What surface finishes are available for CNC machining?
What are the types of surface finishes for CNC machining? A post-processing or surface finish will improve the wear resistance, cosmetic properties, and surface roughness of metal parts. Get an understanding of how CNC parts are commonly finished and how to select the right method.
Part with Anodizing Type II
Moisture and weather will always cause metals to rust. The last thing you want is an uneven layer of corrosion when creating custom parts.
It’s a problem that has been around for centuries, but new solutions are available. For example, coating your part is one of the best solutions for preventing corrosive wear.
You may have heard terms like as-machined, powder coating, anodizing, or media blasting when discussing CNC machining surface finishing options. Prior to diving into the details of these processes, we should clearly define two terms that are frequently misused: surface finish and surface finishing.
Surface Finishes
During the manufacturing process, surfaces produce some characteristics: lay, roughness, and waviness. These characteristics are called “surface finishes” and measure a character’s distinctive (on a micro level). These characteristics might need to be targeted at specific values according to the product’s function.
Usually, when discussing surface finish, roughness is the most significant factor, which is also what we refer to in this article as machined.
Surface finishing
Surface finishing refers to the process of protecting and improving surfaces by adding, removing, or using heat, electricity, or chemicals. Such as, anodizing, powder coating, and black oxide.
What surface finishing do we offer for CNC Machining?
Our platform offers the following most common surfaces. All of these are for METAL parts.
As machined
The surface roughness Ra 3.2 µm and tool marks are typical for parts that come right off the machine. The Ra can be increased up to Ra 0.4 µm.
Conversion coating
Metal oxide conversion coatings are used to protect metal surfaces from corrosion, rust, and wear and provide valuable material properties. Conversion coatings include anodizing, alodine, passivation, and black oxide.
Powder Coating
It provides wear- and corrosion resistance to almost all metals by adding a specific material to their surfaces. It is more impactful than anodizing and comes in a variety of colors.
Bead Blasting
Parts that have been bead-blasted have a matte finish and a light texture. Its primary purpose is to enhance a part’s visual appearance.
Surface Finishes explained
As Machined
Different applications require different roughness values. The specification of lower surface roughness should be limited to situations where it is necessary. A lower Ra value requires more machining effort/operations and quality control because it requires more machining effort. They can significantly increase the cost and time of machining. Specific surface roughness values aren’t usually achieved through post-processing operations. These operations may affect the dimensional tolerance of parts since they can’t be controlled precisely.
In CNC Machining, we typically specify four levels of surface roughness.
- 3.2 μm Ra
- 1.6 μm Ra
- 0.8 μm Ra
- 0.4 μm Ra
3.2 μm Ra
Commercial machines are usually finished this way. There are visible cut marks on this material, but it is suitable for most consumer parts and sufficiently smooth. As a default, it applies to all surfaces unless otherwise specified.
1.6 μm Ra
With this option, cut marks are usually barely visible. For fast-moving and light load-bearing surfaces, this Ra rating is suitable for tight fits. However, it is not ideal for fast rotating parts or vibration-sensitive parts. Under controlled conditions, high speeds, fine feeds, and light cuts produce this rough surface.
0.8 μm Ra
To produce this surface finish, very close control is required, which leads to a higher price tag. Stress concentration exposes parts to this requirement. It can be used as a bearing in periodic motion and light loads.
0.4 μm Ra
Surface roughness of this quality is the best available. Parts with high tension or stress can benefit from it. A bearing or shaft that rotates rapidly requires it as well. This surface roughness should only be specified when smoothness is of primary importance.
| 3.2μm | 1.6μm | 0.8μm | 0.4μm | |
| Pros | Tighest dimensional tolerances | Tighest dimensional tolerances | Tighest dimensional tolerances | Tighest dimensional tolerances |
| Cons | Visible tool marks | Slightly visible tool marks | barely tool marks | No tool marks |
| Protection | Weak | Weak | Weak | Weak |
| Suitable for | All machinable materials | All machinable materials | All machinable materials | All machinable materials |
| Cost | No added cost | 5% Added cost | 10% Added cost | 15% Added cost |
Tips to achieve well as machined finishes
What steps can we take to ensure a better surface finish? Your dilemma is understandable. As a result, we have compiled a list of significant surface finishes that will assist you in improving the quality of your CNC machined parts. Please take a look at them!
1.Measurement of Surface Finish
Knowing the surface you’re supposed to work on is a vital part of the process. We measure surface finish using various techniques and characteristics, including profiling, area, and microscopy, emphasizing roughness peaks (Ra) and their separation (D). We need to understand which technique fits in most effectively to get desired results without spending a lot of time and effort.
2. Feeds and speeds need to be adjusted correctly
If you have to pick a speed and feed for finishing your machined parts, stick to the pre-defined and proper feeds and speeds. You don’t want anything to turn out wrong. Finishing machining should be performed at a faster surface foot per minute (SFM) and a slower inch per revolution (IPR). The former aids in prolonging tool life, while the latter prolongs insert life.
