Surface finish is an essential part of precision machining. A high-quality and complex finish goes beyond aesthetics and physical appearance. It influences functionality, durability, and compliance with industry standards. CNC milling meets most surface finish requirements and produces parts with exceptional accuracy and reduced incidence of flaws. 

 

However, achieving complex finishes using CNC milling requires conscious efforts. It involves optimizing each step in an operation. This article explains factors that affect surface finish, techniques to get high-quality cuts, and ways to reduce post-machining surface roughness.

Factors Influencing Surface Finishes in CNC Milling 

In part manufacturing, some factors affect the quality of the surface finish you would get. Some of these factors are 

  • Material Properties

The intrinsic properties of the workpiece material are a major determining factor. Some properties to look for are machinability, hardness, ductility, and thermal conductivity. Softer metals are easier to machine and produce excellent finishing compared to harder ones. Additionally, materials with high ductility are prone to rougher finishing. Furthermore, metals with high thermal conductivity prevent heat retention, and the cuts are near perfect. 

  • Tool Selection 

The type of tool you use will also determine how smooth the cut will be. You should consider the tool material and choose the most appropriate based on the workpiece in question. For instance, high-speed tools are preferred for softer metals and plastics when aiming to produce ultra-precise finishes. The tool geometry also matters. Cutting edge sharpness, rake angle, and nose radius directly affect surface finish complexity.

  • Cutting Parameters

The cutting parameters used determine how well the tools interact with the workpiece. All milling parameters are important. However, you should look for spindle speed, depth of cut and feed rate. Based on the material, one should optimize the spindle speed. Using the right speed will reduce tool marks. Furthermore, a slower feed rate and lighter depth of cut are recommended for high-quality finishing.

  • Coolant and Lubrication

Coolants and lubricants play significant roles in achieving complex surface finishes. The common types of CNC coolants are water and oil-based. Water-based coolants have excellent heat dissipation capabilities. On the other hand, oil-based coolants provide additional lubrication properties. Employing the right type and quantity of coolant is important for friction reduction.

Strategies for Achieving High-Quality and Complex Surface Finishes

Achieving high quality and complex surface finishes is not a magic as there are some strategies one should employ. Let’s look at important CNC milling strategies every machinist should keep in mind.

  • Toolpath Optimization 

You must carefully design and optimize the movement of the cutting tool relative to the workpiece. This entails ensuring smooth and continuous movements. Sudden stops or changes in direction can lead to tool marks and inconsistent finish. Toolpath optimization also involves minimizing unnecessary movements to reduce the risk of imperfections. Additionally, smaller stepovers and adaptive toolpaths can be used to produce finer surface textures. 

  • Use of High-Speed Machining (HSM)

Consider using high-speed machining for your task to get a complex and top-notch surface finish. You do this by moving the spindle and cutting tool faster to cut clean cuts. High-speed machining helps reduce cutting forces and minimize tool deflection. It also improves tool dissipation and maintains stable cutting conditions. This is especially useful when working on tough materials such as hardened steel, where precision is critical.

  • Tool Wear Monitoring

The condition of the cutting tool directly affects the surface quality you would get. Worn-out tools are dangerous and can lead to everything from roughness to dimensional inaccuracies. You should always watch out for common signs of tool wear including increased cutting forces and formation of built up edges. You can monitor the cutting tools in-process using sensors. Alternatively, inspect and conduct visual checks during a routine inspection.

  • Multi-Axis Machining

Multi-axis machining makes everything better in CNC milling. Using 5-axis CNC machining helps to produce complex-shaped parts with high-quality surface finishes. Multi-axis machining improves accessibility. Hence, the cutting tool can approach from different angles in a single set up. Additionally, this approach enables you to cut with shorter tools, minimizing tool deflection. All these optimize cutting angles and create smooth cuts on contoured surfaces.

  • Specialized Tooling 

Utilizing specialized tooling is another way to get a complex and top-notch finish. For example, consider ball-nose and tapered end mills when cutting detailed areas. Additionally, use tools with embedded abrasives to achieve ultra-smooth finishes. Furthermore, tool coating is important in CNC milling operations. Tools coated with titanium aluminum nitride or diamond-like carbon are generally preferred. They reduce friction and heat generation, leading to better finishing.

  • Vibration Reduction

Reducing vibrations and unnecessary movements makes all the difference in CNC milling. Sudden shifts in the workpiece can cause poor finishes and flawed cuts. Proper use of work holding jigs and fixtures is key. Jigs secure the cutting tools and direct them. Conversely, fixtures hold the workpiece in place and prevent movement. Other ways to minimize vibrations include using vibration dampeners and going for CNC machines with high structural rigidity.

Post Machining Surface Enhancement

There are times when the machined finish is not suitable for aesthetics and performance. In this case, you have to use suitable techniques to enhance the workpiece surface. Below is an extensive discussion of the available post-machining surface enhancement operations.

  • Deburring

This involves removing sharp edges and protrusions from the machined surface. You can deburr manually for small-scale operations or mechanically when bulk processing parts are needed. Cleaning burrs is important for parts that need to be further assembled. Additionally, it ensures safety during handling and usage.

  • Anodizing

The anodizing process is another ideal way of improving surface finish. Anodizing creates a thin protective oxide layer on the workpiece surface. This improves corrosion and wear resistance. It also makes parts more appealing. However, you can only use anodizing for conductive metals. 

  • Painting 

It is possible to paint a machined part for surface enhancement purposes. One can use different types of paints, and they can be classified into liquid and powder paint. They are versatile and come in different colors. When painting, clean the workpiece surface properly and control environmental conditions. 

  • Electroplating 

Another means of improving surface quality is electroplating. This process uses an electrolytic bath to deposit metal ions on the workpiece surface. You can plate using nickel, chrome, gold, or zinc.  They improve the mechanical properties of the part and add aesthetic value.

Conclusion

CNC milling is a powerful technology used in modern manufacturing. It produces complex-shaped parts with exceptional quality. However, some factors, such as material properties, type of cutting tools, and milling parameters, affect the surface finish of the parts. Complex finishes are possible with CNC milling, even when working on hard materials. Consider using high-speed machining and proper workholding fixtures to overcome the challenges of cutting complex surfaces. For further clarifications, reach out to a reliable CNC milling company to get all your answers.