Are you facing challenges with the quality of your machined components? Surface roughness determines the workability and your project’s durability. If the machining surface finish is not well managed, the performance is compromised and the wear can be relatively higher, which means replacement/maintenance costs.
The optimal surface roughness should be used for the best-performing material. Achieving the desired surface finish requires careful selection of machining processes and their parameters. Applying the right strategies will enable the organization to achieve the set standards in the industry as well as meet customers’ expectations. Knowing how variables such as machining speed, feed rate, and tooling affect roughness is helpful in your production line. Read more about how making small changes can result in better finishes and guarantee your project’s success.
Surface Roughness in CNC Machined Parts
The surface roughness of CNC machined components plays a crucial role in determining their performance in their environment. Normally, a finish described as ‘as machined’ is attributed a roughness average (Ra) of 3.2; but the part has visible lines made by the cutting tool. The intended roughness is normally acceptable in most applications; however, certain designs may require a somewhat smoother finish. The parts surface that slide across one another should be smoother because it helps reduce friction and increases the wear characteristic.
A desired finer finish is normally obtained after carrying out post-finishing operations such as polishing. The smoother the surface of a manufactured part, the higher the manufacturing cost. Therefore, there is always a trade-off between surface finish and production expenses for certain components. For instance, a bicycle seat post requires a high friction coefficient to not slip. It may be difficult to realize the required roughness with a higher level of accuracy using conventional machining techniques. Thus, there is always a need for secondary processes such as bead-blasting or tumbling. Several procedures can generate a specific surface roughness because both polishing and cutting operations contribute to the final effect.
Why Surface Finish Matters in Engineering?
The improved surface finish plays a major role in defining how a product can in fact interface with its surroundings. A product’s finish can be said to be a direct representation of the functionality of an end-use product. Moreover, the degree of roughness helps in determining the total performance of the products. Engineers as well as manufacturers need to focus on achieving good surface finishes, more often. This focus is important to obtain reliable operations and good output.
So, surface roughness measurements are vitally important in manufacturing control. As can be observed, different surface finishes give different results. It is easy to use finish comparisons to get to established surface standards as a way of getting the intended outcome.
The advantages of adequate metal surface finishing include:
- Improved resistance to corrosion and chemicals.
- The cosmetic enhancement enhances the look of the product.
- Better bonding for coatings and paints.
- Reduction of surface defects to improve the quality of the product.
- Improved current carrying capacity and superior electrical characteristics.
- Higher wear resistance and at the same time lower friction.
Surface Roughness Terminology
- Ra – Ra is a mathematical mean of the peaks and valleys measured across the surface profile length. It is also called the Center Line Average (CLA).
- Rz – Rz is assessed as the mean absolute deviation of the surface profile, defined as the average vertical distance between the peaks and valleys in the surface profile. It takes into account multiple heights, measures the distance between the five major peaks and the five major valleys, and finds the average.
- Rp – Rp describes the height measured from the highest point of the profile to the average line drawn across the assessment length.
- Rv – Rv means the height from the lowest point of the valley to the mid-line in the specified measurement length.
- Rmax – Rmax measure is the single largest deviation that is observed between the highest peak and the lowest valley over the evaluation length.
- RMS – The Root Mean Square value is found within the evaluation length. It’s a square root of the mean of the squares of the deviations of the signal from the mean line.
Typical Surface Roughness Chart Sheet
| Use Case | Material Type | Average Roughness (Ra, µm) | Mean Roughness Depth (Rz, µm) | Maximum Roughness (Rmax, µm) |
| Interior of Engine Cylinder | Cast Iron | 0.4 – 1.6 | 2.5 – 6.3 | 8 – 20 |
| Surface of Bearing | Steel | 0.05 – 0.2 | 0.3 – 1.2 | 1 – 3 |
| Cavity of Mold | Steel | 0.1 – 0.4 | 0.6 – 2.5 | 2 – 8 |
| Component Made from Sheet Metal | Steel | 1.6 – 6.3 | 10 – 40 | 32 – 125 |
How To Measure Surface Roughness?
Surface roughness measures the deviation from the reference line of a surface. Understanding surface geometry involves three essential components: roughness, waviness, and lay. All of these factors affect the general properties of a surface. Multiple systems exist for assessing surface roughness, including:
- Direct Measurement Methods
- Non-Contact Methods
- Comparison Methods
- In-Process Methods
Direct Measurement Methods use a stylus. It is used to slide on the material surface. If the stylus is placed perpendicular to the surface, it records the contour, which in turn enables machinists to establish the roughness parameters.
Non-contact methods involve light or sound instead of a stylus. Optical-based instruments include white light and confocal systems for measurement. For instance, ultrasonic devices transmit pulses to the surface and, using the waves reflected from the surface, deduce the roughness parameters.
Comparison Methods include set standard roughness specimens. Using tactile and visual analyses manufacturers compare the achieved results with desired parameters to evaluate the accuracy.
In-process techniques use electromagnetic energy to gauge surface roughness such as the Inductance gauge. An inductance pickup measures the distance from the sensor to the surface and thus calculates relative roughness parameters.
Typical Types of Measuring Surface Roughness
Different measurement methods can be classified into three main categories:
Profiling Techniques use higher-resolution probes for surface measurement. These methods present a need for a sensitivity comparable to phonograph needles since conventional CNC probes may not be precise enough.
Area Techniques measure a limited surface portion and provide arithmetical means of high and low points. Some common techniques for measuring include ultrasonic scattering, optical scattering, and capacitance probes which enable automation and are easy to perform.
Microscopy Techniques involve the measurement of contrast and thus are mainly qualitative. These techniques are useful in estimating the high and low of the surface.