3.Invest in chip breakers.
To get a good surface finish, the chips produced should be controlled before making contact with the workpiece. The chips produced have a great deal of influence on the whole machining process.
You might find cutting and re-cut these troublesome chips easier using proper chip breakers. If you have been using one, you should change its style as it isn’t compelling enough. By reducing the chips into bits that are easier to fall into the cutting zone, chip breakers reduce cutting pressures and facilitate the evacuation of even long, string-like chips.
4.Minimize chatter and Deflection
Make sure your tool doesn’t deflect or chatter for a smooth finish. This will result in a messy and chippy finish that we typically throw out. It is critical to make your tool rigid and effective, so you do not waste time, money, or resources.
5.Reduce vibration with balanced tooling
To reduce vibration when finishing, it’s important to use balanced tooling techniques. This step becomes even more essential when you have a high RPM.
6.Ensure minimal run-outs
In addition to poor surface finishes, a high run-out can cause a tool to spin in an eccentric path instead of the desired circle, leading to incorrect operation. If you want a better and lower run-out, you may switch to an ER collet chuck over a setscrew-style end mill holder.
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Surface Finishes explained
Conversion coating
Your metal parts are coated with conversion coatings through a chemical reaction. The metal oxides can be applied to almost all metals; however, if they are formed in a controlled environment, they can harden to protect metal parts by creating an inert surface impenetrable to uneven corrosion. The conversion coatings we will discuss in this section are anodizing, alodine, passivation, and black oxide.
Anodizing
A standard electrolytic passivation method for aluminum is anodizing. An electrochemical current is applied to parts after stripping off any corrosion built up in a sulfuric acid bath. The surfaces of these molecules attract oxygen molecules that are negatively charged and very reactive. A coating of different thicknesses and densities can be created by varying the electric current, anodizing time, consistency, and temperature of the solution.
Type I anodizing
As the thinnest layer, chromic acid anodize does not change the dimensions of parts. A gray anodize is less absorbent than other colors and appears grayer in color.
Type II anodizing
In type two anodizing, the coating thickness can range from 4 um to 25 um, and it provides good corrosion resistance and limited wear resistance to the parts it is applied to. Type two anodizing is mainly used for smoother surfaces and provides superior corrosion resistance but limited wear resistance.
Type III anodizing
The Type III anodizing process produces highly resistant ceramic coatings of up to 125 um thickness suitable for functional applications. The layers provide excellent corrosion and wear resistance, making them ideal for practical applications. Due to its close process control requirements, it is more expensive than the other two types of anodizing.
3.Invest in chip breakers.
To get a good surface finish, the chips produced should be controlled before making contact with the workpiece. The chips produced have a great deal of influence on the whole machining process.
You might find cutting and re-cut these troublesome chips easier using proper chip breakers. If you have been using one, you should change its style as it isn’t compelling enough. By reducing the chips into bits that are easier to fall into the cutting zone, chip breakers reduce cutting pressures and facilitate the evacuation of even long, string-like chips.
Tips for anodizing
Anodic coating grows equally in both. In other words, after anodizing, a cylinder with 2 mils in diameter will have a diameter of 2.50 mils or approximately 62.5 um thick above the original surface. MIL-A-8625 provides more details on anodizing.
| Type I | Type II | Type III | |||
| Pros | Durable cosmtic coating | Easily applied to small parts and cavities | Top-of-the-line engineering coating with high wear resistance | ||
| Cons | Brittle | Only availalbe with aliminum and titaninum alloys | Type III anodizing is expensive | ||
| Thickness | 4-8 µm | 8-25 µm | 50-125 µm | ||
| Protection | 2/5 | 3/5 | 4/5 | ||
| Apperance | Clear | Clear/ color | Clear/ dyed black | ||
| Conductive | NO | NO | NO | ||
| Suitable for | Aluminum & Titanium | Aluminum & Titanium | Aluminum & Titanium | ||
| Price | Stanard | Standard | Expensive |
Alodine
Alodine, aka chem file or iridite, creates its oxidative layer without using electricity as anodizing does. A highly reactive chromium atom instead drives the purely chemical reaction. A chromium bath is used to soak the parts; then, they are raised to dry. Coatings begin as soft, gel-like gels but harden into conductive, productive layers as chemical reactions continue. Alodine does not alter the dimensions of the skin.
