Surface Finish Standards: Ra, Rz & ISO 21920 Guide

Why Surface Finish Matters in Grinding

Surface finish is the first thing engineers think about when they specify grinding. A part called out at "Ra 0.4" looks tight but is actually routine on a CNC cylindrical grinder. A part called out at "Ra 0.05" needs a polishing or superfinishing step on top of grinding.

The challenge is that surface finish is specified in many different ways — Ra, Rz, Rt, RSm, and a dozen others — and the standards themselves have changed over the years. An old drawing from 1995 calls out ISO 1302; a 2026 drawing calls out ISO 21920. Buyers and engineers both need to know what they are looking at.

This guide explains the common parameters, the standards that govern them, and how to read surface finish specifications on drawings. For related topics, see Grinding Wheel Selection and Cylindrical vs Centerless Grinding.

The Main Parameters in Plain Language

Surface finish is a profile. Most parameters are statistical summaries of that profile:

Ra (arithmetic average roughness). The average of the absolute vertical deviations from the mean line. Ra 0.8 means the average deviation is 0.8 μm. This is the most common parameter worldwide.

Rz (mean peak-to-valley height). The average of the five largest peak-to-valley distances in the evaluation length. Rz is more sensitive to scratches, pits, and isolated defects than Ra. Common in European and ISO drawings.

Rt (total peak-to-valley height). The single largest peak-to-valley distance in the evaluation length. Sensitive to a single bad spot on the surface.

RSm (mean spacing of profile irregularities). The average horizontal distance between profile peaks. Important for surfaces that need to retain lubricant (cross-hatch on cylinder bores).

Rq (root-mean-square roughness). Similar to Ra but statistically more sensitive to outliers. Used in research; rare on drawings.

Of these, Ra and Rz are by far the most common on grinding drawings. RSm comes in on cylinder bores (honed surfaces). Rt is sometimes specified as a "no single defect larger than X" check.

Ra: The Default Parameter

Ra is the parameter you will see on most drawings. It is calculated as:

Ra = (1/L) ∫0L |z(x)| dx

where L is the evaluation length and z(x) is the deviation from the mean line. In practice, the profilometer samples the surface and computes this average automatically.

Ra is easy to specify, easy to measure, and correlates reasonably well with how a surface performs in sliding contact. It is not perfect — two surfaces with the same Ra can feel very different if one is wavy and the other is textured — but for grinding it is the standard.

Common Ra ranges for ground surfaces:

Ra 3.2–6.3 μm: rough grinding, often replaced by milling or turning on parts that can take it.

Ra 1.6–3.2 μm: standard cylindrical grinding finish on bearing journals and general shaft work.

Ra 0.8–1.6 μm: fine cylindrical grinding, standard for hydraulic shafts, motor shafts, transmission shafts.

Ra 0.4–0.8 μm: precision cylindrical grinding, standard for bearing rings, fuel injector bodies, precision gears.

Ra 0.2–0.4 μm: mirror-grade cylindrical grinding, requires rigid machine, fine grit, slow feed. Often followed by superfinishing.

Ra 0.05–0.2 μm: optical-grade, typically requires superfinishing or polishing in addition to grinding.

Rz: The European Standard

Rz is the average of the five largest peak-to-valley heights in the evaluation length. It was the standard parameter in Germany and much of Europe for decades, and it remains common on European drawings. Rz is also the parameter most commonly used in ISO 21920.

Rz is roughly 4–7 times larger than Ra for a typical ground surface, because Rz captures the worst-case peaks and valleys while Ra averages everything. The exact ratio depends on the surface texture — scratchy surfaces have a higher Rz/Ra ratio than smooth ones.

For a quick conversion (rough rule of thumb for ground surfaces):

Ra 0.4 μm ≈ Rz 2.0 μm

Ra 0.8 μm ≈ Rz 4.0 μm

Ra 1.6 μm ≈ Rz 8.0 μm

Ra 3.2 μm ≈ Rz 16 μm

These are approximations. Real conversions depend on the actual profile. Use a profilometer to verify, not a rule of thumb.

