How are sizes and acceptable variation added to a drawing so a part is made and inspected correctly?
Dimensioning and tolerances: the rules for dimension and projection lines, leaders and arrowheads, dimensioning circles, radii, diameters and angles, datum and chain dimensioning, and stating tolerances (limits, bilateral and unilateral).
An SQA Higher Graphic Communication answer on dimensioning and tolerances, covering dimension and projection lines, arrowheads and leaders, dimensioning diameters, radii and angles, datum versus chain dimensioning, and stating tolerances as limits.
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What this key area is asking
The SQA wants you to dimension a drawing to British Standards and state tolerances correctly: the rules for dimension lines, projection lines, leaders and arrowheads, dimensioning diameters, radii and angles, the difference between chain and datum dimensioning, and giving a tolerance as limits (bilateral or unilateral). Dimensions turn a shape into a part that can actually be made and inspected.
The rules for dimension and projection lines
Good practice keeps dimensions outside the outline where possible, does not repeat a dimension, dimensions each feature once, and places dimensions so they are not crossed by other lines. Overall sizes go furthest from the part, smaller detail dimensions nearer.
Dimensioning circles, radii and angles
- A diameter is dimensioned across the circle through the centre and prefixed with the diameter symbol O (for example O20).
- A radius is dimensioned from the centre to the arc with a single arrowhead and prefixed with R (for example R8).
- An angle is dimensioned with a curved dimension line between the two faces and given in degrees.
- A hole can be dimensioned by a leader to a note (for example "O10 THRO" for a hole right through).
Datum and chain dimensioning
The choice matters because of tolerance accumulation. In a chain, the tolerance on each link adds up, so the position of the last feature can drift well out. Datum dimensioning keeps each feature's position independent of the others, so where several features must be accurately located (for example a row of holes that must line up with another part) the datum method is used.
Stating tolerances
Tolerances exist because no manufacturing process can hit an exact size every time. A tighter tolerance costs more to achieve, so the designer specifies only as tight as the part's function demands, balancing fit and reliability against cost.
Worked example
Examples in context
Tolerancing is where graphics meets real manufacturing cost. A shaft and a hole that must turn freely are toleranced to give a clearance fit; a part pressed permanently into place is toleranced for an interference fit. The same datum thinking governs CNC machining, where the datum is the origin the machine measures from.
Try this
Q1. State the symbol placed before a diameter dimension. [1 mark]
- Cue. The diameter symbol, O.
Q2. A size is given as 25 with limits 25.05 and 24.95. State the tolerance. [1 mark]
- Cue. 0.10 mm (25.05 minus 24.95).
Q3. State one advantage of datum dimensioning over chain dimensioning. [1 mark]
- Cue. Tolerances do not accumulate, so feature positions stay accurate.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher (style)3 marksExplain the difference between chain dimensioning and datum (parallel) dimensioning, and state one advantage of datum dimensioning.Show worked answer →
In chain dimensioning each feature is dimensioned from the previous feature, so the dimensions run end to end in a chain. In datum dimensioning every feature is dimensioned from one common reference edge or point (the datum).
The advantage of datum dimensioning is that errors do not accumulate. In a chain, the small permitted error (tolerance) on each link adds up, so the last feature can be a long way out of position. Measuring everything from one datum keeps each feature's position independent and within its own tolerance, which is why datum dimensioning is preferred where several features must be accurately located.
Markers reward: chain = feature to feature, datum = all from one reference, and that datum dimensioning prevents tolerance build-up (errors accumulating).
SQA Higher (style)4 marksA shaft diameter is given as 40 with an upper limit of 40.02 and a lower limit of 39.98. State the tolerance, explain what bilateral means here, and explain why a tolerance is given rather than a single exact size.Show worked answer →
The tolerance is the difference between the upper and lower limits: 40.02 minus 39.98 = 0.04 mm. The shaft is acceptable if its actual diameter falls anywhere between 39.98 and 40.02.
It is bilateral because the variation is allowed on both sides of the nominal size 40 (plus 0.02 and minus 0.02). A unilateral tolerance would allow variation in one direction only (for example 40 plus 0.04, minus 0).
A tolerance is given rather than one exact size because no manufacturing process can hit an exact size every time. Stating limits defines how much variation is acceptable while the part still works, so parts can be made economically and still fit and function. Tighter tolerances cost more to make, so the designer specifies only as tight as the function needs.
Markers reward: tolerance = 0.04, bilateral = varies both sides of nominal, and a tolerance is needed because exact sizes are impossible and it controls cost versus fit.
Related dot points
- Orthographic projection in third-angle: the six principal views, the front elevation, plan and end elevation, how they line up and project, and the use of the projection symbol and auxiliary views for complex features.
An SQA Higher Graphic Communication answer on orthographic projection, covering third-angle projection, the front elevation, plan and end elevation, how the views project and line up, the third-angle symbol, and auxiliary views for sloping faces.
- British Standards (BS 8888) line types and conventions: continuous thick outlines, thin lines for dimensions and projection, dashed hidden detail, chain centre lines, cutting planes and the conventional representation of repeated features.
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