How to Ensure the Surface Finish of CNC Parts?

Part rejection due to poor surface finish is an outcome no manufacturer desires. Even if parts meet all dimensional specifications and function as intended, customers can reject them if the surface finish is imperfect. This is especially critical for parts used in visible end products or as mold components. Due to the nature of Cnc Machining, visible microscopic tool marks will always be present on machined parts. However, factors like excessive vibration between the tool and workpiece, and incorrect feed rates, can also lead to poor surface finish. This article discusses techniques, tools, and processes applicable both during and after machining to limit the visibility of tool marks and ensure excellent machined surface quality every time.

Why is Poor Surface Finish Unacceptable?

Beyond being unsightly, poor surface finish adversely affects corrosion resistance and makes certain manufacturing processes, such as painting, more difficult and time-consuming. With a poor surface finish, it is harder to adequately prepare parts for post-machining finishing processes like anodizing or Powder Coating. Coatings may struggle to adhere – leading to pitting in uncoated areas and potential corrosion spread across exposed metal sections. Consequently, the part becomes more susceptible to corrosion. Poor surface finish also increases friction effects between the part and other objects, and can reduce strength and wear resistance.

What is an "As-Machined" Surface?

When a finished part is removed from the CNC machine, its surface (before any post-process surface finishing) will appear dull and exhibit visible tool marks. This is called the as-machined surface. The quality of the finished surface is quantified by measuring the part's Average Surface Roughness (Ra). A standard commercial machine surface roughness for parts is 3.2 μm Ra. This is often the cheapest surface finish option as it requires no additional processing. While some customers may accept an as-machined surface, others may demand a lower surface roughness (Ra).
This article focuses solely on tips related to improving the as-machined surface finish during CNC machining operations. However, several other surface finishing processes can be applied to parts after machining to achieve the desired surface roughness.

Four Tips for Improving Machined Surface Quality

Achieving a better machined surface quality requires revisiting machining fundamentals. By modifying tool setups and optimizing toolpaths and cutting parameters, manufacturers can ensure their parts have excellent surface quality. Here are 4 tips on how to avoid poor surface finish on machined parts:

1.Increase Speed, Reduce Feed: To achieve the desired machined surface, the CNC machine must operate within the specified feed and speed ranges for the material being cut. Increasing the tool's spindle speed reduces contact time between the tool and workpiece. This reduces Built-Up Edge (BUE) and extends tool life. Rapid material removal is the goal for roughing paths, while slow removal for precise dimensions and superior surface quality is the goal for finishing paths. By reducing the feed rate, lighter, shallower cuts can be taken, resulting in a more uniform surface finish. By combining increased spindle speed with reduced feed rate, BUE on the part is minimized, thereby improving surface finish.

2.Minimize Tool Chatter and Deflection: Chatter refers to vibrations occurring on the tool during CNC operations due to forces. Deflection, on the other hand, is the distance the tool tip travels away from the tool holder axis. Both are significant causes of poor surface quality. While not entirely avoidable, here are two tips to minimize them:
* Ensure the correct tool and tool holder, suited to the job and material, are used to guarantee rigidity. Ensure cutting parameters and feed rates are within recommended ranges.

*Use the correct tool holder to minimize tool overhang, thereby minimizing deflection. Ensure the tool is securely clamped in the collet and the workpiece is firmly fixtured. Both minimize the chance of the tool or workpiece moving or vibrating during the machining operation.
Applying these two tips will significantly reduce tool chatter and deflection, resulting in high-quality machined surfaces.

3.Control Chip Evacuation: Incorrect chip breaking and removal can adversely affect surface finish, as chips can score the workpiece or even damage the tool. To address this, consider using chip breakers. Chip breakers are features built into or clamped onto the tool's cutting face. They promote chip breaking by deliberately bending the chip, making it shorter, less stringy, and easier to break. Reducing chip load lessens the forces acting on the tool. By controlling chip evacuation, manufacturers come closer to achieving high-quality machined surfaces, avoiding BUE, and extending tool life.

4.Increase the Rake Angle: The rake angle is the angle between the tool's rake face (the surface chips flow across after being cut) and a plane perpendicular to the direction of the tool's velocity vector. Increasing the rake angle helps promote chip evacuation and minimize BUE. By positively increasing the rake angle, cutting forces are reduced, and chips form and evacuate continuously, which also helps reduce the occurrence of poor surface finish.

Conclusion

Poor surface finish on machined parts is a significant headache. Despite being machined to precise dimensions, parts with poor finish are rarely accepted and can erode company profits. However, by applying the techniques discussed above, meeting customer surface finish requirements in the as-machined state becomes achievable.