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Engineering Graphics and CAD · Lesson 11 of 35

Dimensioning principles and presentation (ISO 129-1)

Add size and location information so a part is fully and legibly defined, following ISO presentation rules.

Readiness check

Learning objectives

By the end of this lesson you can:

Identify the elements of a dimension: extension line, dimension line, termination (arrowhead), and value.
Place dimensions using ISO 129-1 general rules.
Distinguish size dimensions from location dimensions and provide both.
Avoid dimensioning to hidden lines.
Choose units and decimal presentation consistently.

Check your starting point

Five to ten minutes.

On a drawing you have seen, were the numbers written on top of the part outline, or on lines placed off the part?
What is the difference between saying "the hole is 10 across" and "the hole is 15 from the left edge"?
If a drawing is in millimetres, does every dimension need "mm" written after it?

Interpretation.

Q1: Off the part, on dedicated dimension lines. Numbers on the outline are hard to read. This lesson formalizes placement.
Q2: The first is a size (how big the hole is); the second is a location (where it sits). Both are needed, as Skill 11.3 explains.
Q3: No; units are stated once (usually a general note), not repeated on every value. Skill 11.4 covers this.

You need Part I and II (views, sections, auxiliaries), because you dimension the views you can now produce and read.

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 5, Lesson 10, then return.

3 or more weak itemsWork through the prerequisite examples before continuing.

The core idea

What it is. Dimensioning is adding numerical size and location information to views, using standard elements and placement rules, so that a part is completely and legibly defined. ISO 129-1 sets the presentation rules.

Why an engineer needs it. Views show shape; dimensions give size and position. Without dimensions a drawing is only a picture. With clear, standard dimensions it becomes a definition a maker can build to and an inspector can measure against.

What problem it solves. It communicates exact sizes and positions in a form every reader interprets identically, placed so the drawing stays readable.

What goes wrong when it is ignored. Dimensions crowded onto the part, attached to hidden lines, or duplicated, become unreadable or contradictory. Missing location dimensions leave features unplaced. Inconsistent units cause errors.

A simple mechanical example. A plate with a hole needs its overall length and width (size), its thickness (size), the hole diameter (size), and the hole's distance from two edges (location). Each is placed on a dimension line off the part, so the outline stays clean.

The elements of a dimension (ISO 129-1):

Extension (projection) lines: thin lines extending from the feature, with a small gap from the outline, between which the dimension is measured.
Dimension line: a thin line between the extension lines, ending in terminations (arrowheads).
Value: the number, placed on or above the dimension line, readable from the bottom (and, by convention, from the right).
Leader: a thin line with an arrow pointing to a feature, used for notes and diameters.

Core placement rules:

Dimension visible outlines, not hidden lines.
Place dimensions between the views they relate to, and off the part.
Leave a small gap between the extension line and the outline.
Do not duplicate a dimension.
State units once (millimetres by default) and keep decimals consistent.

Part 3: Dimensioning and technical definition.

Check: explain the decision in your own words before using a CAD command.

The concept. A complete dimension set communicates both size and location without repeating values or crowding the geometry.

The skills, taught in order

Skill 11.1 - Build a dimension from its elements

Concept. Every dimension is made of extension lines, a dimension line with terminations, and a value. Terminology. Extension line, dimension line, termination (arrowhead), value, leader. Procedure. Draw thin extension lines from the feature (with a gap), a dimension line between them with arrowheads, and place the value. Reasoning. Standard elements are read identically by everyone; the gap keeps the outline clean. Failure mode. Extension lines touching the outline, or values buried on the part. Check. Label the four elements on a dimension.

Skill 11.2 - Place dimensions off the part and between views

Concept. Dimensions live in the space around and between views, not on the geometry. Terminology. Placement is where a dimension sits relative to the views. Procedure. Put each dimension off the outline, between the related views where possible, grouped and aligned, with larger dimensions outside smaller ones to avoid crossing. Reasoning. Off-part, grouped placement keeps the drawing readable and the outline clear. Failure mode. Dimensions on the part or crossing each other. Check. Confirm no dimension sits on the outline.

