Lesson 10

SECTIONING

W E L C O M E !

LESSON 9

LESSON 11

LESSON 10 OUTLINE

InShot_20220713_234044035.mp4

OBJECTIVES

at the end of the lesson the students should be able to;

to define what is Sectioning
to enumerate the kinds of section
to obey sectioning principles
illustrate sectioning appropriately

SECTIONING

Sectioning is the process of representing the inside view or structure or an object. This inside view is also known as a sectional view or merely a section. A section is also an orthographic view. The object to be sectioned is imagined to be cut whole or in part, and the cut portion is shown with visible lines. Another purpose of sectioning., aside from showing the inside view of the object, is to indicate the kind of material it is made of.

SECTIONAL VIEW

The basic method of representing parts for designs by views or projections. By means of a limited number of carefully selected views, the external features of the most complicated designs can be thus fully described. However, we are frequently confronted with the necessity of showing more or less complicated interiors of parts that cannot be shown clearly by means of hidden lines. We accomplish this by slicing through the part much as one would cut through an apple or a melon. A cutaway view of the part is then drawn; it is called a sectional view, a cross section, or simply a section.

ALPHABET OF LINES

Border Line – Are generally the heaviest or darkest lines in a drawing. The border surrounds a drawing and is usually rectangular in shape. It makes the onlooker feel as if the parts of the drawing are combined into a single unified graphic representation.

Visible Outline or Object Line – Represents the visible edges of an object. A heavy stroke of the soft pencil is considered a visible line in sketching.

Hidden Line – A hidden line was formerly called invisible or dotted line. The original line was composed of fine dots. A hidden line is now composed of small or short dashes about 3-4 millimeters long with gaps between dashes, each 1-2 millimeters wide. It is use to represent the hidden edges of an object.

Dimension Line – Is used to indicate the measurements of an objects. It consists of three parts: arrowheads, fine line, and number or measurement. The fine line has the same thickness or weight as the projection line. The arrowheads are short heavy strokes (called flares) placed at the extremities of this fine line. The shape of an arrowhead is like the point of an ordinary drawing pen. The number or figure is usually positioned at the middle of the fine and its axis is perpendicular to it.

Projection Line or Extension Line – Is slightly heavier than a center line and is composed of long equal dashes about 7 millimeters to 4 centimeters long and with gaps of about 1-2 millimeters. This line is used to project one view to another and to determine the limit of a dimension line.

Limiting Line – Is a long break. The thickness of this line is about half that of the visible line. It consists of broken and straight lines alternately drawn. The line is used for limiting the length of an elongated object so that it can be shown without changing the size of its view.

Reference Line – Is an irregular fine, curved line with an arrowhead at one end. It is often used to label the parts of an object. Both arrowheads in the leader and reference lines point to and touch the parts labeled or not measured.

Center Line – Is considered the lightest of finest line in a drawing. It represents the axis or center of objects with symmetrical shapes such as washers, gears, pulleys, rivets. It consists of one long and one short dash alternately drawn. The gaps between dashes are of the same distances as those in the hidden lines.

Leader Line – Is a fine broken line with an arrowhead at one end. The broken line usually consists of a short horizontal line and an inclined line drawn at any convenient angle. It is also used for indicating the measurement and name of a part.

LINES AND SYMBOLS USED IN SECTIONING

1. Cutting Plane Line.

The line consists of two short and one long dash repeated till the required length, terminating in arrows. These arrows point in the direction on which the object is views, when the section is taken. When numbers of cutting planes are passing

through an object each section plane is identified by letters at extremities.

2. Section Lines (Crosshatching Lines).

Section lines are usually drawn at an angle of 45⁰ with thin dark lines to contrast with the heavier object lines. The spacing of section lines

should be uniform to give even tint in about 1/16 in. apart (on small drawing) and 1/32 in.

(for large drawings).

3. Short Break Lines.

Short break line is a heavy irregular curved line used in partial sectioning. When a small part of the object requires exposure, a short break line (limiting line) is commonly used.

4. Sectional Symbols for Various Materials.

Different metals and materials are conventionally shown in section by different types of section lines as shown and described below. In addition, names and other particulars of metals and materials should be specified on the drawing sheet by means of notes.

KINDS OF SECTION

There are five recognized kinds of section: the full section, half section, detail or removed section, revolved section, and broken-out section.

SYMBOLS FOR MATERIALS AND SECTIONS

The symbols for materials, as shown in sectional views, are made up of section lines.

In metal objects these lines are generally straight and are drawn at 45 .The combination of parallel lines varies with every metal. Figure 65 shows the different symbols for cast iron, wrought iron, steel, etc. The distances between parallel cross hatching lines in metal objects are optically calculated instead of being measured.
Wooden parts which are in section are cross-hatched with light curved or nearly straight lines. Such lines represent the end grains or annual rings of wood.

In cross section, the cross-hatching lines of wood are curved radial lines. These lines are nearly straight in a longitudinal section.

SECTIONING RULES or PRINCIPLE

To standardize drafting practices regarding sectioning., most draftsmen follow certain rules which include:

1. The cutting plane line need not be a continuous straight line. It may change in direction depending on the desired position of the sectional view. This lines may be drawn in colored ink to distinguish it from the other lines in the drawing

2. Rivets, bolts, screws, keys, ribs, and spokes, which lie along or in line with the cutting plane line should not be sectioned.

3. Any section which makes the view unsightly should be conventionalized

4. Cross-hatching or section lines for adjacent metal parts must be made in opposite directions even if they are made of the same material.

5. In half-sections, always show the limiting line or long break.

6. All detail or removed sections should be labeled with capital letters.

7. V-threads in section should be conventionalized. This rule also applies to tapped holes or holes having threads.

8. Hidden lines beyond the sectional view may be omitted.

9. Always indicate the arrowheads at the extremities of broken cutting plane lines so that the section can be easily checked as to the proper direction of the sectional view.

10. The section lines for metals must be uniformly drawn not only in weight or thickness but also in distances between them.