Standard Surface Finish in Machining
In machining, the standard surface finish is defined as the surface texture produced in a defined manufacturing process by the material removal process.
In most cases, the surface roughness of a machined part is specified to be 3.2 μm Ra. This finish is the least expensive of all and is the roughest recommended for parts that undergo vibrations, heavy loads, or stress. It allows for visible tool marks but diminishes manufacturing costs as well as time since the parts are machined at high rates.
A finishing cutting pass is also possible to reduce the surface roughness with a lower Ra value. However, this process may increase costs, create more machining operations, and prolong the production cycle time.
As-machined components usually conform to precision machining roughness of 3.2μm Ra and 1.6 μm Ra.
Types of Machining Finishes
The finish depends on the machining type, the material used, and the final result that a constructor wants to achieve. Below are common types of machining finishes:
- As-Machined Finish: As-machined does not require any subsequent operations after the machining operation has been finished. This type of surface usually has tool marks being used and no sign of sleekness.
- Smooth Finish: Smooth finish can merely acquired through grinding or honing. It’s ideal for applications where a smooth coating is needed for functionality.
- Textured Finish: Some particular applications require the surface of the molded product to have a certain texture to enable it to be gripped or for visual appeal. Such a finish may be developed by processes like knurling or bead blasting.
- Mirror Finish: The mirror finish is smoother and mirror-like than the textured one. The effect can be attained by extensive polishing and is applied mostly to ornamental parts.
- Anodized Finish: In metals such as aluminum, anodizing forms a barrier of oxide protection to the metal. This process improves the strength, and in addition, it can add color to the component too.
Choosing the Right Surface Roughness for Your Project
The desired Ra value can differ depending on the product’s intended use, how long it needs to last, if the part will be further treated like with polishing or painting, and how critical it is to retain dimensions and your wallet.
Economical projects may use a surface roughness of 3.2 μm Ra although subsequent finishing may be carried out to improve the roughness. If you wish to reduce the number of tool marks visible while keeping the costs down, a 1.6 μm ra surface finish might be the best for you.
In case, your project demands a finer surface finish – 0.8 μm Ra or 0.4 μm Ra, the cost is going to be higher. Nevertheless, these finishes are necessary for applications where the tolerance is critical and/or where the parts are likely to be loaded at particular points since, for example, these finishes have no cut/tool marks. As such, they should only be specified when the requirement for smoothness and high precision of the task at hand cannot be overemphasized.
Factors Affecting Surface Finish in Machining
Machined part surface finish is affected by numerous factors, vitally involved in producing parts and components.
Type of Coolant Utilized
Coolant applied during machining greatly affects both the surface and the tools’ optimum movement. Probably, the most crucial use of coolant in CNC machining help reduce the friction between the cutting tool and the material during the process to control temperatures. High temperatures cause changes in the material characteristics; therefore, the products’ surfaces are rough.
High-pressure coolants can improve the surface finish greatly but they may not be very effective for operation at the lower cutting speed. This strategy not only helps to reduce cost but also helps to reduce waste, minimizes pollution level of the environment, and maximizes the productivity of machining parts due to less tool wear.
Cutting Parameters
The particular cutting parameters being used during machining have a direct correspondence to the surface roughness. These factors include feed rate, cutting depth, and cutting speeds. Increasing cutting speed, while keeping other CNC parameters constant, improves surface finish.
On the other hand, an increased depth of cut results in higher surface roughness. Interestingly, adjusting the feed rate has a complex relationship: as the feed increases the average surface roughness decreases. These fluids not only cool the tool but also reduce wear.
Furthermore, the penetration of the fluid in the machining area also helps in minimizing the adhesion between the tool and the workpiece which also signifies the surface quality.
Type of Machining Process
The level of surface finish is highly dependent on the machining process used. It is governed primarily by two key parameters: feed rate and cutting speed.
In CNC turning, a high feed rate indirectly increases surface roughness. Conversely, a higher value of cutting speed results in a better surface finish of the workpiece. Therefore, it finds the right machining process and these parameters are crucial to the quality output of the final product.
Impact of Vibrations
Machining vibrations due to worn or unworn cutting tools are known to affect the finishing of the machined components. When these tools become dull they can cause surface roughness due to the inconsistencies produced. Unused tools can produce frequency vibrations with sine waves of varying amplitudes. In general, the amplitude of these vibrations increases with the average surface roughness. This knowledge can be used to improve the efficiency of machining to achieve the best outcomes.
How to Improve Surface Roughness?
Improving the surface finish of products, parts, or equipment is always a goal in manufacturing. To improve the surface roughness, several techniques are employed mainly to minimize friction and vibration during the machining operation. Here are some key strategies to achieve better surface finishes:
- Improve Cutting Conditions: The cutting conditions can be regarded as the most important factor that affects the surface roughness. Consider the following improvements:
- Increase Cutting Speed: Generally, a higher speed of machining can give an improved finish.
- Reduce Feed Rate: Decreasing the feed rate reduces the surface roughness as well.
- Use High-Quality Cutting Fluids: The choice of cutting fluid plays a great role in minimizing friction between the tool and workpiece as well as cooling the tool.
- Implement Ultrasonic Vibration Cutting: By following the ultrasonic vibration technique, better surfaces can be obtained since there are minimal marks that can be made by tools.
- Select the Right Processing Technique: The ineffectiveness of the process affects the speed and quality of production. So, it’s essential to carefully choose methods that suit the specific requirements of the part being produced.
Conclusion
To sum up, it’s desirable to obtain the best surface finishes in various product applications as they contribute to longevity as well as overall performance. Adequate surface finish specification coupled with viable strategies is mandatory to achieve the desired surface finish which would also be economical. Surface roughness charts help designers and manufacturers understand material characteristics. By using these charts, they can select the right finishing methods for their products.
