BNG 101 – Engineering Graphics
Slide Set 1 – Alphabet of Lines and
Precedence of Lines
An introductory point
Please read Pages 1-1 through 1-15 of the Planchard text on your own
(covers some introductory concepts that should be review)
Drawing Vocabulary
• Drawing Lines
• Lettering
ABC 
• Measurement Systems


mm
Inch
• Scale
We will focus primarily on the use of lines in engineering drawings
Alphabet of lines
• The lines used in drafting are often referred to as
the Alphabet of Lines
• Line types and conventions for mechanical
drawings are covered in ANSI Standard Y14.2M
• There are four different distinct thicknesses of
lines in orthographic projection:
–
–
–
–
Very Thick
Thick (0.5 – 0.6 mm)
Medium (0.35 – 0.45 mm)
Thin (0.3 mm)
won’t follow these
explicitly when
sketching in freehand
Alphabet of lines
• Types of lines in order of preference:
–
–
–
–
–
–
–
–
Visible (object/feature) lines
Hidden lines
Cutting plane lines
Centerlines
Break lines
Dimension lines
Extension lines/lead lines
Section lines/crosshatch lines
Alphabet of lines
• Visible lines / Feature
lines: Visible lines
(object lines) are
continuous lines used to
represent the visible
edges and contours
(features) of an object
• Since visible lines are the
most important lines,
they must stand out from
all other secondary lines
on the drawing
• Line weight – thick
Alphabet of lines
• Hidden lines: Hidden
lines are short-narrow
dashed lines. They
represent the hidden
features of an object.
• Hidden lines should
always begin and end
with a dash, except when
a dash would form a
continuation of a visible
line.
• Line weight – medium thick
Alphabet of lines
• Extension lines:
Extension lines are used to
indicate the termination of
a dimension
• An extension line must not
touch the feature from
which it extends, but
should start approximately
(2 – 3 mm) from the
feature being dimensioned
• They can also show the
extension of a surface to a
theoretical intersection
point
Alphabet of lines
• Extension lines:
When extension lines
cross other extension
lines, hidden lines,
leader lines, or
centerlines, they are
usually not broken.
• For circular features,
centerlines are used as
dimension lines
• Extension lines should
not cross dimension
lines
Alphabet of lines
• Do NOT use object lines as extension lines!
http://prin617.blogspot.com/2007/08/dimensioning-and-tolorancing.html
Alphabet of lines
• Dimension lines:
Dimension lines are used to
show the extent and the
direction of dimensions.
• All dimension lines terminate
with an arrowhead on
mechanical engineering
drawings; a slash, or a dot in
architecture.
• If possible, dimension lines
are aligned and grouped for
uniform appearance.
Note: In SolidWorks, inserted dimensions in the drawing are displayed in
gray. Imported dimensions from the part are displayed in black.
Dimension lines in architecture
Alphabet of lines
• Leader lines: A leader
line is a continuous straight
line that extends at an
angle from a note, a
dimension, or other
reference to a feature
• An arrowhead touches the
feature at that end of the
leader
• At the note end, a
horizontal bar (6 mm) long
terminates the leader
approximately (3 mm)
away from mid-height of
the note’s lettering, either
at the beginning or end of
the first line
note
note
Alphabet of lines
• Leaders should not be bent to
underline the note or dimension.
Unless unavoidable, leaders
should not be bent in any way
except to form the horizontal
terminating bar at the note end
of the leader.
• Leaders usually do not cross.
Leaders or extension lines may
cross an outline of a part or
extension lines if necessary, but
they usually remain continuous
and unbroken at the point of
intersection.
• When a leader is directed to a
circle or a circular arc, its
direction should be radial.
Alphabet of lines
• Break lines: Break lines are applied to
represent an imaginary cut in an object, so the
interior of the object can be viewed or the object
can be fitted to the sheet. Line weight is thick
(0.5 – 0.6 mm).
Alphabet of lines
• Centerlines: Centerlines
are thin, long and short
dashes, alternately and
evenly spaced, with long
dashes placed at each end
of line
• Centerlines are used to
represent the axes of
symmetrical parts of
features, bolt circles, paths
of motion, and pitch circles
• Every circle, and some arcs,
should have two centerlines
that intersect at their
center of the short dashes –
“centermark”
Alphabet of lines
• Phantom lines: Phantom lines consist of medium-thin,
long and short dashes. They are used to represent
alternate positions of moving parts, adjacent positions of
related parts, and repeated details.
• They are used to show the cast, or the rough shape, of a
part before machining. The line starts and ends with the
long dash of (15 mm) with about (1.5 mm) space between
the long and short dashes. Line weight is usually (0.45
mm).
Alphabet of lines
• Section lines: Section
lines are thin, uniformly
spaced lines that indicate
the exposed cut surfaces of
an object in a sectional
view.
• Spacing should be
approximately (3 mm) and
at an angle of 45°C.
• The section pattern is
determined by the material
being “cut” or sectioned.
• Section lines are commonly
referred to as “crosshatching”
Alphabet of lines
• Section lines: The
section pattern is
determined by the
material being “cut”
or sectioned. Section
lines are commonly
referred to as “crosshatching”.
Alphabet of lines
• Cutting Plane lines:
Cutting Plane lines show
where an imaginary cut
has been made through
an object in order to view
and understand the
interior features.
• The line type is phantom.
• Arrows are located at the
ends of the cutting plane
line and the direction
indicates the line of sight
into the object.
Section view – General
• A section view in
general is used to
create a new drawing
view that is defined by
cutting an existing view
with a cutting plane
line
• Drawing view: a
defined, oriented view
of a part or model used
for manufacturing
Full section view – Assembly: Example 1
Partial section view
Precedence of line types
• Whenever lines coincide in a view, certain ones
take precedence
• Since the visible features of a part (object lines)
are represented by thick solid lines, they take
precedence over all other lines
• If a centerline and cutting plane coincides, the
more important one should take precedence
• Normally, the cutting plane line, drawn with a
thicker weight, will take precedence
• The following list gives the preferred precedence
of lines on your drawing:
Precedence of line types
1.
2.
3.
4.
Visible/Feature (object) Lines
Hidden (dashed) Lines
Cutting plane Lines
Centerlines
Precedence of line types
5.
6.
7.
8.
Break Line
Dimension
Extension Lines / Leader Lines
Section Lines / Crosshatch Lines
Example 1
• Identify the line types in the drawing:
Visible lines, hidden lines, extension lines, dimension lines
Example 2
• Identify the line types in the drawing:
visible lines, hidden lines, extension lines, dimension lines
Leader line, break line, centermark