How to accurately select surface roughness to create top-quality parts for CNC Machining?

CNC machining technology has a high degree of accuracy and precision and can produce fine parts with tolerances as small as 0.025 mm. This machining method belongs to the category of subtractive manufacturing, which means that during the machining process, the required parts are formed by removing materials. Therefore, tiny cutting marks will remain on the surface of the finished parts, resulting in a certain degree of surface roughness.

What is surface roughness?

The surface roughness of the parts obtained by CNC machining is an indicator of the average fineness of the surface texture. In order to quantify this characteristic, we use a variety of parameters to define it, among which Ra (arithmetic mean roughness) is the most commonly used one. It is calculated based on the tiny differences in surface height and low fluctuations, usually measured under a microscope in microns. It is worth noting that surface roughness and surface finish are two different concepts: although high-precision machining technology can improve the smoothness of the surface of the part, surface roughness specifically refers to the texture characteristics of the surface of the part after machining.

 

How do we achieve different surface roughness?

The surface roughness of the parts after machining is not randomly generated but is strictly controlled to reach a specific standard value. This standard value is pre-set, but it is not something that can be assigned arbitrarily. Instead, it is necessary to follow the Ra value standards that are widely recognized in the manufacturing industry. For example, according to ISO 4287, in CNC machining processes, the Ra value range can be clearly specified, ranging from a coarse 25 microns to an extremely fine 0.025 microns to suit a variety of different application requirements.

We offer four surface roughness grades, which are also typical values for CNC machining applications:

3.2 μm Ra

Ra1.6 μm Ra

Ra0.8 μm Ra

Ra0.4 μm Ra

Various machining processes have different requirements for the surface roughness of parts. Only when specific application requirements are specified will lower roughness values be specified because achieving lower Ra values requires more machining operations and more stringent quality control measures, which often increase costs and time. Therefore, when a specific roughness is required, post-processing operations are usually not selected first because post-processing processes are difficult to control accurately and may have an adverse effect on the dimensional tolerances of the part.

In some machining processes, the surface roughness of a part has a significant impact on its function, performance, and durability. It is directly related to the friction coefficient, noise level, wear, heat generation, and bonding performance of the part. However, the importance of these factors will vary depending on the specific application scenario. Therefore, in some cases, surface roughness may not be a critical factor, but in other cases, such as high tension, high stress, high vibration environments, and where precise fit, smooth movement, rapid rotation, or as a medical implant are required In components, surface roughness is crucial. In short, different application conditions have different requirements for the surface roughness of parts.

Next, we’ll take a deeper dive into roughness grades and provide you with all the information you need to know when choosing the right Ra value for your application.

3.2 μmRa

This is a widely used surface preparation parameter that is suitable for many parts and provides sufficient smoothness but still with obvious cutting marks. In the absence of special instructions, this surface roughness standard is usually adopted by default.

 

3.2 μm Ra machining mark

For parts that need to withstand stress, load, and vibration, the recommended maximum surface roughness value is 3.2 microns Ra. Under the condition of light load and slow movement speed, this roughness value can also be used to match moving surfaces. In order to achieve such roughness, high-speed cutting, fine feed, and slight cutting force are required during the processing.

1.6 μm Ra

Typically, when this option is selected, the cut marks on the part will be quite light and unnoticeable. This Ra value is well suited for tightly fitting parts, parts subject to stress, and surfaces that move slowly and are lightly loaded. However, it is not suitable for parts that rotate quickly or experience severe vibration. This surface roughness is achieved by using high cutting speeds, fine feeds, and light cuts under strictly controlled conditions.

In terms of cost, for standard aluminum alloys (such as 3.1645), choosing this option will increase production costs by approximately 2.5%. And as the complexity of the part increases, the cost will increase accordingly.

 

0.8 μm Ra

Achieving this high level of surface finish requires very tight control during production and is, therefore, relatively expensive. This finish is often used on parts with stress concentrations and is sometimes used on bearings where movement and loads are occasional and light.

In terms of cost, choosing this high level of finish will increase production costs by approximately 5% for standard aluminum alloys such as 3.1645, and this cost increases further as the part becomes more complex.

 

 

0.4 μm Ra

This finer (or “smoother”) surface finish is indicative of a high-quality surface finish and is suitable for parts that are subject to high tension or stress, as well as for fast-rotating components such as bearings and shafts. Because the process of producing this surface finish is relatively complex, it is only selected when smoothness is a critical factor.

In terms of cost, for standard aluminum alloys (such as 3.1645), choosing this fine surface roughness will increase production costs by approximately 11-15%. And as the complexity of the part increases, the required costs will rise further.