Surface Finish and Roughness in Machining – Ultimate Guide (with Chart)

Surface finish is one of the most important aspects of CNC machining as it improves appearance and influences functionality. There are several types of surface finishes, but the common types include:

1. As Machined

As-machined parts are just off the production line and contain light tool marks, an average amount of surface abrasion, and a particular texture or completed appearance. According to the use case, a component’s average surface roughness (Ra) is the difference between its real and ideal machining surface roughness.

Leaving the surface of a CNC machined part “As machined” is quite beneficial, especially when achieving tight dimensional tolerance and consistency across multiple units.

However, on the downside, parts or products with this CNC machined surface finish often have visible tool marks on their surface. They are often exposed to environmental forces due to their distinct lack of protective coating.

2. Bead Blasting

Bead blasting is a finishing method that uses a pressured air gun to blast components with tiny glass beads. This procedure primarily enhances aesthetics by producing a matte, satin, or light-textured finish. It’s a primary finish type that mechanically modifies the surface to change or erase machining marks by eliminating extra material, leaving a smooth surface in its wake.

The even and textured finish of bead blasting is a benefit. This machined surface finish has little impact on tolerances, but it can affect critical dimensions of the surface, which is why it is best not to cover key surface features.

3. Powder Coating

Powder coating is the method of applying powder by static electricity to the surface of components. The major difference between powder coating and spray painting is using dry powder rather than liquid. The powder used is uniformly adsorbed on the machined components’ surface before being baked into the pieces in the oven.

This produces a robust, wear- and corrosion-resistant coating that is more long-lasting than conventional spray coating techniques. Powder coatings are durable and highly environmentally friendly. It gives the surface a drip-free uniform finish, improving its mechanical strength, corrosion, and aging resistance.

4. Anodizing Surface

Anodization is a metal surface finish that involves thickening the natural oxide layer of a CNC machined part. Aside from thickening, this process also makes the material more durable, denser, and electrically non-conductive.

The first step in the process involves submerging the component which acts as an anode into an acid electrolyte bath. The next step is placing a cathode into the tank that contains the electrolyte bath and introducing electricity. With the introduction of electrolytes, the combination of the atoms from the alloy and oxygen ions from the electrolyte covers the surface of the parts. This improves the wear resistance of the parts.

5. Polishing Surface

Three main types of polishing are used on machined parts surface finish; conventional polishing, fire polishing, and vapor polishing.

Conventional polishing involves smoothing a surface by using an abrasive. It greatly improves the durability of parts, although there is a risk of chipping or cracking when using this surface finish.

Fire polishing involves using an open flame at a specific temperature and angle to melt a part’s surface layer. Fire polishing facilitates the reduction of ridges and bumps formed during machining.

Vapor polishing is an ideal machined plastic surface finish, as it helps clarify opaque and dull plastics. The final result after vapor polishing is often a shiny and smooth surface. It is best to sand machined parts with extreme imperfections before applying the vapor polish surface finish.

It is important to note that polishing generally requires skill not to damage the product, so it is best to leave it to skilled professionals.

Surface finishing is usually not straightforward as there are factors that affect the outcome of the surface finished part. These factors include;

Machined Material

The possible surface finish for a material depends chiefly on the machined material type. Hard materials, like metals, are smooth; in contrast, the surface finish of softer materials, like plastics, is rougher.

Feed Rate And Cutting Tool

While the CNC fixture holds the workpiece in place, the feed rate impacts the CNC machining surface finish since it increases the number of passes the tool makes over the material. In other words, when the feed rate is high, it often results in a rougher surface finish, while a lower feed rate results in a smoother finish.

In addition, CNC machining parts with blunt CNC cutting tools often produce a poor surface finish compared to cutting with a sharper tool.

Cutting Depth

The cut depth impacts the workpiece’s surface finish. A shallow depth of cut results in an even or smoother surface finish, whereas a large cut could result in a more irregular finish.