Guide
How do you read and write a surface finish symbol on a drawing?
A surface finish symbol is the check-mark-shaped graphic that a drawing uses to state the texture a surface must have: whether material has to be removed to make it, which roughness parameter and limit apply, how the tool marks may run, and how the result is judged. In the US it is defined by ASME Y14.36; on ISO drawings by ISO 21920-1, which replaced ISO 1302 in 2021.
By Saola Manufacturing engineering team · Published · Updated
This guide is about the notation, not about choosing the value. It covers the three basic symbol forms, where each piece of a callout sits around the symbol, the lay symbols, how to write a roughness callout a supplier cannot misread, and the mistakes that most often turn a finish requirement into a dispute at incoming inspection. Which value a seal groove or a bearing seat needs is covered in the surface finish chart guide.
On this page
What do the three basic surface finish symbol forms mean?
The bare check mark means any manufacturing method is allowed, a bar closing the short leg means material must be removed, and a circle inside the V means material must not be removed. Everything else in the callout hangs off one of these three forms.
The basic symbol is two straight lines of unequal length meeting at a point that touches the surface, or an extension line or leader from it. Its point always sits on the surface it controls, from outside the material. On its own, with no value attached, the bare symbol says very little: it only has meaning when it carries a requirement, or when it is used as a key referring to a note. On a drawing made to ISO 21920-1, the symbol also carries a short dash above the triangle on its left side, which marks it as a profile specification under the new series; the three forms and their meanings are unchanged.
The two modified forms matter more than drafters often realize. The closed (triangle) form tells the supplier the surface has to be cut, ground or otherwise machined, so an as-cast, as-forged or as-stamped face does not conform even if its texture happens to meet the number. The circle form says the opposite: the surface must be left as it came from the previous process, which is how a drawing protects a cast datum face or a rolled sheet surface from a well-meaning cleanup pass. If you leave the bare form on a casting drawing, the supplier is free to choose either route.
| Symbol form | Meaning | Typical use |
|---|---|---|
| Basic check mark | Any process permitted | General note, or a surface where the route does not matter |
| Check mark with bar (closed triangle) | Material removal required | Machined seal lands, bearing seats, datum faces cut after casting |
| Check mark with circle in the V | Material removal prohibited | As-cast or as-rolled faces that must not be touched |
| Any form with a horizontal extension line | Requirements follow | Carries parameter, limit, process note and lay |
| Any form with a circle at the corner | All around | Same requirement on every surface around the outline in that view |
Where does each value go around the symbol?
Each entry has a fixed position, labeled a through e: the parameter and limit at a, a second requirement at b, the process at c, the lay symbol at d and any machining allowance at e. Because the positions are fixed, a reader always knows what each entry means.
The positions are those defined in ISO 1302:2002. The 2018 revision of ASME Y14.36, which lists ISO 1302:2002 among its references, moved US practice much closer to that layout. Older US prints, drawn to Y14.36M-1996 or the earlier ANSI standard, often show nothing more than a bare number tucked into the V of the check mark. By the convention of those editions, that bare number is an Ra limit, in microinches on an inch drawing, but nothing on the symbol itself says so, which is why converted or redrawn legacy prints are a common source of finish disputes.
| Position | What goes there | Example entry |
|---|---|---|
| a | First (or only) requirement: parameter, limit, and filter or length data if not default | Ra 3.2 |
| b | Second requirement, when two apply | Rz 12.5 |
| c | Manufacturing method, treatment or coating | ground |
| d | Lay symbol | C |
| e | Machining allowance, in millimeters on ISO drawings | 0.5 |
Position a is the only one most drawings need. Positions c and d are where a drafter can say what the number cannot: that a bore must be honed rather than just bored, or that a face-seal land must carry circular lay. Use the process position sparingly, though. Naming a method locks the supplier into it, and a note such as “ground” on a surface that could be finish-turned to the same texture can add an operation without improving the part.
What do the lay symbols on a surface finish callout mean?
