Surface roughness is one of the most common workpiece parameters. It is an important factor that determines the quality of the part surface. The relevant content and tables of surface roughness will be introduced in this article. The tables are also a comparison table commonly used by many engineers, including myself. It can help us make better decisions on part surface design and avoid various problems such as over-design and processing feasibility. Finding the best balance between part performance and cost is very important.
Quick Answer
- Ra is the most commonly used surface roughness parameter in CNC machining.
- 1 µm = 39.37 µin, so Ra 0.8 µm ≈ 32 µin, Ra 1.6 µm ≈ 63 µin, and Ra 3.2 µm ≈ 125 µin.
- Ra values in µm and µin can be converted directly, because they represent the same parameter in different units.
- Ra cannot be converted exactly into Rz, Rt, or Rq/RMS. Their relationship depends on the actual surface profile and manufacturing process.
- In CNC machining, Ra 3.2 µm is a common general-machined reference, while Ra 1.6 µm and Ra 0.8 µm are typically used when a finer surface is required.
- Always follow the surface roughness parameter and acceptance requirement specified on the engineering drawing, rather than relying only on approximate conversion tables.
What is Surface Roughness in Machining?
Part surface roughness is an important parameter in the machining industry. Surface roughness is a measure of microscopic geometric shape errors, essentially representing the minute irregularities left on a workpiece’s surface by cutting tools after machining. A good surface finish is important for many applications, especially in industries such as aviation, automotive, and medical. The roughness and precision of parts determine their quality.
Why does Machining Inevitably Produce Roughness?
Unlike processes such as mold injection molding or investment casting, CNC machining is a subtractive manufacturing process. All parts are formed on machine tools using cutting tools to remove excess material from the workpiece. The main causes are the following three points:
Tool Geometry: Cutting tools have a tip radius, so when the tool moves along a straight line, it cannot cut a perfect plane, but will leave rows of tiny arc-shaped grooves on the workpiece.
Feed Rate: This is a key parameter that determines roughness. Feed directly affects the geometric marks left by the cutting edge. In general, a higher feed per revolution or feed per tooth tends to produce a rougher theoretical surface. However, actual results also depend on tool geometry, radial runout, material, machine rigidity, and process stability.
Chatter: Insufficient rigidity in the machine–tool–workpiece system, excessive tool overhang, unstable cutting conditions, tool wear, etc. can cause vibration and chatter. These vibrations may leave irregular ripples or chatter marks on the machined surface and significantly increase the measured roughness.
Surface Roughness Conversion Chart
Below are several “golden treasures” that I have collected, which will allow you to understand various parameters and data comparisons at a glance, and can bring more decision-making references to your work.
Current Surface Roughness Standards in 2026
ISO 4287, ISO 4288, and ISO 1302 are still commonly found on legacy engineering drawings, but they have been withdrawn. The current ISO profile surface-texture framework is ISO 21920-1:2021, ISO 21920-2:2021, and ISO 21920-3:2021. ASME B46.1-2019 (R2026) remains an important reference for U.S. engineering practice.
Master Surface Roughness Ra Conversion Chart
The table below provides direct conversions between Ra values in micrometers (µm) and microinches (µin). These are unit conversions of the same roughness parameter and can therefore be converted accurately. I believe that through it, you can understand and compare different surface roughness standards well, thereby ensuring precision in manufacturing and CNC machining.
| Legacy N Grade | Ra (µm) | Ra (µin) | Common Reference |
|---|---|---|---|
| N1 | 0.025 | 1 | Ultra-fine surface |
| N2 | 0.05 | 2 | Ultra-fine surface |
| N3 | 0.10 | 4 | Very fine surface |
| N4 | 0.20 | 8 | Fine finishing |
| N5 | 0.40 | 16 | Precision finishing |
| N6 | 0.80 | 32 | Fine machined surface |
| N7 | 1.60 | 63 | General precision machining |
| N8 | 3.20 | 125 | Common general-machined reference |
| N9 | 6.30 | 250 | Coarse machined surface |
| N10 | 12.5 | 500 | Rough machined surface |
| N11 | 25 | 1000 | Very rough surface |
| N12 | 50 | 2000 | Very rough surface |
Exact unit conversion:
1 µm = 39.37 µin
* N grades are included as a legacy reference. In modern engineering drawings, we recommend directly marking the required surface roughness parameters, such as Ra 0.8 µm or Ra 1.6 µm, rather than using only N grades.
