Basic Blueprint Reading

🧭 Overview

🧠 One-sentence thesis

Different line types—varying in thickness, pattern, and purpose—form a visual language that conveys edges, hidden features, dimensions, and internal structure on blueprints, and mastering this language is essential for interpreting and building objects accurately in the trades.

📌 Key points (3–5)

• What lines communicate: form (shape) and "weight" (thickness/width); together they provide the information needed to understand a print.
• Why it matters: interpreting blueprints and building objects accurately is a core skill in all trade crafts; it requires time and practice.
• Line variety: visible/object lines show outer edges; hidden lines show unseen features; center, dimension, extension, and leader lines convey measurements and notes; section and cutting-plane lines reveal internal structure; break lines indicate interruptions.
• Common confusion: line weight (thickness) and pattern (solid, dashed, long-short-long) distinguish one line type from another—don't confuse a thick continuous object line with a thin continuous dimension line, or a medium dashed hidden line with a very heavy cutting-plane line.

📏 Lines that define shape and visibility

🔲 Object line (visible line)

A visible line, or object line, is a thick continuous line used to outline the visible edges or contours of an object.

• Purpose: shows what you can see—the outer boundaries and contours.
• Appearance: thick and solid (no gaps).
• Example: the outline of a box's front face on a drawing.

🔳 Hidden line (hidden object line)

A hidden line, also known as a hidden object line, is a medium weight line made of short dashes about 1/8" long with 1/16" gaps, to show edges, surfaces, and corners which cannot be seen.

• Purpose: reveals features behind or inside the object that are not visible from the current view.
• Appearance: medium thickness, dashed (short dashes with small gaps).
• When used: sometimes added to make a drawing easier to understand; often omitted in isometric views.
• Example: a hole drilled partway into a block, shown from the side where the hole opening is not visible.

🎯 Lines for centers and symmetry

⊕ Center line

Center lines are used to indicate the centers of holes, arcs, and symmetrical objects. They are very thin (size), long-short-long kinds of lines.

• Purpose: mark the centerpoint or axis of circular features and symmetrical shapes.
• Appearance: very thin, alternating long and short segments (long-short-long pattern).
• Example: a vertical center line through a cylindrical shaft to show its axis of rotation.

📐 Lines for dimensions and notes

📏 Dimension line

Dimension lines are thin and are used to show the actual size of an object. There are arrowheads at both ends that terminate at the extension lines.

• Purpose: indicate the measured distance or size.
• Appearance: thin, with arrowheads at each end touching extension lines.
• Don't confuse: dimension lines are thin (not thick like object lines) and always have arrowheads.

📐 Extension line

Extension lines are also thin lines, showing the limits of dimensions. Dimension line arrowheads touch extension lines.

• Purpose: define the start and end points of a dimension without cluttering the object outline.
• Appearance: thin lines extending from the object.
• Relationship: dimension line arrowheads terminate at extension lines.

🗨️ Leader line

Leaders are more thin lines used to point to an area of a drawing requiring a note for explanation. They are preferably drawn at 45° angles.

• Purpose: connect a note or label to a specific feature on the drawing.
• Appearance: thin, typically at a 45° angle.
• Example: a leader pointing to a small hole with a note "Ø 0.25 in."

🔪 Lines that reveal internal structure

✂️ Cutting plane line

A cutting plane line (very heavy) helps to show the internal shape of a part or assembly by slicing through the object.

• Purpose: indicates where an imaginary cut is made to create a section view.
• Appearance: very heavy (thickest line type mentioned).
• Why it matters: shows the viewer where the object has been "sliced" so the internal features can be seen.

🧱 Section line

Section lines are used to show the cut surfaces of an object in section views. They are fine, dark lines. Various types of section lines may indicate the type of material cut by the cutting plane line.

• Purpose: fill the area that has been cut by the cutting plane, making it clear which surfaces are exposed.
• Appearance: fine (thin), dark lines; pattern may vary by material type.
• Example: diagonal hatching on the cross-section of a metal part.

🔗 Break line

• Purpose: indicate that part of the object has been omitted or broken away (the excerpt mentions three kinds but does not detail them further).
• Note: the excerpt text cuts off before explaining break line types fully.

📊 Line type comparison

Line type	Weight (thickness)	Pattern	Primary purpose
Object (visible)	Thick	Continuous solid	Outline visible edges
Hidden	Medium	Short dashes (1/8" dash, 1/16" gap)	Show unseen edges/surfaces
Center	Very thin	Long-short-long	Mark centers of holes, arcs, symmetry
Dimension	Thin	Continuous solid, arrowheads at ends	Show actual size
Extension	Thin	Continuous solid	Define dimension limits
Leader	Thin	Continuous solid, preferably 45°	Point to notes/labels
Cutting plane	Very heavy	(not specified)	Indicate where section cut is made
Section	Fine (thin), dark	Varies by material	Fill cut surfaces in section views
Break	(not specified)	(not specified)	Show omitted or broken portions

🛠️ Skill development note

• The excerpt emphasizes that interpreting blueprints is a skill that takes time and practice to master.
• Success in all trade crafts depends on accurately reading and building from prints.
• Don't expect instant proficiency—like other trade skills, fluency with line language develops through repeated use.

🧭 Overview

🧠 One-sentence thesis

Different line types—each with distinct thickness, pattern, and purpose—work together to convey the complete information needed to interpret blueprints and build objects accurately.

📌 Key points (3–5)

• What lines encode: form (shape/pattern) and weight (thickness/width) combine to communicate different kinds of information on technical drawings.
• Why line variety matters: each line type (object, hidden, center, dimension, etc.) serves a specific function; recognizing them is essential for reading blueprints.
• Common confusion: weight vs. pattern—some lines differ by thickness (e.g., object lines are thick), others by dash pattern (e.g., hidden lines are dashed), and some by both.
• Skill development: interpreting blueprints and building from them accurately requires practice and time to become proficient.
• Foundation for visualization: understanding line types is the first step before learning to "think in three dimensions" with orthographic and pictorial drawings.

