Surface roughness directly affects friction, sealing, wear, coating performance, and the fit of CNC-machined parts. Choosing the wrong roughness parameter can lead to unclear drawings, inconsistent inspection results, unnecessary finishing, or functional problems after assembly.
Ra represents average profile deviation, while Rz emphasizes peak-to-valley height and localized vertical irregularities. This guide explains how both parameters are calculated and measured, what affects their values, how CNC processes influence them, and how to specify them correctly on engineering drawings.
What Is Surface Roughness?
Surface roughness refers to the small, closely spaced peaks and valleys on a manufactured surface. On CNC-machined parts, these variations mainly result from tool movement, feed marks, vibration, material response, and finishing processes. Unlike waviness or form errors such as flatness, straightness, and roundness, roughness is quantified through surface roughness parameters, typically reported in micrometers (µm) or microinches (µin).
Common surface roughness parameters include:
- Ra (Arithmetical Mean Roughness): Average deviation of the profile from its mean line.
- Rz (Maximum Height of the Roughness Profile): Peak-to-valley height within the roughness profile.
- Rq (Root Mean Square Roughness): Places greater emphasis on larger profile deviations.
- Rt (Total Height of the Roughness Profile): Full vertical range across the evaluation length.
- Rp (Maximum Profile Peak Height): Highest peak above the mean line.
- Rv (Maximum Profile Valley Depth): Deepest valley below the mean line.
- RSm (Mean Width of Profile Elements): Average spacing between profile features.
- Sa (Arithmetical Mean Height): Average height variation across a three-dimensional surface area.
What Is Ra Surface Roughness?
Ra measures the average absolute deviation of a surface profile from its mean line across the evaluated length. Combining deviations above and below the mean line into a single value provides a practical indication of the overall roughness level and is widely specified on CNC machining drawings. However, Ra does not reveal the exact height, shape, or spacing of individual peaks and valleys, so surfaces with similar Ra values may still have different profile characteristics.
In general, smaller Ra values correspond to finer surface textures, while larger values indicate greater overall profile variation. For instance, Ra 0.8 μm indicates a smoother average texture than Ra 3.2 μm, although both values provide only an overall indication of roughness rather than a complete representation of the surface profile, which is why a surface finish chart is often used for practical comparison.
What Is Rz Surface Roughness?
Rz focuses on the vertical difference between the highest peak and deepest valley in an evaluated roughness profile. Its precise calculation can vary with the applicable measurement standard. Since Rz reflects extreme vertical features rather than averaging all profile deviations, it responds more clearly to pronounced peaks, deep valleys, and localized surface irregularities. However, the value does not define the spacing, direction, or distribution of those features.
A smaller Rz value generally means that the surface has less peak-to-valley variation, while a larger value points to more pronounced height differences. Rz 6.3 μm, for example, represents a smaller vertical range than Rz 25 μm, but the number alone does not show how the peaks and valleys are arranged across the surface.
Are Ra and Rz Values Directly Related?
Ra and Rz often move in the same direction, but they do not have a fixed mathematical relationship. Both values come from the same surface profile, yet they respond to that profile differently. Ra averages all absolute deviations from the mean line, while Rz is more sensitive to the vertical distance between pronounced peaks and valleys. As a result, two surfaces can have similar Ra values but noticeably different Rz values when one contains deeper grooves, isolated scratches, or uneven tool marks.
Approximate Ra-to-Rz ratios may support early estimates when the material, machining process, tool path, surface pattern, and measurement conditions remain consistent. However, changes in feed marks, tool wear, vibration, profile shape, cutoff length, or evaluation length can alter the relationship. Ra and Rz should therefore be treated as related but independent parameters rather than values that can be converted through one universal ratio.
Key Differences Between Ra and Rz Surface Roughness
The main difference between Ra and Rz lies in the surface information each parameter emphasizes. Ra summarizes the average roughness level across the evaluated profile, while Rz focuses on the vertical distance between peaks and valleys. This difference affects how each value responds to scratches, grooves, tool marks, and other localized surface features.
