Dimensions and tolerances

Why it matters

No part is made exactly to size. A machine, a saw or a concrete form always varies a little. Tolerances say how much variation is acceptable, and dimensions say what the target is and where it is measured from. Get them right and parts from different shops fit together; read them wrong and a shaft will not go into its bearing.

Size and location dimensions

• Size dimensions give how big a feature is: length, width, diameter, radius, depth.
• Location dimensionsDimensions that say where a feature sits, measured from an edge, a centre line or a datum. give where a feature is, measured from an edge, a centre line or a datum.

Common symbols

Tolerance formats

1. Limit dimensions: both limits shown, larger on top. 25.05 / 25.00 means any size from 25.00 to 25.05 mm is acceptable.
2. Bilateral (plus/minus): 25.00 ±0.05 means 24.95 to 25.05. Unequal versions exist: 25.00 +0.10 / -0.05.
3. Unilateral: variation in one direction only: 25.00 +0.05 / 0.
4. General tolerancesDefault tolerances in the title block that apply to any dimension without its own, often set by the number of decimal places shown. in the title block: dimensions with no toleranceThe total allowed variation in a dimension; for 25.00 ±0.05 it is 0.10 mm. of their own use the block. Example (mm):

So on that drawing 40 means 39.5 to 40.5, while 40.00 means 39.95 to 40.05. The number of decimal places carries meaning. On inch drawings the same idea applies (.X, .XX, .XXX).

The tolerance itself is the total allowed variation: for 25.00 ±0.05 the tolerance is 0.10 mm.

Chain versus baseline dimensioning

How dimensions are arranged changes how errors add up.

• Chain dimensioningDimensioning each feature from the previous one, so the tolerances of successive dimensions add up.: each feature is dimensioned from the previous one. Tolerances stack up.
• Baseline dimensioningDimensioning every feature from one common edge or datum, so each location keeps its own tolerance.: every feature is dimensioned from one common edge or datum. Each location keeps its own tolerance.

Worked example: tolerance stack-up

Three holes are chain-dimensioned from the left edge: 20 ±0.1, then 30 ±0.1, then 30 ±0.1.

• Nominal distance from the edge to the third hole = 20 + 30 + 30 = 80 mm.
• Worst-case variation = 0.1 + 0.1 + 0.1 = ±0.3 mm, so the third hole could be anywhere from 79.7 to 80.3 mm.

If the third hole were baseline-dimensioned at 80 ±0.1 from the edge, it would stay within 79.9 to 80.1. Choose the scheme that controls what matters to the fit.

Fits between parts

When a shaft goes into a hole, the fit depends on both tolerances:

• Clearance fitA fit in which the shaft is always smaller than the hole, so it can slide or rotate.: the shaft is always smaller than the hole (slides or rotates).
• Interference (press) fit: the shaft is always larger (pressed in, stays put).
• Transition fit: could be either.

Worked example: hole 20.00 to 20.05, shaft 19.95 to 19.98. Smallest clearance = 20.00 - 19.98 = 0.02 mm; largest = 20.05 - 19.95 = 0.10 mm. Always positive, so it is a clearance fit. Standard fit systems such as ISO 286 (for example H7/g6) are tabulated so designers do not reinvent them.

Dimensioning rules you will see followed

• Each feature is dimensioned once; no repeated or conflicting dimensions.
• Dimensions are placed on the view that shows the feature's true shape.
• Dimensions are not taken to hidden lines where avoidable.
• Extension lines do not touch the part; a small gap remains.
• Units are stated once in the title block, not after every number.

Common mistakes

• Treating 40 and 40.00 as the same.
• Adding up chain dimensions without the stack-up.
• Reading 4X Ø6 as one hole.
• Using a reference dimension (in brackets) to inspect a part.
• Confusing the tolerance (total band) with the plus/minus value.

Recap

• Size dimensions say how big; location dimensions say where.
• Tolerances: limit, bilateral, unilateral, or the title-block general tolerance (decimal places matter).
• Chain dimensions stack tolerances; baseline dimensions do not.
• Fits: clearance, interference, transition, worked out from the hole and shaft limits.