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Views: 50 Author: Site Editor Publish Time: 2024-04-17 Origin: Site
This article aims to offer a comprehensive overview of surface finish choices tailored to CNC machined metal parts. It seeks to assist you in navigating through the available options and selecting the suitable finishes for your specific applications.
Freshly CNC machined metal parts often exhibit noticeable tool marks, sharp edges, and burrs. These imperfections not only diminish the visual appeal but may also result in inaccuracies in dimensions and tolerances. To ensure the delivery of finely finished parts, surface finish treatments are highly advisable following CNC machining.
These post-processing procedures serve to enhance not just the material's strength but also confer anti-corrosive attributes while refining surface roughness.
In the CNC machining manufacturing process, heat treatments (such as annealing and case hardening) and surface treatments (like bead blasting and powder coating) stand out as the two prevalent post-processing operations. By selecting appropriate surface finishes, surface roughness can be enhanced alongside enhancing both the visual appeal and functional attributes of CNC machined parts.
The process of heat treatment enhances the physical and mechanical characteristics of metal through a controlled heating and cooling process. This procedure can modify various properties such as strength, hardness, ductility, malleability, formability, and elasticity. Operations associated with heat treatments include:
The annealing process is characterized by heating metal to the point where recrystallization starts, without inducing stress changes. Following heating, the metal is either cooled gradually in an oven or placed in sand. This slow cooling process enhances metal elasticity by reducing its hardness, consequently enhancing its ability to withstand cold working.
Primarily employed for steel alloys, annealing can also be applied to copper, aluminum, and brass.
Tempering, a heat treatment technique, primarily aims at reducing metal hardness. The process entails heating the metal to a temperature below its critical point. The specific temperature adjustment depends on the desired level of hardness reduction, varying according to the metal type.
Typically, lower temperatures diminish brittleness while preserving much of the metal's hardness. Conversely, higher temperatures reduce hardness while enhancing elasticity and plasticity, albeit at the expense of some yield and tensile strength.
To prevent steel from cracking, it's advisable to heat gradually. Subsequently, the metal is maintained at this temperature for a specified duration. Steel alloys are frequently subjected to tempering to optimize their properties.
Case-hardening, also known as carburizing, is a heat treatment technique that involves hardening the surface of metal while maintaining the underlying metal's softness. During carburizing, carbon or nitrogen is introduced into low-carbon alloys at high temperatures to enhance hardenability.
This surface treatment can be implemented on the part either before or after the CNC machining process.
Through hardening, in contrast to case hardening, involves uniformly increasing the hardenability of the entire alloy rather than just the surface. This is achieved by introducing carbon into the alloy matrix and subjecting it to repeated quenching in brine, water, or sometimes oil.
Bead blasting is a process that involves directing a pressurized stream of tiny beads of media, typically plastic or glass, from a nozzle onto the surface of the part. This effectively removes burrs and imperfections, resulting in a smooth finish. The end result often resembles a uniform satin finish.
Typically conducted within a closed chamber, bead blasting with glass beads is a common choice for treating CNC machined parts. The matte surface achieved through bead blasting is particularly suitable as a preliminary treatment for painting or dying. Additionally, bead blasting prior to anodizing serves as an excellent surface preparation method, creating a non-reflective surface.
For optimal results, it is recommended that parts fall within the size range of 6-600 mm in both height and width. Smaller parts may present challenges in terms of handling and achieving an even surface finish.
Electroless nickel or nickel phosphorus plating is a process where a uniform layer of nickel-phosphorus alloy is deposited onto the surface of a solid substrate, such as aluminum or steel. This is achieved by immersing the substrate into a water solution containing nickel salts and phosphorus-reducing agents.
In contrast to electroplating, electroless nickel plating ensures a uniform deposition across the substrate. Electroplating often encounters issues like uneven current density and substrate resistance, which can result in non-uniform plating.
The powder coating process entails applying a free-flowing dry powder onto a substrate. Unlike traditional liquid paint, which relies on evaporating solvent, powder coating involves electrostatic application followed by curing under heat or UV light. The powder used can consist of either thermoplastic or thermoset polymer materials.
Compared to conventional coatings, powder coating typically results in a tougher and more uniform finish. It is particularly effective for coating metals like aluminum and steel with polymer powders.
Electropolishing stands in contrast to electroplating, where the substrate serves as the cathode, and ions from the sacrificial anode deposit onto it. However, in electropolishing, this process is reversed: the substrate functions as the anode, and its ions migrate towards the cathode.
This method effectively smoothens micro peaks and valleys on the surface, making it ideal for deburring and reducing overall surface roughness.
Passivation primarily targets stainless steel, composed mainly of iron, chromium, and nickel. Chromium in the alloy is pivotal for its anti-corrosion properties, as it naturally forms a chromium oxide layer on the surface. However, free iron can accelerate corrosion.
During passivation, stainless steel is immersed in an acid bath, typically nitric acid, to dissolve free iron and enhance the durability of the chromium oxide layer, thus mitigating corrosion.
Anodizing, akin to passivation, enhances the thickness of the oxide layer to fortify the metal surface against corrosion, a prevalent practice for aluminum in CNC machining. It diverges from passivation by employing an electrolytic process, hence referred to as electrolytic passivation.
During anodizing, an aluminum alloy is immersed in a sulfuric acid electrolyte and serves as the anode. When electricity is applied with a cathode (typically made of inert materials like stainless steel, nickel, or carbon), oxygen migrates to the anode and reacts, forming an anodic oxide layer. This layer primarily consists of aluminum oxide, which acts as a corrosion barrier.
Common electrolytes used include:
A 10–15% sulfuric acid solution at 25°C, yielding an oxide layer thickness of approximately 25µm/hr.
A blend of oxalic and sulfuric acids at 30°C, producing about 30µm/hr.
A 10% chromic acid solution operating at 38–42°C, resulting in a thickness of about 15µm/hr.
Moreover, anodizing induces a coarse, grainy surface texture on the metal, enabling dye retention within the oxide layer, thereby imparting a nearly permanent coloration.
Zinc coating, commonly linked with steel, particularly in a process called hot-dip galvanizing, involves submerging steel in a bath of molten zinc, effectively enveloping the steel with multiple layers of zinc-iron alloy and zinc metal.
This method relies on a chemical reaction between the zinc and the ferrous elements within the steel, prompting the diffusion of the coating perpendicular to the surface, resulting in the formation of a uniformly thick layer along the steel surface.
Surface finish plays a crucial role in CNC machining, enhancing the overall quality of manufactured parts. While heat treatment enhances the metal's properties, surface finishes primarily focus on increasing its resistance to corrosion.
At Zonze, we provide CNC machining services, including CNC turning and CNC milling processes. Our skilled team is dedicated to assisting you with your projects. Simply upload your files to Zonze, and you'll receive a quote for your CNC machining projects within hours.
