Modern Welding, 12th Edition - Chapter 2 Print Reading

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Modern Welding

Chapter 2
Print Reading

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Learning Objectives

After studying this chapter, you will be able to:

Identify the various views on a mechanical drawing or sketch and understand the concept of orthographic projection.
Obtain needed dimensions from a mechanical drawing or print.
Identify various types of lines in each view of a mechanical drawing.
Compute the dimensions of an object to be welded that is drawn at a reduced or enlarged scale.
Compute the sizes of parts when those sizes are not provided directly on the drawing.
Obtain needed information from the title block of a drawing.

Everything that is manufactured must first be drawn. Creating a sketch or a drawing is the best way to communicate for an engineer or designer. Sketching typically refers to freehand drawing, whereas, drawing means using drawing instruments, including anything from drafting tools to computers, which provide the capability of adding precision and accuracy to drawings. Parts and assemblies of parts used in industry are produced from mechanical drawings. These drawings are called mechanical drawings because they were traditionally drawn by mechanical means using drafting tools such as compasses, triangles, and a T-square. Today, mechanical drawings are produced on a computer using computer-aided design (CAD) software, although they can still be made using traditional drafting instruments. A mechanical drawing is also called a print.

By making a drawing, a designer can show the exact shape and size of a part or assembly so others can see what the object will look like. All parts can be checked to see if they fit properly. Measurements of rotating or sliding parts can be checked to make certain that those parts properly clear all other parts. A proper mechanical drawing accurately communicates a designer’s ideas to the people who will manufacture the part or assembly.

All of the size information needed to make the part must be given on the mechanical drawing or print so the parts will be made according to exact design specifications. These sizes are called dimensions. Dimensions are given in US Customary System units (inches) or the International System of Units (meters), also known as SI metric units or simply metric units.

Print reading refers to the skill needed to read and understand all of the special symbols, lines, and instructions on mechanical drawings so a manufacturer can create the parts shown with precision. Besides the part itself, mechanical drawings also include detailed technical information regarding the tools, machinery, processes, and specific materials to be used in manufacturing the parts, along with any special notes.

2.1 Types of Drawings

A drawing used in a shop to produce objects is known as a working drawing. Working drawings focus on providing essential information needed by the craftsmen who produce the parts in the shop. 26Like mechanical drawings, working drawings are created using CAD software programs. Working drawings are produced in two forms—the assembly drawing and the detail drawing.

2.1.1 Assembly Drawings

An assembly drawing shows the object to be made as it would appear in a fully assembled, ready to use form. Important location dimensions and overall size dimensions are shown on the assembly drawing. However, not all dimensions required to make each piece in the assembly are shown on the assembly drawing.

2.1.2 Detail Drawings

Individual detail drawings are made of each different part in the assembly. These drawings are typically made on separate sheets; however, multiple detail drawings may be made on one large print. Every view required to make a part is shown on the detail drawing, along with every dimension required to produce the part.

Understanding how a drawing is made helps a person find various areas, lines, holes, welds, and other details on a drawing. This knowledge also helps in finding the sizes of parts on a detail drawing or determining the type of weld to make and where to place it.

To be able to read a print, you must understand the concept of orthographic projection. Almost all assembly and detail drawings are made using orthographic projection.

2.2 Orthographic Projection

To see all the dimensions, holes, depressions, spaces, curves, and angles in an object, you may need to view it from various sides. In a drawing, all these sides and views must be shown on a flat sheet of drawing material. To make multiple views possible in a flat plane, the orthographic projection method of drawing was developed. Orthographic projection, also called multiview projection, is a two-dimensional representation of a three-dimensional object.

To make an orthographic drawing, the object to be drawn is imagined as being located and viewed inside a clear glass box. Each side of this imaginary clear box consists of what is known as the projection plane. See Figure 2-1. All points and lines are imagined as projected onto the six sides of the box, just as they would be seen by a person viewing the object through each of the projection planes. The object is then drawn on the various projection planes, Figure 2-2. 27Note that the lines of sight from the object to the projection plane are parallel to each other, Figure 2-3. This is known as parallel projection, and it is the projection type of choice for making working drawings. When the sides of the clear box are folded out flat, all sides of the object can be seen in one plane and drawn on a single print. See Figure 2-4.

