Factors Affecting Surface Quality in Milling

Practical experience has shown that several common surface quality issues in CNC milling services, mainly caused by chatter, can be analyzed and addressed with specific preventative measures. By applying appropriate technical means and process improvements, surface quality issues can be prevented, effectively avoiding rework, ensuring product quality, maintaining production schedules, and enhancing a company’s competitiveness.

Chatter and Over-Cutting in Fillet Milling

The root fillet, where the sidewall and the bottom surface of the part intersect, can present different challenges depending on the geometry:

1. Sidewall is vertical, and bottom surface is horizontal.
2. Sidewall is vertical, and bottom surface is a freeform surface.
3. Sidewall is a freeform surface, and bottom surface is horizontal.
4. Both sidewall and bottom surface are freeform surfaces.

Surface quality issues with root fillets are most common when the sidewall is vertical, and the bottom surface is a freeform surface.

In practical production, some parts exhibit non-smooth root fillets, with wavy, sawtooth-like marks and occasional pitting. Additionally, the sidewalls may show noticeable chatter marks.

Analysis of Causes:

1. Tool Path Issues: The tool path often approaches and then moves away from the sidewall rather than following the root fillet continuously. Observing the tool marks on the bottom surface of the part, it is evident that the tool path trajectory includes several instances of leaving and returning to the sidewall, particularly when the bottom surface is a freeform surface. During contour milling, the tool path cannot follow the root fillet precisely if the bottom is a freeform surface. These irregularities correspond to the small pits observed on the part, indicating that the pits are reflected in the tool path. When the tool leaves the sidewall, the cutting force suddenly decreases, and when it returns to the sidewall, the cutting force abruptly increases. This sudden change in cutting force can induce tool vibration.
2. Excessive Stock in the Root Fillet: When there is a significant amount of excess material in the root fillet, the cutting force increases due to the larger cutting volume. This exacerbates the sudden changes in cutting force, intensifying tool vibration.
3. Matching Radii of Tool and Root Fillet: When the radius of the tool’s fillet matches the radius of the part’s root fillet, the entire fillet of the tool is engaged with the root fillet of the part. This increased contact area can cause a sudden increase in cutting force, leading to tool vibration and resulting in over-cutting and small pits on the part.

Solutions:

1. Optimize Tool Path Programming: Avoid tool paths that repeatedly cut into and out of the root fillet. Instead, adopt a strategy where the tool path follows the fillet, preventing abrupt changes in cutting force that cause tool vibration. When the tool path follows the fillet, it yields excellent results with no pits in the root fillet. However, when the bottom surface is a freeform surface, a three-step process should be employed:

Step 1: During the finish milling of the sidewall, leave a 0.2mm allowance on the bottom surface and do not mill down to the bottom or the fillet.

Step 2: During the finish milling of the bottom surface, leave an allowance on the sidewall and do not mill to the sidewall or the fillet.

Step 3: Perform finish milling of the fillet following its direction (commonly known as fillet cleaning). This approach prevents tool vibration caused by sudden changes in cutting force.

2. Reduce Excess Stock in the Root Fillet: Implement a rough fillet cleaning step first, leaving a 0.05mm allowance, followed by a finish fillet cleaning step. This prevents excessive stock from causing tool vibration.
3. Use Tools with Smaller Fillet Radius: Utilize tools with a fillet radius smaller than that of the part’s root fillet. This reduces the contact area between the tool and the part, minimizing the risk of tool vibration.