Measurement Principle and Calculation
Ra and Rz are calculated from a filtered roughness profile after the mean line has been established. Ra averages all absolute profile deviations, while Rz evaluates the vertical distance between the highest peaks and deepest valleys. For Ra, deviations above and below the mean line are converted into absolute values and averaged across the evaluation length:
Ra = (1/L) × ∫₀ᴸ |z(x)| dx
In these formulas, L is the evaluation length, x is the position along the profile, and z(x) is the profile height relative to the mean line. dx represents a small interval along the measured profile. In the digital form, N is the number of sampled points, while zᵢ is the height of each point relative to the mean line.
Rz follows a different calculation principle. Within an individual sampling length, the maximum profile height comes from the highest peak above the mean line and the deepest valley below it:
Rzᵢ = Rpᵢ + Rvᵢ
Here, Rpᵢ is the highest peak height and Rvᵢ is the deepest valley depth within sampling length i. The exact evaluation of Rz depends on the applicable standard, as some methods assess a single sampling length while others average values across multiple sections. These differences, together with the analytical and digital calculation forms for Ra, are outlined in height parameters. Ra therefore represents overall average roughness, whereas Rz responds more directly to pronounced peak-to-valley variation.
Surface Feature Sensitivity
Ra and Rz differ in their sensitivity to surface features because they evaluate profile variations in different ways. Ra is sensitive to overall changes in surface texture, including consistent tool marks, feed patterns, and general machining roughness across the measured profile. However, Ra has lower sensitivity to isolated peaks, deep valleys, or individual surface defects because their influence is averaged across the measured profile.
Rz can respond more strongly to pronounced vertical features because it is determined by peak-to-valley height within the applicable sampling structure. As a result, localized scratches, vibration marks, burrs, or machining defects can cause a larger change in Rz compared with Ra. Ra is therefore more representative of general surface texture, while Rz provides additional information about extreme surface irregularities that may not be reflected clearly by average roughness values.
Application Focus
Ra and Rz support different types of surface evaluation in CNC quality control. Ra is mainly used to track the overall roughness level and compare surface consistency between parts, machining operations, tools, or production batches. Since it summarizes the full profile as an average, Ra provides a practical basis for process monitoring and general finish comparison, but it may not clearly reveal isolated vertical extremes.
Rz is primarily applied where quality control must limit pronounced peak-to-valley variation rather than average texture alone. It places greater emphasis on abnormal grooves, raised peaks, chatter marks, and tool-related surface changes that may have only a limited effect on Ra. Comparing Rz across parts or production batches provides a direct basis for verifying whether localized surface height remains controlled. Ra focuses on overall texture consistency, whereas Rz supports closer control of pronounced vertical irregularities.
| Comparison | Ra | Rz |
| Calculation basis | Average absolute profile deviation | Peak-to-valley height under the applicable standard |
| Surface representation | Overall texture level | Pronounced vertical variation |
| Feature sensitivity | Less affected by isolated defects | More sensitive to deep grooves and high peaks |
| Result stability | Generally more stable across repeated measurements | More affected by measurement location and local extremes |
| Primary focus | General finish and process consistency | Functional risks from localized irregularities |
| Drawing specification | Common for routine surface requirements | Selected for sealing, contact, fit, or critical surfaces |
How Are Ra and Rz Surface Roughness Measured?
Ra and Rz are measured by analyzing surface profile variations obtained from different measurement methods. The two most common approaches are contact measurement and optical measurement, each using different principles to capture surface characteristics. The appropriate method depends on factors such as surface condition, measurement requirements, feature size, and the purpose of inspection.
Contact Measurement
Contact measurement is commonly performed with a stylus profilometer, also known as a contact profilometer or surface roughness tester. A diamond-tipped stylus moves across the surface at a controlled speed and records its vertical displacement as it follows the profile peaks and valleys. The instrument then filters the recorded profile, establishes the mean line, and calculates Ra, Rz, and other roughness parameters over the defined sampling and evaluation lengths.
Stylus profilometers are widely used for routine inspection of accessible CNC metal parts. However, the stylus tip may not fully enter valleys narrower than its radius, while the contact force can affect soft materials, thin coatings, or delicate surfaces. Since each trace covers only one line, the measurement location and direction should reflect the machining lay. The calibration and adjustment of these instruments are covered by ISO 12179:2026, supporting more consistent and traceable profile measurements.