| Pros | Can provide both corrosion resistance and conductivity. | |
| Cons | Not suitable for decorative purposes | |
| Skin-irritating | ||
| Thickness | 0.25-1.0 µm | |
| Protection | 2/5 | |
| Apperance | Clear/ Gold | |
| Conductive | Yes | |
| Suitable for | aluminium, zinc, copper and more | |
| Price | < Anodize |
Passivation
Conversion coatings are often called passivation. Steel and stainless steel parts are commonly coated with passivation processes. Anti-corrosion properties have made stainless steel the material of choice. Passivation is easy since it contains a high amount of chromium. Cleaning parts creates a protective barrier formed by alloyed chromium reacting with oxygen near the surface.
| Pros | Removes Surface Contamination | |||||||
| Increases Corrosion Resistanc | ||||||||
| Reduces the Risk of Product Contamination | ||||||||
| Cons | Non-smooth surface | |||||||
| Require pre-cleaning process | ||||||||
| Thickness | 5-15 µm | |||||||
| Protection | 3/5 | |||||||
| Apperance | Glossy | |||||||
| Conductive | NO | |||||||
| Suitable for | Steel/ stainless Steel and more |
Black oxide
Black oxide is a type of conversion coating that produces magnetite, a conductive iron oxide layer. As a result of its black oxide finish, the metal’s surface reflects very little light and the part’s dimensions barely change. Oil or wax can seal a black oxide finish, which increases lubricity and prevents galling.
| Pros | Dimensional changes are minimal | ||
| Surface Aesthetics | |||
| Corrosion Resistance | |||
| Cons | Black oxide layer is easily rubbed off | ||
| Thickness | < 0.75 µm | ||
| Protection | < passivation | ||
| Apperance | Smooth, Matte black | ||
| Conductive | Yes | ||
| Suitable for | Steel/ stainless Steel and more |
Powder Coating
With powder coating, the part’s surface is protected by a thin layer of polymer. With this finish, you can create parts with smooth surfaces and excellent corrosion resistance that are compatible with all metals and can be combined with other surface treatments to create parts that are wear-resistant and strong.
A thicker coating can be created by applying multiple layers, and the thickness varies from 18 to 72 μm . A variety of colors is available.
| Pros | Multiple colors availalbe |
| Competible with metals availalbe | |
| strong corrosion resistant | |
| Cons | weak dimensional control |
| only for big size compoents | |
| Thickness | 18-72 µm |
| Protection | > anodizing |
| Apperance | variety colors |
| Conductive | NO |
| Suitable for | All metals |
Bead Blasting
The primary purpose of bead blasting is to enhance the aesthetics of parts rather than their functionality. It is a manual process, so the operator’s skill will partly determine the outcome. The main parameters are the air pressure and the size of the glass beads. Glass beads come in different sizes, just as sandpaper has different grades. Huge parts are generally blasted with a grid size of #120.
| Pros | environmentally friendly |
| Excellent for delicate components | |
| Cons | Dimensions and surface rougness are affected |
| Protection | 1/5 |
| Apperance | matte surface |
| Suitable for | all materials |
Tips for choosing the suitable surface finishing for your CNC Machining.
Surface finishes have advantages and disadvantages, so picking one based on your desired features depends on your application and parts requirements.
Your decision will be influenced by your part’s functional and aesthetic features and the aspect of how your part interacts with adjoining components. It would help if you also considered whether your pair might be subject to repeated or intense friction.
As machined: When 0.8 mm Ra is applied to the surface finish, it works very well with tolerance control and surface roughness.
Type I anodizing: This surface provides a thin coating but a high level of corrosion resistance and parts come out with natural matte colors.
Type II anodizing: This treatment allows aluminum and titanium alloys to be coated with a clear or colored protective layer.
Type III anodizing: Their higher oxide layer thickness in engineering applications makes them suitable for corrosion and abrasion resistance.
Alodine: The anti-corrosive properties of alodine will remain intact even if a part is scratched. There are alternatives to anodizing.
Passivation: A mild oxidant is used to remove free iron or other foreign matter from stainless steel, such as a nitric acid solution
Black oxide: This surface treatment can minimize light reflection to your part, which can apply to ferrous materials, zinc, copper and its alloys, silver solder, and powdered metals.
Powder Coating: powder coating can apply to almost every metal material providing a high impact strength. This is a good solution if you can’t anodize your parts.
Bead Blasting: It creates a smooth, uniform surface that may be dull, satin, or rough, as well as cleaning the part’s surface.
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Frequently asked questions
In CNC machining, surface finishing is the last step. Surface finishing improves the appearance of parts by eliminating defects and flaws. As a result, the material will be more resistant to wear and tear, have improved electrical conductivity, and more.
Surface finishes serve the purpose of protecting and/or decorating the underlying surface of a product. Material can either be added to the surface or removed from it, or both can be done at once. It is possible to salvage or repair parts using some of these techniques in limited circumstances
You can indicate surface finishes in your drawings or inform your project manager during communication if your parts require surface finishes.
You can combine different types of surfaces to benefit from varying properties. Anodizing a part can be improved by bead blasting it first.