Common CNC Reference: Ra 3.2 (N8) vs. Ra 0.8 (N6)
Ra 3.2 (125 µin): This is a common general-machined reference level for CNC machined parts.
Ra 0.8 (32 µin): This level of roughness is considered precision machining. Ra 0.8 µm may require fine machining, polishing, grinding or controlled finishing depending on material and geometry.
Quick Approximation
If you sometimes don’t have this table at hand, you can also do a quick mental calculation by using the following empirical formula:
µm to µin: Ra (µm)×40≈Ra (µin)
µin to µm: Ra (µin)÷40≈Ra (µm)
※ You can also use the gadget below to instantly convert surface roughness values：
DSR Surface Roughness Converter
Convert Ra values between micrometers (µm) and microinches (µin).
Can Ra Be Converted to Rq, Rz or Rt?
Ra, Rq, Rz, and Rt describe different characteristics of surface profiles. However, you should be clear that apart from Ra being a unit conversion between µm and µin for the same parameter, there is typically no universal and accurate fixed conversion relationship between these different roughness parameters.
| Relationship | Exact Conversion? | Notes |
| Ra µm ↔ Ra µin | Yes | Same parameter, different units |
| Ra ↔ CLA | Generally equivalent terminology | CLA is an older term for arithmetic average roughness |
| Ra ↔ Rq / RMS | No | Relationship depends on profile distribution |
| Ra ↔ Rz | No | Strongly depends on peak and valley structure |
| Ra ↔ Rt | No | Rt is highly sensitive to extreme peaks and valleys |
| Ra ↔ Cut-off Length | No | Cut-off is a measurement/filter setting, not a roughness parameter |
In some engineering estimations or early process planning, similar to:
- Rq ≈ 1.1 × Ra
- Rz ≈ 4–7 × Ra
Such empirical relationships.
However, these values can only be used as a rough reference and cannot be considered official parameter conversion relationships, nor can they replace the roughness requirements explicitly specified in engineering drawings.
CNC Machining Processes & Typical Roughness Ranges
Many times, there are specific surface roughness targets on the drawings, and this table will help you better select the appropriate machining process.
| Manufacturing Process | Typical Ra Range (µm) | Typical Ra Range (µin) | Cost Impact |
| Flame Cutting | 12.5 – 25 | 500 – 1000 | Low |
| Sawing | 6.3 – 25.0 | 250 – 1000 | Low |
| Drilling | 3.2 – 12.5 | 125 – 500 | Low |
| Milling (Rough) | 3.2 – 12.5 | 125 – 500 | Low-Med |
| Milling (Finish) | 0.8 – 3.2 | 32 – 125 | Medium |
| Turning (Rough) | 3.2 – 12.5 | 125 – 500 | Low-Med |
| Turning (Finish) | 0.4 – 3.2 | 16 – 125 | Medium |
| Reaming | 0.8 – 3.2 | 32 – 125 | Medium |
| Surface Grinding | 0.2 – 1.6 | 8 – 63 | High |
| Honing | 0.1 – 0.8 | 4 – 32 | High |
| Lapping | 0.025–0.2 | 1 – 8 | Very High |
| Polishing | 0.05–0.4 | 2–16 | High–Very High |
Common Surface Finishes & Roughness Compatibility
In actual production, surface treatment is generally required after parts are processed. This comparison table on the relationship between post-processing and roughness will show you detailed data.
