Surface Roughness: Ra and Rz Parameters, Grades, Measurement and Drawing Symbols

Surface roughness is a property you cannot see with the naked eye, yet it determines friction, sealing, fatigue life and durability of a part, and it quietly drives the machining price as well. In this guide we explain how surface roughness requirements are specified, what the basic parameters Ra and Rz really say, how surface roughness measurement works, and what finish turning, milling or grinding can realistically achieve.

What surface roughness and surface texture are

No machined surface is perfectly smooth. Under a microscope even a mirror finish turns out to be a landscape of tiny peaks and valleys. These micro-irregularities, left behind by the cutting edge or the abrasive grain, are what we call roughness. Surface roughness is defined as the finest, closely spaced irregularities of a surface, which together with waviness and form errors make up what the standards call surface texture. Roughness is a fingerprint of the process itself: turning leaves one pattern, milling another, grinding yet another.

Why does the topic matter so much? Because surface roughness affects nearly every function of a part: friction and wear in sliding contact, the tightness of sealed joints, fatigue strength (the valley bottoms act as notches), coating adhesion and appearance. A rough surface is sometimes desirable, for example under adhesive bonding or paint. On the other hand, smoothness costs money: the lower the required roughness parameter, the more operations and time the surface needs. That is why roughness should be treated like any other dimension: specified deliberately, where it is needed, not everywhere "just in case".

Roughness, waviness and form errors

A measured surface profile is a superposition of three components of different scale, separated by filters during analysis.

Waviness

Waviness consists of irregularities with a larger spacing than roughness, usually caused by vibrations of the machine, tool and workpiece system. Surface waviness can let a medium leak through a joint that looked tight according to Ra alone, which is why sealing faces are checked for both components.

Form errors

Form errors are deviations at the largest scale: ovality, taper, convexity of a flat face. Formally they do not belong to roughness, but they are easy to confuse with it during measurement, as we discuss below.

Typical roughness parameters

Profile parameters are defined by EN ISO 4287 (currently being superseded by the EN ISO 21920 series, which tidies up the definitions without changing the essence). Every parameter value is calculated from the roughness profile over a sampling length, a standardized measuring distance chosen according to the expected size of the irregularities. This is the foundation of any roughness measurement: values quoted without the sampling length are not comparable.

Ra, the average roughness

Ra is the arithmetic mean deviation of the profile, colloquially the average roughness. What does Ra mean in practice? Ra 1.6 says that the average deviation of the profile from the mean line is 1.6 micrometers. Ra is the world's most popular parameter because it is stable and repeatable, but it averages: a single deep scratch can almost disappear in it.

Rz and the height of the roughness profile

Rz is the roughness height calculated from the highest peak and the deepest valley within the sampling length, in practice averaged over several lengths. Rz reacts to individual peaks of the roughness profile, so it is used where a single summit or scratch matters. As a rule of thumb Rz is 4 to 7 times larger than Ra for the same surface, though the ratio depends on the machining process.

The material ratio curve and other parameters

The standards define more quantities: Rt, Rp, Rv, RSm and Rmr, the material ratio of the roughness profile, which describes how much bearing material exists at a given depth. The material ratio curve (Abbott-Firestone curve) is invaluable for sliding surfaces, because it tells you how much of the surface actually carries the load. Day to day, however, 90% of drawings use Ra and Rz, while areal parameters (Sa, Sz) appear mainly with optical measurement.

Roughness grades

Roughness grades are conventional smoothness classes. Historically, Polish documentation used triangle classes (∇1 to ∇14), still found in older drawings and in the classic time-norm handbooks of the 1970s. The ISO grades N1 to N12 map each class to an Ra value: N6 is Ra 0.8, N7 is Ra 1.6, N8 is Ra 3.2. In practice most drawings simply state the required value, e.g. Ra 1.6, and the correct roughness value should follow from the function of the given surface, not from habit. The required roughness parameter is a designer's decision: tight for functional faces, looser for everything else.

Indicating surface roughness on a technical drawing

A technical drawing leaves no room for guesswork: surface roughness requirements are written with a standardized symbol.

The roughness symbol

The surface roughness indication is the characteristic check-shaped mark placed on an edge or a leader line. The symbol with a value (e.g. Ra 3.2) defines the requirement for the indicated surface, while a symbol in the corner of the drawing sets the general requirement for all surfaces not marked individually. A closed triangle requires material removal, a circle in the symbol forbids it. The indication can also specify the lay, the direction of the machining marks (symbols =, ⊥, X, M, C, R), which matters for seals and guideways: the direction of the roughness pattern relative to the movement of the mating part can decide whether a joint seals at all. A complete indication therefore covers the value, the parameter, and optionally the process and the lay.

Units of surface roughness

The units are always micrometers. Keep that in mind with documentation from the US market, where microinches (µin) appear: Ra 32 µin is roughly Ra 0.8 µm.

How to measure surface roughness

Roughness measurement is done most often by the contact method, and increasingly optically. In workshop conditions, testing the roughness of a surface takes literally seconds today.

