Surface finish of CNC machined parts and tips for improving it
Date: 2022-09-20
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Surface finish of CNC machined parts and tips for improving it

Due to the decreasing characteristics of CNC, small tool marks will appear on the surface of the workpiece after machining. As a result, many parts require additional machining to obtain the desired surface characteristics or performance for aesthetics, corrosion resistance, wear resistance, or other purposes.

Here, we'll discuss the most common metal CNC finishes to help you choose the best finish for your application.

This is our standard finish. Since the machined parts will have slight visible tool marks, the standard value of the machined surface roughness (Ra) is 3.2μm. Finishing passes can be applied to reduce surface roughness to 0.8, 0.4 μm or less. This will increase the production cost of the part due to additional machining steps and tighter quality control.

Because the processed parts can also be ground or polished to reduce the surface roughness, thereby improving their surface quality and aesthetics. Grinding and polishing removes some material that affects part dimensional tolerances.

Sandblasting will add a uniform matte or satin surface finish to machined parts, eliminating tool marks. In the process of bead blasting, compressed air is used to form a high-speed jet beam, and the material is sprayed to the surface of the workpiece to be processed at a high speed, which can remove part of the material and make the surface smooth. Critical surfaces or features (such as holes) can be covered to avoid dimensional changes, which will incur some additional cost.

It can remove the small burrs on the surface of the workpiece, improve the smoothness of the workpiece, make the workpiece show a uniform metallic color, and make the appearance of the workpiece more beautiful.

Bead blasting is mainly used for visual purposes. This is a manual process, so results are somewhat dependent on the skill of the operator. The size of the air pressure and the size of the glass beads are the main process parameters. Glass beads come in different sizes (coarse to very fine), just like sandpaper comes in different sizes and grades.

MT-11010 Light Bead Sandblasting - removes light knife marks and light sandpaper scratches, leaving a smooth and even sheen.

MT-11020 Heavy-Duty Sandblasting - Removes heavy-duty knife marks and scratches left by sandpaper with a light-colored appearance.

Anodizing is an electrolytic oxidation process. During electrolysis, the part is immersed in a dilute sulfuric acid solution and a voltage is applied between the part and the cathode.

An electrochemical reaction consumes material on all exposed surfaces of the part and converts it into duralumin or titanium oxide. Masks can be used on surfaces with critical dimensions (such as threaded holes) or surfaces that must remain conductive to prevent anodizing, and can improve the surface hardness and wear resistance of aluminum parts for longer service life.

Anodizing is the most widely used surface treatment for aluminum and titanium machined parts. Anodized parts can be dyed in different colors - black, red or gold are the most common

Coatings of different thicknesses and densities can be formed by changing the current of the solution, anodizing time, concentration and temperature.

Type II Anodizing

Type II anodizing, also known as "standard" or "decorative" anodizing, produces coatings as thick as 25 microns. Typical coating thickness depends on color. Thickness can be between 8-12 microns for black-dyed parts and 4-8 microns for clear (undyed) parts.

Type II anodizing is primarily used to produce smooth and aesthetically pleasing parts and provides good corrosion resistance with limited wear resistance.

Anodized Type III (Hard Coated)

Type III anodizing, also known as "hard coat" anodizing, produces coatings up to 125 μm thick. Typical Type III anodic coatings are 50 μm thick unless otherwise specified.

Type III anodizing produces a high density thick ceramic coating with excellent corrosion and wear resistance for functional applications. Compared to Type II anodizing, it requires tighter process controls (higher current density and constant solution temperature close to 0oC) and is therefore more expensive.

Powder coating is the process of spraying powder onto the surface of a part under the action of static electricity. The process of powder coating is similar to spraying, but the "paint" is a dry powder rather than a liquid. The powder is evenly adsorbed on the surface of the aluminum workpiece, and then baked into the part through the oven. This produces a strong, abrasion and corrosion resistant coating that is more durable than standard spray methods. Various colors can be used to achieve the desired aesthetic of the part. Powder coatings are superior to wet process coatings in terms of mechanical strength, adhesion, corrosion resistance and aging resistance. It can reach 100% high utilization rate and is very environmentally friendly.

Powder coating is a strong, wear-resistant coating that is compatible with all metallic materials and can be used in combination with sandblasting to produce parts with smooth, uniform surfaces and excellent corrosion resistance.

Multiple layers can be applied to produce thicker coatings, with typical thicknesses ranging from about 18 μm to a maximum of 72 μm. There are many colors to choose from.

SANS offers a variety of surface treatments to ensure your machined part meets all of its surface properties. The most common surface treatments include sandblasting, anodizing and powder coating. We also offer chemical films (conversion coatings), electroless nickel, gold or silver, and custom finishes.

We cannot ignore the importance of impeccable surface finish, so how to improve the finish of machined parts?

The correct way to handle finishing is to increase surface feet per minute (SFM) and decrease inches per revolution (IPR). The former helps reduce built-up edge (BUE), helping to extend tool life, while the latter extends insert life.

When machining parts with a milling machine, there are two methods: clockwise milling and counterclockwise milling. Clockwise milling has a better surface finish than counterclockwise milling.

The efficiency and performance of CNC machine tools are inseparable from tools. For the machining of high-precision parts, it is not only necessary to focus on perfect craftsmanship, but also to select and use new and efficient tools.

Optimize CNC machining programs, provide the best CNC data for machining, and improve quality. Unnecessary pauses and pauses can also get in the way of proper tidying up.

For good results, you need to make sure your tool doesn't skew or wobble.

Learn to reserve roughing tools for roughing and finishing tools for finishing. We may want to cut costs and use them in different processes, but this may not provide the highest quality finishing.


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