Lay is the direction of the dominant tool-mark pattern, and the lay symbol states it relative to the view in which the callout is drawn. Specify lay only where direction changes function, such as a sealing face or a sliding surface.
The direction matters because a roughness average says nothing about which way the marks run. Radial scratches across a gasket or O-ring face give fluid a path from inside to outside, while circular marks of the same roughness run parallel to the seal and do not. The symbol is read against the drawing view: the “=” and “⊥” symbols refer to the plane of projection of the view that carries the callout, so moving the callout to another view can change what it asks for.
| Symbol | Lay direction | Where it typically comes from |
|---|---|---|
| = | Parallel to the plane of projection of the view | Planing, shaping, surface grinding; direction set by the view |
| ⊥ | Perpendicular to the plane of projection of the view | The same processes, with the marks running the other way in that view |
| X | Crossed in two oblique directions | Honing |
| M | Multidirectional | Lapping, some milling paths |
| C | Approximately circular about the center of the surface | Facing on a lathe |
| R | Approximately radial from the center of the surface | Some face grinding |
| P | Particulate, non-directional or protuberant | Blasting, electrical discharge machining |
How do you write an Ra callout that cannot be misread?
Write the parameter by name, the limit with its unit or a drawing-wide unit note, and say whether the value is a maximum, an upper and lower pair, or a max-rule limit. Then name the standard and edition in the title block so the defaults behind the callout are fixed.
Every callout has defaults hiding behind it: the filter cutoff, the evaluation length and the rule for comparing measured values with the limit. Per ISO 1302:2002 and ISO 4288:1996, a plain upper limit is judged by the 16% rule, meaning up to 16% of measured values may exceed it, and adding “max” to the parameter switches to the max rule, where none may. The ISO 21920 series, which replaced ISO 1302 and ISO 4288 at the end of 2021, changed those defaults. Under ISO 21920-3, which holds the new default specification operators, the max rule is the default and the 16% rule has to be called for explicitly. A symbol carrying the ISO 21920-1 profile dash is read under the new series; one without it is read under ISO 1302 and ISO 4288. A callout copied from an old print onto a new-series symbol can therefore quietly change meaning, and the title block reference is what tells the inspector which set of defaults to use.
| Requirement in words | ISO-style entry at position a | Legacy US print |
|---|---|---|
| Machined; Ra no more than 3.2 µm, default comparison rule | Ra 3.2 on the removal-required symbol | 125 in the V (µin) |
| Machined; no reading may exceed Ra 0.8 µm | Ramax 0.8 | No direct equivalent; add a note |
| Dynamic seal surface; Ra between 0.2 and 0.4 µm | U Ra 0.4 over L Ra 0.2 | 16 over 8 (µin), stacked |
| As-cast face; no machining allowed | Circle-in-V symbol, with or without a value | Circle-in-V symbol |
| Face-seal land; circular lay required | Ra 1.6 with C at position d | 63 in the V, C beside the symbol |
On a US drawing that works in microinches, write the unit once in a general note and keep every callout in that unit. Per ASME B46.1, which defines the parameters themselves, Ra and Rq (often still written “RMS”) are different calculations, so a callout reading “125 RMS” is not the same requirement as an Ra limit of the same number. If you inherited RMS callouts, decide which parameter you mean and rewrite them; do not leave the supplier to guess.
For most drawings the cleanest approach is one general requirement plus a few local callouts. Per ISO 1302:2002, the general requirement is placed near the title block, followed by the basic symbol in parentheses, which reads as “this applies everywhere except where another symbol is shown.” Where the same complex requirement repeats on many surfaces, a letter on the symbol, such as a “y” or “z” defined in a note, keeps the views readable.
What are the most common surface finish symbol mistakes?
Missing units, an unnamed parameter, the wrong symbol form for the process, and callouts on surfaces that do not need them. Each one either adds cost or leaves the supplier and the inspector reading the drawing differently.
- A bare number with no unit or standard. A value that is reasonable in microinches would be a very rough surface if read in micrometers, and the reverse. Put the unit in a general note and the standard in the title block.
