Issues with Over-Cutting on Freeform Surfaces in Milling Parts

Several factors can contribute to surface quality issues in milling services, including part material, structural rigidity of the part, tool material, tool clamping rigidity, part clamping rigidity, machine tool rigidity, cooling system, and machining parameters (which can cause tool wear and chatter). However, a significant cause of these issues is tool chatter.

When milling tools experience chatter, they can vibrate at high frequencies and oscillate, leading to surface damage on the part. Surface quality problems commonly occur in the root fillets, sidewall corners, vertical sidewalls, and freeform surfaces of parts.

Cause Analysis:

Freeform surfaces refer to non-planar, non-standard curved surfaces (also known as B-surfaces). When machining these surfaces, ball-end mills or bull-nose end mills are typically used in a reciprocating cutting process. Quality issues often manifest as surface roughness, burrs, or irregular grooves (also called drag marks).

1. Programming Precision: Insufficient precision in toolpath programming can result in anomalies or slight fluctuations in the Z-axis coordinates by a few micrometers. Experiments conducted by Siemens showed that poor surface quality was associated with uneven toolpath point distribution, while good surface quality had uniformly distributed toolpath points.
2. Machine Axis Backlash: Significant backlash in the machine’s coordinate axes, especially in the Z-axis, can cause the machine to jerk or shift unexpectedly.
3. Improper Machining Parameters: Inappropriate machining parameters in the program can cause the machine to momentarily pause or experience abrupt feed rate changes.
4. Dull Cutting Tools: A dull cutting tool can lead to sporadic material adhesion to the tool.
5. Cooling Issues: Dirty coolant or insufficient cooling can cause chips and debris to be pressed against the part surface.

Solutions:

1. Improve Programming Precision: Set the programming tolerance to ±0.003, which is more appropriate. Use a 45-degree toolpath direction (along the direction with the largest curvature changes) to avoid micrometer-level fluctuations in the toolpath points.
2. Check and Adjust Z-Axis Backlash: Inspect the machine’s Z-axis backlash. If the backlash is small, machine parameters can be compensated. If the backlash is large, the machine needs to be repaired.
3. Use Sharp Cutting Tools: Replace with new, sharp tools to ensure clean cutting edges. This prevents the tool from becoming dull, which can lead to hard pressing against the part surface, causing material adhesion and surface roughness.
4. Maintain Clean and Adequate Coolant: Ensure the coolant is clean and sufficient in flow. This prevents debris from being pressed against the machined surface and avoids inadequate cooling, which can cause material adhesion and surface roughness.