When it comes to CNC machining, surface roughness is more than just a detail — it plays a significant role in how well parts function and integrate into larger assemblies. Knowing how to evaluate and choose the right surface roughness helps engineers and designers ensure parts are reliable and suitable for their intended use. Typically, surface roughness in CNC machining is categorized into four main levels. In this article, we’ll break down those levels, compare them side by side, and explain the common measurement systems, including a helpful conversion chart.
What is CNC Machining Surface Roughness?
Surface roughness in CNC machining refers to the tiny textures and irregularities left on a part’s surface after it’s been cut or shaped. Imagine zooming in with a microscope — you’d see small bumps, lines, or grooves that aren’t visible to the naked eye. Even after processes like polishing, blasting, or other surface treatments, some level of roughness is usually still present. That’s because CNC machining is a subtractive process, where material is carefully removed using cutting tools, and those tools naturally leave behind micro-marks on the surface.
These surface characteristics aren’t just cosmetic — they directly affect how a part performs. Surface roughness can influence things like friction, noise, wear, heat buildup, and how well components stick or slide against each other. Ever notice that some parts fit together smoothly while others seem slightly off? That often comes down to surface roughness and overall tolerances.
Interestingly, surface roughness isn’t always something to eliminate. Some parts require a certain level of roughness to meet design standards or enhance performance. For example, a slightly rough surface may help coatings adhere better or improve function in high-friction environments. That’s why it’s important for engineers and manufacturers to choose the right roughness level for each application — it’s about balancing form and function.
Common CNC Machining Surface Roughness Levels
Understanding the different surface roughness levels can help you choose the right finish for your project. The table below outlines the typical roughness types, their ideal applications, the impact on machining time, and the related cost implications. This quick overview will guide you in making informed decisions for your manufacturing needs.
| Roughness (Ra) | Typical Use | Machining Time | Cost Impact |
| 6.3 μm | Non-critical parts, rough cuts, prototyping | Fastest | None |
| 3.2 μm | General structural parts, brackets, and covers | Minimal | Baseline finish |
| 1.6 μm | Light-load moving parts, fasteners, and housings | Moderate | +~2.5% |
| 0.8 μm | Precision parts, valves, and sealing surfaces | Higher | +~5% |
| 0.4 μm | High-speed or stress-sensitive parts, molds | Long | +~11–15% |
| 0.2 μm | Optical parts, medical components | Longest | +~20% or more |
How Post-Processing Affects Surface Roughness
Once a CNC part is produced, it often undergoes additional steps known as post-processing to improve its final look, feel, or performance. These operations include techniques like bead blasting, electropolishing, anodizing, vapor smoothing, and powder coating—all of which can significantly impact the part’s surface roughness.
For instance, bead blasting leaves a matte, textured finish by bombarding the surface with tiny particles. Electropolishing, on the other hand, smooths out microscopic peaks by removing a thin layer of material, resulting in a shiny and ultra-smooth surface. Anodizing adds a protective layer that can slightly increase roughness, while vapor smoothing melts the outer surface of plastics just enough to create a sleek, glossy feel.
Choosing the right post-processing method isn’t just about aesthetics—it directly affects how parts fit together, resist corrosion, or withstand stress. For industries like aerospace, automotive, and medical, getting the surface finish just right can make or break the product’s performance.
How Is Surface Roughness Measured?
Surface roughness tells us how smooth or bumpy a part’s surface really is, even if it looks flawless to the naked eye. Measuring it accurately is essential for ensuring parts perform well, especially when tight tolerances, sealing surfaces, or moving components are involved.
One common way to measure roughness is with a contact profilometer, which gently drags a diamond-tipped stylus across the surface and records its movement over bumps and valleys. For more delicate or hard-to-reach parts, non-contact methods like laser triangulation or optical profilometry use light to map the surface texture. For extremely fine measurement, like on mirrors or semiconductor parts s Atomic Force Microscopy (AFM) provides nanometer-level detail.
Each measurement technique helps assign a specific value to the surface, most commonly an Ra value, which stands for Roughness Average. This gives manufacturers a standard way to check if a part meets the required finish for its intended function, making the process both repeatable and reliable.
CNC Machining Surface Roughness Indicators
Surface roughness indicators help determine how suitable a machined part is for specific applications, both functionally and aesthetically. Each indicator captures a different aspect of surface texture. Here’s a breakdown:
Ra (Average Roughness)
Definition: Ra is the arithmetic mean of absolute deviations of the surface profile from the mean line over a specified length.
Significance:
- Most commonly used indicator worldwide.
- Gives a general view of surface texture.
- Not sensitive to individual peaks or valleys.
Applications: General-purpose parts in machining, casting, and grinding.
Formula: Ra = (1/L) * \u222b |y(x)| dx over length L
Rp, Rv, L Explained
| Symbol | Description |
| Rp | Maximum height frthe om mean line to the peak |
| Rv | Maximum depth from the mean line to the valley |
| L | Evaluation length |
These values provide context to Ra but are not always used individually.
Rz (Average Maximum Height)
Definition: Rz is the average of the highest peak-to-lowest valley measurements in five sampling lengths.
Significance:
- More detailed than Ra.
- Highlights extreme height differences.
Applications: Sealing surfaces, load-bearing parts, and areas prone to wear.
Rt (Total Roughness)
Definition: Total height difference between the highest peak and lowest valley across the evaluation length.
Significance:
- Sensitive to outliers.
- Used in quality control to spot surface defects.
RMS (Root Mean Square Roughness)
Definition: RMS is the root mean square of height deviations from the mean line.
Significance:
- Weighs large deviations more heavily than Ra.
Applications: Precision engineering and optics.