📏 Fundamental line properties

📏 Form and weight

• Form: the definite shape or pattern of a line (continuous, dashed, long-short-long, etc.).
• Weight: the thickness or width of a line.
• These two properties work together to distinguish line types and convey different information.
• Example: a thick continuous line means something different from a thin dashed line, even if both outline parts of the same object.

🎯 Purpose of line variety

• Lines provide "part of the information needed to understand the print."
• No single line type can communicate everything; the combination of different lines gives the full picture.
• Don't confuse: lines are not decorative—each type has a functional role in technical communication.

🔲 Lines that define object geometry

🔲 Object line (visible line)

A visible line, or object line, is a thick continuous line used to outline the visible edges or contours of an object.

• Thickness: thick (heavy weight).
• Pattern: continuous (no breaks).
• Purpose: shows what you can see when looking at the object—the outer boundaries and edges.
• Example: the outline of a box's front face would be drawn with object lines.

🔳 Hidden line (hidden object line)

A hidden line, also known as a hidden object line, is a medium weight line made of short dashes about 1/8" long with 1/16" gaps, to show edges, surfaces, and corners which cannot be seen.

• Thickness: medium weight.
• Pattern: short dashes (approximately 1/8" dash, 1/16" gap).
• Purpose: reveals internal or obscured features—edges behind the visible surface.
• When used: sometimes included to make a drawing easier to understand; often omitted in isometric views.
• Example: a hole drilled partway into a block from the back would be shown with hidden lines when viewing the front.

⊕ Center line

Center lines are used to indicate the centers of holes, arcs, and symmetrical objects. They are very thin (size), long-short-long kinds of lines.

• Thickness: very thin.
• Pattern: long-short-long repeating sequence.
• Purpose: marks the axis or center of circular features and symmetrical shapes.
• Example: the centerline of a cylindrical shaft or the center of a circular hole.

📐 Lines for dimensions and notes

📐 Dimension line

Dimension lines are thin and are used to show the actual size of an object. There are arrowheads at both ends that terminate at the extension lines.

• Thickness: thin.
• Pattern: continuous, with arrowheads at both ends.
• Purpose: indicates measurements—the actual size of features.
• The arrowheads touch the extension lines to define the span being measured.

📐 Extension line

Extension lines are also thin lines, showing the limits of dimensions. Dimension line arrowheads touch extension lines.

• Thickness: thin.
• Purpose: extends from the object to define where a dimension starts and ends.
• Relationship: dimension lines terminate at extension lines; together they form the measurement annotation.

📐 Leader line

Leaders are more thin lines used to point to an area of a drawing requiring a note for explanation. They are preferably drawn at 45° angles.

• Thickness: thin.
• Pattern: continuous, typically at 45° angle.
• Purpose: connects a note or label to a specific feature on the drawing.
• Example: a leader might point to a hole with a note specifying "drill 0.25 inch diameter."

✂️ Lines for sections and breaks

✂️ Section line

Section lines are used to show the cut surfaces of an object in section views. They are fine, dark lines. Various types of section lines may indicate the type of material cut by the cutting plane line.

• Thickness: fine (thin).
• Appearance: dark.
• Purpose: fills in the area where an object has been "cut" to reveal internal structure.
• Material indication: different section line patterns can represent different materials (e.g., metal, wood, plastic).

✂️ Cutting plane line

A cutting plane line (very heavy) helps to show the internal shape of a part or assembly by slicing through the object.

• Thickness: very heavy (thickest line type).
• Purpose: indicates where an imaginary cut is made to create a section view.
• Relationship: the cutting plane line shows where to cut; section lines show the cut surface itself.

✂️ Break line

There are three kinds of break lines used in drawings. They are used to remove, or "break out" part of a drawing for clarity, and also to shorten objects which have the same shape throughout their length and may be too long to place on the drawing.

• Three types: short break, long break, and cylindrical break.
• Purpose:
  • Remove portions of a drawing to reveal internal features.
  • Shorten repetitive or very long objects that won't fit on the sheet.
• Usage by shape:
  • Short and long break lines: for flat surfaces.
  • Cylindrical break lines: for rods, dowels, and other round objects.
• Example: a 10-foot-long uniform rod can be drawn shorter with break lines, since the middle section is identical throughout.

🔄 Lines for movement and boundaries

🔄 Phantom line

Phantom lines are long-short-short-long lines most often used to show the travel or movement of an object or a part in alternate positions. It can also be used to show adjacent objects or features.

• Thickness: thin.
• Pattern: long-short-short-long repeating sequence.
• Purpose:
  • Illustrate motion or alternate positions of a moving part.
  • Show nearby (adjacent) objects or features for context.
• Example: a hinged door might be shown in both open and closed positions, with the open position drawn in phantom lines.

🔄 Border line

Border lines are very thick, continuous lines used to show the boundary of the drawing or to separate different objects drawn on one sheet. They are also used to separate the title block from the rest of the drawing.

• Thickness: very thick.
• Pattern: continuous.
• Purpose:
  • Define the edge/boundary of the drawing area.
  • Separate multiple objects on the same sheet.
  • Separate the title block from the main drawing.
• Example: a rectangular border around the entire drawing sheet.

🧠 Building visualization skills

🧠 From lines to three-dimensional thinking

• Understanding line types is the foundation; the next step is developing "visualization abilities."
• Visualization: the ability to "see" technical drawings and "think in three dimensions."
• The excerpt emphasizes this as "the most important part" of learning to read blueprints.

🧠 Pictorial vs. orthographic drawings

• The excerpt introduces two categories:
  • Pictorial drawings: perspective, oblique, and isometric—easier to visualize because they look more like the actual object.
  • Orthographic projection (multi-view drawing): the most common type on engineering and architectural prints; will be emphasized in later study.
• Don't confuse: pictorial drawings are easier to "see" but orthographic projection is the standard for technical work.

🧠 Practice and proficiency

• Interpreting blueprints and building accurately is "a needed skill to become successful in all trade crafts."
• Like other skills, it "will take time and practice to fully understand and become proficient."
• The excerpt includes quizzes and exercises (identify line types, complete figures) to build this skill.

