HOW TO ACHIEVE BRILLIANT SURFACE FINISHING FOR CNC MACHINING
In the CNC machining process, minor tool marks remain on the visible surface. This requires post-processing and surface finishing procedures to improve the surface of the workpiece. It obviously enhances the visual properties of qualified CNC machined components.
CNC machined parts have a milled surface finish. Minor tool marks appear on the surface of the component. This is because of the shrinking nature of the CNC (computerized numerical control). So, many components need further processing. The main purpose is to get the desired surface uniqueness. And performance to achieve;
- Corrosion resistance
- Wear resistance
- And some other purposes
Here, I will talk about the most familiar metal CNC surface finishes. Then, it will help you to select the most appropriate surface finish for your application.
Standard CNC machining finishes:
The detail of the same is as under;
1. Standard, as machined:
It is a standard surface finish as CNC machined parts will have minor visible tool marks. And the average value of the machined surface roughness (Ra) is 3.2 μm. We use the finishing process to diminish surface roughness by 0.8, 0.4 μm, or less. This will boost the production cost of the components. All we have to do is apply some extra processing methods. We cannot ignore the stricter quality control process too.
Polishing is also used to reduce the surface roughness of CNC machined parts. In this way, we also maximize the surface quality of the components. Polishing and grinding end some materials that affect the dimensional tolerances. Some parts only need grinding and polishing for a better surface finishing.
2. Sandblasting or bead blasting process:
Bead blasting adds consistent matte or satin surface finishing to CNC machined components. So, this process removes tool marks from the surface. In the bead spraying procedure, compressed air makes a high-speed jet beam. That sprays the material on the surface of the components. The components gone through at high speed during the process. Afterward, it helps to eliminate some materials from the part’s surface. And also provides smooth surface finishing.
There are other steps that you can adopt to prevent dimensional changes. But, it will incur some extra costs. Here are some findings of the process;
- It can clean tiny burrs available on the surface of the component
- It improves the surface finishing of the workpiece
- It exposes the component to a uniform metallic color
- And it makes the component look more beautiful
The main use of spray beads is for visual purposes. This is not an automatic but a manual process. So, the outcome depends to some extent on the operator’s skill. The critical process parameters are;
- The size of the air pressure.
- And the size of the glass beads. (Glass beads have many sizes (from thick to very thin)). Like sandpaper that also has different sizes.
MT-11010 Light Bead Sandblasting-can end light tool marks. Moreover, it eliminated light sandpaper scratches. Finally, it provides smooth and even glosses.
MT-11020 heavy bead blasting eliminates deep cutter marks. And also eliminates scratches left by the sandpaper. As a result, it provides a light-colored appearance.
Anodizing is an electrolytic oxidation method. We plunges the part in a dilute sulfuric acid solution in the electrolysis procedure. Then there is will be a voltage connection between the cathode and the workpiece. The electrochemical reaction devours the material on all naked surfaces of the component. And it converts it into hard titanium or aluminum oxide.
The masks are essential for the surfaces with critical dimensions. And on surfaces that keep moving to avoid anodizing. As a result, it improves the surface hardness of the aluminum components. It also maximizes the wear resistance of aluminum components. Moreover, it is also helpful to prolong the service life of the workpiece.
For the surface treatment of aluminum parts, the anodizing process is very famous. Anodizing is also used for the surface treatment of titanium machined parts. We can dye anodized parts in following many colors;
- Urban grey
- Coyote brown
We can form layers of various thicknesses and densities by the anodizing process. Yet, there is a need to change the current and anodizing time to do the same. Moreover, there is a need to change the concentration and temperature of the solution.
a. Anodizing (Type II):
The anodizing (Type II) is also known as “standard” or “decorative” anodizing. It can create coatings up to 25 μm thick on the CNC machined parts. The thickness of the layer depends on the color. For parts dyed in different colors, the coating thickness will be as follow;
- For dyed black, the thickness will be between 8 – 12 μm
- And for transparent (undyed) components, the thickness will be between 4 – 8 μm
The primary use of anodizing (type II) is to produce smooth and attractive components. Type-II anodizing offers the best corrosion and limited wear resistance to the parts.
b. Anodizing (Type III), or hard coating:
The anodizing (Type III) is also known as “hard coating” in the industries. It can create coatings up to 125 μm thick on the workpiece. The anodizing (Type III) surface treatment creates a high-density thick ceramic layer. This layer will be excellent in corrosion and wear resistance. Moreover, it will also be appropriate for functional applications. Anodizing (type III) requires stricter progression control than type-II anodizing. It means higher current concentration and stable solution temperature. So, this type of anodizing is higher in cost.
3. Powder coating:
The powder coating provides a thin protective polymer layer on the workpiece’s surface. The layer works as wear-resistant and shows good results. We make optional phosphating or chromatizing coats on the surface of the parts. The purpose of this is to increase the corrosion resistance of parts.
But the only difference is that paint is a desiccated powder rather than a liquid. So, the powder adsorbed on the surface of the CNC machined aluminum parts. It baked into the components with the help of the oven.
The outcome of the powder coating process is very excellent. It results in a robust, corrosion-resistant, and wear-resistant coating. Moreover, it is more robust than standard spray coating procedures. We can use different colors to get the desired aesthetics of the components. Powder coating is much better than wet coating procedures. Along with surface finishing in the following terms;
- Mechanical strength
- Corrosion resistance
- Aging resistance, powder coatings are better than wet coatings
The powder coating process can meet a high consumption rate of 100% and is very eco-friendly. Powder coating is a burly, wear-resistant layer suitable with all metal materials. We also can use this in combination with sandblasting or bead blasting process. We mix them to create smooth and uniform surfaces components.
We can apply various layers to create thicker coatings. The average thickness range of the coating is from 18 μm to a largest of 72 μm. We can select many colors to perform the powder coating process.
Some tips and tricks to improve CNC machining finishes:
I cannot overlook the importance of perfect surface finishing of CNC machined parts. So, the following are some tips about how to improve the machined jobs finishing;
1. Accurate feed and cutting speed:
The accurate way to handle surface finishing is;
- To raise surface feet per minute (SFM)
- And decrease inches per revolution (IPR)
The former provides help to diminish built-up edge (BUE). That gives support to lengthen the life of the tool. So, must have accurate knowledge about feed and cutting speed.
2. Selecting the best cutting procedure:
When the parts are on CNC milling at the machining stage, there are two ways to cut;
- Clockwise milling: It provides a better surface finishing to the parts.
- Counterclockwise milling: It does not provide better surface finishing to the components.
So, we have to use a clockwise milling process to get better surface finishing. Yet, we can use counterclockwise milling for roughing purposes.
3. Selection of the tooling:
Try to use excellent and new cutting tools to better surface finishing to the parts. If you apply old tools, they will give poor surface finishing. I know it will maximize your costs, but it is a better option.
The CNC program is also essential in the excellent surface finishing of parts. First, the CNC programmer must create a separate best program for this purpose. Then, after making the program, he must deliver the instruction to set proper feed and speed. Yet, programs that provide better surface finishing take a little longer.
I am hopeful that now you have a lot of knowledge about surface finishing. I have provided all essential details about the same in exact words. The main goal of writing in simple terms is to understand in a better way.
First, I covered typical CNC machining finished in detail. And, I explained about sandblasting or bead blasting, anodizing (Type II & Type III). Moreover, I provided details about powder coating as well. Afterward, I explained some tips and tricks to improve CNC machining finishes. I will be waiting for your valuable feedback.
Link 1: dimensional tolerance
Link 2: metal materials