Drafting Practices

§1The drawing as a contract

A production drawing succeeds when a maker who has never met the designer can build the part, check it, and prove it conforms — with nothing left to ask.

That standard settles most style arguments. A complete sheet carries: enough views to fix every feature; every dimension exactly once; tolerances on everything that matters (and a general-tolerance note for everything that doesn’t); material, finish and treatment; and a title block owning the sheet — number, revision, scale, projection symbol, drawn/checked signatures. Revision control is part of the contract: a changed drawing gets a new revision letter and a change note, never a silent edit. In Australia the governing conventions are the AS 1100 series; internationally, the ISO drawing standards — the practices below are common ground.

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§2Projection — first and third angle

Both systems unfold a part’s faces onto flat paper; they differ only in which side of the front view each unfolded face lands on.

In third-angle projection — the common practice in Australia and North America — each view is placed on the side of the part it is seen from: the top view sits above, the right-side view sits to the right. In first-angle — traditional European practice — the view is projected through the object to the far plane, so everything lands opposite: top view below, right view on the left. The two are mirror bookkeeping of the same information, and confusing them puts holes on the wrong side of real parts, which is why the frustum symbol in the hero drawing is mandatory in the title block. Read the symbol before reading the sheet.

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§3The language of lines

Line weight and pattern carry meaning before a single dimension is read.

Line conventions
Line	Appearance	Says
Visible outline	thick, continuous	edges you can see
Hidden detail	thin, short dashes	edges behind the surface
Centre line	thin, long-short chain	axes and symmetry — also the anchor for position dimensions
Dimension / projection	thin, continuous, arrowed	measurement scaffolding, never part of the shape
Cutting plane	thick-ended chain with arrows	where a section is taken and which way it looks
Section hatching	thin, 45° equal-spaced	solid material cut by the plane
Phantom / alternate position	thin, long–double-short chain	moving parts’ other positions, adjacent parts
Break line	thin, ruled with zig-zags	a long uniform piece shortened to fit the sheet

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§4Views and sections

Use the fewest views that leave nothing ambiguous — and when the inside matters, cut the part open on paper instead of drowning it in hidden lines.

Most parts need two or three orthographic views; simple turned parts often need one plus a diameter symbol. When interiors dominate, a section replaces hidden dashes with cut, hatched material: a full section cuts straight through; a half section shows half cut and half whole on a symmetric part; an offset section jogs the cutting plane through features that don’t line up; revolved and removed sections drop a cross-section right on or beside a long member — the standard way to show a spoke or rib profile. Two conventions save confusion: ribs, webs, shafts and fasteners are not hatched when the plane runs along them (hatching would fake solidity), and hatching on one part keeps one direction so adjacent parts in an assembly can alternate.

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§5Dimensioning discipline

Dimension the function, once, from a datum — three habits that between them prevent most shop-floor arguments.

Once: every size appears exactly one time; duplicates eventually disagree. Function: dimension what the part must do (hole centres from the mating face), not what is easy to draw. Datum: measure features from a common reference, not from each other, because chained tolerances add up:

Example 1 — chain versus datum, priced

Three steps each dimensioned 20 ± 0.1 in a chain put the last face at 60 ± 0.3 — the errors stack. Dimensioned from one datum face as 20, 40, 60 each ± 0.1, no face is worse than ± 0.1. Same part, same tolerances, three times the certainty — for free. (Chain dimensioning is chosen deliberately only when the step itself is the function.)

Housekeeping that reads as competence: dimensions outside the outline, on the view that shows the feature’s shape; smaller dimensions inside larger; leaders to a hole’s edge pointing at its centre; diameters as Ø and radii as R; and no dimensioning to hidden lines — cut a section instead.

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§6Sheets and scales

ISO paper is a geometric series: every size is the previous one halved, and every size has the same √2 proportions — which is why a drawing photocopies between sizes without distortion.

ISO A-series, computed by successive halving from A0 (mm; area m²)
Size	Dimensions	Area
A0	841 × 1189	1.000
A1	594 × 841	0.500
A2	420 × 594	0.249
A3	297 × 420	0.125
A4	210 × 297	0.062
A5	148 × 210	0.031
Each size is the previous one halved across its long edge; the proportion stays 1 : √2 throughout, and A0 is defined as one square metre — which is why every A-size photocopies onto the next without distortion.

Example 2 — reading a scaled dimension

On a 1 : 5 sheet a slot measures 37 mm on paper: the feature is 37 × 5 = 185 mm. But the printed dimension figure always states true size — scaling off a drawing with a rule is the last resort, and modern practice marks it DO NOT SCALE. Preferred scales run 1:2, 1:5, 1:10 … for reduction and 2:1, 5:1, 10:1 … for enlargement, with 1:1 the default whenever the part fits.

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§7Quick reference

The working core of the page on one card rack.

Projection

third angle: view on the side seen

check the frustum symbol first

Sections

full · half · offset · revolved

ribs & shafts left unhatched

Dimensioning

once · functional · from a datum

chain of n: tolerance × n

Paper

A-series: halve to descend

A4 = 210 × 297, ratio √2

Contract test

a stranger can build and check it

revise by letter, never silently

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Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Drafting Practices. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Drafting Practices by beginning with the duty, not the component or software command. Convert the key ideas—drawing, contract, projection, views, sections—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

What function and failure consequence govern this decision?
Which inputs are measured, specified, assumed or illustrative?
How will conformity be demonstrated and recorded?
What change would invalidate the current evidence?
Are mandatory requirements distinguished from recommendations and illustrative values?
Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Drafting Practices?