Principles of CNC Machining Technology

The principles of CNC machining technology emphasize the overall process rather than the nature of a single operation or the processing of a particular surface. For example, some positioning datum surfaces need to be machined accurately during the semi-finishing or even roughing stages. Sometimes, to avoid dimensional chain conversions, certain secondary surfaces may undergo semi-finishing in the finishing stage.

Once the machining methods and stages for part surfaces have been determined, the operations for each surface within the same stage can be combined into several machining steps.

1. Methods for Dividing Machining Operations

For parts machined on CNC machines, operations are generally divided according to the principle of process concentration. The division methods include:

1). Division by Tool Usage: Group processes completed with the same tool as one operation. This method is suitable when the workpiece has many surfaces to be machined and is commonly used in machining centers.

2). Division by Number of Workpiece Setups: Group processes that can be completed in a single setup as one operation. This method is suitable for parts with limited machining content, ensuring all machining is completed in one setup while maintaining part quality.

3). Division by Rough and Finish Machining: Separate rough machining processes from finish machining processes. This method is applicable when parts require heat treatment due to strength and hardness requirements or need to effectively relieve internal stresses and minimize deformation after machining.

4). Division by Machining Area: Group processes that complete identical features as one operation. For parts with multiple and complex surfaces, the sequence of CNC machining, heat treatment, and auxiliary operations should be reasonably arranged, ensuring proper coordination between operations.

Principles for Dividing Machining Operations

Parts consist of multiple surfaces, each with its own precision requirements and inter-surface accuracy requirements. To meet design specifications, the machining sequence should adhere to certain principles:

1). Rough Before Finish Principle: Surfaces should be machined in the order of roughing, semi-finishing, finishing, and honing. The aim is to gradually improve the precision and surface quality of the part. If all surfaces are machined on CNC machines, operations are generally sequenced as roughing, semi-finishing, and finishing. During roughing, most material is quickly removed, followed by precision machining of each surface. This improves efficiency and ensures part accuracy and surface finish. For surfaces requiring high positional accuracy, this sequence may vary; for instance, surfaces with tight dimensional tolerances may be processed separately in roughing, semi-finishing, and finishing stages. Parts should ideally be set aside between rough and finish machining stages to allow internal stresses to dissipate, reducing deformation and enhancing machining accuracy.

2). Benchmarks First Principle: Always start by machining the surfaces used as precision benchmarks. Accurate benchmark surfaces minimize clamping errors. Therefore, any part's machining process begins with roughing and semi-finishing the benchmark surfaces, and if necessary, finishing them. For example, shaft components typically undergo roughing and semi-finishing of the center holes before precise machining of hole systems and other surfaces. If there are multiple precision benchmarks, they should be processed in the order of benchmark conversion and gradually improving machining accuracy.

3). Faces Before Holes Principle: For parts like housings, brackets, and bodies where flat dimensions are large, machining should start with faces before holes. This provides a stable and reliable surface for subsequent operations and makes hole machining easier, enhancing hole accuracy. Typically, simpler geometries are machined first, followed by more complex ones; lower-precision areas are machined before higher-precision ones; and faces are machined before holes.

4). Interior Before Exterior Principle: For precision sleeves, where high concentricity between the outer diameter and inner hole is required, follow the principle of machining the hole first, then the outer diameter. Using the outer diameter as a reference for machining the hole, followed by using the precisely machined hole as a reference for machining the outer diameter, ensures high concentricity and simplifies fixture design.

5). Minimize Tool Changes Principle: In CNC machining, arrange the machining sequence based on the order in which tools enter the machining position to minimize tool changes.