Engineering drawing: symbols and sheet sizes

November 3, 2026  ·  Indumor

The drawing came in on a Friday from a customer in Córdoba: a flange with a step on one side. On Monday the machinist made it exactly as drawn, and the part came out perfect and backwards, step on the wrong side. Nobody had looked at the small truncated cone next to the title block: the drawing was in first-angle projection, the ISO convention, and a US shop reads every print in third angle.

Sheet sizes, margins and title block

The ISO 216 A series follows one rule: A0 is one square meter and each size is half the previous one, with sides always in the ratio 1 to 1.414. ISO 5457 sets what surrounds the drawing: a frame 10 mm from the edge and 20 mm from the left edge, four centring marks, a grid of 50 mm fields so you can say "the detail is in C4", and the title block, 180 mm long, in the bottom right corner. A4 is always portrait; the other sizes are landscape.

ISO 7200 says what goes in the title block: eight mandatory fields and eleven optional ones. Scale, projection symbol and general tolerance are not on that list, but custom puts them there. US drawings use the ASME Y14.1 inch series, which starts from the 8.5 × 11 in letter sheet; a B sheet is the closest thing to an A3.

Scales

Scales come from ISO 5455: there is no standard 1:3 or 1:4, whatever the CAD system allows, and the standard leaves intermediate scales for exceptional cases. Dimensions always state the real size of the part, at any scale; measuring the print with a rule is the oldest mistake in the trade. The calculator does the two usual sums.

Lines, views and projection

ISO 128-2 uses two line widths in a 2 to 1 ratio. The usual choice: on A4 and A3, wide lines of 0.5 mm and narrow lines of 0.25; on A1 and A0, 0.7 and 0.35. Width says as much as the pattern: a narrow long-dashed dotted line is a centre line, and the same line drawn wide marks a cutting plane or a zone to be hardened. Lettering follows ISO 3098, 3.5 mm on the small sheets.

Both projection systems show the same part; what changes is where each view lands. In first angle (the old ISO method E, a name still used in Argentine IRAM 4501; used in Europe and Argentina) the part sits between the viewer and the projection plane: the top view is drawn below the front view and the view from the left goes on the right. In third angle (method A, used in the United States and Canada) the plane sits between the viewer and the part, like a glass box with the views drawn on its faces: the top view goes above and the right-side view on the right. The truncated cone says which is which.

Dimensioning symbols

ISO 129-1 symbols go in front of the number, and in front of all of them goes the tolerance nobody writes: a dimension without ± is governed by the general tolerance in the title block. ISO 2768-mK means medium class (m) for linear and angular dimensions and class K for flatness, perpendicularity and symmetry; the values are in chapter 3. Holes carry the diameter plus depth ↧, counterbore ⌴ and countersink ⌵, which ASME Y14.5 introduced. Threads are named by their designation, and their tables start in chapter 4. ISO drawings customarily use a decimal comma; ASME drawings, a point.

Geometric tolerances and surface texture

A geometric tolerance frame reads left to right: the characteristic symbol, the tolerance value (with Ø in front when the zone is a cylinder) and the datum letters in order of precedence. ISO 1101 has fourteen symbols; the four form tolerances take no datum, the two profiles may or may not, and the others do. ASME Y14.5 uses the same glyphs but dropped concentricity and symmetry in 2018; ISO keeps both.

Surface texture is called out with the ISO 21920-1 symbol (formerly ISO 1302): the check mark with the long leg, the Ra value in micrometers (US drawings often use microinches: 125 µin is Ra 3.2 µm), the process if required and the lay. Old drawings carry N grades (N7 is Ra 1.6) and the triangle marks of the old DIN 140 and DIN 3141. What roughness each process leaves is in chapter 14.

Welding symbols

A welding symbol always has the same anatomy: an arrow pointing at the joint, a reference line that carries the elementary symbol and a tail with the process. ISO 2553 adds a dashed identification line: what sits on the continuous line is welded on the arrow side, what sits on the dashed line goes on the other side. AWS A2.4 has no dashed line: below the line is the arrow side, above it is the other side. A fillet is sized by throat (a5) or leg (z7) in ISO and by leg in inches in AWS; the process goes in the tail as an ISO 4063 number (111 stick, 135 MAG, 141 TIG) or as AWS letters (SMAW, GMAW, GTAW). How those joints are prepared is in the joints and positions lesson of the MIG-MAG course.

Drawing abbreviations

Spanish drawing abbreviations are not standardized and every drawing office has its own; the table lists the ones that recur on Argentine drawings next to their English counterparts, most of them from ASME Y14.38. Two to watch: TYP, which applies to every identical feature even if the dimension appears once, and REF, an informative dimension nobody will inspect.

Frequently asked questions

What is the difference between first and third angle projection?

Where each view sits. In first angle (ISO, Argentina, Europe) the top view goes below the front view and the view from the left is drawn on the right; in third angle (United States) the top view goes above and the right-side view on the right. The truncated cone in the title block tells you which one you have.

What does ISO 2768-mK mean on a drawing?

That dimensions without a written tolerance follow the medium class (m) of ISO 2768-1 for sizes and angles, and class K of ISO 2768-2 for flatness, perpendicularity and symmetry. That second part was withdrawn in 2021 and replaced by ISO 22081, but it still turns up on many drawings. The values are in chapter 3.

How big is an A3 sheet and how much of it can you draw on?

An A3 sheet is 297 × 420 mm. With the ISO 5457 frame (20 mm on the left, 10 mm on the other sides) the drawing space is 277 × 390 mm, and the 180 mm title block takes the bottom right corner.

Sources: ISO 216:2007; ISO 5457:1999; ISO 7200:2004; ISO 5455:1979; ISO 128-2:2020; ISO 128-3:2022; ISO 5456-2:1996; ISO 129-1:2018; ISO 1101:2017; ISO 2692; ISO 21920-1:2021 and ISO 1302:2002; ISO 2553:2019; ISO 4063; AWS A2.4:2020; ASME Y14.5-2018; ASME Y14.1; ASME Y14.38; ISO 6410-1; ISO 15786; ISO 3040; ISO 3098; IRAM 4501, 4502, 4504, 4505 and 4508.