Skill 11.3 - Give size and location

Concept. Every feature needs both a size and a location (except the overall part, which needs size). Terminology. A size dimension gives how big; a location dimension gives where, measured from a reference. Procedure. For each feature, state its size (diameter, length) and its location (distances from chosen references). Provide overall sizes too. Reasoning. A feature with size but no location is unplaced; with location but no size is undefined. Failure mode. Giving hole diameter but not its position. Check. For a hole, give both its diameter and its two location distances.

Skill 11.4 - Units and decimals

Concept. Units are stated once and decimals kept consistent. Terminology. The general note states the default unit (mm). Procedure. State millimetres once (a note or title block). Keep the number of decimals consistent and appropriate to the tolerance. Reasoning. Repeating units clutters; inconsistent decimals confuse precision. Failure mode. Writing "mm" on every value, or mixing 10, 10.0, and 10.00 arbitrarily. Check. State how many times "mm" should appear on a millimetre drawing (answer: once, as a note).

Worked example 1: fully dimension a plate with two holes

Problem. A rectangular plate is 80 long, 50 wide, 10 thick, with two holes of diameter 12, each located 20 from the nearer short edge and centred across the width. Dimension it fully and legibly.

Planning. Give overall sizes, then hole sizes, then hole locations from chosen references, placing everything off the part.

Solution.

Overall sizes. Length 80 and width 50 on the front view (off the part); thickness 10 on the side view.
Hole size. Diameter 12, given once with a leader and the diameter symbol; a note "2 holes" (or the count) shows it applies to both, avoiding duplication.
Hole location across width. Both holes are centred across the 50 width, so a single dimension of 25 (or a centre line with a symmetry note) locates them vertically.
Hole location along length. Each hole is 20 from its nearer short edge. Locate them from a chosen reference (for example both from the left edge: 20 and 60), or symmetrically; avoid also dimensioning the 40 gap, which would duplicate and risk conflict (see L12).
Units. State millimetres once as a note. Keep values as whole numbers (consistent decimals).

Result. A fully dimensioned plate: overall 80 by 50 by 10, holes diameter 12 located 20 from the edges and centred across the width, all off the part, no duplication.

Why the method works. Separating size from location and choosing single references gives a complete, non-redundant definition.

How to verify independently. Count: can every feature be made from the dimensions given, with none missing and none repeated? If yes, the scheme is complete and non-redundant.

Worked example 2: clean versus crowded placement

Problem. The same plate is dimensioned by a beginner who attaches dimensions to hidden lines, crowds them between the views, and repeats the hole location two ways. Show a clean placement beside the poor one and explain the consequences. The complication is that the information is all present but placed badly.

Planning. Compare readability and inspection, not just correctness of numbers.

Solution.

Poor placement issues. (a) A thickness dimension attached to a hidden line in the front view instead of the visible side view. (b) Overall and feature dimensions stacked so extension lines cross. (c) Hole location given both as "20 from the left" and "40 between holes" and "20 from the right," over-defining the 80 length.
Why each is a problem. (a) Dimensioning to hidden lines invites misreading and violates ISO 129-1. (b) Crossing extension lines make values ambiguous. (c) The three location dimensions plus the overall 80 form a closed, over-defined chain: if the tolerances do not add up, the drawing is contradictory (L12).
Clean version. Thickness on the visible side view; dimensions grouped with larger ones outside smaller; hole location given once from a single reference; overall length given once.
Consequence. The clean version is read and inspected without ambiguity; the crowded version risks wrong parts and inspection disputes.

Comparison. Both carry the same nominal sizes, but only the clean placement is unambiguous and inspectable. Presentation is not cosmetic; it determines whether the definition is usable.

Result. The clean placement (visible-line dimensioning, grouped, single references) is correct; the crowded version is technically "complete" but unreadable and over-defined.

Independent check. Trace each extension line: none should start on a hidden line or cross another. If any do, the placement needs fixing.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Dimension to hidden lines to save a view."	It avoids adding a view or section.	ISO 129-1 says dimension visible geometry; hidden-line dimensions mislead.	The dimensioned edge is dashed.	Dimension a visible view or a section.	"Is this dimension on a visible edge?"
"Write mm after every number."	It looks explicit.	Units are stated once; repetition clutters.	Every value carries "mm."	State units once in a note.	"Is the unit given once as a note?"
"More dimensions make it safer."	It feels thorough.	Duplicated dimensions over-define and can conflict (L12).	The same distance is given two ways.	Give each dimension once.	"Is this distance already defined elsewhere?"