Figure 2-1. The orthographic projection concept is shown by imagining the part enclosed in a clear glass box.

Figure 2-2. Six views of an object are projected onto the sides of the orthographic box.

Figure 2-3. Orthographic projection, also called multiview projection, makes use of parallel lines of sight that are perpendicular to the projection plane as shown.

Figure 2-4. The developed views can be folded out from the orthographic box to form a flat surface.

2.2.1 Views

When discussing orthographic drawings, the direction from which the object is viewed is given a name. Generally, the view that shows the most about the overall shape of the object is called the front view. The view to the right of the front view (as the orthographic box is unfolded in your imagination), is the right side view. The view to the left of the front view is the left side view. The view above the front view is the top view, and the view below the front view is the bottom view. Refer to Figure 2-2. A back view (also referred to as a rear view) is possible on an orthographic drawing. However, it is seldom used.

To produce an accurate drawing of an object, it is seldom necessary to show all six views. A working drawing typically shows three views of an object, which is sufficient to describe its shape and show all the required dimensions.

To obtain all essential information from a drawing, print reading often requires you to follow a line or a point from one view to another. For example, a straight line has two end points. The end points of straight lines on a drawing may be identified by letters or numbers. Note that when viewed from the end, a line will look like a point. See Figure 2-5.

2.2.2 Special Views

Within parallel projection, there is an important subcategory of drawings known as pictorial drawings, or pictorials. These drawings make use of special or unique views other than the six views previously discussed (front, right, left, top, bottom, and rear). Pictorial drawings are commonly used for technical illustrations because they effectively depict several faces of an object as seen from a special view or vantage point. Using parallel projection, all three directions of space (also known as axes) can be revealed in one drawing using axonometric projection and oblique projection.

Figure 2-5. A three-view orthographic drawing of a wire. This drawing shows that a point in one view can appear as a line in two other views.

28Axonometric projection is derived from the Greek words, axon, meaning axis, and metric, meaning measure. Axonometric projection rotates an object on any of its three axes relative to a projection plane in order to create a pictorial view, Figure 2-6. CAD programs can display objects from any viewpoint and have revolutionized the ability to create pictorial drawings. An infinite number of axonometric views can be created.

Depending on the angles chosen for use in an axonometric pictorial, there are three types of axonometric views—isometric, trimetric, and dimetric. See Figure 2-7. If a cube is drawn in an isometric view, the three edges converging at the corner closest to the viewer are of equal size, as are the three angles between those edges. 29This is achieved by starting with a standard orthographic front view and rotating the object by 45° about the vertical axis followed by rotating the object 35.264° about the horizontal axis. In a trimetric view, there are no equal angles or edges. In a dimetric view, two of the three angles and edges are equal.

Figure 2-6. Axonometric projection rotates the object on one or more of its axes and uses parallel projection to form a pictorial view on the projection plane. A—Depicts an object rotated on one axis. B—Depicts how objects can be rotated on two or three axes to create an infinite number of pictorial views.

Figure 2-7. Isometric, trimetric, and dimetric views are created based on how much the object is rotated on its three axes to form equal or unequal angles.

In oblique projection, parallel projection rays are not perpendicular to the viewing plane (unlike in multiview and axonometric projection). Rather, oblique projection lines strike the projection plane at an angle other than 90° See Figure 2-8. This results in unfavorable distortions, which is why oblique drawings are rarely used in professional design work.

Section views and detail views are two other special views commonly included in working drawings. A section view depicts a cross section of the object in an imaginary cut plane. This enables the inner features of the object to be pictured with more clarity than regular projections can show.

A detail view depicts a portion of the parent working drawing in much greater detail and typically appears as an inset, which allows for enlargement and better visibility of the selected area of the drawing. Detail views usually provide annotations and measurements that cannot be clearly shown on the parent working drawing.

