How to read dimensions and tolerances on a drawing before machining

A poorly interpreted drawing is the cause of most defective parts. Before machining, it helps to understand the nominal dimension and the tolerance. At MASA Mecanizados (Subirats, Barcelona) we help you review them.

Basic elements of a machining drawing

The nominal dimension is the theoretical measure (e.g. Ø20). No part comes out exactly at 20.000 mm, so the drawing states how much it may vary. The tolerance defines that margin. A dimension with ±0.05 tolerance accepts any value between 19.95 and 20.05 mm.

What tolerance means

Tolerances are given two ways: with limits (±0.05) or with a tolerance field (h7, H7, f7). If you don't know ISO fields, read our guide on what the H7 fit is: the H7 hole is always greater than or equal to the nominal, and is the basis of precise, repeatable fits.

Surfaces and roughness

The Ra symbol indicates surface roughness. Ra 1.6 requires a fine finish (sometimes a facing pass or a radius tool); Ra 3.2 accepts more tool mark. State it whenever friction or sealing matters.

Tips to send your drawing to MASA

• Send the drawing in PDF or DWG with dimensions in mm.
• Mark the critical dimensions and their tolerance; don't leave them implicit.
• State material and quantity: they affect the process, see our guide to machinable materials.
• If in doubt, ask us first: a call saves a scrapped part.