Methods for Addressing Milling Part Surface Roughness Issues

Electromechanical products consist of one or more parts, and the surfaces of these parts are often shaped by metal machining processes, which are crucial in determining product quality.

The characteristic surfaces of CNC-milled parts include horizontal planes, vertical sidewalls, inclined planes, grooves, bosses, cavities, root radii, sidewall corners, freeform surfaces, and various holes.

Each type of part feature has its own unique machining process characteristics.

Issues with Chatter and Overcutting in Vertical Sidewall Milling

Vertical sidewalls are surfaces where the part’s sidewall is parallel to the tool’s axis (i.e., perpendicular to the horizontal plane), allowing the entire sidewall to be cut with the side edge of the tool in one pass. There are typically two strategies for machining these sidewalls:

1. Layered Milling as a Freeform Surface:

The vertical sidewall is treated as a freeform surface, milled in layers along the Z-axis, with each layer cutting approximately 0.2mm. This method can produce a rough surface finish. However, if the layers are made finer, around 0.05mm per layer, a much higher surface quality can be achieved, though at the cost of reduced machining efficiency.

• Single-Pass Side Milling:

The side edge of the tool is used to cut the entire sidewall surface in a single pass. This strategy can achieve high surface quality and reasonable machining efficiency. However, if the machining process is not well-controlled, surface quality issues such as machining chatter marks can occur on the vertical sidewall.

Both methods have their pros and cons, and the choice of strategy depends on the specific requirements for surface quality and machining efficiency. Proper selection and control of cutting parameters are crucial to minimize surface roughness and achieve the desired quality.

Cause Analysis:

1. Tool Chatter at the Root Fillet:

When the tool experiences chatter while machining the root fillet, it can damage the sidewall.

• Tool Chatter on the Bottom Surface:

Chatter occurring during the machining of the bottom surface can also lead to sidewall damage.

• High Spindle Speed Settings:

Setting the spindle speed too high can cause the tool to resonate, resulting in sidewall damage.

• Poor Tool System Rigidity:

Inadequate rigidity of the tool system (including issues with tool overhang, tool material, and tool edge design) can cause chatter, leading to sidewall damage.

Solutions:

1. Leave Allowances for Sidewalls and Fillets During Finishing:

When finishing the sidewalls, leave an allowance of around 0.15mm for the bottom surface and fillet. This prevents chatter when machining the sidewalls, which can otherwise damage the sidewalls.

• Leave Allowances for Sidewalls and Fillets When Machining the Bottom Surface:

When machining the bottom surface, leave an allowance of about 0.15mm for the sidewalls and fillets. This avoids tool contact with the sidewalls during the final bottom surface milling. There are three situations where tool contact with the sidewalls can cause damage:

1. Program Accuracy: The tool path theoretically touches the sidewall, but due to program precision issues, geometric errors might occur, causing the tool to either cut into or not reach the sidewall.
2. Machine Positioning Accuracy: If the program is accurate, the tool theoretically touches the sidewall, but machine positioning errors can cause the tool to cut into the sidewall (even a few microns can leave a mark).
3. Tool Chatter at Fillets: When the tool is in contact with the sidewall and the tool’s fillet matches the part’s root fillet, chatter may occur, leading to sidewall damage.
4. Separate Fillet Machining:

After finishing both the sidewalls and bottom surfaces, machine the fillets separately. This ensures the vertical sidewalls are smooth and meet the surface quality requirements.

• Reduce Spindle Speed:

Lower the spindle speed to avoid tool resonance.

• Increase Tool System Rigidity:

Improve the rigidity of the tool system to prevent chatter.

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