📊 Line type comparison

Line Type	Weight (Thickness)	Pattern	Primary Purpose
Object (Visible)	Thick	Continuous	Outline visible edges
Hidden	Medium	Short dashes (1/8" dash, 1/16" gap)	Show hidden edges/surfaces
Center	Very thin	Long-short-long	Mark centers of holes/arcs/symmetry
Dimension	Thin	Continuous with arrowheads	Show actual size
Extension	Thin	Continuous	Define dimension limits
Leader	Thin	Continuous (45° angle)	Point to notes/labels
Section	Fine (thin), dark	Varies by material	Show cut surfaces
Cutting Plane	Very heavy	(Not specified)	Indicate where section cut is made
Break (short/long)	(Not specified)	Jagged or wavy	Shorten/remove parts of flat surfaces
Break (cylindrical)	(Not specified)	S-curve	Shorten/remove parts of round objects
Phantom	Thin	Long-short-short-long	Show movement/alternate positions/adjacent features
Border	Very thick	Continuous	Define drawing boundary/separate areas

🧭 Overview

🧠 One-sentence thesis

Orthographic projection is the most important visualization method for technical drawings because it clearly presents dimensions and details for complex objects through multiple flat views, even though pictorial drawings are easier to understand at first glance.

📌 Key points (3–5)

• Three main pictorial types: perspective (most realistic), oblique (front parallel, side angled), and isometric (both sides at 30°)—used for simpler objects.
• Orthographic projection uses multiple flat views (typically top, front, right side) that don't show depth directly but allow precise dimensioning and detail for complex objects.
• Hidden lines reveal internal features: dashed lines show surfaces that would normally be invisible, letting readers "see inside" objects.
• Common confusion: curved vs. sharp surfaces—sharp corners produce visible lines in other views, but smooth curves do not.
• Why orthographic matters: it handles complexity better than pictorials for dimensions, notes, and configuration details.

🎨 Pictorial drawing types

🖼️ Perspective drawing

Perspective is the most realistic form of drawing where objects grow smaller as they recede into the horizon.

• Artists use one-point, two-point, or three-point perspective to create visual depth.
• Used by architects and for industrial pictorials of plan layouts and machinery where realism is required.
• Example: a building drawn in two-point perspective shows walls converging toward two vanishing points on the horizon.

📐 Oblique drawing

Oblique drawings have one plane (front) of the object parallel to the drawing surface, with the side drawn at 30° or 45°.

• Only the side/receding part is angled; the front face remains flat and parallel to the page.
• Receding lines are drawn at a different scale than vertical and horizontal lines, making the drawing seem "out of shape."
• Not used very often in industry due to this distortion.

🔷 Isometric drawing

Isometric drawings have both visible surfaces drawn at 30°, creating less distortion than oblique.

• Used more frequently in industry, especially in the piping industry.
• Requires practice to fully understand but best represents what is being built from different sides with one drawing.
• Example: a cube in isometric shows three faces, each angled at 30° from horizontal.

🔍 How to distinguish pictorial types

Type	Front face	Side/depth	Realism	Industry use
Perspective	Converges to horizon	Converges to horizon	Most realistic	Architecture, layouts
Oblique	Parallel to page	30° or 45° angle	Distorted	Rarely used
Isometric	30° angle	30° angle	Less distortion	Common (piping)

📏 Single and two-view drawings

📄 Single-view drawings

• Sometimes one view is all that's needed for complete visual explanation when combined with dimensions and material notes.
• Most one-view drawings are of flat objects (sheet metal, gasket stock).
• Spherical objects also need only one view plus a diameter note.
• Example: a flat gasket can be fully described with one view showing its outline plus thickness and material specifications.

📑 Two-view drawings

• Two views may be sufficient to show an object's shape, especially for cylindrical objects like pipe.
• One view shows length, the other shows diameter.
• Without both views, the shape could be mistaken for something else (square tube, channel, etc.).

🔲 Orthographic projection system

🗂️ What orthographic projection is

Orthographic projection is a drawing system that usually has three views: typically top, front, and right side.

• Other views (left side, bottom) are possible but less common.
• The arrangement/order of views on the print is fixed and must be memorized.
• Does not show depth directly—objects appear flat—but readers learn to "read" depth by scanning all three views together.

🔗 How the three views relate

• Projection lines connect the views, showing height, width, and depth relationships.
• The front view is the key because it most clearly shows the object's shape.
• Each surface in one view corresponds to surfaces in the other two views.
• Example: if the front view shows an "L" shape, the top and side views reveal which parts are taller or closer to the viewer.

🎯 Why orthographic beats pictorials for complex work

• Pictorial drawings (isometric, oblique) are easier to visualize but only work for relatively uncomplicated objects.
• For complex objects, orthographic projection provides clearer presentation of:
  • Dimensions
  • Notes
  • Configuration details
• Don't confuse: pictorials are better for quick understanding; orthographic is better for precision and complexity.

👁️ Hidden surfaces

🔍 What hidden lines show

• Hidden lines (dashed lines) represent surfaces that normally could not be seen—the inside or back of an object.
• This is a major advantage of orthographic projection: the ability to "see inside" objects.
• Becomes very useful with complicated objects.

📦 Examples of hidden features

• A hidden line in the right side view can represent an entire flat surface between two higher sides.
• Hidden lines can result from a square hole through the middle of an object.
• Hidden lines appear when part of a corner is cut away or "recessed."
• Each hidden feature in 3D space produces specific dashed lines in one or more views.

🌊 Curved and inclined surfaces

🌀 Curved surfaces

Curved surfaces show the curve in only one view; other views require visualization to understand the curve.

• Key principle: sharp corners produce visible lines in other views; smooth curves do not produce lines.
• When there is a sharp change of direction (like at a corner), a line appears in another view.
• When the change of direction is smooth (like a curve), no line is seen in other views.
• Don't confuse: the absence of a line doesn't mean nothing is there—it means the surface curves smoothly.

