Engineers place tolerances on dimensions to define how much a part is allowed to vary from its nominal size while still fitting, functioning, and being economical to produce.

What tolerances are

  • A tolerance is the permitted variation around a target dimension, usually given as upper and lower limits (for example, 10.00 mm ± 0.05 mm).
  • It recognizes that no real manufacturing process can make a part at an exact, single value every time.

Why tolerances are necessary

  • To ensure parts still fit together and work correctly despite inevitable manufacturing variation, especially for mating features like shafts and holes.
  • To guarantee interchangeability, so any part from any batch will assemble correctly with its matching components.

Cost and manufacturability

  • Tight tolerances demand more precise machines, slower processes, and stricter inspection, which increases cost and lead time.
  • Looser tolerances are cheaper and easier to manufacture but, if overdone, can cause poor fit, leaks, vibration, or outright failure.

Communication and quality control

  • Tolerances clearly communicate to manufacturing and inspection what is acceptable and what is scrap, providing objective pass/fail criteria.
  • They highlight which dimensions are most critical to function, guiding process planning, tooling choice, and inspection strategy.

Big picture

  • Without tolerances, designers would either demand impossible “perfect” parts or leave everything ambiguous, leading to assembly problems, rework, and safety risks.
  • Proper tolerancing balances function, fit, and cost , which is why it is a core skill in engineering drawings and GD&T practice.

Information gathered from public forums or data available on the internet and portrayed here.