Surface Finish Chart: A Guide to Understanding Surface Finishing

Processes involving the use of machines impose characteristic irregularities on their products’ surfaces. The degree of roughness on the product surface affects its durability and performance during usage. This necessitated the need for the surface finish chart.

Continuous use often causes the product to wear and tear as time goes by. This is often due to the large friction established between the product’s surface and the surface it is used against. In contrast, smooth surfaces with surface finishing are more durable for use with little or no friction.

If you’re wondering why surface finishing is important for your product, this guide is for you. Continue reading for a complete understanding of surface finish.

What is Surface Finish?         

Surface finish is a parameter that determines the physical characteristics (appearance) of a component. It is a means of altering material surfaces through processes involving adding, removing, or reshaping the metal surface.

 Three (3) major characteristics can define the surface finish. They are surface roughness, waviness, and lay.

The measure of the total space irregularities on the surface metal is known as Surface Roughness. It also depicts the number of peaks and valleys on a surface. The lower the number, the lower the irregularities, implying low surface roughness and a better surface finish. When professionals refer to surface finish, it is often ‘surface roughness.’

Surface finish measurement can also be in terms of waviness. Waviness is caused by deflection, warping, or vibration of particles. It can be measured if a surface has more space between irregularities and compared with other surfaces.

Finally, lay refers to the direction of the major surface patterns.

Why Does Surface Finish Matter?

Surface finishing plays a key role in custom machining for determining how a product reacts with its immediate environment. Therefore, it is crucial to evaluate the durability and effectiveness of the product during usage.

The surface finish can affect several abilities of the product part to resist wear and tear. These include the ability to assist or destroy lubrication, increase or decrease friction with contact parts, and resist corrosion.

There are different surface finishes with varieties of effects on the product. Surface finish help in the following ways and more:

How to Measure Surface Roughness?

Measuring surface roughness involves calculating the relative smoothness of a product’s surface profile. It uses Ra as the numeric parameter.

As mentioned earlier, the three basic components of surfaces are roughness, waviness, and lay. These components are vital in surface geometry characteristics.

There are several systems for measuring surface finish. These surface finish measurement systems include the following:

Contact Methods (Stylus Probe Instrument)

Contact method measures the surface finish of a product with the use of a stylus probe instrument. First, the stylus is drawn along the surface of the product, followed by registering the perpendicular motion of the stylus.

The registered profile of the stylus then helps to calculate the three basic parameters of surface roughness. This method requires interruption of the machine, and the stylus tip might make little scratches on the product surface.

To measure accurately the surface roughness of a delicate object, ensure the diameter of the stylus tip is as small as possible with low contact pressure on the surface.

Non-Contact Methods (Optical Light, Laser, or X-Ray)

The non-contact methods involve using optical instruments such as X-rays, lasers, etc. instead of a stylus pen to measure the degree of surface roughness. These methods offer a high speed of measurement without the need to contact the product’s surface.

Optical scattering or ultrasonic scattering is one of the most useful methods here. The optical instrument will send an ultrasonic impulse to the surface of the product. This impulse alters, interferes, and causes a reflection of the back to the instrument to occur. Now you can assess the reflected wave in the instrument to determine the surface roughness parameters.

Microscopy techniques also help to examine microscopic peaks on material surfaces. Again, these methods give consistent results.

Comparison Methods

This method compares a product surface finish with a surface with known surface roughness parameters. This known surface serves as a standard for calibrating the surface of the manufactured product.

Manufacturers use visual and tactile senses to compare a product surface with a surface of known surface finish parameters.

In-Process Surface Finish Measurement

There are several methods to measure surface finish parameters on-process, and some of the methods are:

• Inductance Method

This method measures the surface roughness of a magnetic surface. In this method, the inductance pickup uses its electromagnetic energy to estimate the distance between the pickup and the surface. The parametric value obtained helps to determine the comparative surface roughness parameters.

• Machine Method

In this method, a digital system serves as a light source to illuminate the surface of the product. After illumination, data is sent to the computer for analysis. Relating the analyzed data with the surface roughness chart help to obtain the actual roughness of the surface.

• Ultrasound Method

In this technique, a spherical ultrasonic sensor is placed at a non-incident angle to send impulses to the surface. The sent impulse is analyzed by the computer for analysis and calculations of the surface roughness parameters.

Surface Finish Units From RA To Rz

When you see a machining surface finish chart, you may find it difficult to understand the abbreviations. It is difficult to measure surface roughness without understanding these units adequately.

The following surface roughness parameters and measurement units will help you understand the surface roughness chat better. We included their SI units in micro-inches for better understanding.

RA – Average Surface Roughness (Center Line Average)

Some designers and engineers also refer to it as Center Line Average or Arithmetic average on the surface roughness chart. It is an absolute value that depicts the extent of deviation of a roughness profile from the mean line.

RA roughness average is the most commonly used parameter to measure surface roughness. However, it has some drawbacks that make other surface finish units indispensable when defining, assessing, and replicating surface topology.

RA and RMS (Root Mean Square) are the top representations on the surface roughness chart.