| Finish Type | Typical Ra (µm) | Typical Ra (µin) | Effect on Surface | Visual Appearance |
| As Machined | Process-dependent | Process-dependent | Baseline | Visible tool marks, consistent texture. |
| Smooth Machined | 0.8 – 1.6 | 32 – 63 | Baseline | Minimal tool marks, smooth to touch. |
| Bead Blasted | Process-dependent | Process-dependent | Usually increases or normalizes Ra | Matte, non-reflective, uniform “grainy” look. |
| Brushed | ≈0.8–3.2 | ≈32–125 | Directional | Fine parallel lines, satin finish. |
| Vapor Polished | Process-dependent | Process-dependent | Reduces Ra | Glossy, transparent (primarily for plastics). |
| Electropolished | ≈0.2–0.8* | ≈8–32* | Reduces Ra | Bright, microscopically smooth, deburred. |
| Mechanical Polish | ≈0.05–0.4* | ≈2–16* | Reduces Ra | Mirror-like, high reflectivity. |
| Anodized (Type II) | Depends on base finish and pretreatment | Depends on base finish and pretreatment | Usually preserves underlying texture; Ra may change | Changes color, retains original texture. |
| Powder Coated | N/A | N/A | Masks Surface | Thick coating hides original machining marks. |
Notes:
1. The Bead Blast Trap
Many people must think that the sandblasting process can make parts smooth. sandblasting usually increases the Ra value because it creates numerous tiny pits on the surface of the part. The part looks smooth, but the directional knife lines on its surface are eliminated, forming a uniform diffuse reflection.
2. Anodizing Pre-req
Anodizing typically retains rather than masks underlying machining textures. Therefore, tool marks and surface defects visible before treatment may remain clearly visible after anodizing.
However, the final measured roughness does not necessarily equal the original Ra value. Pretreatment, chemical etching, anodizing type, coating thickness, and the initial surface condition can all influence the final surface roughness.
Surface Roughness Symbols and Abbreviations
In the surface roughness parameters, various symbols and abbreviations appear, all of which follow ISO 21920 standards. We often mark them on the drawings, so that we can easily communicate with factories and suppliers. Below, I will explain to you these surface roughness terms.
Surface Roughness Abbreviations
Ra – Arithmetical Mean Height
Ra is the arithmetic mean of the absolute height deviations of the roughness profile from its mean line over the specified evaluation length. It is one of the most commonly specified surface roughness parameters. This is because it provides a simple and repeatable characterization of the average surface texture.
Rq – Root Mean Square Roughness
Rq is the root mean square of the roughness-profile height deviations over the evaluation length. Because the deviations are squared during calculation, Rq gives greater weight to large peaks and valleys than Ra.
Rz – Mean Roughness Depth
Under ISO 4287:1997 terminology, Rz is the vertical distance between the highest profile peak and the deepest profile valley within a sampling length.
However, there is one point you should note: historical standards and instrument settings may use different Rz definitions. Therefore, when inspecting legacy drawings, you should confirm the standards they reference.
Rt — Total Height of the Roughness Profile
Under the ISO 21920 standard framework, Rt represents the total height of the roughness profile within the entire assessment length.
Rsk – Skewness
Rsk describes the asymmetry of the roughness profile height distribution. When RSK >0, the surface resembles a mountain peak; and when Rsk < 0, the surface looks like a canyon.
Rku – Kurtosis
Rku describes the shape of the roughness-profile height distribution and its sensitivity to extreme profile features. When Rku > 3, the surface will be very sharp; when Rku < 3, the surface will be rounded.
Ra vs Rz difference
Regarding the definitions of both, I have already mentioned them above. To put it simply, Ra focuses on the average level of the overall texture of the part, and its value is generally small, such as 3.2 or 0.8. Rz focuses on the most prominent defects on the surface of the part, and its value is generally relatively large, usually 4 to 7 times that of Ra.
In addition, Ra is usually used for general structural parts, appearance parts, etc. Its measurement repeatability is good and the value is relatively stable. Rz is generally applied to sealing surfaces and components subjected to fatigue stress. It has large measurement fluctuations and is greatly affected by the sampling position.
