Surface Roughness For Custom CNC Machining Parts

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Surface Roughness For Custom CNC Machining Parts

Engineers and manufacturers use various methods to predict the performance of a component. The surface roughness of the component is one of the most popular of these methods. Surface roughness measurement is common in quality assurance in virtually every manufacturing process. Other parameters that go alongside surface roughness are strength and tolerances.

Surface roughness can affect product performance, visual appeal, and durability. Therefore, understanding this parameter and how to measure surface roughness is paramount. In this guide, we take a deep dive into the definition of surface roughness, measurement approaches, terminologies used, and much more. We hope this coverage will help you better understand the essence of surface roughness on your CNC machined products.

What is Surface Roughness in Machining?

Surface Finishing

CNC machined surfaces are prone to irregularities due to the operation of the cutting tool and the CNC machine in general. It could be a result of a tool overlap or any other problem. These irregularities are the definition of surface roughness in custom CNC machining.

Zoomed for a closer look, the surface of a CNC machined part resembles a graph. This graph comprises peaks and valleys. In surface roughness circles, this appearance is referred to as a surface profile.

Some people will also refer to these irregularities as surface finish, but scientifically, the two terms are not interchangeable. Surface roughness is a quantifiable parameter that measures the irregularities of a surface. Surface finish is a qualitative parameter that is mostly appearance-based.

Depending on the type of CNC machined product, a particular level of surface roughness may be part of the product requirement. To reach the specified level of surface roughness, manufacturers use several techniques to remove the irregularities. That’s where the need to measure surface roughness comes in.

Measuring Surface Roughness

surface-roughness-measurement

Irregularities on a component surface are triggers of nucleation sites that promote erosion. This can cause such a surface to wear faster than a smooth surface.

Rough surfaces can also enhance surface adhesion to another one. All these results will depend on the type and level of roughness present. A part’s mechanical performance can be affected by the surface roughness.

In engineering applications, quality requirements are usually stringent. Therefore, surface roughness must be accurately measured to make sure it aligns with the quality standards.

It is upon the manufacturer to control this parameter as much as possible. Unfortunately, controlling surface roughness sometimes leads to increased component cost. As a manufacturer, you want to strike a balance between controlling roughness and the resulting component cost.

Measuring surface roughness is done through the following techniques: Direct, non-contact, In-process, and comparison. In the direct method, a sapphire or diamond stylus moves across the surface and records the roughness values. The non-contact methods replace the stylus with sound or light. For in-process measurement, techniques such as inductance are applied to provide real-time measurements. The fourth method, comparison, relies on surface roughness samples. A popular tool for the comparison method is the surface roughness comparator.

Units of Measurement for Surface Roughness

It is not easy to identify one unit of surface roughness measurement because there are several out there. The following units are used for measured surface roughness: Rmax, Ra, Rpm, Rq, Rt, Rz, and Rv.

However, some feature more often than others do. You will regularly come across Rz and Ra as the units of measurement for surface roughness of custom CNC machined parts. Of the two surface roughness parameters, Ra is more popular. We will cover these surface roughness values and terminologies in more detail below.

Common Surface Roughness Terminology Explained

When you come across a surface roughness chart for the first time, some terminologies may confuse you. Here is an explanation of a few surface roughness terminologies.

Arithmetic Mean Surface Roughness (Ra)

Arithmetic mean surface roughness (Ra)

You can call Ra average roughness. It is a general presentation of the surface texture height. It is a measure of the average of deviations from the mean line, as the figure above shows.

Pros and Cons of Ra as the Unit for measuring surface roughness

Ra is an important measure in manufacturing processes. It is a reliable indicator of the surface texture, albeit in average terms. Machinists have for ages used this roughness average to determine the surface roughness of components in automotive and other industries.

One big advantage of this method of determining average surface roughness is that it is inexpensive. This makes it suitable for quality control in shops.

Notably, roughness average values fail to show several surface texture characteristics. Owing to this drawback, there is a potential for manufacturing quality check errors.

Consider the fact that it treats all peaks and valleys as equal. The reality is that some peaks and valleys are larger than others. This reality can bring warranty and quality issues to the manufacturer. Some areas of the component can wear and leak prematurely, despite the component passing Ra value checks for surface roughness.

Ra is also incapable of showing the spatial position of peaks and valleys on the surface. A surface could have features concentrated on one corner. Another could have them scattered evenly. However, the Ra value for the two surfaces can be the same.

Such measurements can lead to serious issues with manufactured components. For instance, surfaces coming together may develop uneven loading and eventually, faster wear and tear.

Nevertheless, since Ra presents some substantial benefits in CNC manufacturing, it is common in the United States and other parts of the world.

What's the Best Ra Value?

A low Ra value is preferable for several reasons. First, it makes it easier to clean the machined part. Microorganisms and dirt thrive where the surface roughness is high. Cracks, porosity, and general surface coarseness will encourage the proliferation of these things.