ISO 21920: The Current Standard (2021)

ISO 21920 was published in 2021 and replaces the older ISO 1302 (for graphical indication) and parts of the ISO 4287/4288 series (for parameter definitions). In 2026, ISO 21920 is the current international standard for surface finish specification on technical drawings.

Key changes from the older ISO 1302:

The graphical symbol changed — a checkmark with a horizontal bar through it, instead of the older triangle-with-bars.

Parameter definitions are explicitly aligned with ISO 21920-1 and ISO 21920-2.

The standard introduces the concept of "surface texture set" — a complete surface specification including roughness, waviness, and lay.

The standard supports digital annotation in CAD systems.

For older drawings still using ISO 1302 or ASME Y14.36M, the old markings remain valid — the standards have not been retroactively changed. But for new drawings, ISO 21920 is preferred.

Reading a Surface Finish Symbol

The ISO 21920 surface finish symbol is a checkmark (✓). Modifiers around it tell you the rest:

Position a: upper limit of roughness, e.g. Ra 0.8

Position b: lower limit of roughness (optional)

Position c: sampling length or evaluation length, e.g. 0.8 mm

Position d: roughness symbol modifier (production method, lay, etc.)

Position e: machining allowance

Position f: other roughness values (Rz, Rt, RSm)

Example: ✓ 0.8 / Rz 4.0 means Ra upper limit 0.8 μm and Rz upper limit 4.0 μm.

The lay (direction of the surface pattern) is shown as a small symbol next to the checkmark. For cylindrical grinding, the lay is parallel to the grinding direction (circumferential on a cylindrical grinder, axial on a centerless grinder). For honing, the lay is a cross-hatch.

Conversions and Common Confusions

Three conversions you will see on old drawings:

Rz (old) vs Rz (ISO 21920). Old ISO standards defined Rz differently. The old Rz was a single peak-to-valley measurement; the new Rz is the average of five. The new Rz is roughly 80% of the old Rz for the same surface. Old drawings with high Rz values may need reinterpretation.

Ra vs CLA vs AA. Center Line Average (CLA) and Arithmetic Average (AA) are older American terms for the same parameter as Ra. They are interchangeable.

μm vs μin vs Ra "grade" numbers. American drawings sometimes use grade numbers (e.g. "grade 32" = Ra 32 μin = Ra 0.8 μm). The grade system is from the old ISO 1302; new drawings use μm directly.

How Surface Finish Is Measured

Three methods are in common use:

Contact profilometer (most common). A diamond stylus with a 2 μm or 5 μm tip radius traces the surface. Outputs Ra, Rz, and a full profile trace. Examples: Taylor Hobson Surtronic, Mahr Federal, Mitutoyo SJ-series. Accuracy: ±5% or better.

Optical profilometer (non-contact). Uses white-light interferometry or confocal microscopy. No contact, no stylus wear, works on soft materials. Examples: Bruker, Zygo, Keyence. Accuracy: comparable to contact, slower to use.

Visual and tactile comparison. Visual comparison against a roughness standard (a set of reference samples of known Ra). Quick and dirty. Useful for shop-floor sampling but not for certification.

For grinding customers, the contact profilometer is the standard. A benchtop unit costs $3,000–$10,000 and measures in 5–15 seconds per location. Most production grinding shops have one.

Sample Length and Evaluation Length

Ra and Rz depend on the evaluation length. A typical ground surface measurement uses:

Sample length (cut-off): 0.08 mm, 0.25 mm, 0.8 mm, 2.5 mm, or 8.0 mm. Selected based on the expected roughness.

Evaluation length: typically 5 sample lengths. The Ra value is the average of 5 measurements over 5 sample lengths.

Sample length selection per ISO 21920:

Ra 0.006–0.02 μm: 0.08 mm

Ra 0.02–0.1 μm: 0.25 mm

Ra 0.1–2.0 μm: 0.8 mm

Ra 2.0–10.0 μm: 2.5 mm

Ra 10.0–80.0 μm: 8.0 mm

For typical grinding (Ra 0.2–1.6 μm), use 0.8 mm sample length. Wrong sample length gives wrong Ra.