2.3 Using a Working Drawing

To find the size of a line, hole, surface, or other feature, it is necessary for the person reading a print to be able to follow a given point or line from one view to another. In Figure 2-9, each point on the object is designated by a number or letter. Each point on the upper surface has been given a letter; each point on the lower surface has been given a number. Lines are defined by the letter or number at each end, such as (h-i). The number or letter to the left side or closest to the viewer is stated first. Points are defined as (a,b) and surfaces by all points that define the surface in a clockwise or counterclockwise order from any starting point, such as (b,a,l,c). It should be noted that a line in one view (k-d) is a line in one other view (k-d) and a point or corner in the third view (k,d). Also, remember that a point or corner in one view (1,2) is a line (1-2) in the other two views, as shown in Figure 2-5. And finally, a surface in one view (b,a,l,c) becomes a line in the other two views (1-a) and (a-b).

Figure 2-8. In oblique projection, parallel projection rays strike the projection plane at an oblique angle (other than perpendicular), resulting in distortion. For example, a circle is projected as an ellipse.

2.3.1 An Exercise in Defining Points on a Drawing

In the oblique view of the object in Figure 2-9, each end of each line has been given a letter or number. Working with a friend or in class, verbally define each line, point, and surface in each of the three orthographic views. See your instructor for the correct answers.

A surface is identified by all the letters on the corners of that surface. The surface letters or numbers are given consecutively in either a clockwise or counterclockwise order, starting at any point. However, lines first must be given the letter or number to the left or closest to the viewer, then the point to the right or farthest away is given. Example: (k-d) in the right side view. Note that the numbers or letters of a given line are separated by a dash. Points are defined in the same way (the closest letter or number first), but separated by a comma. Example: (a,b) in the front view.

Figure 2-9. An object with all corners identified by letters or numbers.

Employability

Decision Making and Problem Solving

Employers value workers who have the ability to make sound decisions and solve problems that arise. The processes for making decisions and solving problems are similar:

Identify the problem or issue to be decided.
Brainstorm possible solutions.
Decide which solution to implement.
Implement the solution.
Evaluate the results.

Having the ability to solve problems on the job shows an employer that you are able to handle more responsibility. Solving problems as a group can strengthen the team and help employees feel more pride in their work.

Making decisions and solving problems both require critical thinking skills. These are higher-level skills that enable you to think beyond the obvious. You learn to interpret information and make judgments. Supervisors appreciate employees who can analyze problems and think of workable solutions.

2.3.2 Calculating Dimensions on a Working Drawing

Working drawings must include all of the information needed to produce the object being illustrated. For this reason, prints feature vast amounts of critical notes, annotations, data, measurements, and dimensions. In many complex drawings, there is no space available to insert redundant or unnecessary information. This means that the dimensions for many features on a drawing will not be provided. However, all of the necessary dimensions and information are included if the drawing was properly created. The challenge is for you to calculate the missing dimensions. These can be determined from the dimensions that are shown.

This section focuses on finding the dimensions that have not been provided in a drawing. A welder uses basic math skills to determine those missing dimensions. The ability to accurately read a print drawing may require a welder to do some basic math, including addition and subtraction. For example, he or she must be able to add two dimensions to get a total dimension or subtract one dimension from another in order to determine a third dimension.

Knowing basic geometry may also be necessary to calculate some dimensions on a working drawing. A welder needs to know that the diameter of a circle is its width and the radius (the distance from the center of the circle to its circumference) is one-half of the diameter. See Figure 2-10. 31Also, the circumference of a circle is 2π times the radius of the circle, and a circle has 360°. Remember that π is a constant number that equals approximately 3.1416.

Figure 2-10. The geometry of a circle. The terms diameter, radius, perimeter, and circumference are used to define the size of a circle.

Diameter	= width of circle going through the center point of the circle = 2 × r (r is the radius)
Radius	= distance from the center point of a circle to the circle’s perimeter = ½ × d (d is the diameter)
Perimeter	= the closed curve bounding the circle area
Circumference	= distance around the circle = 2 × π × r (π is the relationship between the diameter of a circle and the circumference. It is about 3.1416.)

A welder will find it useful to remember that the three angles in any triangle add up to 180°. A right triangle has one 90° angle. An acute triangle is a triangle with each of its angles less than 90°. An obtuse triangle is a triangle with one of its angles greater than 90°.

Any single triangle can be divided into two right triangles by drawing a line from the vertex of the widest angle in the triangle to the perpendicular of the longest side of the triangle. Any square or rectangle can be divided into two right triangles. See Figure 2-11.

