Surface Roughness Chart: Ra, Rz, RMS and CNC Finish Guide

What Does Surface Roughness Actually Measure?

No machined surface is perfectly flat. Under magnification, it contains peaks, valleys, feed marks, and small irregularities left by the tool and material. Surface roughness describes the closely spaced texture of that profile. It is different from waviness, which covers broader undulations, and from form error, such as taper, bow, or out-of-roundness.

This distinction matters because a part can meet dimensional tolerance and still fail in service. A seal may leak across deep tool marks. A sliding shaft may wear quickly if high peaks carry the load. A painted surface can show machining lines even when its size is correct. Roughness is therefore a functional requirement, not simply a cosmetic grade.

Measurements are normally reported in micrometres (µm) or microinches (µin). The instrument, filter, cut-off, sampling length, measurement direction, and evaluation rule all influence the reported result. When the function is important, a bare number such as “Ra 0.8” is better than “smooth,” but it may still need a stated standard and measurement direction.

Ra vs. Rz vs. RMS: The Practical Difference

Ra: arithmetic average roughness

Ra is the arithmetic mean of the absolute profile-height deviations from the mean line over the evaluation length. It compresses a large amount of profile data into one stable, familiar number. That makes it useful for general drawing control, supplier communication, and process comparison.

Its limitation is equally important: very different profiles can have the same Ra. A surface with many small, regular marks and one with a few sharp valleys may produce a similar average. When isolated defects or sealing performance matter, Ra alone may not describe enough.

Rq or RMS: root mean square roughness

Rq is the root mean square of the profile deviations and is commonly called RMS roughness. Squaring the deviations before averaging gives greater influence to larger peaks and valleys. Rq is therefore normally a little higher than Ra for the same profile.

You may see the shortcut Rq ≈ 1.11 × Ra, or Ra ≈ 0.90 × Rq. It can be a reasonable estimate for certain regular or near-Gaussian profiles, but it is not a universal conversion. The actual relationship depends on the surface-height distribution. Do not use the approximation to accept or reject a production part.

Rz: peak-to-valley sensitivity

Rz responds more strongly to profile extremes than Ra, which makes it useful for detecting pronounced tool marks, scratches, or valleys that could affect sealing and wear. The catch is that “Rz” has not meant exactly the same thing in every historical ISO, DIN, JIS, or ASME context. Modern ISO 21920 defines the parameter within its current profile framework, while older drawings and instruments may follow earlier conventions.

If a drawing specifies Rz, state the applicable standard or confirm it with the customer. Converting Rz to Ra using a fixed multiplier can hide the very peaks and valleys that Rz was chosen to control.

Surface Roughness Chart: Ra, Microinch and N Grades

The chart below converts common Ra values between micrometres and microinches and shows the traditional N-grade reference. The process descriptions are planning guidance, not guaranteed capability. Real results depend on the machine, tool, material, geometry, access, and measurement method.

What Surface Finish Can CNC Machining Produce?

For many custom parts, Ra 3.2 µm is a practical as-machined requirement. A finer Ra 1.6 µm surface is also common on functional faces when the geometry and tool access support a finishing pass. Ra 0.8 µm may be achievable by controlled turning or milling on suitable features, but it demands more attention to setup rigidity, tool edge, material behavior, and inspection. Below that level, grinding, honing, lapping, or polishing often becomes the more predictable route.

Those values should never be treated as universal machine limits. A broad, accessible turned diameter behaves differently from the floor of a deep milled pocket. Thin walls can chatter. Long, slender parts can deflect. Interrupted cuts, difficult alloys, small internal radii, and inaccessible faces may require a different process plan.

Factors that change the result

• Feed and tool geometry: feed marks and nose radius strongly influence the theoretical profile.
• Tool wear and built-up edge: a tool can hold size while leaving a poor surface.
• Rigidity: chatter from the machine, fixture, tool, or workpiece creates periodic marks.
• Material: gummy aluminum, stainless steel, hardened steel, and engineering plastics cut differently.
• Coolant and chip control: recutting chips can scratch an otherwise acceptable finish.
• Measurement direction: measuring across the lay often gives a different result from measuring along it.

How to Specify Surface Roughness Without Overpaying

The safest drawing begins with function. Ask what the surface must do: seal, slide, retain lubricant, accept paint, locate another component, or simply look clean. Then apply the requirement only to the affected faces. A blanket Ra 0.8 µm note can add finishing and inspection work to every pocket and wall even when most of them do not need it.

State the parameter, numerical limit, and units—for example, Ra 1.6 µm max. For critical applications, also state the governing standard, measurement direction, cut-off or filtering requirement, evaluation length, and acceptance rule. If Rz is used, identifying the standard is especially important. A drawing note should be measurable by both supplier and customer in the same way.

A finish requirement also needs space in the process plan. Finishing can remove material, soften edges, change dimensions, or alter coating thickness. Coordinate roughness with dimensional tolerance, edge-break notes, heat treatment, plating, and final inspection. The best result is not the smallest Ra number; it is the least costly controlled surface that reliably performs its job.

A useful RFQ checklist

• Mark only the faces that require a controlled finish.
• Specify Ra, Rz, Rq/RMS, or another parameter intentionally.
• Write the units and whether the value is a maximum or a range.
• Identify the applicable ISO, ASME, DIN, or customer standard.
• Include the measurement direction when the machining lay matters.
• Separate cosmetic expectations from measurable roughness limits.
• Share mating, sealing, wear, coating, and lubrication requirements with the supplier.