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 • Measurement Systems • Scale

A B C

mm Inch

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 “cross-hatching”

Alphabet of lines

• Section lines: The section pattern is determined by the material being “cut” or sectioned. Section lines are commonly referred to as “cross-hatching”.

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. Visible/Feature (object) Lines 2. Hidden (dashed) Lines 3. Cutting plane Lines 4. Centerlines

Precedence of line types

5. Break Line 6. Dimension 7. Extension Lines / Leader Lines 8. 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