Figure 2-11. Knowledge of the geometry of triangles is helpful to welders. A—A right triangle (one angle equal to 90°), an acute triangle (each angle is less than 90°), and an obtuse triangle (one angle is greater than 90°). B—Acute and obtuse triangles can be made into two right triangles by drawing a line from the vertex of the widest angle in the triangle to the longest side of the triangle. The line must be perpendicular (90°) to the longest side. C—Any rectangle or square can be bisected into two right triangles.

Welders may also need to apply the Pythagorean theorem to determine missing dimensions. The Pythagorean theorem can be used to find the hypotenuse of a right triangle or the length of the diagonal line that bisects a rectangle or square. See Figure 2-12.

It is also useful for a welder to be able to convert fractions to decimals, which is done by dividing the top number in the fraction (numerator) by the bottom number (denominator). The bar or the line in a fraction means “division” or “divided by.” As an example, the fraction 3/4 is correctly understood as 3 divided by 4, and .75 is the correct decimal conversion to this mathematical equation.

Figure 2-12. The Pythagorean theorem can be used to solve for one of three sides of a right triangle, or the sides or diagonal of a square or rectangle. A—The Pythagorean theorem formula is a² + b² = c². B—Side a of the triangle shown has a length of 3 and side b has a length of 4. The mathematical equation for determining the length of c is shown using the Pythagorean theorem.

Figure 2-13. This dimensioned parts drawing shows correct line types and line weights.

2.4 Types of Lines Used on a Drawing

Various types of lines are used on a drawing, Figure 2-13. The thickness and shape of the lines vary, depending on their purpose on the drawing. Line types differ so a person looking at a drawing first sees the outline of the object, which stands out because of the heavy lines used. Then, by studying the other various lines and points from view to view, the person reading the print can accurately visualize the object and determine all of the dimensions needed to produce the object.

Today, CAD programs can precisely create all of the various line weights in mechanical drawings. Previously, draftsmen carefully created line thicknesses using mechanical pencils with pencil leads of varying diameters.

2.4.1 Object Lines and Hidden Lines

The outline of the object is shown with a heavy solid black line called an object line. Lines hidden from view by material in front of them are called hidden edge lines. Hidden edge lines are made up of a series of 1/8″ (3 mm) long dashes spaced 1/16″ (1.5 mm) apart.

2.4.2 Centerlines

The center of a radius, circle, or cylinder is marked by a centerline. It is made up of a series of 3/4″ (19 mm) long and 1/8″ (3 mm) short dashes with a 1/16″ (1.5 mm) space between them. Two centerlines run perpendicular to each other and intersect at the exact center of a circular part or hole. See the top view of Figure 2-13 and note that a “+” marks the point of intersection and shows the exact center point.

2.4.3 Extension Lines

The ends or terminal points of a dimensioned line are marked by extension lines. They are the same thickness as a centerline, but are solid lines. The size of a given line or feature on the object is usually placed between extension lines. On very small dimensions, when a number cannot be placed between the dimension lines, the dimension is placed at the end of one of the extension lines.

2.4.4 Dimension Lines

Dimension lines and dimensions are placed about 1/2″ (13 mm) away from the outer edge of the object. A long, thin arrowhead appears at each end of a dimension line. Dimension lines generally stretch between extension lines, with a gap in the center for the dimension. When multiple dimensions are required in the same area or in overlapping areas, each dimension line is placed farther from the object at 1/2″ (13 mm) intervals. The dimension lines with shorter spans are placed closer to the object, and dimension lines with greater spans are placed progressively farther from the object. This arrangement is used so the dimension lines do not interfere with each other.

2.4.5 Cutting Plane Lines

Occasionally, it is necessary to show the inside features of an object. To do so, it may be necessary to theoretically cut through the object at some point. The direction of the cutting line is shown with a cutting plane line. This is the thickest line type used in drawings. It is made up of a series of 3/4″ (19 mm) long lines and pairs of short lines, each 1/8″ (3 mm) long. Whenever a cutting plane line is used, there must be a section view provided on the print. A section view is a special view on a drawing that shows the hidden area and internal detail of an object where it has been theoretically cut in half. See the section view labeled Section A-A in Figure 2-13.

