Surface Finish Roughness Chart: A Complete Guide

Surface roughness refers to irregularities on a surface. These irregularities deviate from the ideal form. CNC machining companies use a surface roughness chart to make the surface smooth. So this guide will explain:

1. What is surface roughness?
2. Why does it matter?
3. Different parameters used to describe it
4. How does surface roughness impact the product manufacturing process?

What is Surface Roughness?

Let’s make it very simple by giving you an example from your life. Imagine you have a piece of paper. You feel it rough when rubbing because there are tiny peaks. Similarly, surface roughness is a way to measure how smooth or bumpy is the surface of any part.

A flat surface will be smooth like a clean mirror. However, most of the surfaces have imperfections. Not significant but some deviations. Even after the polishing, you will find some irregularities.

Importance of Surface Roughness

Surface roughness looks very trivial because the roughness details are small. However, the tiny details have a big impact on both product performance and manufacturing process.

For example, if a part of a car engine has a rough surface, it is vulnerable to wear out faster. On the other hand, a part with minimum roughness will endure for maximum time. 

The manufacturing process has a significant impact on part surface roughness. For instance, if you choose die casting manufacturing, die casted parts will have more roughness. On the other hand, CNC machining provides a smoother surface. 

Choosing the right manufacturing process will impact the entire project outcomes. 

The Role of Surface Roughness in CNC Parts 

The quality of any part surface significantly impacts its performance. For example, if car engines have rough surface finishes, they keep rubbing each other. Ultimately, they will wear out quickly and will also lead to engine breakdown. Moreover, the loyalty of the car manufacturing companies comes at high risks. 

Therefore, surface roughness might seem trivial but it can ruin or make a successful entire project. 

Surface roughness is important in several ways:

• Friction: Rough surfaces are bumpy which lead to more friction. It means the parts will generate more heat and cause wear and tear. Parts with poor surface roughness will hinder the performance. 
• Sealing: Parts like gaskets or O-rings, require certain roughness. Too rough, and the seal won’t be tight. Too smooth, and it would be vulnerable to loose grip.
• Wear and Tear: You have read the first point. Since parts like gears or bearings are involved in regular movement, they need to be very smooth.

Ra Surface Roughness and Manufacturing Processes

How the part is made also decides the surface roughness. 

• Machining: Milling, turning, and drilling all have different levels of surface roughness. For example, a roughing cut will result in a rougher surface than a finishing cut.
• Grinding and Polishing: Processes like grinding and polishing are incorporated to make the surface very smooth.
• Other Factors: There are other factors too such as what type of material you are using, cutting tools, and what is the machine condition. They all have separate impacts on the part’s surface roughness.

There is another point to remember that CNC parts manufacturers need to keep a balance. For instance, with poor surface roughness, the part won’t deliver the best results. Similarly, too smooth surface roughness will lead to extravagant costs. So the balance of both things are crucial. 

To learn more about cnc parts surface finishining, feel free to read the guide: Surface Finish in CNC Machining

How to Measure Surface Roughness?

To better understand the topic surface roughness chart, you need to learn how to measure it first. There are two methods of measuring surface roughness:

1. Contact Method
2. Non Contact Methods 

Contact methods: In contact methods, experts check the surface physically incorporating a probe to measure peaks and valleys.

Non-contact methods: These are light-based methods to measure the surface without touching it. In the non contact method, there are further three techniques involved. These are: 

• Interferometry
• Optical profilometry
• Confocal microscopy

Each roughness measurement method has its own advantages and disadvantages. Let’s explore each in more detail.

1. Contact Methods

Stylus Profilometry

This is the most common method to measure surface roughness. Its process is:

1. A diamond tip: A very small super-hard diamond tip is attached to a very sensitive arm.
2. Surface Tracing: The tip is dragged across the entire surface of the part.
3. Measuring up and down: As the tip moves over the part surface, it goes up and down following the peaks and valleys. This tip reads the measurement of rough surface. A computer records the entire movement.
4. Creating a profile: In the next step, a computer creates a graph profile which explains the shape of the surface. For example at which point the tip recorded the bumps and smooth surface.
5. Calculating roughness: With incorporating mathematical formulas, the computer calculates different roughness values, such as Ra, Rz, and Rmax.

Advantages of Stylus Profilometry:

• Very accurate
• Wide range of surfaces can be measured 
• Comparatively inexpensive

Disadvantages of Stylus Profilometry:

• Can damage delicate surfaces
• The tip can be damaged
• It is time-consuming

2. Non-Contact Methods

Interferometry 

In this method, a light reads the measurement without touching the surface. Here are its steps:

1. Light is shone on the surface: A beam of light is directed at the surface of the part to measure the bumps and rough surface areas.It can cover the entire area perfectly so the measurement will be significantly accurate. 
2. Light reflects: The light bounces off the surface and recombines with the original light beam.
3. Interference patterns: This interference leads to combined patterns that display a surface’s shape.
4. Measuring height differences: A computer analyzes these drawn patterns. It calculates the height difference which means how much is the roughness available on the part. 

Advantages of Interferometry:

• Doesn’t touch the surface, so it won’t damage it
• Can measure very small features
• It can also create 3D images

Disadvantages of Interferometry:

• Vibrations can affect the reading 
• It is expensive
• It is not suitable for all types of surfaces

Optical Profilometry

Optical profilometry incorporates light to generate a 3D image of the surface. However, it is similar to interferometry. But uses different methods to collect the surface information.