How Grinding Parameters Affect Surface Finish

Surface finish on a cylindrical grinder is set by five parameters:

Wheel grit. Finer grit = smoother finish. 46 grit = Ra 1.6–3.2 μm. 60 grit = Ra 0.8–1.6 μm. 80 grit = Ra 0.4–0.8 μm. 100 grit = Ra 0.2–0.4 μm. 120 grit = Ra 0.1–0.2 μm.

Wheel dressing. Finer dressing (smaller dress depth, slower dress feed) = smoother wheel face = smoother ground surface.

Infeed rate. Lower infeed = smoother surface. 0.002 mm/pass vs 0.010 mm/pass can mean Ra 0.4 vs Ra 0.8.

Workpiece speed. Slower workpiece speed = smoother surface, with diminishing returns above a certain point.

Coolant flow. Adequate coolant flow prevents burn, which leaves a rougher surface. Insufficient coolant flow shows up as burn + roughness, not as a smooth burn-free surface.

For deeper reading on how wheel selection drives surface finish, see Grinding Wheel Selection for Cylindrical Grinding.

What Surface Finish Numbers Actually Mean in Practice

A few examples from common applications to anchor the numbers:

Hydraulic cylinder rod (Ra 0.4 μm): smooth enough to seal against a wiper seal; rough enough to retain lubricant on the contact surface. Common in mobile hydraulics and machine tools.

Bearing journal (Ra 0.4–0.8 μm): smooth enough for a fluid film to develop under load; not so smooth that the bearing balls skid on start-up. Specified by bearing manufacturers (SKF, FAG, NSK) on most bearing seats.

Fuel injector body (Ra 0.2 μm): smooth enough to hold the tight clearance between needle and body. Affects injector flow and spray pattern.

Medical implant (Ra 0.05–0.2 μm): polished to reduce bacterial adhesion and tissue irritation. Requires superfinishing on top of grinding.

General shaft (Ra 1.6 μm): standard for transmission shafts, motor shafts, and similar. Easy to hit on any cylindrical grinder.

If you are unsure what Ra to specify, ask what the mating part requires, not what the grinder can deliver. A Ra spec that is tighter than the application needs adds cost and cycle time for no benefit.

Surface Finish vs Form vs Tolerance

Three different geometric specifications often get confused:

Surface finish = how rough the surface is at the micron scale (Ra, Rz). Independent of roundness, cylindricity, or size.

Form = how the part deviates from its ideal shape (roundness, cylindricity, flatness). Measured at the millimetre to part-whole-scales.

Tolerance = how far the actual size can be from the nominal size (size tolerance). Measured at the part-whole scale.

A part can have Ra 0.4 (excellent finish) and still fail cylindricity 0.005 mm. A part can have cylindricity 0.001 mm (excellent form) and still fail Ra 0.4 because of chatter marks. They are independent specifications.

When writing a drawing, specify all three. When evaluating a grinding process, check all three. When buying a grinder, verify all three can be met.

Common Drawing Mistakes

Specifying Ra without specifying the cut-off length. The profilometer may use the wrong cut-off length and give a misleadingly low or high Ra. Always specify the evaluation length.

Specifying Ra without specifying the lay direction. For seal surfaces, the lay direction matters (parallel to the seal lip vs perpendicular). For aesthetics, perpendicular lay shows scratches more.

Specifying Ra when the application really cares about Rz. Bearing surfaces are often specified in Rz because Ra averages out the rare scratch that the bearing cannot tolerate.

Specifying both Ra and Rz with the same limit. Ra and Rz are different parameters with different values for the same surface. Specifying both makes the surface spec ambiguous unless the limits are carefully chosen.

Specifying Ra tighter than the process can deliver. A drawing called out at Ra 0.05 on a standard cylindrical grinder is scrap. Match the spec to the process or change the process.

Surface Finish in Different Standards

Standard	Region	Active in 2026	Notes
ISO 21920	International	Yes (since 2021)	Replaces ISO 1302
ISO 1302	International	Withdrawn	Old standard, still seen on drawings
ISO 4287/4288	International	Replaced by ISO 21920	Old parameter definitions
ASME Y14.36M	USA	Yes	American standard for surface finish symbols
GB/T 131	China	Yes	Chinese standard, aligned with ISO 21920
DIN 4762/4768	Germany	Replaced	Old German standard, replaced by ISO
JIS B 0601	Japan	Yes	Japanese standard, aligned with ISO

Most new drawings in 2026 use ISO 21920 (or its national adoption — GB/T 131 in China, JIS B 0601 in Japan, ASME Y14.36M in the US). Old drawings may use the older standards.