Practice ladder

Level A - Recognition

Task. Label the four elements of a dimension on five examples and flag any dimensioned to a hidden line. Deliverable. Five labelled examples with flags. Success criteria. Elements correct; hidden-line dimensions flagged. Answer guidance. Extension line, dimension line, termination, value. Common errors. Confusing extension and dimension lines. Difficulty. Low.

Level B - Guided application

Task. Add the missing size and location dimensions to a partly dimensioned bracket, with prompts. Deliverable. The completed dimensioning. Success criteria. Every feature has size and location; nothing on hidden lines; no duplication. Answer guidance. Work feature by feature: size, then location. Common errors. Adding size but not location. Difficulty. Medium.

Level C - Independent application

Task. Fully dimension a supplied bracket drawing from scratch to ISO 129-1 presentation. Deliverable. A fully dimensioned drawing. Success criteria. Complete, non-redundant, legible, off the part, units once. Answer guidance. Separate size and location; choose single references. Common errors. Crowding and duplication. Difficulty. Medium.

Level D - Transfer and design

Task. Take an undimensioned part and produce a complete, legible dimensioning scheme, then explain two placement choices you made. Deliverable. The dimensioned drawing plus a short rationale. Success criteria. Complete and legible; rationale shows deliberate reference and placement choices. Answer guidance. Anticipate L12: choose references that will tolerance well. Common errors. Placing dimensions by convenience rather than clarity. Difficulty. Medium to high.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to list ISO 129-1 placement rules and check against this lesson.
Ask for a dimensioning checklist to apply to your drawing.
Ask it to critique a described dimensioning scheme.

Limits:

A text assistant cannot see whether your dimensions are on visible lines or crowded.
It may suggest duplicating dimensions for "clarity."

Verify AI output against: ISO 129-1 rules (visible geometry, off the part, units once), and the no-duplication principle.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "Should I dimension the hole depth to the hidden line in the front view?" and the assistant replies: "Yes, dimension it to the hidden line so you do not need a section."

This violates ISO 129-1. Detect it with the rule: dimension visible geometry, not hidden lines. The evidence is practical: hidden-line dimensions are ambiguous and often misread. Correct conclusion: add a section or use a view where the feature is visible, then dimension it there.

Retrieval and spaced review

Name the four elements of a dimension.
Where are dimensions placed relative to the part and the views?
What is the difference between a size and a location dimension?
Why not dimension to hidden lines?
How often are units stated?
Why avoid duplicating a dimension?
Cumulative (L10): Where do you dimension a feature on an inclined face, and why?
Reconstruction task: From memory, dimension the two-hole plate from Worked Example 1.

Answers. 1: extension line, dimension line, termination (arrowhead), value. 2: off the part, between the related views. 3: size gives how big; location gives where, from a reference. 4: hidden-line dimensions are ambiguous and against ISO 129-1. 5: once (a note or title block). 6: duplication over-defines and can conflict. 7: in the true-shape auxiliary, because principal views foreshorten it.

Suggested review intervals. 1 day, 3 days, 7 days.

Reference mapping and next step

Read further

Giesecke ch.10
ISO 129-1:2018.

Standards details must be checked against the current official edition used by your institution or employer.

Finish the lesson

You can now: build dimensions from their elements; place them off the part and between views; give size and location; avoid hidden-line dimensioning; and handle units and decimals.

Self-assessment checklist.

I can name and draw the four dimension elements.
I place dimensions off the part, grouped.
I give both size and location for each feature.
I never dimension to hidden lines.
I state units once and keep decimals consistent.

Next lesson: L12 - Functional, non-redundant dimensioning. Why it follows: you can now place dimensions correctly; next you learn to choose which dimensions to give, from function, so the scheme is non-redundant and tolerances behave, the difference between a tidy drawing and a manufacturable one.

Required files or submissions: submit your Level C fully dimensioned bracket. Optional extension: dimension a household part and check that every feature has both size and location with no duplication.