⛰️ Inclined surfaces

Inclined surfaces are those at an angle or slanted—neither horizontal nor vertical.

• Common on technical prints and require alertness when reading drawings.
• An inclined surface in one view often creates visible or hidden lines in other views.
• Example: an object with two inclined surfaces will show angled lines in the front view and corresponding features in top and side views.

🧭 Overview

🧠 One-sentence thesis

Technical sketching is a practical skill that enables tradespeople to communicate design ideas and modifications quickly through freehand drawings without needing artistic talent or professional drafting tools.

📌 Key points (3–5)

• Why sketching matters: enables effective communication of problems, repairs, and modifications in the field, making tradespeople more valuable.
• Three main methods: oblique (one plane parallel to paper, side at 30–45°), isometric (all angles at 30°), and orthographic (multiple aligned views showing different sides).
• Real-world context: field sketches often happen on napkins, cardboard, or any flat surface—not just formal paper.
• Common confusion: oblique vs isometric—oblique has one flat face parallel to paper with flexible angles; isometric requires strict 30° angles on all axes.
• Core principle: legibility and clear communication matter more than artistic perfection.

✏️ Basic sketching fundamentals

✏️ Tools and technique

• Start with minimal equipment: a soft pencil (#2 or F) and paper.
• Keep the pencil sharp but not too sharp; hold with firm but relaxed grip.
• Draw lightly at first to allow easy erasing without smudging; darken lines once the sketch takes proper shape.
• Turn the pencil slowly while drawing to maintain even line width and keep the point sharper longer.

📝 Lettering requirements

The most important requirement of good lettering is legibility.

• The entire alphabet can be formed from straight and curved lines practiced in warm-up exercises.
• If a person cannot read your writing on a drawing, the drawing becomes useless.
• Lettering practice is required for work ahead and tests unless your printing is already similar to examples and easily readable.

🎨 Oblique sketching method

🎨 What oblique projection is

Oblique sketches are a type of pictorial having one plane parallel to the drawing surface, and the visible side sketched at an angle.

• One face of the object appears flat and parallel to the paper (no distortion on this face).
• The receding side is drawn at an angle, usually 30° to 45° or somewhere in between.
• You can choose to show either the left or right side depending on what needs to be communicated.

⚠️ Common beginner mistake

• Beginners often have trouble keeping the 30° or 45° lines at the same angle throughout the sketch.
• If angles vary, the sketch will look distorted and confusing.
• Example: Start with a cube—draw the front face as a square, then extend all receding lines at the same consistent angle.

🔧 Three-step process

The excerpt shows how to build an oblique cube systematically:

1. Draw the front face (appears as a true square).
2. Extend receding lines at chosen angle (30–45°).
3. Complete the visible edges and add hidden lines if needed.

📐 Isometric sketching method

📐 What makes isometric different

• Unlike oblique's flexible angles, isometric must maintain an angle very close to 30° on all axes.
• No face appears flat or undistorted—all three visible faces are equally foreshortened.
• Requires more precision with angles to avoid a distorted appearance.

📦 The "boxing in" technique

Generally, it's recommended that you start at the bottom of the object and "box it in", thereby enclosing it within a rectangular framework.

• Start by creating a rectangular frame using 30° guidelines.
• Build the object's features within this framework in an organized way.
• Remove unnecessary construction lines once the object is complete.
• For beginners, this framework prevents "losing" the sketch as complexity increases; experienced sketchers may skip guidelines for simple objects.

⭕ Circles become ellipses

• Circles and arcs become elliptical in shape when drawn in isometric.
• Using a true circle will appear distorted.
• Three positions exist for ellipses depending on which surface (picture plane) contains the circular feature.
• Example: A cylindrical hole appears as an ellipse, not a circle, in isometric view.

📊 Orthographic sketching method

📊 Why orthographic is most common

Of all the methods of making drawings, orthographic projection is the most commonly used by draftsperson.

• Shows parts of an object more completely than other methods.
• Uses a system of projecting from view to view to graphically describe the object.
• Standard views: front, top, and right side, each showing different faces without distortion.

🔗 Critical alignment principle

The important thing to remember in orthographic sketching is the alignment of views.

Alignment rule	What it means
Top view	Projected directly above the front view
Side view	Lines up horizontally with front view
Dimensions	Height, width, and length must remain the same from view to view
Spacing	Enough distance between views to prevent crowding and leave room for dimensions

🎯 Projection exercise

The excerpt emphasizes practicing projection: converting pictorial views (oblique or isometric) into orthographic views, and vice versa.

• Example: Given an isometric drawing, sketch it in oblique; given orthographic views, visualize and sketch the 3D object.
• Don't confuse: Each orthographic view shows only what is visible from that specific direction—no perspective or angle distortion.

🔨 On-the-spot sketching practice

🔨 Real-world application

"On-THE-SPOT" sketching is a standard industry practice.

• Used when machinery needs changing, supports need adding, or pictorial information requires documentation.
• Involves sketching physical objects you can touch and examine, not just copying from paper drawings.
• Requires producing both an isometric view and an orthographic drawing with as many views as needed to show all details for construction.

🏗️ Field conditions

• Many field sketches are drawn on napkins, cardboard, wood scraps, or any flat surface.
• While professionally produced drawings exist for many projects, tradespeople must work with what they have available.
• The goal is to describe with lines what needs to be built, repaired, or modified—this takes practice and is a very important skill to develop.

🧭 Overview

🧠 One-sentence thesis

Scaling allows drawings to represent objects at sizes different from their actual dimensions—most commonly smaller—so that blueprints remain convenient to use while preserving accurate proportional relationships.

📌 Key points (3–5)

• Why scaling exists: drawings are usually made smaller than the object for user convenience (e.g., a locomotive drawing at full size would be impractical).
• Three common scales: full size (1"=1"), half size (1/2"=1"), and quarter size (1/4"=1"); different occupational groups use different scales.
• Critical rule: never measure dimensions directly from a print if a dimension is unclear or missing, because prints shrink, stretch, and may not be drawn to scale.
• Common confusion: reading fractional dimensions on reduced scales—you must locate the whole number first, then count backwards to zero to add the fraction.

