CNC Surface Roughness Ra vs Rz: Chart, Differences, and How to Choose the Right Finish

A CNC-machined part can look smooth but still have microscopic peaks, valleys, and scratches that affect its performance.

These surface variations are measured using parameters such as Ra and Rz. Choosing the right surface roughness matters for sealing, sliding contact, wear, coating quality, and appearance.

This guide explains the difference between CNC surface roughness Ra and Rz, common roughness values, and how to specify the right finish without paying for unnecessary polishing or machining.

Surface roughness describes the small peaks and valleys left on a part after machining.

It is different from flatness or dimensional tolerance.

For example, a mounting plate can meet its required length and thickness but still have a relatively rough machined surface. Conversely, a smooth-looking surface may contain an isolated scratch that affects sealing or contact.

Surface roughness is normally expressed in micrometers (µm) or microinches (µin).

Two commonly specified parameters are Ra and Rz.

Ra measures average roughness, while Rz describes peak-to-valley height variation over a defined evaluation length.

Imagine two aluminum surfaces that both measure Ra 1.6 µm.

The first surface has fairly uniform machining marks. The second has similar overall texture but contains one deep scratch.

Their Ra values may be similar because Ra averages the profile deviations. However, the second surface may have a higher Rz because the deep scratch creates a larger peak-to-valley variation.

This is why Ra alone may not be sufficient for critical sealing or sliding surfaces.

Important: Ra and Rz are different parameters and cannot be converted using one universal mathematical ratio. Their relationship depends on the actual surface profile, manufacturing process, and measurement standard.

The following values are commonly used as practical references when specifying CNC-machined surfaces.

These are reference values, not guaranteed capabilities for every material or geometry. Actual results depend on the cutting tool, feed rate, material, machine condition, machining strategy, and secondary finishing process.

For many general-purpose CNC parts, Ra 3.2 µm is a practical starting point. Tighter roughness should be specified when the function requires it.

The right value depends on what the surface needs to do.

For brackets, mounting plates, and non-contact housing surfaces, Ra 3.2 µm may be sufficient when no finer requirement is needed.

For parts that contact or move against another component, a finer finish such as Ra 1.6 or Ra 0.8 µm may be appropriate. The actual requirement depends on the fit, load, lubrication, material, and operating conditions.

For O-ring seats, hydraulic components, and other sealing interfaces, do not select the finish based only on appearance.

A deep scratch or isolated surface valley may create a leak path even when the average Ra value appears acceptable. Depending on the seal design, both Ra and Rz may need to be specified.

Ra 0.4 µm or lower may be required for specialized applications, but achieving such a finish can require additional finishing operations.

Do not assume that every precision CNC part needs a mirror-like surface. Specify the roughness required for the actual function.

Surface roughness is commonly measured using a profilometer, which traces or scans a surface and calculates parameters such as Ra and Rz.

Two common measurement methods are:

Contact profilometer: A fine stylus traces the surface profile.

Optical measurement: An optical instrument evaluates the surface without direct stylus contact, where appropriate.

A caliper or CMM is useful for dimensional inspection, but it does not replace a dedicated surface roughness measurement.

For a reliable inspection result, the drawing or inspection plan should identify:

1:Required parameter, such as Ra or Rz

2:Limit value and units

3:Applicable measurement standard

4:Measurement location

5:Relevant inspection conditions for critical surfaces

For example, writing “smooth finish” is subjective. Specifying “Ra 1.6 µm maximum” gives the supplier a measurable requirement, subject to the agreed standard and measurement method.

Surface roughness depends on the machining process and the condition of the part before finishing.

Factors include:

1:Cutting speed and feed rate

2:Tool geometry and tool wear

3:Material properties

4:Machine and fixture rigidity

5:Finishing passes

6:Polishing or abrasive treatment

For aluminum parts, polishing can reduce visible machining marks, while sandblasting creates a more uniform matte appearance. Anodizing provides a protective oxide layer but does not automatically remove the texture left by machining.

The underlying surface preparation can remain visible after anodizing. For cosmetic parts, the required appearance and preparation process should be agreed before production.

Our recent guide, Surface Treatments for CNC Machined Parts: Anodizing, Polishing & Sandblasting, explains how the different processes affect appearance and surface properties.

You can also read Powder Coating vs. Anodizing: Which Metal Finish Is Right for Your Parts? when deciding how to specify the final coating or finish.

In a documented project for an Indian customer, our factory manufactured a 6061 aluminum plate containing repeated elongated slots, small holes, and perimeter mounting holes.

The manufacturing process included three-axis CNC machining, manual abrasive surface finishing, dimensional inspection, and black anodizing. The customer also visited the facility to review the tolerance and anodized surface requirements.

The project highlights an important issue: surface appearance and surface roughness are related, but they are not interchangeable measurements.

Manual abrasive finishing can reduce visible tool marks, but excessive finishing around slots and holes can round edges or affect small dimensional features.

For this reason, the required visual appearance, critical dimensions, and inspection criteria should be established before production.

The project did not disclose an Ra or Rz acceptance value, so no numerical roughness result is claimed. You can see the manufacturing details in our case study, Controlling Slot Dimensions and Surface Finish in a Black Anodized 6061 Aluminum CNC Plate.

Usually, yes, when a lower Ra value requires additional finishing operations.

A finer finish may involve slower cutting, sharper tooling, additional finishing passes, polishing, grinding, or more intensive inspection.

For example, a bracket with a non-contact external surface may not need the same roughness requirement as a sealing face.

If both surfaces are specified at Ra 0.4 µm without a functional reason, the bracket may cost more without gaining meaningful performance.

The most economical approach is to specify the coarsest surface roughness that still meets the part's functional and appearance requirements.

When requesting a quote, provide:

1:2D drawing and 3D CAD file

2:Material and grade

3:Required Ra or Rz value

4:Surfaces where the requirement applies

5:Surface treatment

6:Quantity

7:Inspection requirements

8:Delivery requirements

If the roughness requirement applies after anodizing or another finishing process, make this explicit.

Our How to Prepare a Drawing for CNC Machining guide explains how to communicate material, critical dimensions, tolerance, and surface finish requirements clearly.

For a more complete quotation package, see What to Include in a CNC Machining RFQ.

Our Custom CNC Machining Services support drawing-based components with machining and surface finishing requirements evaluated according to the project.

Ra and Rz measure different aspects of surface roughness.

Ra describes average surface texture, while Rz highlights peak-to-valley variation. For most general CNC applications, Ra provides a practical starting point. For critical sealing, sliding, or fatigue-sensitive surfaces, Rz and other requirements may also be necessary.

Choose the finish according to function, specify it clearly on the drawing, and confirm how it will be measured.

This helps balance surface performance, inspection requirements, and manufacturing cost.

Have a CNC-machined aluminum, stainless steel, or brass component that needs a defined surface finish?

Send your 2D drawing, 3D CAD file, material, quantity, and required Ra or Rz values.

Request a Quote

Our team can review the part geometry, surface treatment, and inspection requirements for your custom CNC machining project.