3. Importance of Calibration and Standards

Why are calibration and standards important for surface roughness measurement? 

Calibrate equipment regularly: There are certain standards to check the equipment measurement reading. It shows that your readings are correct and can be trusted. 

Follow industry standards: Automotive, medical, and aerospace each industry has different standards. These standards help manufacturers and project owners to understand the particular protocols. 

Surface Finish Roughness Units (Ra, Rz and Rmax)

So this is the main topic to understand. How to measure surface roughness in different units? It is measured in micrometers (µm) or alternatively in nanometers (nm). Which is the best unit for surface roughness measurement. The answer is your best roughness features you are interested in. 

For example, for a machining part surface measurement micrometers work best. Subsequently, measuring an automatically smooth surface, a nanometer will be suitable. 

Common Roughness Parameters

There are different parameters to measure surface roughness. However, they are slightly different.

Ra (Average Roughness)

Ra, is the arithmetic average roughness. It is a fundamental parameter that shows the average deviation of the surface profile from its mean line within a defined sampling length.Ra gives a general idea of the overall roughness level.

How to calculate Ra?

1. Measure the surface profile. You can use a profilometer to generate a data set. It should carry the height of the surface at multiple points.
2. Make a sampling length. This is the length of the surface profile. Its purpose is to calculate surface roughness.
3. Calculate the mean line of the profile. This is the average height of the profile. It remains within the sampling length.
4. For each data point within the sampling length, calculate the absolute value. It should carry the deviation from the mean line. This data shows the distance between a specific point on the profile and the average line.
5. Average all the absolute deviations calculated in step 4. This is the average roughness, or Ra.

Rz (Average Maximum Height)

Rz is the ten-point height. It demonstrates the average value of the five highest peak-to-valley heights within the evaluation length. It is used to give an indication of the larger peaks and valleys on the surface. Subsequently, it is pretty sensitive to extreme features compared to Ra.

Here’s how Rz is calculated:

1. Measure the surface profile.
2. Draw an evaluation length. It resembles the sampling length used for Ra.
3. Choose the five highest peaks and the five deepest valleys. It should remain within the evaluation length.
4. Calculate the height difference between each peak and its corresponding valley (peak-to-valley height).
5. Average the five peak-to-valley heights obtained in step 4. This is the average maximum height, or Rz.

Rmax (Maximum Peak-to-Valley Height)

Rmax signifies the largest distance between the highest peak and the deepest valley. It takes place within a specified evaluation length. Rmax shares a quick indication of the maximum roughness variation on the surface. However, it’s important to note that peaks and valleys impact the Rmax. So it is not sufficient to give complete data of the texture surface.  

Use cases and limitations:

• Rmax is good for extreme height variations. For example sealing or bearing surfaces.
• Can also be incorporated for quality control parameters. The purpose is to identify surfaces with excessive roughness.
• However, Rmax should be used with parameters, like Ra, to get a complete picture of surface roughness.

Typical Surface Roughness Ranges

Ra, Rz, and Rmax Graph Illustration

Surface Roughness Comparison Chart

To understand different roughness parameters, a surface finish chart is a visual tool. It helps manufacturers like CNC machinists and engineers to read these parameters.

First, to get a desired surface finish, we need to compare the roughness of various machining processes. Thus, a surface roughness comparison chart could give you a lot of help:

Surface Roughness Convension Chart

Apart from roughness comparison chart, we also need to compare surface roughness values for different ISO Grades. In view of this, a “Surface Roughness Conversion Chart” could be helpful:

Overview of Surface Roughness Values for Different ISO Grades

• Ra: Average roughness in micrometers 
• RMS: Root Mean Square 
• CLA: Center Line Average 
• Rt: Total roughness in microns
• N: New ISO (grade) scale number
• Cut-off Length: Length required for the sample

Factors Affect Surface Finish

The quality of the surface finish is significantly affected by different factors. These are: 

Temperature

Variations in temperature can damage the material. Even if it is slight, it will lead to an uneven surface which ultimately increases surface roughness. Therefore, machining companies ensure a standard temperature range following different materials.

Cutting Techniques

Technological advancements have expanded cutting methods beyond traditional metal blades. Laser and water jet cutting often produce superior surface finishes compared to mechanical alternatives. Laser cutting excels in precision and reducing surface roughness, while water jet cutting is particularly effective for small parts.

Material Removal Rate and Feed

Material removal rate, speed, and feed, the tool’s movement also impact the surface finish. Inappropriate settings will lead to compromised surface quality.

Cutting Instruments and Parameters

Machinery, cutting speed, feed, and depth all have their separate role in surface finishing. After analyzing these factors will help to achieve best surface quality. Excessive cutting speed will expand the surface finishing time and deteriorate the surface too. 

Conclusion

 Surface roughness is a very critical factor that is crucial for product performance. By understanding the main parameters like Ra, Rz, Rmax, you can improve the entire manufacturing process to get the best product with optimum surface roughness. Similarly, a roughness chart is here to understand the basic concepts visually. 

 At Yijin Hardware, we provide complete dimensional inspection to give your parts excellent specifications. Our expert finishing services include anodizing, electroplating, bead blasting, polishing, brushing, and more. 

On-demand Machining Services with
Complete Surface Finish Options

Request A Quote