What you'll learn
An engineering drawing is only useful if it tells the person making the part exactly how big every feature should be — and how much variation is allowed. That is the job of dimensioning and tolerancing. For CSEC Technical Drawing you need to understand the rules for placing dimensions on a drawing, the different methods of dimensioning, what tolerance means, and why it matters in manufacturing. This guide covers the conventions for dimensioning, the main dimensioning systems, the meaning of tolerance and limits, and good practice. By the end you should be able to dimension a drawing correctly and explain how tolerance is applied.
Key terms and definitions
Dimensioning — Adding measurements to a drawing to show the size and position of features.
Dimension line — A thin line with arrowheads showing the extent of a measurement.
Extension (projection) line — A thin line extending from the feature to the dimension line.
Leader line — A line pointing to a feature to add a note or dimension.
Tolerance — The permitted amount of variation in a dimension.
Limits — The maximum and minimum allowable sizes of a dimension.
Nominal size — The intended, ideal size of a feature before tolerance is applied.
Datum — A reference surface or edge from which dimensions are measured.
Core concepts
Why dimensioning matters
A drawing shows the shape of a part, but the person making it needs to know the exact sizes. Dimensioning adds these measurements clearly and unambiguously, so the part can be made correctly. Good dimensioning follows agreed conventions so that anyone reading the drawing understands it the same way, wherever they are.
The lines used in dimensioning
Dimensioning uses several types of thin line:
- Dimension lines run parallel to the feature being measured, with arrowheads touching the extension lines at each end, and the measurement written on or above the line.
- Extension (projection) lines extend from the feature outwards, with a small gap between the line and the feature, so the dimension line can be placed clear of the drawing.
- Leader lines point to a specific feature (such as a hole) at an angle, to add a note or a diameter.
All these are drawn as thin lines so they do not obscure the outline of the part, which is drawn with thick lines.
Rules of good dimensioning
There are important conventions to follow:
- Dimensions should be placed outside the outline of the view wherever possible, so they do not clutter the drawing.
- Each dimension should appear only once — do not repeat the same measurement.
- Smaller dimensions are placed nearer the view and larger overall dimensions further out, so lines do not cross.
- Measurements are usually given in millimetres, and the unit is stated once rather than after every figure.
- Dimension figures should be readable from the bottom or the right-hand side of the drawing.
Methods of dimensioning
There are different systems for arranging dimensions:
- Chain dimensioning places dimensions end to end in a line. It is simple but errors can add up along the chain.
- Parallel (datum) dimensioning measures every dimension from a single datum (reference edge), so errors do not accumulate.
- Dimensioning circles and holes uses the diameter symbol (⌀) for a full circle and the radius (R) for an arc, with a leader line pointing to the feature.
Tolerance and limits
No part can be made to a perfectly exact size, so a tolerance states how much variation is acceptable. The nominal size is the ideal size, and the tolerance gives the limits — the maximum and minimum sizes that are still acceptable. For example, a dimension of 50 mm with a tolerance of ±0.1 mm means the part is acceptable anywhere between 49.9 mm and 50.1 mm.
Tolerance matters because:
- A tighter tolerance (smaller allowed variation) gives a more precise part but is more expensive and harder to make.
- A looser tolerance is cheaper and easier but allows more variation.
Parts that must fit together, such as a shaft in a hole, need carefully chosen tolerances so they fit correctly — this is called a "fit".
Applying tolerance on a drawing
Tolerance can be shown in several ways: as a plus/minus value (for example 50 ±0.1), or by stating the upper and lower limits directly (50.1 and 49.9). A general tolerance may also be stated in the title block to apply to all dimensions that do not have their own specific tolerance.
Types of fit
When two parts are made to go together, such as a shaft in a hole, the relationship between their tolerances determines the fit. There are three main types:
- Clearance fit — the hole is always larger than the shaft, so the parts move or slide freely (for example, a wheel turning on an axle).
- Interference fit — the shaft is always slightly larger than the hole, so the parts are forced together tightly and do not move (for example, a bearing pressed into a housing).