The surface-wearing process is also linked to the Ra value. The higher the value, the faster the surface degenerates. A big factor in this increased wear rate is the higher coefficient of friction. Anything that tries to move across the surface of the component requires a lot of force.

Some industries are extremely sensitive so the surface roughness measurements in Ra must be taken seriously. There are strict surface roughness standards for industries such as pharmaceuticals, food processing, and semiconductors.

A roughness conversion table helps convert roughness grade numbers to the corresponding Ra values. Below is the table showing these conversions.

Document
Roughness Grade Numbers Roughness Values Ra Micrometers (μm) Roughness Values Ra Micro-inches ((μm)
N12 50 2000
N11 25 1000
N10 12.5 500
N9 6.3 250
N8 3.2 125
N7 1.6 63
N6 0.8 32
N5 0.4 16
N4 0.2 8
N3 0.1 4
N2 0.05 2
N1 0.025 1

Surface Finish Grade Numbers and Ra Values

Maximum Surface Roughness (Rz)

Rz represents the maximum height average for a part’s surface in the evaluation length. The sizes of the peaks and valleys are taken, and the five biggest between these elements are used to calculate Rz.

This measurement can be more accurate than Ra because it takes care of the surface extremes. It is more recognized internationally.

Maximum Profile Peak Height (Rp)

This refers to the height of the largest profile peak within the evaluation length in a roughness profile.

Maximum Profile Valley Depth (Rv)

Root mean square surface roughness (Ry)

As shown in the figure above, Rv refers to the deepest valley in a surface roughness profile within the evaluation length. As usual, this distance is taken from the mean line in the evaluation length.

Maximum Roughness Depth (Rmax)

In a surface roughness profile, there is a value called Rti. It is the vertical distance from the highest peak to the lowest valley in a sampling length of a roughness profile.

Successive values of this parameter (Rti) can be taken and compared. The highest of these values is called Rmax.

What machining CNC surface finishes does Aria Manufacturing offer?

CNC Machined Part

As the final stage in the CNC machining process, surface finish plays several roles. It not only enhances the visual appeal of the product but also improves corrosion resistance, adds strength, and modifies the electrical conductivity of the material.

At Aria Manufacturing, we have several machining CNC surface finishes that you can consider for your product. They include:

 

  • As-Machined

  • Bead Blasting

  • Brushing

  • Textured finish

  • Polishing

  • Anodizing

  • Powder coating

As Machined

This is the CNC surface finish option to go for if you wish to take the least turnaround time. It comes immediately after the CNC machining process.

The result of an as-machined surface finish is a surface that is smooth and clean. However, there could be tool marks on the surface. Like in painting, this surface finish is typically completed by hand.

The exact process varies depending on the material. For metals, it mostly involves oiling and waxing. This is essential in the warding off of the wear and tear of the component. If the material is plastic, an as-machined surface finish includes using adhesives to merge different parts. In the case of wood, the application of varnish is a popular exercise.

The average surface roughness (Ra) gives the quality of the surface finish in this context. This surface finish is given a standard rating of 3.2 μm (125 μin), but it can be improved to 0.4 μm.

The as-machined surface finish presents the advantage of strict dimensional tolerance. However, it has the notable challenge of tool marks that can affect the overall component visual appeal.

Bead Blasting

Bead blasting is based on abrasion whereby marbles or beads are used to achieve the required surface finish. The marbles or beads strike the component surface at high speed causing a high friction coefficient. A machine called a bead-buster generates high pressure that propels the marbles or beads onto the part surface.

The method is ideal for hard materials such as plastic and metal. Once bead blasting is complete, it gets shiny and smooth surfaces. This CNC machining surface finish can also help in removing surface imperfections and rust. Manufacturers often use it to remove dents and scratches.

Since the method removes only small amounts of material at a time, machinists like it for the capability to generate the preferred part shape. Alternatives such as grinding and grinding are usually more difficult to use for this. Bead blasting is also ideal for preparing the part surface for coatings such as paint.

Beads come in different forms, but the most popular ones are glass and steel. When the surface is extremely hard or the contaminants tough, steel beads are preferable. Otherwise, the glass beads are used. The glass beads are more variable making them popular for different qualities of surface finish.

Here are some benefits of bead blasting a CNC machine surface:

  • It is an affordable surface finish method

  • It creates a shiny surface without altering the base color

  • It is an eco-friendly method that doesn’t result in toxic residues

  • The bead materials can be used for a long time before replenishment

Nonetheless, bead blasting may be undesirable due to its reductive nature. Where stringent tolerances have to be observed, this can be a problem. Equally concerning is the fact that the method must be a manual process done by an expert. Note also that bead blasting may take a long time to complete on a tougher surface.