📏 Why drawings use scale

📏 Convenience and practicality

• A drawing may be the same size as the object (full size), larger, or smaller.
• In most cases, drawings are made smaller than the actual object.
• Example: a full-size drawing of a locomotive would be unmanageable to carry; a building might be drawn at 1/48 size (1/4"=1'-0"), and a map at 1/200 size (1"=100'-0").
• Conversely, very small objects (e.g., a wristwatch gear) may be drawn larger than actual size (e.g., 10"=1") to show detail.

⚠️ The cardinal rule: do not measure the print

If you need a dimension that is unclear or is not given, do not measure the print!

• Prints shrink, stretch, and may not be drawn to scale.
• Measuring directly from paper can produce very inaccurate dimensions.
• Always rely on labeled dimensions, not physical measurement of the drawing itself.

📐 Full size scale

📐 What full size means

Full scale is simply letting one inch on a ruler, steel rule, or draftsman's scale equal one inch on the actual object.

• One unit on the measuring tool = one unit on the real object.
• Rules are usually divided into 1/16" or 1/32" increments.
• This is the baseline measurement skill; if you can measure accurately at full scale, other scales build on the same principle.

📐 Reading full-size increments

• The excerpt provides a "big inch" divided into 1/32" spaces for practice.
• Each space represents 1/32 of an inch.
• Practice involves locating specific fractions (e.g., 1/32", 3/16") on the scale and marking them with arrows.

📉 Half size scale

📉 How half size works

The principle of half size measurements on a drawing is simply letting a unit, such as 1/2" on the scale, represent a larger unit such as 1" on the drawing.

• If a drawing is labeled HALF SIZE or 1/2"=1', each half-inch on the scale equals one inch on the actual object.
• This scale is used when the object is too large to draw at full size but detail is still needed.

📉 Reading fractional dimensions on half size

• Key technique: to measure a distance like 2-3/16", first locate the whole number (2), then go backwards to zero and count off the fraction (3/16).
• Whole numbers without fractions are measured in the usual forward direction.
• Don't confuse: you measure fractions by counting backwards from the whole number to zero, not forwards.
• Example: for 2-3/16", find "2" on the scale, return to "0", and count 3/16 from there.

📉 Practice and paper instability

• The excerpt includes exercises to measure lines to the nearest 1/32" using a half-size scale.
• Important reminder: exact answers cannot be given because paper is dimensionally unstable due to humidity—another reason not to measure from prints.

📊 Quarter size scale

📊 How quarter size works

Quarter size is used and read in a similar way to half size except that each unit, such as a quarter of an inch, represents a larger unit, such as one inch.

• If labeled QUARTER SIZE, QUARTER SCALE, or 1/4"=1", each quarter-inch on the scale equals one inch on the object.
• Used for even larger objects or when more reduction is needed.

📊 Reading quarter size

• The technique is similar to half size: locate the whole number, then count backwards to zero for fractions.
• Practice involves drawing lines to required lengths and having them checked for accuracy.
• A 1/4"=1" scale tool is available in the lab for exercises.

🧪 Assessment and application

🧪 Quiz structure

• Students are given a physical object to measure with a ruler or tape measure.
• Record measurements at full scale.
• Then draw each length at both 1/2 scale and 1/4 scale.
• This tests the ability to translate real-world measurements into scaled drawings accurately.

🧭 Overview

🧠 One-sentence thesis

Dimensioning completes a technical drawing by adding size information and notes to the shape views, enabling others to manufacture the object exactly as the designer intended.

📌 Key points (3–5)

• Two complete stories: A drawing must show both shape (through views) and size (through dimensions and notes) to be complete and usable.
• Legibility is critical: Incorrectly or carelessly made numbers can be misinterpreted, leading to costly mistakes in materials and time.
• Placement principles: Dimensions should be clear, not crowded, placed where features appear most clearly, and shown only once (no duplication).
• Common confusion: Dimensions under 72 inches are given in inches; over six feet are usually shown in feet and inches—mixing formats (e.g., 4'-8" vs 48") can cause misreading.
• Different drawing types need different approaches: Oblique, isometric, and orthographic drawings each have specific dimensioning rules to maintain clarity.

📏 Fundamental dimensioning elements

📏 Dimension lines

Dimension line: a fine, dark, solid line with arrowheads on each end that indicates direction and extent of a dimension.

• In machine drawings, the line is usually broken near the middle to provide space for dimension numerals.
• In architectural drawings, numerals are usually above an unbroken line.
• Spacing guidelines:
  • First dimension line: approximately 1/2 inch away from the object
  • Additional dimension lines: approximately 3/8 inch apart
• Don't confuse: The goal is not exact spacing but uniform, uncrowded placement where dimensions cannot be confused with surfaces they don't describe.

📏 Extension lines

Extension lines: fine, dark, solid lines that extend outward from a point on a drawing to which a dimension refers.

• Should have a gap of about 1/16 inch where they would meet the object outline.
• Should extend beyond the outermost arrowhead approximately 1/8 inch.
• Should have no gaps where extension lines cross each other.
• Usually meet dimension lines at right angles.
• Example: Larger dimensions are placed outside or beyond shorter dimensions for clarity.

➡️ Arrowheads

• Correctly made arrows are about 1/8 to 3/16 inch in length.
• About three times as long as they are wide.
• Usually have a slight barb, like a fishhook.
• Use the same style throughout the entire drawing for a clean appearance.

🔢 Dimension numerals and fractions

• Regular numerals: normally about 1/8 inch in height.
• Fractions: approximately 1/4 inch in height total, with slightly smaller fractional numbers to allow space above and below the fractional line.
• The excerpt emphasizes that legible numbers are critical—sloppy numbers cause expensive mistakes.

📝 Notes and dimensioning rules

📝 Notes on drawings

• Purpose: provide supplementary information.
• Should be brief and carefully worded to avoid misinterpretation.
• Located in an uncrowded area of the sketch.
• Leader lines should be kept short.
• Usually added after dimensioning to avoid interference with dimensions.

