Technical Drawing Training Lesson 9: Geometrical Tolerances — The 14 Symbols, Frame and Datums

A size tolerance says how big a feature may be. It says nothing about whether a face is flat, an axis straight, or two bores in line. A shaft can be exactly Ø20 all the way along and still be banana-shaped. Geometrical tolerancing — ISO 1101 (TS 1304) — is what controls form, orientation, location and run-out.

The tolerance frame

Every geometrical tolerance is written in a rectangular frame divided into two or three compartments:

• First compartment — the symbol (what kind of tolerance).
• Second compartment — the tolerance value in millimetres, preceded by Ø if the zone is cylindrical.
• Third compartment — the datum letter or letters, where a datum is needed.

The frame is connected by a leader with an arrow to the feature it controls. If the arrow touches the outline, the tolerance applies to that surface. If it is in line with a dimension line, it applies to the axis or median plane of that feature.

A datum is marked with a filled or open triangle on the reference surface and identified by a capital letter in a box. Datums are the frame of reference the tolerance is measured against — they are chosen from the functional surfaces of the part.

The fourteen symbols

What a tolerance zone actually is

This is the point most beginners miss. A geometrical tolerance is not a dimension. It is the width of a zone that the real feature has to stay inside.

• Perpendicularity 0,05 to datum A means the real surface must lie between two parallel planes 0,05 mm apart, both exactly perpendicular to A.
• Position Ø0,1 means the real axis must lie inside a cylinder 0,1 mm in diameter centred on the theoretically exact position.
• Flatness 0,02 means the whole surface must fit between two parallel planes 0,02 mm apart, in any orientation.

Theoretically exact dimensions

When a position tolerance is used, the location itself is given as a boxed dimension — a theoretically exact dimension, with no tolerance of its own. All the permitted variation is contained in the tolerance frame instead. A boxed 40 with a position tolerance of Ø0,1 is far clearer than 40 ±0,05 in two directions, and it defines a round zone rather than a square one.

Test yourself

1. Why is a size tolerance not enough?
   Because a feature can be the right size and still be out of shape, out of square or out of position.
2. Which tolerances need no datum?
   The four form tolerances: straightness, flatness, roundness, cylindricity.
3. What does ⊥ 0,05 A mean?
   The toleranced surface must lie between two parallel planes 0,05 mm apart, both perpendicular to datum A.
4. What does the Ø before a tolerance value mean?
   The tolerance zone is a cylinder, not a pair of parallel planes.
5. What is a boxed dimension?
   A theoretically exact dimension with no tolerance of its own; the variation is controlled by the geometrical tolerance instead.
6. Where does the arrow of the tolerance frame point for an axis?
   It is placed in line with the dimension line of the feature, not on the outline.
7. What is the difference between circular and total run-out?
   Circular run-out is measured in one plane at a time; total run-out covers the whole surface as the part rotates.