Optical Measurement
Optical measurement is performed with equipment such as optical profilometers, white-light interferometers, focus-variation microscopes, and laser scanning confocal microscopes. The system directs light onto the surface, scans the selected area, and converts changes in the reflected optical signal into height data for individual surface points. White-light interferometers calculate height from light-path interference, focus-variation systems locate the sharpest focal position, and confocal microscopes detect focused reflected light at different vertical levels.
Compared with contact methods, optical systems can capture larger surface areas and generate three-dimensional surface maps without touching the part. This makes them suitable for soft materials, thin coatings, polished surfaces, and microfeatures that may be affected by a stylus. However, reflectivity, transparency, steep slopes, optical resolution, and data-processing settings can influence the result. ISO 25178-606:2026 defines relevant design and metrological characteristics for focus-variation instruments.
What Affects Ra and Rz Measurement Results?
Ra and Rz results depend on both the actual surface profile and the conditions used to evaluate it. Changes in measurement direction, profile length, filtering, inspection location, or instrument capability can produce different values from the same part. These effects are especially important for Rz because a single high peak or deep valley can noticeably change the result.
- Measurement direction: A trace taken across the machining lay usually crosses more feed marks and vertical changes than one taken parallel to the lay. This often produces higher Ra and Rz values, especially on turned or milled surfaces with directional tool marks.
- Cutoff and filter settings: Filters determine which profile variations are treated as surface roughness rather than waviness or short-scale noise. Changing the cutoff can include or exclude broader surface features, causing the reported Ra and Rz values to increase or decrease.
- Sampling and evaluation length: A longer profile covers more tool marks and has a greater chance of capturing scratches, chatter, high peaks, or deep valleys. Ra generally changes gradually as more data are averaged, while Rz may rise sharply when an extreme feature enters the evaluated section.
- Inspection location: Roughness can vary near edges, tool-entry and exit areas, transitions, or regions with unstable tool engagement. Measurements taken from different positions on the same part may therefore produce different results even when the overall machining process remains unchanged.
- Instrument capability: A large stylus tip may bridge narrow valleys instead of reaching their full depth, while limited optical or lateral resolution may miss fine surface details. These limitations can lower the recorded roughness, particularly the peak-to-valley height reflected by Rz.
- Surface cleanliness and condition: Dust, coolant residue, and loose chips can introduce false height variations, while oxidation, burrs, or handling damage change the actual measured surface. Both conditions may increase Ra or Rz or create inconsistent readings.
- Applicable standard: Different standards may apply different filters, evaluation lengths, and parameter definitions to the same profile data. Using a consistent standard, such as ISO 21920-3:2021, helps prevent procedural differences from changing the reported results.
How Do Different CNC Machining Processes Affect Ra and Rz Surface Roughness?
CNC machining processes affect Ra and Rz by creating different surface patterns and changing peak-to-valley characteristics. Milling creates cutter marks, turning forms helical grooves, and grinding produces finer abrasive textures. General process changes affect both values similarly, while isolated defects often increase Rz more significantly.
CNC Milling
CNC milling leaves overlapping cutter marks as rotating cutting edges engage and disengage from the workpiece. Feed per tooth influences the material removed by each edge, while step-over controls how closely adjacent tool paths overlap. A higher feed per tooth generally enlarges the feed marks, and a wider step-over can leave taller residual scallops, causing both Ra and Rz to increase. Finer settings usually reduce the profile height, although cutter geometry, tool-path direction, and material response also shape the final texture.
Tooth-to-tooth variation can disturb this repeated pattern. Cutter runout may cause one edge to cut deeper than the others, while worn edges, unstable engagement, or chatter can create isolated grooves and ridges. Ra may change only moderately when these features occupy a small portion of the evaluated profile. Rz often rises more sharply because an unusually deep groove or high ridge directly expands the local peak-to-valley height.