Contact profilometers

A contact profilometer (also called a profilograph, colloquially a surface roughness tester) drags a diamond stylus across the surface and records its vertical deflections, then filters out waviness and calculates the parameters. Limitations: the stylus needs access to the measured spot, and it can leave a mark on very soft materials.

The surface roughness tester in practice

A portable surface roughness tester is standard equipment of any inspection room: measuring a single part takes a moment, and the result immediately shows Ra, Rz and the profile chart along the traverse. Checking the roughness of a bearing seat or a sealing face comes down to placing the drive unit and reading the values. Optical profilometers (interferometric, confocal) measure without contact and map a whole area instead of a single line, which enables areal parameters. They cost more, but they are irreplaceable for delicate and very smooth surfaces.

How to avoid errors when measuring roughness

Measuring roughness is simple, but a reliable result is easy to spoil:

• Choose the sampling length for the expected finish (the roughness standards provide selection tables). Too short a length understates the result, too long a length drags waviness into it.
• Measure perpendicular to the machining marks. Measuring along the lay can show values several times lower than reality.
• Filter out waviness and form errors. On a convex or oval surface without filtering you will measure the shape, not the micro-irregularities.
• Keep it clean. Dust, coolant residue and a fingerprint all falsify the profile.
• Measure several times in different places. Roughness is not uniform across a surface; the standards recommend averaging.
• Check calibration against a reference specimen. A misadjusted tester disqualifies measurements at requirements below Ra 0.8.

Roughness of machined surfaces: turning, milling, grinding

How do you know what roughness a part will have after machining? Every method has its characteristic range, so the finish of the target surface can be predicted at the process planning stage. Machining follows a simple rule here: roughing removes material quickly at the expense of finish, finishing does the opposite. The ranges below for mechanically machined surfaces come from the process selection handbook by Swift and Booker:

Turning

Turning leaves parallel circumferential grooves and a distinctly directional texture. Roughing gives Ra 6.3 to 25, while careful finishing with a reduced feed brings the target surface down to Ra 0.4. Feed rate and insert nose radius decide the profile a finishing pass leaves behind.

Milling

Milling leaves arc-shaped marks from the cutter teeth and typically achieves Ra 1.6 to 3.2 in finish machining. The direction of the marks depends on the strategy, which matters when the target surface has a required lay.

Grinding and finishing operations

Grinding produces a fine, dense texture and Ra 0.2 to 0.8 in typical conditions. When the requirement drops below Ra 0.1, finishing operations such as honing and lapping take over. Every such threshold means an additional operation, often on a separate machine.

Roughness, finish, function and cost

Finish requirements should follow from function. Functional surfaces, bearing seats, guideways or the sealing faces in hydraulics must be smooth, because their operation depends on it. Functional surfaces are usually a minority of the part, though; the requirements for free surfaces that touch nothing can stay at Ra 3.2 or coarser. A blanket "safe" roughness across the whole drawing is one of the most common silent price inflators, which we covered in our article on how tolerances drive machining cost. Every step down, from Ra 3.2 to 1.6 and below, means slower parameters, extra passes or a separate operation, much like setup times do. Tight requirements also mean more inspection: the roughness of the finished part then has to be confirmed by measurement, sometimes on every piece.

The good news: this cost can be calculated instantly. In MetronQ quoting, the required finish acts as a multiplier on machining time, so you can see what stepping down a grade costs before the drawing ever reaches the shop. What the tool itself costs is in the pricing.

Frequently asked questions about surface roughness

How is surface roughness measured? Most simply with a contact tester: the stylus travels perpendicular to the machining marks over a standardized sampling length, and the device calculates Ra and Rz. Optical profilometers are used for delicate parts.

What does Ra 3.2 mean? That the mean deviation of the profile from the center line does not exceed 3.2 micrometers. It is the typical requirement for ordinary machined surfaces and the standard result of proper milling or turning.

What is the difference between roughness, waviness and form errors? Scale and cause. Micro-irregularities are tool marks, waviness usually comes from vibration, and form errors (ovality, taper) are deviations of the whole geometry. Filters separate them during analysis.

What is the typical roughness of machined surfaces? After finish turning and milling Ra 1.6 to 3.2, after careful finishing down to Ra 0.4, after grinding Ra 0.2 to 0.8, after honing and lapping below Ra 0.1.

When should I specify Rz instead of Ra? When a single peak or scratch matters rather than the average: on contact and sealing faces. Nothing prevents you from specifying both.

What are the roughness requirements when a surface seals? For typical static seals Ra 1.6 to 3.2 is enough, for dynamic seals and metallic contact faces the requirement drops to Ra 0.2 to 0.8, always with waviness and lay under control. Final values come from the seal manufacturer.

How do I avoid measurement errors? Measure perpendicular to the marks, over the correct sampling length, with clean and calibrated equipment, several times in different places, with proper filtering. Details in the section on measurement errors above.