Comparison of Surface Roughness Indicators
| Indicator | Usage Frequency | Focus | Sensitivity to Outliers | Use Case |
| Ra | Most common | Average deviation from the mean line | Low | General-purpose machining, casting, and grinding |
| Rz | The second most common | Max peak-to-valley in sample sections | Moderate | Sealing, load-bearing, wear-prone parts |
| RMS | Used in precision work | Root mean square of height deviations | Moderate to High | Precision and optical applications |
| Rt | Less common | Highest peak to lowest valley overall | High | Detecting extreme defects in quality control |
N – Roughness Grade Numbers (DIN ISO 1302)
Definition: A standardized scale (N1 to N12) indicating surface finishes, commonly used in engineering drawings.
Connection with Ra and Rz:
- Ra to N: Directly mapped (e.g., N6 = Ra 0.8 µm).
- Ra to Rz: Estimated using statistical range (not exact).
Chart of Relationships:
| Roughness Grade | Ra (\u00b5m) | Rz (\u00b5m Range) | Ra (\u00b5in) | Rz (\u00b5in Range) |
| N1 | 0.025 | 0.1-0.9 | 1 | 4.6-36.5 |
| N2 | 0.05 | 0.2-1.5 | 2 | 8.8-61.3 |
| N3 | 0.1 | 0.4-2.6 | 4 | 17.1-103.1 |
| N4 | 0.2 | 0.8-4.3 | 8 | 32.9-173.5 |
| N5 | 0.4 | 1.6-7.3 | 16 | 63.7-291.7 |
| N6 | 0.8 | 3.1-12.3 | 32 | 123.0-490.6 |
| N7 | 1.6 | 5.9-20.6 | 63 | 237.6-825.1 |
| N8 | 3.2 | 11.5-34.7 | 125 | 458.9-1387.7 |
| N9 | 8.3 | 21.8-57.7 | 250 | 873.4-2306.4 |
| N10 | 12.5 | 80.9-162.1 | 500 | 1674.6-3855.8 |
| N11 | 25 | 156.2-272.6 | 1000 | 3235.1-6484.6 |
| N12 | 50 | 156.2-272.6 | 2000 | 6249.8-10905.7 |
Roughness Grades Achievable by CNC Machining
Some roughness grades are more easily achieved with CNC machining. Bead blasting, for example, creates matte textures while electropolishing can smooth surfaces significantly.
| Roughness Grade | Typical CNC Method |
| N6-N8 | Milling, turning |
| N9-N10 | Grinding |
| N1-N5 | Lapping, polishing |
| N11-N12 | Sand casting, roughing |
How To Choose the Most Suitable Surface Roughness
Here are some ways you can try to choose a suitable surface roughness.
1. Functionality
Smooth surfaces improve wear resistance, reduce friction, and are essential in parts like seals or optical lenses.
2. Cost & Lead Time
Finer finishes take longer and cost more. Choose rougher finishes for internal or non-critical parts.
3. Aesthetics
Visible parts (e.g., consumer products) benefit from smoother, shinier surfaces.
4. Material Type
Harder materials (like steel) are harder to polish. Softer metals (like aluminum) can reach finer finishes more easily.
5. Geometry
Intricate parts require more careful machining to maintain finish quality without damaging features.
Ordering CNC Machined Parts with Confidence
Understanding surface roughness helps you specify the right quality level. From aerospace gears to automotive housings, choosing the correct Ra or Rz ensures reliability, efficiency, and cost-effectiveness.
Xometry offers CNC services tailored to your exact needs. Upload your CAD files for an instant quote, and our team will guide you to the best surface finish for your application.
Use Our Free Conversion Tool: Easily convert Ra, Rz, and N values using our online calculator.
Conclusion – CNC machining surface roughness
Getting the right surface roughness is important for a part’s performance, appearance, and price. Based on the requirements of an application, different roughness indicators such as Ra, Rz, R,t, and RMS provide important insights into the surface. DIN ISO 1302 grades (N1 to N12) make it possible to discuss and compare surface finishes in the same manner.
Selecting the right roughness grade allows the manufacturer to achieve the right balance between cost, usage, and look. The right surface roughness specifications guarantee the best part function, lower risks, and increased customer happiness. Useful are conversion charts and easily read roughness specifications on drawings, since these ensure the same level of quality is used, and misunderstandings are less likely.
FAQs- CNC machining surface roughness
Q1: What is the most commonly used surface roughness indicator in CNC machining?
A: Ra (Average Roughness) is the most widely used indicator because it provides a simple, general measurement of surface texture suitable for most applications.
Q2: How do Ra and Rz differ, and when should I use each?
A: Ra measures the average height deviations, while Rz measures the average maximum peak-to-valley height, making Rz more sensitive to surface extremes. Use Ra for general purposes and Rz for surfaces where peak heights and valleys critically affect performance, such as sealing or bearing surfaces.
Q3: Can I achieve very fine surface finishes like N1 or N2 using standard CNC machining?
A: No, very fine finishes like N1 or N2 usually require secondary finishing processes such as polishing or lapping. Standard CNC machining can typically achieve grades from N6 to N9, depending on tooling and parameters.
Q4: How does surface roughness affect the function of my machined part?
A: Surface roughness impacts friction, wear, sealing, fatigue resistance, and appearance. A rough surface can lead to poor sealing or faster wear, while a very smooth surface may be necessary for optics or high-precision parts.
Q5: How should I specify surface roughness on technical drawings?
A: Use the Ra value or the corresponding roughness grade (N number) and include it in the surface finish callout on the drawing. For example, specify “Ra 0.8 µm” or “Surface finish N6”.
Q6: Is there a difference between Ra measured in micrometers and microinches?
A: Yes, Ra can be expressed in micrometers (µm) or microinches (µin). 1 µm = 39.37 µin. Make sure to specify the unit to avoid confusion.
