What surface finishes are available for CNC machining? You can apply finishing and surface treatments to improve the surface roughness, cosmetic properties and wear resistance of metal parts. Learn about the most common ways to finish CNC machined parts and how to select the best methods for your applications.
CNC machining is a subtractive manufacturing process that can produce parts with tight tolerances (up to ± 0.025mm) and fine detail from a vast range of metals and plastics. However, due to the subtractive nature of CNC – unlike 3D printing and additive manufacturing – parts often come off milling and turning machines with visible tool marks.
This is where surface finishing comes in. The application of post-treatment and the right surface treatments can improve the surface roughness, cosmetic and visual properties and wear resistance of CNC machined parts. When applied correctly, surface finishing improves functionality and aesthetics, and in many cases both.
In this article, we discuss the most common surface treatments for metal CNC machined parts to help you choose the right one for your application.
All CNC machined parts have marks that follow the path of the cutting tool used during machining. You measure the quality of the surface by the average surface roughness (Ra).Ra is a measure of the average deviation of the machined profile from the ideal surface.
The standard as-machined surface roughness is 3.2 μm (125 μin). You can use a finishing pass to reduce surface roughness to 1.6, 0.8, or 0.4 μm (63, 32, or 16 μin). This increases the cost of producing a part by requiring additional machining steps and tighter quality control.
Once you machine the parts, you can smooth or polish them to improve surface quality and appearance by reducing surface roughness. Smoothing and polishing will remove some material which will affect the dimensional tolerances of the part.
Bead blasting adds a uniform matte or satin finish to a machined part, removing tool marks. It works by bombarding your part with small glass beads using a compressed air gun. This removes excess material and smoothes the surface of the part. Mask critical surfaces or features (such as holes) to prevent dimensional changes.
In general, people use bead blasting for part aesthetics rather than functionality. It is a manual process so the result will depend to some extent on the skill of the operator. The amount of air pressure and the size of the glass beads are the main process parameters. Glass beads come in different sizes (from coarse to very fine), just as sandpaper comes in different sizes and grades. Protolabs Network typically bead blasts parts with a #120 grit.
Anodising adds a thin ceramic layer to the surface of metal parts. This layer protects against corrosion and wear. The anodic layer does not conduct electricity, provides high hardness (Type III), and you can color it. Anodising is only compatible with aluminium and titanium.
Type II & Type III anodising involves immersing the component in a dilute sulphuric acid solution and applying an electrical voltage between the component and the cathode. An electrochemical reaction consumes the material on the exposed surface of the part, converting it to hard aluminium or titanium oxide. You can apply a mask to surfaces with critical dimensions (such as threaded holes) or surfaces that must remain electrically conductive to prevent them from being anodised. Color anodised parts before sealing them, with options such as red, blue, or black. or gold.
You can produce coatings of different thicknesses and densities by varying the electrical current, anodising time, and the consistency and temperature of the solution.
Type II anodising, also known as “standard” or “decorative” anodising, can produce layers up to 25 μm thick. The typical layer thickness depends on the colour and can vary between 8-12 μm for black coloured parts and 4-8 μm for clear (uncoloured) parts.
Type II anodising mainly produces parts with a smoother surface, providing good corrosion resistance and limited wear resistance.
“Hardcoat” anodising, also known as Type III anodising, can produce layers up to 125 μm thick. Unless otherwise specified, the typical Type III anodised coating is 50 μm thick
Type III anodising produces thick, high-density ceramic coatings that provide excellent corrosion and wear resistance suitable for functional applications. Note that it requires tighter process control than Type II anodising (higher current density and constant solution temperature near 0 degrees Celsius), so the cost is higher.
Powder coating adds a thin layer of protective polymer to the surface of the part. The powder coating provides a strong, wear-resistant surface treatments compatible with all metal materials. It can be combined with bead blasting to produce parts with smooth, uniform surfaces and excellent corrosion resistance.
The powder coating process is similar to spray painting, but the “paint” is a dry powder rather than a liquid. Parts are first primed with an optional phosphate or chromate coating to increase corrosion resistance, then coated with dry powder using an electrostatic “spray” gun and cured at high temperature (usually in a 200 degree oven).
You can apply multiple layers to create a thicker coating, with typical thicknesses varying from approximately 18 μm to 72 μm. A wide range of colours are available.
Are you looking for a reputable partner for your parts fabrication and machining projects? Look no further. At PROTO MFG, we specialize in CNC machining and related technologies, including sheet metal fabrication, rapid prototyping, etc. Whether it is a project with a simple design or parts with complex geometries, do not hesitate to contact us today!