- Transition fit — the tolerances overlap, so the fit may be slightly loose or slightly tight, used where an accurate location is needed but the parts must still be assembled.
The type of fit is chosen according to what the parts must do, and it is achieved by carefully selecting the tolerances on the hole and the shaft.
Conventions and the title block
A properly presented engineering drawing also follows standard conventions. Dimensions are given in millimetres without repeating the unit after each figure; a note in the title block usually states that all dimensions are in millimetres. The title block also contains information such as the drawing title, scale, the drawn-by name and date, and often a general tolerance that applies to any dimension without its own stated tolerance. Following these conventions makes the drawing clear and professional, and ensures anyone reading it understands the sizes and allowances in the same way.
Worked examples
Example 1: Reading limits from a tolerance
A dimension is given as 25 ±0.2 mm. What are the maximum and minimum acceptable sizes? Maximum = 25 + 0.2 = 25.2 mm; minimum = 25 − 0.2 = 24.8 mm. Any size between these is acceptable.
Example 2: Choosing a dimensioning method
Why might parallel (datum) dimensioning be chosen over chain dimensioning for a precise part? In chain dimensioning, small errors add up along the chain, so the final dimension can be well out. Parallel dimensioning measures each feature from a single datum, so errors do not accumulate, giving greater accuracy.
Example 3: Dimensioning a hole
How should the size of a circular hole be shown on a drawing? Use a leader line pointing to the hole, with the diameter symbol ⌀ followed by the size, for example ⌀10, meaning a 10 mm diameter hole.
Example 4: Explaining the cost of tolerance
Explain why a very tight tolerance increases the cost of a part. A tight tolerance allows only a very small variation, so the part must be made and checked more precisely, using better machines and more time. This makes manufacturing more expensive than a looser tolerance would.
Common mistakes and how to avoid them
A common error is placing dimensions inside the view or crossing dimension lines. Keep dimensions outside the outline where possible, with smaller dimensions nearer the view, to avoid clutter and crossing lines.
Students often repeat the same dimension in more than one place. Each dimension should be given once only; repeating it can cause confusion and contradictions.
Another mistake is confusing the diameter and radius symbols. Use ⌀ for a full circle (diameter) and R for an arc (radius) — mixing them up gives the wrong size.
When working with tolerance, remember the tolerance is the total permitted variation. For ±0.1, the full tolerance band is 0.2 mm (from −0.1 to +0.1). Take care to add and subtract correctly to find the limits.
Finally, do not forget that dimension and extension lines are thin lines, kept separate from the thick outline of the part, with a small gap where extension lines meet the feature.
Exam technique for "Dimensioning and Tolerancing"
For dimensioning questions, apply the conventions carefully: dimensions outside the view, given once, in millimetres, with thin dimension and extension lines and clear arrowheads. Neatness and following the rules earn marks.
Be ready to explain the difference between chain and parallel dimensioning, and why parallel dimensioning avoids the build-up of errors. Know the diameter (⌀) and radius (R) symbols and how to use leader lines for holes.
For tolerance questions, be able to calculate the upper and lower limits from a nominal size and tolerance, and explain why tolerance is needed (no part is perfectly exact) and the trade-off between precision and cost. Use the correct terms — nominal size, limits, datum, tolerance — throughout.
Quick revision summary
- Dimensioning adds measurements so a part can be made accurately, using thin dimension and extension lines and leader lines.
- Place dimensions outside the view, give each once, in millimetres, with small dimensions nearer the view.
- Chain dimensioning places dimensions end to end (errors accumulate); parallel (datum) dimensioning measures from one datum (errors do not accumulate).
- Use ⌀ for diameter and R for radius, with a leader line pointing to the feature.
- Tolerance is the permitted variation; the limits are the maximum and minimum sizes (e.g. 50 ±0.1 → 49.9 to 50.1).
- Tighter tolerances give more precise parts but cost more; parts that fit together need suitable tolerances.