1. The standard has changed — and a lot of drawings haven't caught up
For decades, ISO 1302 was the standard governing how surface texture is indicated on technical drawings. The most recent version, ISO 1302:2002, itself replaced earlier editions from 1992 and 1978.
ISO 1302:2002 was formally withdrawn in December 2021 and replaced by a new three-part standard:
New standard | Replaces | Covers |
|---|---|---|
ISO 21920-1:2021 | ISO 1302:2002 | Indication of surface texture on drawings (the symbols themselves) |
ISO 21920-2:2021 | ISO 4287:1997 | Terms, definitions, and surface texture parameters (Ra, Rz, and the rest) |
ISO 21920-3:2021 | ISO 4288:1996 | Rules and procedures for measurement and comparison against tolerances |
In practice, this means: a huge number of drawings still in circulation — and many CAD templates and title blocks — reference "ISO 1302" by habit, even though the current governing document is ISO 21920-1. The symbols themselves are largely unchanged in appearance, so this is not usually a source of misread geometry. But it is worth knowing as a buyer, for two reasons: (1) if a supplier's quality system or quote references "compliance with ISO 1302," it's worth asking whether they mean the current ISO 21920 series, since the old designation is no longer current; and (2) some parameter definitions and default evaluation rules were refined in the 21920 series relative to the withdrawn ISO 4287/4288, which occasionally affects how a measured part is judged to pass or fail.
North America uses a parallel, largely equivalent standard, ASME Y14.36M, for surface texture symbols — similar concepts, some different symbol conventions, and inches instead of millimeters by default.
2. Anatomy of a surface texture symbol
The surface texture symbol is built around a checkmark-like graphic (a "tick" shape), with additional marks and text positioned around it to add meaning:
Basic symbol — a bare checkmark. Means a surface texture requirement applies, but the production method is unrestricted (any process — machining, casting, forging — may be used).
Material removal required — the basic symbol with a horizontal bar added across the top, or a circle-and-bar depending on the drafting standard/CAD system. Means the surface must be produced by a material-removal process (machining, grinding), which also signals that extra stock ("machining allowance") must be left on that surface before finishing.
Material removal prohibited — the basic symbol with a circle added at the vertex. Means the surface must be left in its as-produced state (e.g., as-cast or as-forged) with no subsequent machining — often used for surfaces that must retain a scale, coating, or as-formed condition.
All-around symbol — a small circle placed at the junction of the leader line and the symbol, indicating the requirement applies to every surface around the visible outline of the part in that view, not just the single face the leader touches.
Around this basic graphic, several pieces of numerical and textual information can be added in defined positions: the roughness value itself, the measurement parameter, the sampling length or cutoff, the production method or treatment, and the lay direction.
3. The parameters: Ra, Rz, and what they actually measure
The two parameters you'll see on the overwhelming majority of mechanical drawings are:
Parameter | Full name | What it measures | Typical use |
|---|---|---|---|
Ra | Arithmetic mean roughness | The average of the absolute height deviations of the surface profile from the mean line, over the evaluation length | The default, most commonly specified value for general machined surfaces — shafts, housings, mating faces |
Rz | Maximum height of profile (in the current ISO 21920-2 definition, the mean of the peak-to-valley heights over several sampling lengths) | The vertical distance between the highest peak and deepest valley within the sampling length(s), rather than an average of all deviations | Sealing faces, gasket surfaces, and functional contact areas where an occasional deep scratch or high peak matters more than the average texture |
Why the distinction matters to a buyer: Ra and Rz are not simply two names for the same thing, and they are not interchangeable by a fixed conversion factor across all surface types (unlike the leg/throat relationship in welding, there's no universal Ra-to-Rz formula — the ratio depends heavily on the machining process and profile shape). A drawing that calls out "Ra 1.6" and a drawing that calls out "Rz 6.3" may describe surfaces that feel and perform very differently, even though both numbers look "small." If a quotation substitutes one parameter for the other to make a number look more achievable, that substitution needs to be flagged and agreed, not assumed.
Less common but still seen: Rq (root-mean-square roughness, more sensitive to occasional large deviations than Ra) and Rt (total height of the roughness profile across the full evaluation length, capturing the single worst peak-to-valley span).
4. Lay: the direction, not just the depth, of the texture
Roughness value alone doesn't fully describe a surface — the direction of the machining marks (the "lay") also matters, particularly for sealing, sliding, and fatigue-critical surfaces. Standard lay symbols include:
Symbol | Meaning |
|---|---|
| Lay approximately parallel to the surface represented in the view |
| Lay approximately perpendicular to the surface represented in the view |
| Lay angular in both directions (crosshatched) |
| Multidirectional or random lay |
| Lay approximately circular relative to the center of the surface |
| Lay approximately radial relative to the center of the surface |
| Lay particulate, non-directional, or protuberant |
A sealing face against an O-ring, for example, typically needs a circular lay (matching the direction the seal will move) rather than the linear lay left by a straight milling pass — specifying the roughness number alone without the lay symbol can result in a technically "smooth enough" surface that still leaks, because the machining marks run the wrong way relative to the sealing motion.