Surface Texture Symbols on Drawing
1. Core Symbol
| Symbol Shape | English Name | Meaning |
| $$\sqrt{}$$
| Basic Symbol | This symbol only indicates the need to control surface roughness, but does not specify specific processing methods. |
| $$\sqrt{\quad}$$ | Material Removal Required | This symbol means that you have to remove material to obtain the surface. |
| $$\sqrt{\bigcirc}$$ | Material Removal Prohibited | You must maintain the surface created in the previous process and no further cutting is allowed. |
2. How to Read Legacy ISO 1302 Symbol Annotations?
Some of you may not know the meaning of each location label. In legacy engineering drawings based on ISO 1302, each position around the surface texture symbol has a specific meaning. The example below shows the typical annotation layout.
ISO 1302 has been withdrawn and replaced by ISO 21920-1:2021, so this example is mainly intended for interpreting older drawings.
Detailed location details:
- a: Passband or sampling length, together with the surface-texture parameter symbol and value
- b: Second and subsequent surface-texture parameters
- c: Manufacturing method, treatment, or coating.
- d: Surface lay and direction
- e: Machining allowance
3. Lay Symbols
These symbols are used to tell the machining staff the trajectory of the tool, which is particularly important for the sealing and appearance of the part.
| Symbol | Name | Description |
| = | Parallel | Texture is parallel to the plane of projection of the view. |
| ⊥ | Perpendicular | Texture is perpendicular to the plane of projection of the view. |
| X | Crossed | Texture is crossed in two oblique directions (common in Honing). |
| M | Multi-directional | Texture is multi-directional (common in Casting, Lapping). |
| C | Concentric | Texture is approximately concentric to the center (common in Facing). |
| R | Radial | Texture is approximately radial relative to the center. |
| P | Particulate | Texture is non-directional, protuberant, or particulate (common in EDM, Sandblasting). |
Process Surface Roughness Measurement
1. Three Measurement Methods
Contact Measurement Method
This is the most common and authoritative practice in the industry. You need a surface roughness tester. Its principle involves sliding an extremely fine diamond stylus across the part’s surface, sensing surface irregularities, and converting them into electrical signals. Then, you can see a specific value on the instrument. This value is accurate and complies with ISO/ASME standards.
Comparison Measurement Method
The tool required for comparative testing is a surface roughness comparator plate. You first need to lightly scrape your fingernail across the part’s surface, then scrape it across the comparator plate to compare the resistance of the two. This method completely relies on your own feel and vision to judge whether the parts are qualified or not. This method is fast, but it is highly subjective and has no specific data basis.
Non-Contact Measurement Method
This method is mainly aimed at high-end precision machined parts. The tools involved include white-light interferometers and laser confocal microscopes. Its principle primarily relies on light wave reflection to construct 3D topography. It is precisely because there is no contact between the instrument and the parts that there is no risk of surface damage to the parts.
Surface Roughness Measurement Methods Comparison
| Method | Best For | Advantage | Limitation |
| Comparator | Shop-floor quick check | Fast, inexpensive | Subjective |
| Stylus profilometer | General CNC inspection | Quantitative, common | Contact measurement |
| Optical profilometer | Complex/soft surfaces | 3D, non-contact | Higher cost |
2. How to Measure Surface Roughness?
Next, I will introduce the standard measurement process to you step by step.
Step 1: Preparation
Before measurement, the part to be tested is wiped with a lint-free cloth and alcohol. The main purpose of this step is to remove the oil and dust on the surface of the part to avoid affecting the measurement results.
Step 2: Identify the Lay
You also need to observe the direction of the machining texture: is it a circular lay from turning or a straight lay from milling? The measurement direction is typically perpendicular to the predominant lay. Otherwise, the data will be inaccurate.
Step 3: Set Up Parameter
Do not use a single universal cut-off length for all surfaces. You must apply the filter settings and the sampling or evaluation length specified on the drawing and in the applicable standard. Where settings are not specified, you also need to use the standard’s default selection procedure based on the relevant profile type and expected parameter range.