Rmax – Vertical Distance from Peak to Valley

Rmax measures the vertical average height of a surface’s peaks and valleys. It is efficient in measuring burrs, scratches, and other abnormalities on the surface not easily detected with the RA surface finish chart. In addition, Rmax is more efficient in identifying surface texture abnormalities on surfaces.

Rz – Average Maximum Height of The Profile

Rz measures the average maximum height of a  surface profile. It is obtained from the average values of the largest five sampling lengths of peaks and valleys across the whole surface. This parameter helps to ensure accurate measurement and avoid errors. It comes in handy since the center line average (Ra) is often insensitive to some extent, leading to inaccurate measurement.

RA Surface Finishes for Various Manufacturing Processes

 There are several industrial processes manufacturing companies execute to fine-tune the surface of their products. Methods involved in surface finishing vary with different manufacturing processes. Each method has various effects because surface finishing is a broad industrial process.

For example, there are different metal cutting processes, and each process has its surface finish process. This highlights the importance of a surface finish chart.

The first step in choosing surface finishes is identifying an industrial process capable of producing the desired surface texture and predominant surface pattern. Also, the process to be adopted should be cost-effective for commercial purposes.

The image below contains a table showing one example of surface roughness charts. It shows the relative roughness of various manufacturing processes:

Surface Roughness Conversion Chart

An important guide for measuring standard surface finish parameters is the machining process of the surface roughness chart. Manufacturers always use it as reference material to ensure quality in the manufacturing processes.

What are The Factors Affecting Surface Finish?

Surface finishes can be affected by several factors. Some of the factors include the following:

Type of Coolant Used

The use of coolant is an excellent means of enhancing surfaces and tool life in machining processes. It effectively reduces friction on surfaces. During CNC machining processes, heat is produced, affecting surfaces’ physical properties, making the surfaces rough.

The use of high-pressure coolant improves surface finish considerably well, though it is not the most efficient because it is not suitable for lower cutting speed.

Using minimum quality coolant has been accepted as the most efficient method of reducing surface roughness. It is cost-effective, reduces pollution, and enhances part performance. In addition, it decreases tool wear and surface roughness.

Cutting Parameters

Surface finishes are important for the quality of a product during the machining process. However, getting the desired surface finish will depend on cutting parameters such as feed, depth of cut, and cutting speed.

During cutting processes using the CNC machines, an increase in cutting speed decreases surface roughness. This means that the faster the cutting speed, the lesser the surface roughness (provided all other parameters remain constant).

With increasing the depth of cut on the CNC machining process, maximum roughness depth tends to increase. This is only if other parameters involved remain constant. In addition, increasing feed corresponds to a decrease in average roughness in a CNC machining process.

To improve machining processes, cutting fluids reduce surface roughness in all the cutting processes. This occurs by the reduction of tool temperature, which in turn reduces the coefficient of friction. However, fluid penetration into the cutting space also reduces the tool’s and product’s adhesion.

Type of Machining Process

The type of machining process used in manufacturing a product determines the degree of surface roughness of the product. The machining process is governed by two major machining parameters namely; feed rate and cutting speed.

The feed rate and cutting speed used in the metal machining process, such as the turning in the CNC machining process, greatly affect the surface finish of the final product. The surface value increases as the feed rate increases, while the roughness of the surface pattern decreases as the cutting speed increases.

Vibrations

Both unworn and worn tool causes vibrations that will affect the surface finish of a product. When tools get worn, they affect the integrity of the product surface finishes to obtain a smoother surface in unpredictable ways with vibration.

Vibration frequencies and amplitudes obtained using an unworn tool affect the surface finish by adding a simple sine wave vibration. The vibration has various amplitudes and frequencies, implying that the roughness average increases with increasing sine wave amplitude.

You should note that vibration frequency has minimum effect on the roughness of a surface.

How to Improve Surface Roughness

In the manufacturing process, there are several methods of improving the surface finish of products surface, parts, or equipment. Reducing friction and extrusion between the tool and the workpiece is effective. Other techniques involve sharpening the tool edge and ensuring that some materials are heat-treated properly. This way, it is possible to reduce the vibration of the machine tool.

The most effective methods for improving surface roughness include the following:

Improving Cutting Conditions

Reducing surface machining appropriately is a critical cutting condition in the production method. Some improvements to cutting conditions include:

Pick an Appropriate Processing Technology

Selecting an excellent processing technology determines the degree of surface finish of a product. Conversely, wrong or ineffective processes may affect the efficiency and quality of processing.

Choosing the Right Raw Materials

Parts of equipment or machine are made of different materials. Therefore, the selection of different production methods or tools is directly related to the degree of surface finish according to the densities of the different raw materials.

Conclusion

Obtaining optimum surface finishes is essential for the durability and effectiveness of products. Thus, it is crucial to establish exacting surface finish standards. This should go along with the right methodology with cost-effectiveness to generate the desired surface finishes.

Surface finish charts help to understand materials’ surface parameters effectively, be it smooth or rough surfaces. This will help designers and manufacturers select surface finishing techniques that are most effective for products.

Whenever you need to fine-tune the surface of a product, we are always ready to work with you to achieve your goal. In addition, our vast experience helps us offer advice on the best techniques to attain your manufacturing goals.

Contact us today; we have all it takes to give you the best products!