Where is the title block on a drawing?

In the bottom right corner of the sheet, on both US and ISO drawings. It stays visible when the sheet is folded.

What does UNLESS OTHERWISE SPECIFIED mean?

The tolerances in that block apply to every dimension that has no tolerance of its own. A dimension with its own tolerance uses that one instead.

What does ISO 2768-m mean?

General tolerances from ISO 2768-1, class m (medium). They apply to sizes without their own tolerance. The bigger the size, the bigger the tolerance: a 20 mm size gets ±0.2 mm, a 50 mm size gets ±0.3 mm.

Why does the revision matter?

Each change to a drawing gets a new revision letter or number. Making a part from an old revision means making the wrong part.

Drawing basics: lesson 4 of 5

How to read a title block

Answer questions about the title block shown. Get 10 right to finish. Pick ASME (US) or ISO (Europe) in the box below.

A dimension on this drawing reads 5.9 with no tolerance next to it. What tolerance applies?

What is in it

The title block names the drawing and holds the facts that apply to the whole sheet. The layout differs by company, but you will nearly always find the owner, the title, the drawing number, the revision, the scale, the sheet number, the material, and who drew it and when. ISO 7200 lists which of these an ISO title block must have.

General tolerances, US style

US drawings carry a tolerance block that starts with UNLESS OTHERWISE SPECIFIED. The number of decimal places in a dimension picks the row. With the block below, 2.75 has two places and gets ±.01, while 2.750 has three and gets ±.005. The values are set by each company, so read the block on your drawing.

UNLESS OTHERWISE SPECIFIED:
DIMENSIONS ARE IN INCHES
TOLERANCES:  .X ±.1   .XX ±.01   .XXX ±.005
ANGLES ±1°

General tolerances, ISO style

ISO drawings usually name a class from ISO 2768-1, such as ISO 2768-m. The tolerance then depends on the size, not on decimal places.

Size (mm)	0.5 to 3	over 3 to 6	over 6 to 30	over 30 to 120	over 120 to 400	over 400 to 1000
f (fine)	±0.05	±0.05	±0.1	±0.15	±0.2	±0.3
m (medium)	±0.1	±0.1	±0.2	±0.3	±0.5	±0.8

Common mistakes

Dropping trailing zeros on US drawings. 2.5 and 2.500 have different tolerances.
Using the tolerance block for a dimension that has its own tolerance.
Working from an old revision. Check the revision before you start.

Questions

Where is the title block on a drawing?: In the bottom right corner of the sheet, on both US and ISO drawings. It stays visible when the sheet is folded.
What does UNLESS OTHERWISE SPECIFIED mean?: The tolerances in that block apply to every dimension that has no tolerance of its own. A dimension with its own tolerance uses that one instead.
What does ISO 2768-m mean?: General tolerances from ISO 2768-1, class m (medium). They apply to sizes without their own tolerance. The bigger the size, the bigger the tolerance: a 20 mm size gets ±0.2 mm, a 50 mm size gets ±0.3 mm.
Why does the revision matter?: Each change to a drawing gets a new revision letter or number. Making a part from an old revision means making the wrong part.

Explained in our own words and checked against several textbooks. For the full rules, see ASME Y14.2, ASME Y14.3, ISO 128, ISO 5456-2, ISO 7200 and ISO 2768-1.