What is Surface Roughness, and How Can You Determine it?

Surface Roughness

Surface roughness is a crucial property of machined parts that must be maintained within a certain limit. Most industries and corporations use 0.8μm Ra of high-grade surface roughness in manufacturing products to ensure their optimal performance.

Not only this, but the lifespan of a machined part also depends on the surface finishes. Therefore, a deep understanding of this concept is highly recommended if you want perfect results.

This article will cover everything about surface roughness in detail. Above all, you’ll come across surface roughness measurement methods and the best ways to improve roughness. So, let’s dive in!


What is Surface Roughness?

Surface roughness is a measure of the deviations of the material in the direction of the normal vector of an original surface from its ideal form. From this, we can analyze that the more the deviations are from the real surface, the rougher its texture will be. Whereas if the deviations are minor, the surface will be smooth.

Furthermore, surface roughness is commonly involved in determining every minor irregularity on a surface. It also lets you know how a particular object will interact with its environment. The surface texture of a surface mainly consists of three major components: waviness, form, and roughness.


Why There is Surface Roughness?

The cause of surface rugosity simply lies in the deviation from an actual surface or a vector position. Although different scientists have tried to know the fundamental origin of roughness, the main cause is still unknown.

However, surface irregularities on manufacturing parts may be created intentionally or due to some factors. The factors mainly include, the edge bite,  physical properties of the workpiece material, and vibrations due to cutting tools. Generally, in the case of the machining process, the particles of the workpiece detach and leave minor grooves on the surface of the parts. The formation of these grooves leads to surface roughness.

Typically, the surface roughness unit depends upon the setting of the cutting tool. Setting the tool perfectly gives rise to particles of equal size and depth. Ultimately, the surface of the part will be flat. In either case, the component will form a wavy texture on the surface.


Impact of Surface Roughness

Surface roughness scale has both positive and negative impacts on the manufactured parts. Regarding negative impact, rough surfaces mainly have higher friction coefficients and wear more rapidly. Additionally, irregularities on the surface are the fundamental cause of the formation of cracks and corrosion.

Besides this, surface roughness can be aimed intentionally to promote adhesive properties of the surface. Doing so benefits in better surface finishing such as painting, powder plating, or coating. Plus, when lubrication is needed, the rough surface plays a vital role in holding the oil on the surface.


What are the Important Parameters Used for the Evaluation of Surface Roughness?

The surface roughness chart consists of a variety of units and abbreviations, which may create confusion for you. The following are some frequently used surface roughness parameters, along with their symbols:

  • Roughness Average (Ra)

Ra is a significant parameter used to measure surface texture. It is basically the arithmetic average of the heights of all measured surfaces. For different surface roughness profiles, having the same Ra surface finish doesn’t mean they will behave similarly. Thus, to differentiate it, there should be some more parameters.

  • Average Maximum Height of Surface Profile (Rz)

Rz is a parameter that measures the average of the five most enormous differences between valleys and peaks across the entire surface. As surface finish Ra is pretty insensitive to specific extremes, Rz surface finish compensates for the errors caused by it during measurement. Hence, erroneous and imprecise results are prevented.

  • Vertical Distance from Peak to Valley (Rmax)

In addition to measuring the vertical distance between the peak and valley of the surface, Rmax is also ideal for detecting scratches, burrs, and other abnormalities.


How To Measure The Surface Roughness?

To calculate the degree of surface smoothness, you can follow the following methods:

  • Direct surface measurement methods
  • Non-contact methods
  • Comparison methods
  • In-process methods

Let’s discuss each method in detail!

  • Direct Measurement Methods

In order to adopt the direct measurement method to measure surface roughness, you need a stylus. The stylus is drawn perpendicular to the surface. Consequently, manufacturers determine roughness parameters by using a registered profile.

Yet, there is a downside to using direct measurement methods. As the stylus physically contacts the surface, there might be a greater risk of its damage.

  • Non-contact Methods

Unlike the direct method, the non-contact method uses sound or light. White light and confocal are the two primary optical surface instruments that have replaced the stylus. Primarily, both instruments use different measurement principles.

Firstly, the particular instrument delivers an ultrasonic pulse to the target surface. Afterwards, the sound waves reflected back to the instrument due to altering the sound waves are used to derive roughness parameters.

However, in the case of light, a laser beam is targeted on the surface. You can evaluate the surface roughness by measuring the intensity of the reflected light. If more light is dispersed and lower light intensity is reflected, it means the surface is rougher.

Some surface finish examples of non-contact methods include:

  • Electrical capacitance
  • Interferometry
  • Structured light
  • Confocal microscopy
  • Atomic force microscopy
  • Photogrammetry
  • Focus variation
  • Comparison Method

As the name implies, comparison methods utilize surface roughness samples generated by specific surface roughness measurement equipment. The manufacturers use their visual senses to compare the pre-existing samples with known roughness parameters against new surface samples.

  • In-Process Methods

Another surface roughness measuring technique is in-process methods that deal with magnetic materials for the sake of roughness parameters measurement. Usually, inductance pickup uses electromagnetic energy to evaluate the distance to the surface. The ultimate resulting value is then used to find the comparative roughness parameters.

In contrast to other methods, the in-process method is considered the most effective evaluating method as it yields more accurate results. The plus point of this technique is that it continuously examines the surface during operation. Correspondingly, the operator gets the feedback simultaneously and measures the roughness under proper roughness check and balance.


Some other Techniques to Measure Surface Roughness

Let’s have a look at some other popular techniques below:

1. Profiling Technique

Profiling techniques mainly use a high-resolution probe to measure the surface texture. You need to consider a phonograph needle for its sensitivity during this process. Usually, a CNC probe is not regarded as preferable.

2. Area Technique

The area technique is a broader method used to measure the finite area of a specific surface. It is comparatively a more accessible surface roughness formula to implement and automate. Capacitance probes, optical scattering, and ultrasonic scattering methods are commonly used as area techniques.

3. Microscopy Technique

Microscopy technique deals with electron microscopes which can only observe and calculate a tiny piece of surface. Notably, this technique depends upon contrast measurements and provides information about the peaks and troughs of the surface. On the other hand, you need many scans to create surface roughness standard parameters.


What are the Effective Ways to Remove Surface Roughness?

The ultimate method of removing surface roughness is finishing techniques. However, you can prevent or reduce it by following the below-mentioned tips:

  • Reduce Vibrations

Reducing the vibrations of cutting surface measurement tools is the most effective way to remove surface roughness. This is because vibrations may cause fluctuations in the cut’s feed rate, cutting force, and depth. You can simply prevent vibrations by using vibration absorbers and separating the tool from the floor. Adding the cutting fluid and sharpening the tool edge can also be helpful.

  • Reduce Feed Rate

By reducing the tool’s feed rate, you can reduce the surface roughness and get a high. However, it will reduce the material removal rate, which degrades productivity.

  • Prevent BUE Formation

In the case of Built Up Edge (BUE) development, the tooltip adheres to it. As a result, the geometry of the tool is disturbed, and the flow of chips is restricted. You can reduce the probability of BUE formation by using a coolant and appropriate tool angles.

  • Reduce the Wear Rate of the Tool

No doubt, a worn-out tool is the worst for the surface of your workpiece. To reduce the wear, you are recommended to use a suitable coolant to control the cutting temperature.


Final Thoughts

Finally, you might have absorbed the better concept of surface roughness from this article. If you want to prevent surface roughness, make sure you follow all the tips mentioned above. Whereas achieving the precise surface roughness is quite a challenging task. Professional finishing team members can only aim at it.

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