Step 4: Positioning & Measuring
Now you can start the formal measurement. You need to gently place the probe and ensure that it is in perpendicular contact with the surface. Press the “Start” button, and then the probe will move a distance. Remember that there is absolutely no vibration allowed during the measurement.
Step 5: Reading
After the measurement process is completed, you can read the Ra value and Rz value on the screen. You must evaluate the specified roughness parameters according to the measurement conditions and acceptance criteria required by the drawings or applicable standards.
Importance of Surface Roughness in Machining
Engineers and manufacturers have always focused on surface roughness dimensions, but often overlook the importance of surface roughness. Next, I will explain how important it is for the CNC machining industry.
1. Mechanical Performance
Wear & Friction
Surface roughness directly affects friction and wear when parts come into contact, but the relationship is not simply “smoother is always better”. When two parts come into contact, the actual contact typically occurs at the microscopic peaks of the surfaces. Rougher surfaces are more prone to generating localized high contact stress, plowing, or abrasive wear, thereby increasing friction and material loss.
For example, for parts with sliding surface characteristics such as shafts, guide rails, and sliders, excessive roughness is generally detrimental to their stable operation.
Fatigue Strength
Deep pits on rough surfaces are natural stress risers. Cracks can easily initiate and propagate from the microscopic pits on rough surfaces under alternating loads, leading to the fracture of the part.
Sealing Ability
For sealing, with hydraulic or pneumatic parts, if the surface is too rough, gas or liquid will leak into the microscopic grooves.However, for some sealing rings, smoother is not necessarily better. They should also have a slight texture to help retain lubricating oil.
2. Assembly & Fit
Roughness can lead to inaccurate measurement accuracy, which can affect the stability of part assembly.
If you design a press fit with a rough surface, the microscopic peaks on its surface will be flattened during the pressing process.The consequence of this is that the actual effective interference will be reduced, resulting in loose connections.
In addition, there is the issue of measurement error. Calipers or micrometers often measure the peak of a part’s surface. If the Rz value is high, the dimensions you measure include the peaks that are about to be worn away, causing the actual fit gap to be larger than expected.
3. Cost & Efficiency
Cost has always been a major concern for project managers and bosses. Tighter surface roughness requirements generally increase manufacturing costs, especially when additional finishing operations or inspection steps are required.
For example, when machining ordinary turning roughness (such as Ra 3.2), the tool moves quickly, and the efficiency is very high. However, attaining Ra 0.4 requires extremely slow feed speeds and additional processes (such as grinding) are required, and the machining time may be doubled.
In addition, the cost curve also gradually increases as the roughness value becomes smaller, and the smaller the value, the greater the increase. For example, the cost increase from Ra 6.5 to Ra 3.5 is not much. But from Ra 0.8 to Ra 0.2, the manufacturing cost can significantly increase if additional finishing processes such as grinding, honing, or lapping are required.
Therefore, my suggestion is that as long as the surface roughness is sufficient, it can be used. Unless the function of the part requires lower roughness, or the customer has special requirements, you do not need to mark excessive precision requirements on the drawing.
4. Post-Processing
Corrosion Resistance: Pits on rough surfaces can easily hold water vapor, acids, and salts, andare difficult to clean.
Coating Adhesion:If your part needs to be painted or plated, an overly smooth surface is undesirable because coatings will not adhere properly. Parts typically require a certain degree of roughness to provide mechanical interlocking.
Surface Roughness vs Surface Finish
Although in CNC machining shops, surface finish and roughness are often used interchangeably. But strictly speaking, there is a difference between the two. They describe different dimensions of the part surface. Understanding this difference will help you conduct accurate DFM.
Let’s look at the definitions. Surface roughness is a measure of the microscopic geometry of the surface, focusing on the functionality of the part. Surface finish, on the other hand, is a broader term that encompasses the full texture of the surface, including not only roughness, but also waviness, lay, and flaws. And it focuses more on the perception and craftsmanship.