- The bare check mark on a machined datum. If the face must be cut, use the material-removal-required form. Otherwise an as-cast face that happens to meet the number conforms.
- A removal-required symbol on every surface. Every closed-form callout on a casting or stamping is a machining operation the supplier has to plan and price. Use it where the surface seals, locates or carries a bearing.
- No statement about coating. Anodizing and plating change the texture. Say whether the requirement applies before or after the coating, as explained in the before-or-after-coating guide.
- Lay specified on a surface where it does not matter. Lay control can force a particular toolpath or an extra pass; reserve it for sealing and sliding faces.
- Symbol pointing from inside the material, or upside down. The point must touch the surface from outside, and the callout must read from the bottom or the right of the drawing like other text, so each entry is read in its intended position.
- Mixing systems on one print. N-grade numbers, bare microinch values and ISO-style entries on one drawing invite three interpretations. Convert to one form; the ASME vs ISO GPS guide explains why the governing standard should be stated once and followed throughout.
What should the RFQ say about surface finish for machined parts?
The RFQ should carry a drawing where every functional surface has a complete callout and the rest is covered by one general note, plus a short list of the surfaces whose finish you will inspect. That lets a supplier price the finishing passes up front instead of finding them at first article.
In the package, include the governing standard and edition, the unit, the general requirement, and any surface where a lower limit, a max-rule limit or a lay requirement applies. If you will measure finish at incoming inspection, say which surfaces and ask for the measurement method to be agreed before production, since cutoff and evaluation length change the reading. The RFQ package guide lists the rest of what to send.
At Saola, turned parts come off CNC turning and cast housings are finished on CNC milling; both capability pages list a standard as-machined finish, and the milling page asks for a finer finish callout only where a gasket, seal or bearing actually lands. Every drawing is reviewed callout by callout, and you receive a marked-up copy of your own drawing, so an ambiguous finish symbol is raised before tooling rather than after first samples. Plated or coated parts move on to the surface finishing menu of finishes our partner plants apply, which lists the substrate finish expected before plating. When the callouts are complete, send the drawing for a quote.
Frequently asked
Is the surface finish symbol the same in ASME and ISO?
Mostly. Both use the check mark, the bar for material removal, the circle for no removal and the same lay letters, and the 2018 edition of ASME Y14.36 moved the US layout much closer to ISO 1302. ISO 21920-1 adds a short dash above the triangle on the left, and the ISO 21920 series makes the max rule the default. On a modern print the main differences are the unit, the symbol variant and the defaults behind the parameter, which come from the standard named in the title block.
What does a number in the V of the check mark mean on an old drawing?
On older US drawings it is conventionally an Ra limit, in microinches on an inch drawing, treated as a maximum. Confirm it with the drawing owner before quoting if the print has no unit note, especially if it has been converted from another CAD system or redrawn.
Do I need a surface finish symbol on every surface?
No. Use one general requirement for the drawing and local symbols only where a surface seals, locates, slides or carries a bearing. Each extra callout is a surface the supplier has to control and the inspector has to measure.
Can the surface finish symbol specify the machining process?
Yes, at position c, with words such as turned, ground or honed. Use it only when the process itself matters, because naming a method rules out other routes that could reach the same texture.
What does the circle at the corner of the symbol mean?
It is the all-around indication: the requirement applies to every surface around the closed outline shown in that view. It does not cover the front and back faces of the view, so check those separately.
Sources
- ASME Y14.36-2018, Surface Texture Symbols, ASME, 2018
- ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Lay), ASME, 2019
- ISO 21920-1:2021, Surface texture: Profile — Part 1: Indication of surface texture, ISO, 2021
- ISO 1302:2002, Indication of surface texture in technical product documentation, ISO, 2002 (withdrawn 2021)
- ISO 21920-3:2021, Surface texture: Profile — Part 3: Specification operators, ISO, 2021
- ISO 4288:1996, Surface texture: Profile method — Rules and procedures for the assessment of surface texture, ISO, 1996 (replaced 2021)