Surface quality or surface roughness of cnc machined components is of prime importance to manufacturers. The goal is to produce parts that ideally shouldn’t require a major finishing operation to cater to surface deformations.
Realistically that ideal surface finish may not be possible but tool choices can worsen the problem. The tool’s material, size, geometry, wear resistance, and cutting parameters – all influence the surface quality of a CNC machined part. So, the choice of tools must be made after weighing in all those factors.
This article explores all aspects of a cutting tool that affect the surface quality in CNC Machining and the reasons behind that.
Tool Material Choices Surface Finish
The first impacting factor is the tool material. To make things simple, we will consider three types of CNC machine tools: carbide, HSS, and ceramics.
Carbide tools can maintain a sharp edge at high speeds, which helps achieve tighter tolerances on harder materials. Moreover, their high heat resistance allows for faster machining without compromising surface quality.
Ceramic tools offer the highest hardness among all cutting tools and even maintain it over high temperatures. They can achieve fine finishes on hard-to-machine materials like hardened steels and high-strength alloys. However, they are brittle and can break under impact or slow speeds.
HSS cutting tools are suitable for general machining tasks due to their affordability and ease of re-sharpening. However, HSS tools have a lower heat resistance, which limits their performance on hard metals. They are prone to dulling faster under high heat, which can increase surface roughness.
The Effect of Tool Geometry on Surface Finish
Tool positioning and size matter too. A larger diameter increases the tool’s rigidity. But that diameter requires more machine power; it can thermally expand and causes a rougher finish if not properly managed. In contrast, thinner diameters are flexible but prone to deflection.
The rake angle is another influencing factor. A positive rake angle reduces cutting forces, minimizing tool wear and surface smoothness. However, too much positive rake can weaken the cutting edge and chip it off.
The relief angle provides clearance between the tool and the workpiece to prevent unnecessary rubbing. When the relief angle is large, the clearance increases. And so, does the friction and tool wear. However, if it’s too steep, it can weaken the tool edge and chip it off.
A sharper tool edge also results in minimum deformation and a cleaner cut. However, very sharp edges are more prone to wear, especially in harder materials. For such cases, slightly honed or rounded edges provide a balanced approach.
Why Tool Coatings Matter for Surface Quality
Tool coatings improve the tool’s durability, reduce friction, and manage heat—all of which contribute to a smoother finish on the machined surface.
Titanium Nitride (TiN) Coating, gold-yellow in color, is a popular option for cutting tools. Its layer/coating reduces the adhesion between the tool and workpiece, which ultimately minimizes the built-up edge (BUE) – a factor that can degrade surface quality. Moreover, being thermally stable, it allows tools to work at higher speeds.
DLC coatings have exceptional hardness and extremely low coefficient of friction, which makes them ideal for high-precision tasks. The lubricious DLC surface reduces friction, responsible for tool wear. DLC coated cutting tools have shown promising results when machining aluminum.
Tool Wear’s Impact on Surface Roughness
Flank wear and vibration are two primary wear mechanisms that can cause increased roughness.
Flank wear occurs as the tool’s flank face experiences friction against the machined surface, gradually eroding the cutting edge. With time, the tool’s cutting ability declines and you witness rough finishes to inconsistent contact. It also creates micro-grooves and scratches on the surface.
The vibration in CNC machining also stems from tool wear because when the cutting edge deteriorates, it loses its stability. The relation between flank and vibration is linear; the higher the flank wear, the higher the vibration. In such conditions, the chatter marks are quite noticeable on the workpiece.
Cutting Parameters and Their Influence on Surface Quality
Cutting parameters, particularly spindle speed and feed rate, have a role in determine surface quality in CNC machining. They control cutting forces, tool temperature, and chip formation – the factors that govern surface roughness.
Spindle speed is the rate at which the tool rotates. At high speeds, the cutting edge passes over the material more frequently, resulting in smoother cuts and lower surface roughness. Excessively high speeds are disastrous for metals due to heat generation. So, a balance has to be maintained.
The trend is entirely the opposite in the case of feed rate. A lower feed rate often results in a finer surface finish, as the cutting edge engages less material per pass, which creates fewer tool marks. Studies on CFRP composites show that lower feed rates help prevent defects such as delamination and burrs.
Toolpath Strategies and Surface Roughness
The toolpath can influence surface quality by controlling contact time, direction, and cutting load distribution. The example of climb milling can help us here.
In climb milling, the tool cuts in the same direction as the feed, producing less friction and a smoother finish. Whereas, in conventional milling the tool cuts against the feed. This results in more friction, which can cause a rougher surface as the material deforms at the entry point.
Finishing Options Offered by RJCMold
The right tool selection can control the outcome of the surface finish. However, in most cases, achieving a fine, smooth surface is not possible – CNC machined parts have to undergo secondary finishing processes.
At RJCMold, we offer finishing options to enhance the surface quality of CNC machined parts. You can choose between an as-machined (standard) finish, anodizing, powder coating, silver coating, zinc plating, gold plating, and wet painting. In addition, we have multiple finishing options are available for injection molding, 3D printing, and urethane casting.
Contact RJC Mold today to discuss the best finish for your project!