📝 General dimensioning principles

The excerpt provides seven key rules:

Rule	Purpose
Show enough dimensions	Worker shouldn't have to calculate or assume distances
State each dimension clearly	Understood in only one way
Show relationships	Dimension between points/surfaces that have necessary relationships
Avoid accumulation	Prevent buildup of tolerances that cause poor mating
Show each dimension once	No duplication
Dimension where clearest	Place on view where feature appears most clearly in true shape
Specify for availability	Use readily available materials, parts, and tools

🚫 What to avoid

• Don't dimension to hidden lines when possible.
• Don't duplicate dimensions across views.
• Don't dimension on the object itself when it can be avoided (though sometimes necessary for space).
• Don't crowd dimensions—keep them uniform and clear.

🎨 Dimensioning different drawing types

🎨 Unidirectional vs aligned dimensions

Two basic methods for placing dimensions:

• Unidirectional: dimensions read from the bottom of the sketch.
• Aligned: dimensions read from the bottom and right side.
• The excerpt states that unidirectional is usually best because it is more easily read by workers.

🎨 Oblique dimensioning

• Mostly about avoiding dimensioning on the object itself when possible.
• Use common sense principles.
• Usually best to have dimensions read from the bottom (unidirectional).
• Diameter and radius dimensions are often placed on the views if space permits.
• Don't confuse: When space and time are limited, typical rules may be bent—the most important thing is keeping the drawing clean, concise, with all required dimensions but no repeats.

🎨 Isometric dimensioning

• Keep dimensions away from the object itself.
• Place dimensions on the same plane as the surface being dimensioned.
• The excerpt notes this takes practice to do well.
• Notes are different: placed without regard to the same-plane rule (easier to do and read).
• Leader lines for notes should be sketched at angles of approximately 15, 30, 45, 60, or 75 degrees to avoid confusion with other lines.
• Never draw leader lines entirely horizontal or vertical.

🎨 Orthographic dimensioning

• Used when isometric sketches become too cluttered with dimensions.
• Provides the best way to dimension clearly and in detail for complicated objects.
• Dimensions are correctly placed between the views rather than around outside edges.
• Dimension each feature in the view where it appears most clearly and where its true shape appears.

🔧 Practical measurement considerations

🔧 Scale and measurement issues

• Paper is dimensionally unstable due to humidity.
• This is a reminder that it's poor practice to measure from a piece of paper.
• The excerpt mentions half size and quarter size scales used in practice.

🔧 Dimension format standards

• Under 72 inches (six feet): dimensions given in inches.
• Over six feet: usually shown in feet and inches.
• Critical warning: Make sure all dimensions use the same format throughout the entire drawing—mixing formats makes it easy to confuse 4'-8" with 48".
• Example: Use either 4'-5" or 53" consistently; both mean the same thing but mixing creates confusion.

🧭 Overview

🧠 One-sentence thesis

Auxiliary views are additional "helper" views that show the true size and shape of slanted or inclined surfaces that would otherwise appear distorted in standard orthographic drawings.

📌 Key points (3–5)

• Why auxiliary views exist: inclined surfaces cannot be shown without distortion in standard orthographic views, so an extra view is needed to show the true size and shape.
• What an auxiliary view does: it projects the object so the slanted surface appears as it actually is, without foreshortening or distortion.
• Three types by projection source: auxiliary views are projected from the front view, top view, or side view, depending on which surface is slanted.
• Common confusion: slanted surfaces appear shortened in regular orthographic views (distortion), but the auxiliary view shows the true length and shape.
• How to construct one: draw projection lines perpendicular (90°) to the slanted surface and transfer measurements from related orthographic views.

🎯 Purpose and function

🎯 When auxiliary views are needed

• Standard orthographic drawings (front, top, side) cannot accurately show inclined or slanted surfaces.
• Slanted surfaces appear shortened or distorted in regular views because they are not parallel to the viewing plane.
• An auxiliary view provides a more accurate description of any inclined surface.

Auxiliary view: a "helper" view that shows the slanted part of the object as it actually is by turning or projecting the object so the true size and shape of the surface are seen.

🏭 Where they appear

• Auxiliary views are commonly found on many types of industrial drawings.
• They are used whenever objects have inclined surfaces that need accurate representation.

🔧 Three types of auxiliary views

🔧 Front view auxiliaries

• The auxiliary view is projected from the front view of a three-view orthographic drawing.
• Used when the slanted surface appears in the front view.
• Projection lines are drawn perpendicular to the slanted surface in the front view.
• Only the slanted surface is typically shown in the auxiliary view; the rest of the object is omitted (though portions of adjacent surfaces are sometimes included for clarification).

Key observation: The slanted surfaces in the top and side views appear shortened due to distortion, but the auxiliary view shows the actual size.

🔧 Top view auxiliaries

• The auxiliary view is projected from the top view.
• Used when the top view contains the slanted surface.
• Developed the same way as front view auxiliaries, but projection originates from the top view instead.
• The angled surfaces shown in the front and side views are not shown in true length; only the auxiliary view shows true dimensions.

🔧 Side view auxiliaries

• The auxiliary view is projected from the side view.
• Used when the side view contains the slanted surface.
• Drawn using the same method as front and top view auxiliaries.

Selection rule: Which view to project from depends on the position of the object or which surface is slanted.

📐 How to sketch an auxiliary view

📐 Step-by-step construction

1. Start with orthographic views of the object (front, top, side as needed).
2. Add projection lines perpendicular (90°) to the slanted surface.
3. Draw a reference line at any convenient distance from the view containing the slanted surface.
4. Transfer measurements: make distances in the auxiliary view equal to related distances in one of the orthographic views.

Example: If projecting from the front view, a distance marked CB on the auxiliary view is made the same length as the related distance in the side view.

📐 Special cases in practice problems

Round holes on slanted surfaces:

• A round hole centered on a slanted surface and drilled through the object appears elliptical in front and side views (due to distortion).
• The hole appears in its true circular shape only in the auxiliary view.

