape, form, and edges of
objects and can communicate various visual concepts depending on its type,
thickness, and orientation.
Understanding lines is
crucial in drafting, design, and artistic practices because lines are the
building blocks of all drawings. They:
1. Convey Information:
Represent shapes, dimensions, and features of objects.
2. Define Boundaries:
Outline and separate different parts of a design.
3. Indicate Texture:
Create visual effects such as roughness or smoothness.
4. Communicate Design Intent:
Help engineers, architects, and designers share technical ideas.
5. Guide Attention: Direct the viewer's eye to focal points in a drawing or artwork.
Line conventions
Rules for Drafting Lines
·
Lines should be clear and drawn in black
or white (In CAD), depending on the background color.
·
The minimum distance between parallel
lines should ideally exceed 0.7 mm.
·
Dotted lines should ideally intersect at a
dot, while other types of lines should preferably intersect at a dash. (figure
below).
·
When two or more different types of lines
overlap or coincide, the priority for the drawing should be assigned in the
following order:
[1]
Visible outlines and edges
[2] Hidden
outlines and edges
[3] Cutting
planes
[4] Center
lines and lines of symmetry
[5] Centroidal
lines
[6] Projection
lines.
·
For instance, if a visible line overlaps
with a hidden line, only the visible line should be drawn, disregarding the
hidden line. Likewise, if a hidden line overlaps with a projection line, only
the hidden line should be drawn, ignoring the projection line.
DIMENSIONING
Dimensions are specified
on the drawing to define the size attributes, including length, width, height,
diameter, radius, angles, and the position of holes, slots, etc. These should
be directly indicated on the drawing to fully and clearly describe a component
in its final form.
Dimensioning Terminology
Dimension value
It is a numerical value
designated for the size, form, or placement of the feature. These values are
presented in a specific unit on drawings, along with the necessary details.
Dimension lines
These are thin continuous
lines indicating the dimension's direction and extent. Their distance from the
outlines should be 8 to 10 mm. Ideally the dimension values lie close to the center
of the dimension lines.
Projection lines
These are thin,
continuous lines that extend a little (2 to 3 mm) beyond the dimension lines
and should be drawn at a right angle to the feature being measured. Projection
and dimension lines should not cross each other unless it's essential. In some
instances, projection lines can be angled, but they must stay parallel to each
other.
Fig: Dimensioning terminology
Fig: Projection lines drawn obliquely
Leaders or pointer lines
These lines indicate a
feature and notes, represented by thin continuous lines. One end of the leader
terminates either in an arrowhead or a dot. The other end of the leader is
terminated in a horizontal line at the bottom level of the first or the last letter
of the note. Notes and figures are positioned above the extended dimension
lines. Leaders should not be angled at less than 30°, and they should not draw
parallel to nearby dimensions or projection lines to prevent confusion. Leaders
must never be vertical, horizontal, curved, or drawn freehand. They are
typically drawn at convenient angles of 30°, 45°, or 60°. It's best to avoid
using excessively long leaders.
Fig. Leader line (a) Correct method (b) Inclination
less than 30° is not permitted
Arrowheads
Arrowheads are commonly
used to indicate the ends of dimension lines in technical drawings. They come
in various styles, including open angles ranging from 30° to 90°, closed but
unfilled, or closed and filled, as shown in Fig. The closed and filled arrowheads,
which are favored for engineering drawings, typically have a length that is
about three times their width or depth, as depicted in Fig. For smaller
drawings, their length is generally around 3 mm, while for larger ones, it
ranges from 4 to 5 mm. In cases where space is tight and arrowheads cannot fit,
alternatives such as oblique strokes (Fig. below) or points (Fig. below) can be
utilized instead.
Fig: Arrowheads (a)
Various types (b) Closed filled type (c) Oblique stroke (d) Point
PLACEMENT OF DIMENSIONS
Aligned System
Linear dimensioning
All
dimension values are placed above the dimension lines, as shown in Fig.
These values can be read from either the bottom or the right-hand edges of the
drawing sheet. Figure emphasizes the recommended orientation for writing
dimension values on inclined dimension lines. Whenever possible, dimension
lines should be kept out of the 30º zone, which is marked by the hatched area
in Fig.
Fig. Aligned system for linear dimensioning
Angular dimensioning
Angular
dimensions and their deviations are dimensioned in the same way as linear
dimensions. Figure illustrates the preferred direction for writing the
dimension values. In some instances, dimension values can be written
horizontally, as depicted in Fig. if it enhances clarity.