Surface texture includes characteristics such as roughness, waviness, and lay, while surface finish is a broader term used to describe the overall condition of a manufactured surface.
Therefore, when you see Ra 0.5 marked on the drawing, it means that the designer is limiting roughness for functionality. And when you see “Bead Blast” on a drawing, it means the designer is telling you which surface treatment process to use.
Conclusion
Well, that’s all about surface roughness and its various tables. Hopefully, you can have a better understanding after reading the full detailed article. Actually, the perfect surface finish is not the smoother the better, but just right. In addition, I have introduced ISO 2768 Tolerance Standards before. If you are interested, you can also take a look.
Of course, if you’re not sure which finish need for your design? You can send us your drawings with confidence, DSR is a professional parts manufacturer that provides excellent CNC machining services and surface treatment processes. Our engineers will provide you with a free DFM analysis and cost estimate. So get in touch with us!
※ DSR also provides you with the complete surface roughness reference file. You can use it yourself if needed: DSR Surface Roughness Reference.pdf
FAQ
Q1: What is 3.2 Ra surface roughness?
A: Ra 3.2 refers to a surface profile with an arithmetic mean deviation of 3.2 micrometers (µm) and is a common general-machined reference level.
Q2: What is 0.8 RA surface finish?
A: Ra 0.8 is a precision grade finish surface. It is often used as a relatively fine machined surface for precision mechanical parts.
Q3: What is 0.4 RA roughness?
A: Ra 0.4 is an extremely precise finishing surface and is often referred to as a fine grinding grade.
Q4: What RA is 600 grit?
A: There is no universal conversion between abrasive grit size and Ra. In certain controlled stainless steel polishing processes, 600-grit abrasive can produce a surface roughness of approximately Ra 0.1–0.2 µm. However, the actual roughness also depends on factors such as material, initial surface, pressure, speed, etc.
Q5: How do you calculate RA surface roughness?
A: The principle of Ra calculation is that it is the arithmetic mean of the absolute values of the profile deviations within the sampling length.
Q6: What is a number 4 finish in RA?
A: No. 4 is a linearly textured stainless-steel finish rather than one fixed Ra value. Depending on the applicable standard, producer, abrasive process, and starting material, the measured roughness can vary.
Q7: What is the roughness grade number range?
A: Roughness grade numbers N1 to N12 are a legacy reference system used to indicate approximate surface roughness levels. They range from N1 (Ra 0.025 µm), representing a very fine surface, to N12 (Ra 50 µm), representing a very rough surface.
Q8: What is the difference between 2B and #4 finish?
A: The core difference between 2B and #4 lies in their machining processes and surface textures. 2B is mill finish, which is produced by cold rolling, heat treatment, pickling and leveling rolls, and has a smooth surface to the touch. #4 is a brushed finish with a 2B substrate that is mechanically ground and has a clearly visible one-way short fine line and a soft satin sheen.
Q9: What is an N5 surface finish?
A: N5 is a legacy surface-roughness grade corresponding to approximately Ra 0.4 µm (16 µin). It represents a relatively fine surface finish.
Q10: How smooth is a 32 finish?
A: A “32 finish” commonly means Ra 32 µin, which equals 0.8128 µm. It is often paired with the nominal 0.8 µm grade in reference charts.
Q11: What are the differences between ISO 4287 and ASME B46.1?
A: The main difference between ISO 4287 and ASME B46.1 lies in the definition details of applicable regions and default parameters. ISO 4287 is an international standard and uses the sample length as a reference when calculating Ra. While ASME B46.1 is the US national standard, it prefers to treat surface texture as a holistic system, and the default calculation is usually based on evaluating length.
It is important to note that ISO 4287 is still encountered on legacy drawings but was withdrawn in 2021 and replaced by ISO 21920-2.
Q12: What industries require strict surface roughness control standards?
A: These industries include aerospace, automotive industry, biomedicine, food processing, semiconductor and optical manufacturing.