Square holes on slanted surfaces:

• Similarly, square holes cut into slanted surfaces appear distorted in standard views.
• The auxiliary view shows the true square shape.

Don't confuse: The auxiliary view is developed from the view with the slanted surface, not from other views.

🎓 Complexity and applications

🎓 From simple to complex

• The examples presented are very basic and introduce the concept of auxiliary views.
• As objects with inclined surfaces become more complex, auxiliary views become essential for presenting objects in their true size and shape.
• More complex industrial objects may require multiple auxiliary views or combinations with other drawing techniques.

🧭 Overview

🧠 One-sentence thesis

Sectional views eliminate confusing hidden lines by showing objects as if cut apart, revealing internal features more clearly through imaginary cutting planes and standardized section lining.

📌 Key points (3–5)

• Why sectioning exists: complex objects with many hidden lines become clearer when drawn as if cut apart to show internal configuration.
• Core mechanism: an imaginary cutting plane passes through the object; the exposed cut surface is marked with section lining (hatch marks).
• Multiple section types: full sections (cut entirely through), half sections (one-quarter removed), broken-out, revolved, offset, and removed sections serve different purposes.
• Common confusion: section lining appears only on surfaces cut by the plane—existing holes or voids that were already there get no section lining.
• Practical advantage: sectional views show interior details without cluttering the drawing with dashed hidden lines.

✂️ The cutting mechanism

✂️ Cutting plane concept

A cutting plane is an imaginary plane taken through the object at a desired location, as if the object were cut apart.

• Think of cutting an apple in half—you choose where to slice; the same applies to technical drawings.
• The plane is imaginary; the object is not physically cut.
• Example: an engine block can be "sliced" at any location to reveal internal passages and cavities.

📏 Cutting plane line notation

A cutting plane line is a heavy long-short-short-long line terminated with arrows showing the direction of view.

• This line appears on the drawing to indicate where the imaginary cut occurs.
• The arrows show which direction you are looking from (which side of the cut you see).
• The line itself does not appear in the sectioned view—it only marks the location on the original view.

🖊️ Section lining (hatch marks)

Section lining: lines that look like saw marks, indicating the surface exposed by the cutting plane.

• Drawn at approximately 45 degrees, spaced about 1/8 inch apart.
• Very light lines, drawn carefully by eye when sketching.
• Key rule: section lining appears only on surfaces cut by the plane—pre-existing holes or voids (like a square hole already in the object) receive no section lining because they were not changed by sectioning.
• Different hatch patterns identify different materials (cast iron, steel, etc.); when the material is unknown, use the general-purpose symbol (also the cast iron symbol).

🔪 Full and half sections

🔪 Full section

When a cutting plane passes entirely through an object, the resulting section is a full section.

• Generally removes half of the object to show the interior.
• Used when a complete view of internal features is needed.
• The same object can be sectioned from different directions to reveal different internal details.

⚖️ Half section

If the cutting plane passes halfway through and one-quarter of the object is removed, the result is a half section.

• Advantage: shows both inside and outside configurations in one view.
• Frequently used for symmetrical objects.
• Hidden lines are usually omitted on the un-sectioned half unless needed for clarity or dimensioning.
• Important: the cutting plane takes precedence over the center line in the drawing.

Section type	Amount removed	Best use case
Full section	Half the object (cut entirely through)	Complete interior view needed
Half section	One-quarter (cut halfway through)	Symmetrical objects; show inside + outside together

🛠️ Specialized section types

🔨 Broken-out section

• Only a small part of the view is sectioned to show a specific internal detail.
• The section is removed by a freehand break line (irregular, sketched boundary).
• No cutting plane line is needed because the location of the cut is obvious.
• Example: revealing a small internal pocket or feature without sectioning the entire object.

🔄 Revolved section

A revolved section shows the shape of an object by rotating a section 90 degrees to face the viewer.

• Used to show cross-sectional shape changes along the length of an object.
• Example: a spear-like object with three revolved sections showing how its shape changes from one end to the other.
• The section is rotated in place, superimposed on the view.

🔀 Offset section

An offset section includes several features not in a straight line by bending the cutting plane line to pass through them.

• The cutting plane is "offset" (bent) to capture multiple features in a single sectional view.
• Allows showing several important details that don't align on a single straight plane.
• The bend in the cutting plane line is visible in the plan view but does not appear as a line in the section itself.

📤 Removed section

A section removed from its normal projected position and labeled (SECTION A-A, SECTION B-B, etc.) corresponding to letters at the cutting plane line ends.

• Placed elsewhere on the drawing, not in the standard view arrangement.
• May be partial sections.
• Often drawn to a different scale for clarity.
• Useful when the section would clutter the main views or when a larger scale is needed to show detail.

🎯 Key distinctions

🎯 What gets section lining vs what doesn't

• Gets section lining: any solid material surface that the cutting plane slices through.
• Does not get section lining: voids, holes, or cavities that already existed in the object before the imaginary cut.
• Don't confuse: section lining marks the act of cutting, not just "anything that is interior."

🎯 When to use which section type

• Use full section when you need to see the entire interior and the exterior shape is already clear.
• Use half section for symmetrical parts where both inside and outside matter.
• Use broken-out when only one small feature needs revealing.
• Use offset when important features are scattered and don't line up.
• Use removed when space is tight or a different scale is needed.

🧭 Overview

🧠 One-sentence thesis

Machined features are common industry-specific shapes and modifications—such as bevels, bosses, chamfers, and keyways—that appear frequently on technical prints and represent standard manufacturing processes.

📌 Key points (3–5)

• What machined features are: standard terms for shapes and modifications created by manufacturing processes, commonly found on prints.
• Why they matter: understanding these terms is essential for reading technical drawings and communicating about parts.
• Common confusion: distinguishing similar features—e.g., boss vs pad (boss is always circular; pad can be any shape), or keyway vs keyseat (keyway is in the hub/sleeve; keyseat is on the shaft).
• Categories covered: surface modifications (bevel, chamfer, fillet, round), hole modifications (counterbore, countersink, spotface), slots and grooves (dovetail, keyway, keyseat, T-slot), and projections (boss, lug, pad).

