Dimensioning and Tolerances: Complete Beginner's Guide

Dimensioning means writing exact sizes on an engineering drawing using lines, arrows, numbers and symbols, so anyone can make the part correctly without asking questions. Nothing is ever measured off the drawing with a ruler.

Tolerances are the small allowable variations written next to a dimension, because no machine can make a part exactly to size. Together, dimensions and tolerances decide whether parts fit together, how much they cost to make, and whether an assembly will actually work.

Key Concepts

• Nominal size: the target size intended, for example 25 mm for a shaft.
• Limits: the largest and smallest sizes allowed. Upper deviation is the amount above nominal, lower deviation the amount below it.
• Tolerance: the total permissible variation, equal to upper limit minus lower limit. It is always a positive number and has no sign.
• Unilateral tolerance: variation allowed on one side only, such as 25 +0.02 / −0.00. Bilateral tolerance: allowed on both sides, such as 25 ± 0.01.
• Clearance fit: the hole is always bigger, so parts slide freely. Interference fit: the shaft is always bigger, so parts press together. Transition fit: it could go either way.
• IT grades: standard grades from IT01 (extremely fine) to IT16 (very coarse); finer grades cost much more.
• Units: on mechanical drawings all dimensions are in millimetres unless stated otherwise, and the unit symbol mm is not written on each dimension.

Formulas

• Tolerance: tolerance = upper limit − lower limit. Example: 25 +0.02 / −0.01 gives limits 25.02 and 24.99 mm, so tolerance = 0.03 mm.
• Maximum clearance: largest hole − smallest shaft. Minimum clearance: smallest hole − largest shaft. Both are in millimetres and assume the hole and shaft limits are known.
• Maximum interference: largest shaft − smallest hole. Applies only when the fit is interference; the result is the tightest the joint can be.

Worked Example

A hole is specified as 25.00 to 25.03 mm and a mating shaft as 24.96 to 24.98 mm. Determine the type of fit and its clearances.

1. Hole limits: smallest 25.00 mm, largest 25.03 mm. Shaft limits: smallest 24.96 mm, largest 24.98 mm.
2. Hole tolerance = 25.03 − 25.00 = 0.03 mm. Shaft tolerance = 24.98 − 24.96 = 0.02 mm.
3. Maximum clearance = largest hole − smallest shaft = 25.03 − 24.96 = 0.07 mm.
4. Minimum clearance = smallest hole − largest shaft = 25.00 − 24.98 = 0.02 mm.
5. Even the smallest hole (25.00) exceeds the largest shaft (24.98), so this is a clearance fit with clearance from 0.02 mm to 0.07 mm.

Common Misconceptions

• "Tighter tolerance is always better." Tighter tolerances need slower machining and more inspection, raising cost sharply — specify only as tight as the function demands.
• "I can measure the drawing if a dimension is missing." Never. Drawings are not made to an exact measurable scale, and scaling a drawing is forbidden — a missing dimension means the drawing is incomplete.
• "Chain dimensioning is fine." Placing dimensions end to end accumulates every tolerance, so the last feature can drift badly. Dimension critical features from a single datum instead.

Real-World Applications

• Engines: piston clearances of hundredths of a millimetre decide smooth running or seizure.
• Bearings: ball bearings are made to standard fits so any bearing of the right code fits any matching housing worldwide.
• Interchangeable manufacturing: tolerances let parts made in different factories assemble correctly — the basis of mass production.

Dimensioning writes exact sizes; tolerances state the allowed variation, with tolerance equal to upper limit minus lower limit. The right fit and no-tighter-than-needed tolerances make parts assemble correctly at sensible cost.