CNC Turning
CNC turning generates a continuous helical profile through the combined rotation of the workpiece and linear feed of the cutting tool. The relationship between feed per revolution and tool nose radius defines the spacing, depth, and curvature of the turning grooves. A higher feed creates wider and deeper grooves, increasing both Ra and Rz, while a larger nose radius can flatten the profile and reduce both values under stable cutting conditions. Machine rigidity, secure CNC fixtures, and cutting-edge condition determine how consistently this theoretical pattern is reproduced.
Tool wear, built-up edge, material tearing, chip dragging, and vibration can disrupt the regular helical texture. Instead of uniform feed marks, the surface may develop smeared material, uneven ridges, or deep spiral grooves. Ra reflects the overall decline in surface consistency, whereas Rz responds more strongly when one of these defects creates an unusually large vertical variation.
Grinding and Finishing Processes
Grinding removes material through many abrasive grains, producing closely spaced scratches that are usually finer than milling or turning marks. Abrasive grit size, wheel condition, dressing quality, feed rate, and grinding force determine the depth and uniformity of the remaining texture. A fine, properly dressed grinding wheel generally reduces both Ra and Rz by replacing larger machining marks with shallower and more evenly distributed abrasive paths.
Wheel loading, damaged grains, trapped particles, or unstable grinding can leave isolated scratches that keep Rz high even when the surrounding surface has a low Ra. Honing, lapping, polishing, and superfinishing can further remove profile peaks and reduce both values. However, uneven material removal may improve the average roughness while leaving deep residual valleys. In this case, Ra may decrease more noticeably than Rz, indicating that the surface has become smoother overall but still contains localized irregularities.
How to Choose Between Ra and Rz for Your Machined Parts?
Selecting Ra or Rz for a custom CNC precision component should follow the surface function and the type of profile variation that must be controlled. Ra is usually sufficient for general finish control, while Rz takes priority when isolated peaks or valleys could affect sealing, contact, wear, or fit. Critical surfaces may require both parameters to prevent an acceptable average value from hiding local irregularities.
General Surface Requirements
Ra is usually the more appropriate choice when the main objective is to maintain a consistent overall finish across the machined surface. Its averaging method reduces the influence of individual peaks or valleys, making the result relatively stable for production monitoring, supplier comparison, and batch-to-batch quality control.
This approach fits external housings, brackets, covers, cosmetic surfaces, non-critical mounting areas, and surfaces prepared for coating or general assembly. On these surfaces, overall smoothness and visual consistency matter more than a single local irregularity. An additional Rz limit may provide little practical benefit unless scratches or deep tool marks create a specific performance risk.
Functional Surface Requirements
Rz is preferred for surfaces where localized features may affect part performance. A surface may meet its Ra requirement while still containing a deep groove, raised ridge, or isolated machining defect that changes sealing pressure, lubrication behavior, contact stability, or assembly fit.
Sealing faces, bearing seats, shaft journals, sliding interfaces, press-fit areas, and precision mating features often require this closer control. Deep valleys may create leakage paths or interrupt the intended contact area, while high peaks can concentrate load, increase initial wear, or interfere with proper seating. Rz makes these extreme vertical variations more visible than an average value alone.
Critical Surface Control
Ra and Rz are better specified together when the surface must maintain both a controlled average texture and a strict limit on local extremes. Ra confirms that the broader machined profile remains within the intended finish level, while Rz prevents isolated grooves or ridges from being hidden within an acceptable average.
This combined approach is suitable for fluid components, hydraulic components, precision sliding parts, high-performance sealing interfaces, closely fitted assemblies, and surfaces exposed to repeated contact or cyclic loading. It provides stronger functional control, but the requirement should remain justified by actual performance needs because tighter dual limits can increase machining time, finishing effort, and inspection complexity.
How to Specify Ra and Rz Requirements on CNC Drawings?
Surface roughness requirements on CNC drawings should clearly define the required parameter, value, and evaluation standard to avoid different interpretations during machining and inspection. A clear specification helps manufacturers understand whether the requirement focuses on overall surface texture, peak-to-valley variation, or both.