🔧 Surface modifications

🔪 Bevel

A surface cut at an angle.

• Often used as surface preparation for welding.
• Creates an angled face rather than a perpendicular one.

✂️ Chamfer

A process of cutting away a sharp external corner or edge.

• Explicitly not for welding (distinguishes it from bevel).
• Removes sharp edges for safety, ease of assembly, or aesthetics.
• Example: the outer corner of a shaft is cut at 45 degrees to eliminate the sharp edge.

🌊 Fillet

A small radius filling formed between the inside angle of two surfaces.

• Creates a smooth, rounded internal corner.
• Reduces stress concentration and improves strength.
• Don't confuse with round: fillet is internal; round is external.

🔵 Round

A small radius rounded outside corner formed between two surfaces.

• Creates a smooth, rounded external corner.
• Example: the outer edge of a block is rounded instead of sharp.

🕳️ Hole modifications

🔩 Counterbore

To enlarge a drilled hole to a given diameter and depth.

• Creates a flat-bottomed recess.
• Usually done for recessing a bolt head so it sits flush or below the surface.
• Example: a bolt head needs to sit below the surface—counterbore provides a cylindrical pocket.

🔻 Countersink

To machine a conical depression in a drilled hole for recessing flathead screws or bolts.

• Creates an angled, cone-shaped recess (not flat-bottomed like counterbore).
• Allows flathead fasteners to sit flush with the surface.

🎯 Spotface

A round surface on a casting or forging for a bolt head, usually about 1/16" deep.

• Provides a smooth, flat seating area on rough cast or forged surfaces.
• Shallow depth (typically 1/16 inch) distinguishes it from deeper counterbores.
• Example: a rough casting needs a flat spot for a bolt head to seat properly.

🔲 Slots and grooves

📐 Dovetail

A slot of any depth and width, which has angled sides.

• The angled sides create a trapezoidal cross-section.
• Used for sliding assemblies that resist pull-out.

🔑 Keyway vs Keyseat

Feature	Location	Definition
Keyway	In the hub/sleeve (the hole)	A narrow groove or slot cut in the shaft hole of a sleeve or hub for accommodating a key
Keyseat	On the shaft	A narrow groove or slot cut in a shaft for accommodating a key

• Both accommodate a key (a small piece that prevents rotation between shaft and hub).
• Common confusion: the names sound similar, but location differs—keyway is in the part that goes over the shaft; keyseat is on the shaft itself.

🪚 Kerf

The narrow slot formed by removing material while sawing or other machining.

• The width of the cut made by a saw blade or cutting tool.
• Material is removed, creating a gap.

🔧 Neck

To machine a narrow groove on a cylindrical part or object.

• Creates a reduced-diameter section, often for clearance or to define a shoulder.

🪛 T-Slot

A slot of any dimensions to resemble a "T".

• Cross-section looks like the letter T.
• Commonly used in machine tables to hold bolts that can slide along the slot.

🏔️ Projections and raised features

⚪ Boss

A circular pad on forgings or castings, which projects out from the body of the part.

• Must be circular (this is the defining characteristic).
• The surface is machined smooth for a bolt head to seat on.
• Has a hole drilled through to accommodate the bolt shank.
• Example: a casting has a raised circular area around a bolt hole to provide a smooth, flat seating surface.

📦 Pad

A slightly raised surface projecting out from the body of a part.

• Can be any size or shape (not limited to circular like boss).
• Provides a raised area, often for machining or seating.
• Don't confuse with boss: boss is always round; pad can be rectangular, irregular, etc.

🏷️ Lug

A piece projecting out from the body of a part, usually rectangular in cross section with a hole or slot in it.

• Typically used for attachment or lifting.
• Distinguished by rectangular cross-section and presence of hole/slot.

🎨 Surface textures

💎 Knurl

To uniformly roughen with a diamond or straight pattern a cylindrical or flat surface.

• Creates a textured, grip-friendly surface.
• Pattern can be diamond-shaped or straight lines.
• Example: a tool handle is knurled to improve grip.

⚙️ Power transmission features

🦷 Spline

A gear-like serrated surface on a shaft.

• Takes the place of a key when more torque strength is required.
• Multiple teeth/serrations distribute load better than a single key.
• Example: high-torque applications use splined shafts instead of keyed shafts for greater strength.

🧭 Overview

🧠 One-sentence thesis

This section introduces basic print reading using machine drawings as the primary working-drawing format applicable across nearly every trade.

📌 Key points (3–5)

• What this section covers: basic print reading skills using machine drawings as examples.
• Why machine drawings: they are used to some extent in nearly every trade, making them a universal teaching tool.
• Target audience: beginners learning to interpret working drawings.
• Common confusion: "print interpretation" is not limited to machine shops—the skills apply broadly because machine drawings appear across trades.

🗺️ Scope and purpose

🎯 What print interpretation means

Print interpretation: the skill of reading and understanding working drawings.

• The excerpt positions this as a "final section," suggesting it builds on earlier material (likely the machined features covered in the preceding pages).
• The focus is on basic print reading, not advanced or specialized interpretation.

🔧 Why machine drawings are used

• Machine drawings serve as the teaching vehicle because they appear "to some extent in nearly every trade."
• This makes them a practical, cross-disciplinary choice for instruction.
• Don't confuse: the section uses machine drawings as examples, but the print-reading skills themselves are transferable to other types of working drawings.

🏗️ Context and application

🏗️ Working drawings as the foundation

• The excerpt explicitly states that "the working drawings used in this section are all machine drawings."
• Working drawings are the detailed technical documents that communicate how to manufacture or assemble a part.
• Example: A machinist, welder, or inspector might all need to read the same machine drawing to understand dimensions, features, and tolerances.

🌐 Cross-trade relevance

• The excerpt emphasizes that machine drawings are used "in nearly every trade," not just machining.
• This suggests that print interpretation is a foundational skill for anyone working with technical drawings, regardless of specialty.