2.4.6 Section Lines

Section lines are thin lines that show solid, cutaway surfaces in a section view. The section lines are drawn at a 45° angle to the object lines. The section lines in adjacent parts are drawn in opposite directions, and different line styles can be used to indicate different materials. Thin parts, such as sheet metal or gaskets, do not typically include section lines.

2.4.7 Leader Lines

The size of a corner radius or size for a hole is given by using a leader line that points to the edge of the circle or arc and “leads” the reader out to a clear area on the drawing where a diameter or dimension is given. The leader line’s arrowhead touches the arc or circle, and the leader line is aligned with the feature’s radius. See the example in the top view of Figure 2-13.

2.5 Drawings Made to a Scale

If an object is drawn full size, it is said to be drawn full scale. Large objects like airplanes, automobiles, houses, ships, or skyscrapers cannot be easily drawn full size (although this is occasionally done). To get some objects to fit on the size paper being used, however, the object must be drawn to scale.

When an object is drawn to scale, the object is drawn at a reduced (or in some cases, enlarged) size. The object may be drawn one-half its actual size, or half scale. This means that every actual 1″ (25 mm) on the object is drawn only 1/2″ (12.5 mm). Half-scale is shown as follows: 1/2 = 1.

Objects can be drawn to any scale. Some tiny parts, such as those used in a watch or an electronic device, may have to be increased in size on a drawing to be seen and dimensioned more easily. The scale for such a drawing might be 10 = 1. A 1/8″ (3 mm) part at a 10 = 1 scale would be drawn 1 1/4″ (30 mm) in size. A 1″ (25 mm) part drawn at a 4 = 1 scale would be shown as 4″ (100 mm) in size.

The scale of a drawing may be shown somewhere near the actual part drawing or indicated in the drawing’s title block. It is important to determine what scale is being used. All lines and geometric figures in a drawing are drawn to the same scale. Any line and geometric figure that is not drawn to scale will look abnormal on the drawing.

2.6 The Title Block

The title block is a boxed area typically located in the lower right corner of the print that provides general information about the part shown in the drawing. 34Large companies develop a standard title block in their CAD software. That title block is filled in with the appropriate information and printed on each drawing produced by the company. See Figure 2-14.

The following information may appear in a title block:

Drawing Number. Each drawing has a unique number to identify it.
Dash Number. A dash followed by a number may be added to the drawing number to distinguish between right- and left-hand parts or assembly and detail drawings.
Permitted Part Size Tolerances. These are tolerances that define how accurately a part must be made.
Name Given the Part or Assembly. This is a descriptive name that describes the part or its function.
Sheet Number. Many sheets may be required to make all the parts required for a single assembly. If so, each page is numbered as follows: “Sheet 1 of 20,” “Sheet 2 of 20,” and so on.
Drawing Sheet Size. Drawings are made on a variety of sheet sizes. The assembly drawing is normally on a large sheet with detail drawing on smaller sheets. The sheet size is shown as a letter code running from A to E. An “A” sheet is about 9″ × 11″ and “E” is about 36″ × 48″.
Scale of the Drawing. Drawings may be made actual size, or they may be made smaller or larger than the actual size of the object. A drawing that is twice the actual size of the part would show a scale of 2 = 1 or 2:1. A drawing made half the actual size of a part would be in a scale of 1/2 = 1 or simply 1:2. The title block in Figure 2-14 shows the scale of the drawing as 1:1, meaning it is drawn to actual size.
Responsible Signatures Area. Spaces are often provided for signatures of the CAD technician, part designer, drawing checker, supervisor, inspector, or others involved in the production of a part.
Material. The material to be used in making the part is shown.

Other information may be given in a title block, depending on the company that designs and draws the part or assembly. For example, the drawing may include change information. When changes are made to the drawing, the change is briefly described in the title block and a date of the change is listed. Parts must be made according to the latest drawing change.

Figure 2-14. A typical title block with areas or sections identified. A—Drawing Number. B—Dash Number. C—Permitted Part Size Tolerances. D—Name Given the Part or Assembly. E—Sheet Number. F—Drawing Sheet Size. G—Scale of the Drawing. H—Responsible Signatures Area. I—Material.