Fig. Aligned system for angular dimensioning
Unidirectional System
Linear dimensioning
All
dimension values are oriented upright as illustrated in Fig. allowing them to
be read from the bottom edge of the drawing sheet. To insert a dimension value,
the dimension line is interrupted at the center. Figure shows that there are no
limitations on writing dimension values for inclined dimension lines. This
approach is beneficial for large drawings where reading dimensions from the
right-hand side can be challenging.
Fig. Unidirectional system for linear dimensioning
Angular dimensioning
Angular
dimensions and their deviations are dimensioned in the same way as linear
dimensions. Figure illustrates the proper orientation and method for writing
dimension values.
Fig. Unidirectional system for angular dimensioning
ARRANGEMENT OF DIMENSIONS
1.
Continuous or Chain Dimensioning
In
chain dimensioning, dimensions are organized so that the arrowhead of one
dimension connects directly to the arrowhead of the next, as shown in Figure
below. The overall dimension is placed outside the smaller individual
dimensions. This approach should only be used when the potential accumulation
of tolerances does not affect the functional requirements of the part. A
tolerance indicates the precision level needed in the manufacturing of the
product.
Fig. Continuous or chain dimensioning
2.
Dimensioning from a Common Feature
In
this scenario, several dimensions are measured in the same direction from a
shared feature. Clearly, all these dimensions align along a common extension
line. This arrangement is useful when dimensions need to be established from a
specific datum surface.
2.1 Progressive or parallel dimensioning
In
parallel dimensioning, the dimension lines are set up parallel to one another,
with the smallest dimension nearest to the outline. The next smallest dimension
is placed further away, followed by additional dimensions in order, as shown in
Figure below. To enhance clarity when multiple parallel dimensions are present,
they should be staggered.
Fig: Progressive or
parallel dimensioning
2.2 Superimposed running dimensioning
This is a simplified form of parallel
dimensioning, used in situations where space is limited and legibility is not
an issue. All dimensions start from a common origin, marked by a small circle
about 3 mm in diameter, and end with arrowheads indicating where each dimension
concludes. The dimension values are rotated 90° and aligned with the projection
line, as illustrated in Figure below, or positioned above the dimension line
close to the arrowhead.
Fig.
Superimposed running dimensioning
3.
Combined dimensioning
This combined dimensioning is
achieved by using chain dimensioning, parallel dimensioning, and superimposed
running dimensioning all together in one drawing, as illustrated in Figure
below.
Fig.
Combined dimensioning
4.
Coordinate dimensioning
Using
a coordinate table for dimensioning makes the drawing process easier,
especially for parts with many dimensions, since other methods can lead to a
messy look. Figure shows a plate with several holes, demonstrating that
coordinate dimensioning is the best choice, as other styles would overcrowd the
drawing. It's important to clearly mark the origin (X and Y) for the part, and
all features, like the holes, should be numbered. The coordinate table
containing the dimensional information should be placed near the title block.
Fig.
Coordinates dimensioning
SYMBOLS AND NOTES FOR
DIMENSIONING
Circle
A
circle represents circular features like cylinders, holes, or series of holes
in a component, typically specified by its diameter. The dimension value should
be preceded by a symbol 'Ø', and the leader should be a radial line. Dimension
lines should be horizontal or vertical. The size of the circle and available
space dictate the method chosen. The dimension for diameter should be placed on
the most appropriate view for clarity.
Fig.
Methods of diameter dimensioning
Radius
Radius is a curved surface defined by
arcs like fillets and rounds. Its dimension value should be preceded by a
letter 'R' and specified by a leader, a radial line with one arrowhead. The
arrowhead must touch the arc contour. Alternative methods of radius
dimensioning are shown in Fig. The center of the arc should be denoted by a dot
or small cross. If space is limited, the radius's size can be determined from
other dimensioning.
Fig.
Methods of radius dimensioning
Angle
Angular
dimensions are given when a surface's outline is at an angle to horizontal,
vertical, or other radial outlines. To dimension an angle, a curved dimension
line is drawn, forming a circular arc with a center at the vertex. The radius
depends on the space required for dimension vales. Dimension values are
expressed in degrees, minutes, and seconds. Alternative methods of angle
dimensioning are also available.