Brushing

This is yet another effective method of achieving a nice surface finish on the CNC machined component. It is popular for its unmistakable texture and lines. Our machinists achieve this finish by using an abrasive belt or wire brush to smooth the surface. Brushing is applicable for a wide range of metals including aluminum, copper, and steel.

When compared to other surface finishes, brushing is unique in the sense that it produces distinctive fine lines. Instead of being reflective, the surface scatters light. The fine lines are consistent though, hence the unique appearance. The appearance and feel of the surface roughness depend on the force and direction of the brushing action.

To start the process, the machinist prepares the surface by cleaning it. Sanding is usually part of this cleaning stage, whereby irregularities and rust are removed. Needing special attention are surfaces such as aluminum and steel, which can exhibit oxidation and related problems.

Application of the brushing finish involves a brittle brush moved in unidirectionally. The result is a set of fine lines that leave the “brushed” appearance and feel. Based on the surface finish and type of metal, the machinist can choose the brush from among brass, steel, and other types of brushes. After the brushing step, the surface is taken through curing, drying, and finally, post-finishing.

Alongside the brush type and application technique, the following factors can determine the outcome of the brushing surface finish:

  • Environmental conditions

  • Base material

  • Quality of curing and drying

Textured Finish

Surface patterns and surface texture on a CNC machined part are growing in complexity every day. For us at Aria Manufacturing, these are exciting developments because they mean that the surface design options for our clients are wider.

Several techniques are available for actualizing the surface texture finish on your product. Popular ones are knurling, bead blasting, laser etching, sanding, and polishing.

Take our knurling service for example. With this procedure, your products get higher value in terms of aesthetics and grip. We can perform the textured finish on both flat and curved surfaces, and have a wide range of patterns for selection.

We will look at polishing in more detail in the section below.

Polishing

Polishing a CNC machined surface can enhance aesthetics, add a protective layer on the surface, increase functionality, and enhance the cleanliness of the component. Machinists use different methods for polishing including manual, machine polishing, and chemical polishing. The result can be a smooth finish or mirror finish.

Smooth Finish

The smooth finish for CNC machined surface is ideal for removing machining marks, thus forming a smoother surface. This smoother finish reduces roughness so the item is more aesthetically pleasing and easier to clean.

Mirror Finish

To make the surface smoother, we can produce a mirror finish. This polishing finish results in a surface that has high reflectivity, akin to a mirror. It is popular for concealing welding marks. Parts with this kind of surface finish are easy to clean because of the smoother surface. However, the person doing it has to be skilled to achieve flawless results.

Anodizing (Type II)

The other name for the type II anodizing process is sulfuric acid anodizing. It is called so because it involves immersion of the metal part or component in an electrolyte bath composed of sulfuric acid.

The applicable metal in this case is aluminum, which assumes a smooth surface with a layer about 1.8 μm thick. Different colors are achievable through type II anodizing – red, clear, black, and others.

Anodizing (Type III)

Type III anodizing, or hard coat anodizing, forms a thick oxide layer in an electrochemical process. It is also restricted to aluminum and its alloys. While the concept is similar to type II anodizing, the process occurs at a higher voltage and lower temperature.

Powder Coating

As an advanced manufacturing establishment, we also use powder coating for CNC machined surface finishing. The basic definition of powder coating is a finishing process that entails the application of powder or dry paint on the CNC part.

Resin and pigment in fine form are the main constituents of the powder. Powder coating applies electrostatic charges on the powder and uses a spray gun to apply this charged powder on the surface. Once the powder has settled and adhered to the part surface, it is subjected to heat curing for a durable coat.

CNC Machining Surface Finish Standard

CNC machining surface finish standard is useful for drafters, machine shops, and engineering departments. It offers the designer an understanding of the feel and appearance of a CNC machined surface.

Standard As-machined Surface Roughness Standard

At Aria Manufacturing, standard CNC as-machined parts have a surface roughness standard of Ra 3.2μm. This finish typically has visible tool marks and other surface flaws synonymous with CNC machining. However, you may specify if you need lower roughness levels for an as-machined surface.

Send us a quote for a standard as-machined surface roughness finish.

Precision Machining Surface Roughness Standard

The major difference between standard as-machined surface and precision machining surface is the level of tolerance. Precision machining has precise tolerances and usually involves many operations and highly skilled operator(s). Our roughness standard for precision machining is a Ra value of up to 0.4 µm.

Conclusion

Pursuing the right surface roughness for a custom CNC machined surface touches on more than just visual appeal. It ensures that the part works as expected and lasts.

We can all agree that surface roughness is a wide subject. The element can be presented in many ways, but we have had a good start by describing the general ones.

Given the potential of surface roughness subject to confusion, expert involvement during manufacture is always recommended. The team at Aria Manufacturing Limited proudly offers unmatched CNC surface finish services for your product needs and preferences.

We are only a call away. Get in touch today.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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