Surface Finish and Inspection Sampling Plans

For production inspection, sample frequency depends on part criticality:

First article: 100% inspection. Measure at multiple locations, document the values, and approve before production starts.

In-process (every N parts): typically every 10–50 parts for grinding, depending on CpK and customer requirement. Measure 1–3 locations per part.

Final inspection: 100% or sampled depending on customer requirement. Aerospace, medical, and bearing customers usually want 100%.

A sample plan that works for grinding production:

First part of the shift: 3 measurements on each of 3 parts (9 measurements total).

Every 10 parts: 1 measurement on each of 3 parts.

First part after a wheel change: 3 measurements.

First part after a setup change: 3 measurements.

Last part of the shift: 1 measurement.

This catches the usual failure modes (wheel wear, setup drift, operator error) without slowing production.

FAQ

Q: What is the difference between Ra and Rz?

A: Ra is the arithmetic average of absolute deviations from the mean line, across the evaluation length. Rz is the average of the five largest peak-to-valley heights in the evaluation length. For a typical ground surface, Rz is roughly 4–7× larger than Ra. Ra is more common in the US; Rz is more common in Europe and on ISO drawings.

Q: What surface finish can a cylindrical grinder deliver?

A: Standard cylindrical grinding produces Ra 0.4–1.6 μm routinely. Fine grit, slow feed, and rigid setup can push to Ra 0.1–0.2 μm. Below Ra 0.05 μm requires superfinishing or polishing in addition to grinding.

Q: How do I convert Ra to Rz?

A: Rough rule of thumb for ground surfaces: Rz ≈ 4–7× Ra. So Ra 0.8 μm ≈ Rz 4.0 μm. The exact ratio depends on the surface texture. For precise conversion, measure both with a profilometer.

Q: What is the current ISO standard for surface finish in 2026?

A: ISO 21920, published in 2021. It replaces the older ISO 1302 for graphical indication and the ISO 4287/4288 series for parameter definitions. Old drawings still use ISO 1302 markings; new drawings use ISO 21920.

Q: Can the same wheel deliver different surface finishes?

A: Yes, by adjusting parameters. The same 80-grit wheel can produce Ra 0.4 μm at low infeed with fine dressing, or Ra 1.6 μm at high infeed with coarse dressing. The wheel sets the upper limit; the parameters set the actual finish.

Q: What is the difference between Ra and CLA?

A: CLA (Center Line Average) is the older American term for the same parameter as Ra. CLA and Ra are interchangeable; modern drawings and instruments use Ra. AA (Arithmetic Average) is also the same parameter under a different name.

Q: What does ISO 21920 specify?

A: ISO 21920 specifies the graphical symbols for indicating surface texture on technical drawings, the parameter definitions (Ra, Rz, Rt, RSm, etc.), the sampling and evaluation procedures, and the rules for combining multiple parameters in a single specification. It replaced ISO 1302 in 2021.

Q: Should I specify Ra or Rz on my drawing?

A: For most grinding applications, Ra is sufficient and is the easiest parameter to verify. Specify Rz when the application is sensitive to isolated defects (bearing surfaces, seal surfaces) or when the drawing follows European convention. Specify RSm when the lay pattern matters (honed cylinder bores).

Bottom Line

Surface finish is specified in micrometres (Ra or Rz) on a drawing. Ra is the average roughness; Rz is the peak-to-valley average. ISO 21920 is the current standard in 2026, replacing the older ISO 1302.

For most grinding applications, Ra 0.4–1.6 μm is routine and well within the capability of any modern CNC cylindrical grinder. Ra below 0.2 μm requires fine-grit wheels, slow infeed, fine dressing, and possibly superfinishing. Specifying the surface finish tighter than the application needs adds cost without benefit.

For the underlying grinding parameters that drive surface finish, see Grinding Wheel Selection and Grinding Burn. For machine selection, see the full Yelin CNC cylindrical grinder line or contact our engineering team.