Select the Right Parameter
The drawing parameter should reflect the surface characteristic that affects part performance. Ra defines the allowable average texture, while Rz limits pronounced vertical variation. Where both characteristics matter, each parameter should receive its own limit rather than relying on an estimated relationship between them.
Specify Ra for overall finish control, Rz for localized peak-to-valley control, and both when neither requirement can be safely inferred from the other. Avoid adding multiple roughness parameters without a functional reason, as unnecessary controls can restrict manufacturing options and increase inspection work.
Define Clear Callouts
A clear callout should state the parameter symbol, numerical limit, unit, and limit condition, such as Ra 0.8 µm max or Rz 6.3 µm max, to maintain consistency during CNC machining services. It should also identify the exact face, diameter, groove, or contact area covered by the requirement and clarify whether the value applies to the entire surface or only a designated functional zone. Separate callouts are preferable when adjacent surfaces have different roughness requirements.
Measurement direction should be defined when directional machining marks could change the result. Cutoff, evaluation length, filter settings, or inspection location should also be added when standard defaults may not represent the functional area consistently. These details help ensure that machining and inspection evaluate the same surface profile under equivalent conditions.
Specification Considerations
The specified value should balance functional performance with realistic machining and inspection capability. A tighter roughness limit may require lower feed rates, additional surface finishing, more frequent tool control, or a different manufacturing process. Material behavior, feature geometry, measurement access, and production volume should therefore be considered before finalizing the requirement.
Ra and Rz should not be treated as interchangeable or converted through a fixed ratio. Inspection should follow the same parameter definition, standard, direction, filtering conditions, and surface location intended by the drawing. For small, curved, recessed, or interrupted surfaces, confirming that the required profile can be measured reliably is as important as selecting the numerical limit.
Conclusion
Ra and Rz describe different aspects of surface roughness, and the right choice depends on whether the requirement focuses on overall texture, peak-to-valley variation, or both. Ra is commonly selected for general surface finish evaluation, while Rz provides clearer insight into isolated peaks, valleys, and surface irregularities. Reliable results also require suitable machining conditions and clear drawing callouts.
DZ Making supports custom CNC machining projects through surface roughness review, process selection, and inspection planning. Contact us to review your drawings, define appropriate Ra and Rz requirements, and identify machining or finishing solutions that balance part performance, manufacturability, and cost.
FAQs
1. What surface roughness is considered good for CNC machining?
A suitable surface roughness depends on the part function, material, and machining process. Ra 3.2 µm is common for general machined surfaces, while Ra 1.6–0.8 µm may suit closer fits, sealing areas, or surfaces requiring a more refined finish. Lower values should be specified only when they provide a clear functional benefit.
2. What does a surface finish callout mean on a CNC drawing?
A surface finish callout defines the required surface texture for a specific area of the part. It normally identifies the roughness parameter, allowable value, unit, and applicable surface. Additional details may include measurement direction, limit type, cutoff, or inspection location.
3. Can Ra be converted to Rz surface roughness?
Ra cannot be converted accurately to Rz through a fixed universal ratio. Approximate relationships may support early estimates when the same material, process, and surface pattern are involved, but local grooves, scratches, and measurement settings can change the relationship. Each parameter should be specified or measured independently.
4. Should I specify Ra or Rz on CNC machining drawings?
Ra is generally more suitable for controlling overall surface finish, while Rz provides stronger control over localized peaks and valleys. Ra fits general appearance and process consistency requirements. Rz is more relevant for sealing, sliding, contact, or precision mating surfaces, while critical areas may require both.
5. What is the difference between Ra, Rz, and Rt surface roughness?
Ra represents the average deviation of the surface profile, Rz reflects peak-to-valley variation under the applicable evaluation procedure, and Rt measures the total height across the full evaluated profile. Ra describes general texture, while Rz and Rt respond more strongly to pronounced surface features. Rt can be particularly sensitive to one extreme defect within the evaluation length.
6. Is a lower surface roughness value always better for machined parts?
No, a lower value is only better when it supports the intended surface function. Some surfaces require controlled texture for lubricant retention, coating adhesion, sealing behavior, or running-in. Unnecessarily tight roughness limits can increase machining time, finishing cost, and inspection difficulty without improving performance.