5. Machining allowance: the number that changes the process, not just the finish
When the "material removal required" symbol is used, a numerical value can be added to the left of the symbol specifying the machining allowance — the minimum amount of stock, typically in millimeters, that must remain on that surface after a prior operation (casting, forging, rough machining) specifically so that a finishing operation can remove it.
This is one of the most commercially significant — and most overlooked — pieces of information on a drawing for anyone quoting from castings or forgings:
Too little allowance specified (or none, when it's needed): the finishing pass may not fully clean up the as-cast/as-forged surface, causing scrap or rework.
Too much allowance specified: extra material, extra cutting time, extra tool wear, and — if the allowance is uneven due to casting variation — extra risk of breaking through to the raw surface locally.
A buyer comparing quotes from a machine shop and from a foundry-plus-machining supply chain should confirm that the machining allowance called out on the drawing (or added by the supplier if the drawing is silent) is consistent across quotes — it directly affects both raw casting/forging weight and finishing time.
6. The hidden cost of over-specifying finish
This is the section that matters most for a buyer's bottom line: surface finish cost does not scale linearly, and tightening it "just to be safe" is one of the most common sources of avoidable machining cost.
A rough guide to what different Ra bands typically require in practice:
Approximate Ra (µm) | Typical process capable of achieving it | Relative cost impact |
|---|---|---|
Ra 12.5 – 25 | Standard rough milling, turning, drilling | Baseline — essentially free with normal roughing passes |
Ra 3.2 – 6.3 | Standard finish milling or turning pass | Small cycle-time increase; usually absorbed in a normal finishing pass |
Ra 1.6 | Fine finish turning/milling, careful feeds and speeds | Noticeable increase in cycle time and tool selection sensitivity |
Ra 0.8 | Often requires grinding rather than milling/turning alone | Adds a distinct secondary operation, fixturing, and inspection step |
Ra 0.4 and below | Grinding, honing, lapping, or polishing | Substantial added cost: separate process, separate setup, separate quality check, often outsourced |
The jump between roughly Ra 1.6 and Ra 0.8 is where many parts cross from "achievable on the same machine, same setup" to "requires a second process on a second machine, often at a second supplier." That crossing point is where quotations can jump disproportionately relative to how "small" the number looks on paper.
The buyer's practical question isn't "what's the tightest finish available?" — it's "what does this surface actually need to do?" A few honest categories:
Cosmetic/non-functional surfaces: the basic symbol with a generous Ra (often Ra 3.2–6.3) is usually sufficient — tightening this for appearance alone adds cost without functional benefit.
Mating/bearing surfaces, seals, static fits: genuinely need a controlled Ra (commonly Ra 0.8–1.6) and, often, a specific lay — this is where the specification earns its cost.
Fatigue-critical or rolling-contact surfaces: may genuinely require Ra 0.4 or tighter, plus grinding — the added cost here is buying performance, not caution.
"Default tightened" specifications copied from a template or a previous, unrelated part: the most common and most avoidable cost driver. If nobody can explain why a surface needs Ra 0.8 rather than Ra 3.2, it's worth asking the design engineer before the RFQ goes out, not after three quotes come back higher than expected.
7. A buyer's checklist before sending a drawing out for quote
Does the drawing (or supplier's quality documentation) reference "ISO 1302" or "ISO 21920"? Either is fine to see referenced, but be aware ISO 1302 itself is a withdrawn standard as of December 2021 — the current governing document is ISO 21920-1.
For every toleranced surface finish: is the parameter Ra or Rz — and would substituting one for the other change what's actually being delivered?
Is a lay direction specified where the surface function (sealing, sliding, fatigue) actually depends on it — and is it missing where it should be present?
Where "material removal required" is specified, is a machining allowance value given, and is it consistent with the starting process (casting, forging, bar stock)?
For every finish tighter than roughly Ra 1.6, can someone explain the functional reason? If not, it's worth confirming with engineering before quoting, not after.
Are finish requirements consistent with the base process assumed elsewhere on the drawing — e.g., a "no material removal" surface on a feature that also carries a tight positional tolerance is often a drawing error worth flagging back to the design team rather than pricing as-is.
A short review against this list, before a drawing goes out for quotation, is usually enough to catch both the technical misreads and the invisible over-specification that quietly inflates machining cost.