Quick Tips on Machining Quality (4–6)

Due to the complexity of CNC machining—involving diverse machine tools, materials, cutting tools, cutting methods, and parameter settings—it takes a considerable amount of time for personnel (whether in machining or programming) to reach a proficient level. The following is a compilation of practical experience regarding CNC machining processes, operational steps, tool parameter selection, and process monitoring, summarized by engineers over long periods of actual production.

IV. Q: How can a reasonable tool-setting point be determined? What is the relationship between the workpiece coordinate system and the programming coordinate system?

1. The tool-setting point can be located on the workpiece itself; however, it must be a datum point or a surface that has already been precision-machined. Sometimes, the tool-setting point may be destroyed during the first machining operation, making it impossible to locate the reference point for subsequent operations. Therefore, when setting the tool for the first operation, it is advisable to establish a relative tool-setting position at a location with a fixed dimensional relationship to the positioning datum. This allows the original tool-setting point to be recovered based on their relative positions. This relative position is usually set on the machine table or the fixture. The selection principles are as follows:

1) Easy alignment.

2) Convenient programming.

3) Minimal tool-setting error.

4) Convenient inspection during machining.

2. The origin of the workpiece coordinate system is set by the operator; it is determined via tool setting after the workpiece has been clamped. It reflects the positional relationship (distance) between the workpiece and the machine tool’s zero point. Once established, the workpiece coordinate system generally remains unchanged. The workpiece coordinate system and the programming coordinate system must be aligned; that is, they must coincide during machining.

V. Q: How should the tool path be selected?

The tool path refers to the trajectory and direction of the cutting tool relative to the workpiece during CNC machining. Selecting a rational tool path is crucial, as it is closely linked to the part’s machining accuracy and surface quality. The following points should be considered when determining the tool path:

1) Ensuring the part meets machining accuracy requirements.

2) Facilitating numerical calculations and reducing the programming workload. 3) Seek the shortest machining path and minimize air-cutting time to improve machining efficiency.

4) Minimize the number of program blocks as much as possible.

5) Ensure the surface roughness requirements for the workpiece profile are met; the final profile should be machined continuously in the last pass.

6) Carefully consider tool approach and retraction paths to minimize tool dwell on the profile (which can cause elastic deformation due to sudden changes in cutting force and leave tool marks) and to avoid vertical plunging onto the profile surface, which could scratch the workpiece.

VI. Q: How should monitoring and adjustments be handled during machining?

Once workpiece alignment and program debugging are complete, the process enters the automatic machining stage. During automatic machining, the operator must monitor the cutting process to prevent quality issues or other accidents caused by abnormal cutting conditions.

Monitoring the cutting process primarily involves the following aspects:

1. Monitoring the machining process: Rough machining focuses on the rapid removal of excess material from the workpiece surface. During automatic machining, the tool follows a preset path based on defined cutting parameters. The operator should observe changes in cutting load via the load meter and adjust cutting parameters based on the tool’s load status to maximize machine efficiency.

2. Monitoring cutting sound: During automatic cutting, the sound of the tool cutting the workpiece is typically stable, continuous, and crisp, accompanied by smooth machine movement. As machining progresses, instability may arise due to factors such as hard spots in the workpiece, tool wear, or tool slippage. This instability manifests as changes in cutting sound, impact noises between the tool and workpiece, and machine vibration. Cutting parameters and conditions should be adjusted promptly; if adjustments prove ineffective, the machine should be paused to inspect the tool and workpiece.

3. Monitoring the finishing process: Finishing focuses on ensuring dimensional accuracy and surface quality; it typically involves higher cutting speeds and feed rates. Particular attention should be paid to the impact of built-up edges on the machined surface; for cavity machining, one must also guard against overcutting at corners and tool deflection. To address these issues, first, adjust the cutting fluid application to ensure the machined surface remains under optimal cooling conditions at all times. Second, monitor the quality of the machined surface and adjust cutting parameters to minimize quality fluctuations. If adjustments fail to yield significant improvement, halt the operation to verify the validity of the programmed toolpath.

Special care must be taken regarding tool positioning during pauses or shutdowns for inspection. Stopping the machine while the tool is actively cutting—causing a sudden spindle halt—can leave tool marks on the workpiece surface. Ideally, the machine should be stopped only when the tool has disengaged from the cutting zone.

(4) Tool Monitoring: Tool quality largely determines the quality of the machined workpiece. During automated machining, assess the tool’s condition—distinguishing between normal wear and abnormal breakage—using methods such as sound monitoring, cutting time tracking, interim inspections, and surface analysis. Take timely action regarding the tool based on machining requirements to prevent quality issues arising from a failure to address tool-related problems promptly.