Surface finish, thread and weld symbols on engineering drawings

Three families of symbols carry a surprising amount of manufacturing cost on a drawing: surface finish symbols, thread callouts and weld symbols. A single Ra (average surface roughness) value decides whether a face is turned once or sent for grinding. A missing thread depth leaves the machinist guessing. A fillet size read the wrong way can leave a structural weld 30 % undersized.

Each family has its own standards in ISO and in US practice. The traps that catch graduates appear when a drawing crosses between European and US suppliers.

Surface finish symbols: the three basic symbols

Surface finish symbols (also searched as surface roughness symbols) are defined by ISO 21920-1:2021 in ISO practice and ASME Y14.36-2018 (R2024) in US practice. The basic symbol is a tick of two unequal legs at 60 degrees to the surface. Two modifications give three symbols with different process permissions.

Symbol	Meaning	Typical use
Open tick	Any manufacturing process permitted	Cast, forged or machined, the supplier chooses
Tick with closing bar	Material removal required	Machined bearing seats, sealing faces
Tick with inscribed circle	Material removal not permitted	Leave as cast, as rolled or as forged

The requirement text, such as Ra 1.6, sits under the horizontal line extending from the long leg. Further positions around the symbol can carry a second parameter, the manufacturing method, a lay symbol and a machining allowance. Lay is the direction of the tool marks.

What Ra means, and what it hides

Ra is the arithmetic mean deviation of the profile. To find it, trace the surface, draw the mean line, and average the absolute height of every point above or below it. It is stable and easy to measure, which is why it dominates drawings.

Ra is an average, so it cannot tell a surface of sharp peaks from one of deep valleys. A single deep scratch barely moves it. Rz, the maximum height of the profile (peak to valley), reacts to those extremes. Neither tells you the lay. For seals, bearing surfaces and fatigue-critical faces, a checker may ask for Rz or a lay direction in addition to Ra.

ISO 1302 to ISO 21920: what changed

ISO 1302:2002 has been withdrawn and replaced by ISO 21920-1:2021. At the same time ISO 21920-2:2021 replaced ISO 4287:1997 (parameter definitions) and ISO 21920-3:2021 replaced ISO 4288:1996 (specification operators). The practical changes for a designer:

Topic	ISO 1302 era	ISO 21920
Symbol	Basic tick	Same three symbols plus a small extra bar that marks them as ISO 21920-1
Default acceptance	16 % rule: up to 16 % of measured values may exceed the limit	Max rule: no measured value may exceed the limit
Parameter definitions	ISO 4287	ISO 21920-2
Measurement defaults	ISO 4288	ISO 21920-3

The new acceptance rule makes the same number tighter. An old “Ra 1.6” tolerated a few high readings; a new one does not. Keep that in mind when you revise a legacy part to the new standard.

Older drawings: N-grades and microinches

Older drawings may use roughness grade numbers from the obsolete 1992 edition of ISO 1302. US drawings to ASME Y14.36 with ASME B46.1-2019 definitions state Ra in microinches (µin). The conversion is 1 µm = 39.37 µin.

Default surface texture and what processes achieve

Most faces share one finish, so drawings state a default in or near the title block. A typical default is the material removal symbol with Ra 3.2, followed by the basic symbol in brackets. The bracketed symbol means “all other surfaces”: only the faces that differ carry their own symbol on the views.

Machining handbooks quote these typical ranges. They are guidance only, not a standard, and tooling, material and feed move them a lot:

Process	Typical Ra (µm)
Turning	0.8 to 3.2
Milling	1.6 to 6.3
Grinding	0.1 to 1.6

How to draw and write a thread callout

Threads are never drawn as helices. ISO 6410-1:1993 and ASME Y14.6-2001 (R2025) use a simplified convention: the crest is a continuous wide line and the root a continuous narrow line. The crest is the major diameter on an external thread and the minor diameter on an internal thread. The wide and narrow lines therefore swap places between a bolt and a tapped hole.

In the end view the root is drawn as a narrow arc of about three quarters of a circle, preferably open in the upper right quadrant. The chamfer circle is omitted. In a section the hatching (US: section lining) extends to the crest line, which for a tapped hole is the minor diameter.

Metric thread callouts

A metric thread callout to ISO 965-1:2026 gives the letter M, nominal diameter, pitch and tolerance class:

Callout	Reads as
M10x1.5-6H	Internal thread, 10 mm, 1.5 mm pitch, tolerance class 6H
M10x1.5-6g	External thread, same size, class 6g
M10-6g	Coarse pitch may be omitted, so this is the same as M10x1.5-6g
M10x1.25-6H	Fine pitch, so the pitch must be stated

The tolerance class is a grade number followed by a position letter: capitals for internal threads, lower case for external.

For a tapped blind hole, state both depths: the full thread depth, and the tap drill diameter and depth. For example: “M8-6H, thread depth 16, drill Ø6.8 depth 20”. The drill must go deeper than the thread so the tap has room for its lead-in and the chips have somewhere to go. Check that the engagement is enough with the thread engagement length calculator, and set the tightening torque with the bolt torque and preload calculator.

Unified inch thread callouts

Under ASME B1.1-2024, 1/2-13 UNC-2A reads as nominal size 1/2 in, 13 threads per inch, Unified coarse series, class 2, with A for external. The mating tapped hole is 1/2-13 UNC-2B. Classes run 1, 2 and 3, from loosest to tightest.

Weld symbols: ISO 2553 system A, system B and AWS

Weld symbols are governed by ISO 2553:2019 in ISO practice and AWS A2.4:2020 in US practice. ISO 2553 offers two systems, and a drawing must use one only; never mix them.

Feature	ISO 2553 system A	ISO 2553 system B and AWS A2.4
Reference line	Continuous line plus a dashed identification line	Single continuous line
Arrow side weld	Symbol on the continuous line	Symbol below the line
Other side weld	Symbol on the dashed line	Symbol above the line
Symmetrical weld	Dashed line omitted	Symbols on both sides
Fillet size	a (throat) or z (leg), prefixed	AWS: leg size

System A is the long-standing European practice; system B follows the single-line Pacific Rim and US style. A tail at the end of the reference line carries the welding process, a specification or other reference.

Fillet size: a versus z

ISO drawings state fillet size as a, the throat thickness, or z, the leg length, with the letter written before the number. For an equal-leg 90 degree fillet the throat is the height of an isosceles right triangle, so

$$z = a\sqrt{2}, \qquad a = \frac{z}{\sqrt{2}}$$

An a5 fillet therefore has a leg of $z = 5\sqrt{2} \approx 7.1$ mm. AWS A2.4:2020 states fillet size as the leg, with no prefix letter.

Surface finish, threads and welds sit alongside the dimensions and general tolerances from lesson 4. The final lesson, how to read an engineering drawing, puts every symbol from this module together on one annotated drawing.