Fig:
Methods of angle dimensioning
Curved surface
Curved
surfaces cannot be defined by arcs, and their contours are dimensioned using
parallel dimensioning and coordinate dimensioning, as illustrated in Figure
below.
Fig:
Curved surface dimensioning using parallel dimensioning
Fig:
Curved surface dimensioning using coordinate dimensioning
Sphere
Following
figure outlines the method for dimensioning a spherical part, specifying the
dimension value of spherical diameter by a symbol 'SØ', and the dimension value
of spherical radius by a letter 'SR'. The leader, a radial line with one
arrowhead, should be placed horizontally or vertically, and the arrowhead
should touch the arc contour.
Fig:
Dimensioning spheres
Square and hexagonal Cross section
Squares
are machined at the shaft end for turning with a spanner. The dimensioning
method for a square cross section is shown in Figure, preceded by a symbol '□'
and two diagonal lines to indicate the visible flat surface. For a hexagonal
cross section, the dimension value is preceded by a word 'HEX'. The distance
between flat faces is given a dimensional value, but the actual dimension value
represents the length of the hexagonal side. Two continuous diagonal lines are
added to indicate visible flat surfaces.
Fig:
Dimensioning of Square
cross section and Hexagonal cross section
RULES OF DIMENSIONING
[1]
The size of dimensions needs to be clear
letting people understand them in just one way. Numbers and letters should be
large enough to read. This makes sure
everyone can see and understand the dimensions without trouble.
[2]
Typically, a circle is specified using its
diameter, while an arc is defined by its radius. Center lines should not carry
over from one view to another.
[3]
Projection lines are typically drawn at a
right angle to the feature being dimensioned, but they can also be drawn at an
angle, provided they remain parallel to one another.
[4]
Dimension values should ideally be
positioned near the center of the dimension line. If space constraints make
this impossible, they can be placed above the extended part of the dimension
line beyond the arrowheads, preferably on the right-hand side.
Fig: Correct Placing dimension 5 and
10
[5]
Whenever possible, dimensions should be
placed outside the views. If necessary, they may be positioned within the view,
as illustrated in Figure below. However, dimensions should only be placed
within a view if it enhances the clarity of the drawing.
Fig:
Placing dimension
[6]
The lines in a drawing should never act as
dimension lines or intersect with them. Dimension lines need to be consistently
spaced throughout the drawing, placed 8 to 10 mm away from the object's edge
and 6 to 10 mm apart from each other.
Fig: Placing dimension
[7] When there are multiple parallel dimensions, they should be arranged in a staggered manner.
[8]
Dimensions should be added to the view
that best illustrates the corresponding features.
[9]
Dimensions provided in one view do not
need to be repeated in another, unless necessary for identification, clarity,
or both.
Fig. Placing dimension
[10]
Dimensions should be added to the view
that best represents the contour shape. They should reference visible outlines
instead of hidden lines. Dimensions should originate from a baseline,
centerline of a hole, cylindrical parts, or finished surfaces—elements that can
be easily identified and are aligned with design requirements and relationships
to other components, as illustrated in Figure below.
Fig. Placing dimension
[11]
An axis or contour line shouldn't be used
as a dimension line; however, it can serve as a projection line.
Fig. Placing dimension
[12]
Whenever possible, the intersection of
dimension lines should be prevented. However, if two-dimension lines must
intersect, they should remain unbroken.
[13]
When multiple dimensions are placed on the
same side of the drawing, position the shortest dimension closest to the
component. This helps prevent dimension lines from crossing projection lines.
If their intersection cannot be avoided, neither of the lines should be shown
with a break.
Fig: Intersection of projection and
dimension lines is unavoidable
[14]
Overall dimensions should be placed
outside of the intermediate dimensions. If an overall dimension is provided,
one of the intermediate dimensions becomes unnecessary and should not be
included.
Fig.
Dimension placing
[15]
If there is enough space for the arrowhead
termination, it should be placed within the dimension lines' limits. If space
is limited, the arrowhead termination can be shown outside the intended
dimension line limits. However, if the space is too small for an arrowhead, it
may be replaced with an oblique stroke or a dot.
Fig.
Methods for arrowhead termination
[16]
Whenever possible, all dimensions in a
single drawing should be represented using a single unit of measurement. If a
drawing uses different unit a footnote is added in a prominent location to
clarify the unit of measurement.
[17]
When a dimension line cannot be fully
extended to its usual endpoint, the open end should be marked with a double
arrowhead.
Fig. Free end is terminated with double arrowheads