Summary

Print reading refers to the skill needed to read and understand all of the special symbols, lines, and instructions on mechanical drawings.
Working drawings may be assembly drawings or detail drawings. An assembly drawing shows the object to be made as it would appear in a fully assembled form. Individual detail drawings are made of each different part in the assembly.
Orthographic projection is a two-dimensional representation of a three-dimensional object. In orthographic drawings, the direction from which the object is viewed is given a name. Views include the front view, right side view, left side view, top view, bottom view, and rear view.
Axonometric projection rotates an object on any of its three axes relative to a projection plane in order to create a pictorial view. Depending on the angles used in an axonometric pictorial, the three types of axonometric views are isometric, trimetric, and dimetric.
A section view shows a cross section of the object in an imaginary cut plane. A detail view shows a portion of the parent working drawing in greater detail. The detail view typically appears as an enlarged callout.
Lines used on a drawing include object lines, hidden lines, centerlines, extension lines, dimension lines, cutting plane lines, section lines, and leader lines.
An object drawn to scale is drawn at a reduced or enlarged size. Objects can be drawn to any scale.
The title block is a boxed area typically located in the lower right corner of the print that provides general information about the part shown in the drawing.

Technical Terms

assembly drawing

axonometric projection

centerline

cutting plane line

detail drawings

detail view

dimension lines

dimensions

dimetric view

drawn to scale

extension lines

front view

full scale

half scale

hidden edge lines

International System of Units

isometric view

mechanical drawings

object line

orthographic projection

parallel projection

pictorial

print reading

Pythagorean theorem

section lines

section view

title block

trimetric view

US Customary System

working drawing

Review Questions

Answer the following questions using the information provided in this chapter.

Know and Understand

To show all sides and views of an object on a flat sheet of drawing material, the _____ projection method of drawing is used.
1. section view
2. assembly
3. orthographic
4. detail
True or False? It is seldom necessary to show all six views in order to produce an accurate drawing of an object.
All three angles and corners are of equal size in _____.
1. dimetric view
2. trimetric view
3. an isometric view
4. all axonometric views
A(n) _____ view shows a cross section of the object in an imaginary cut plane, enabling the inner features of the object to be more clearly pictured.
1. section
2. dimetric
3. trimetric
4. oblique
True or False? Any square or rectangle can be divided into two right triangles.
The Pythagorean theorem (a² + b² = c²) can be used to _____.
1. find the hypotenuse of a right triangle
2. determine the circumference of a circle
3. find the diameter of a circle
4. convert fractions to decimals
The purpose of an extension line is to _____.
1. show the outline of the object
2. show solid, cutaway surfaces in a section view
3. provide general information about a part shown in the drawing
4. mark the ends or terminal points of a dimensioned line
True or False? An object that is drawn to scale may be drawn at a reduced or at an enlarged size.
36A part drawn at a scale of 1/4 = 1 is shown on a drawing as 1.5″ (38 mm) long. What is the length of the real part?
1. 4″ (102 mm)
2. 6″ (152 mm)
3. 8″ (203 mm)
4. 15″ (381 mm)
A letter code (A, B, C, D, or E) in a title block indicates _____.
1. the first page in the drawing
2. the drawing sheet size
3. the material of the part
4. the scale of the drawing

Analyze and Apply

Refer to the figure below and read or calculate the following dimensions:

What is the distance 1?
What is the distance 2?
What is the distance 3?
What is the distance 4?
What is the distance 5?
What is the distance 6?
What is the distance 7?
What is the distance 8?
What is the distance 9?
What is the distance 10?
What is the distance 11?
What is the distance 12?
Refer to Figure 2-14 for the answers to the following questions:
What is the name of the draftsperson or company who drew this part?
What is the scale of this drawing?
What size paper is this drawing made on?
What is the required accuracy of a size given as a three-place decimal?
What is this drawing number?
How many other sheets are there for this part?
What metal and metal thickness is used for this part?
What is the name of this part?

Critical Thinking

Explain the importance of being able to visualize a 3-D object by reading a 2-D print drawing. Describe how orthographic projection enables this to happen.
Why does a welder who works with three-dimensional materials need the skill of print reading?
Why is the back view (also called a rear view) seldom used on an orthographic drawing?

Experiment

As a class, look at a complex 3-D object carefully. Then, discuss together what views would be needed to draw that object so that someone else could make it by only looking at the drawing.