- Machining Allowance
This is one of the most fundamental and critical considerations. Refers to the extra material left on the die-casting blank for CNC cutting removal.
Key Considerations:
Insufficient Allowance: May fail to fully remove draft angles, surface flow marks, or oxide layers. It could also result in the tool being unable to cut effectively due to dimensional variations in the casting, or even direct tool impact against the hard surface skin, damaging the tool.
Excessive Allowance: Increases machining time, tool wear, and costs, and may expose deeper internal porosity in the casting.
Best Practice: Minimize the machining allowance as much as possible while ensuring the removal of all surface defects and achieving the final dimensions. This requires high stability and consistency in the die-casting process itself.
- Material Characteristics and Internal Defects
The internal condition of die-cast parts directly affects the feasibility and quality of CNC machining.
Hard Spots and Internal Porosity:
Hard Spots: Rapid cooling during die-casting forms a hard “skin” on the surface, which may be harder than the internal material, posing a challenge during initial tool engagement.
Internal Porosity: Shrinkage or gas pores may exist inside the die-cast part. This is one of the most critical risks. When the tool cuts into a porous area, sudden changes in cutting force can lead to:
Tool Chatter or Chipping: Loss of support causes uneven force on the tool.
Poor Surface Quality: Leaves holes on the machined surface, resulting in part rejection.
Considerations: During programming, tool paths should尽量避免 areas with expected high porosity (e.g., the thickest sections that solidify last). For high-demand parts, X-ray inspection can be used to predict internal defects.
- Clamping and Positioning
Stable and precise fixation of the die-cast part is a prerequisite for successful machining.
Challenges: Die-cast parts often have complex shapes, may lack rigidity, and irregular raw surfaces make them difficult to clamp directly.
Considerations:
Design Dedicated Fixtures: For mass production, custom fixtures are typically designed using principles like the “3-2-1 locating principle,” positioning based on a reference surface and non-machined features of the die-cast part to ensure datum consistency and minimize error accumulation.
Avoid Deformation: Clamping force must be sufficient and evenly distributed to prevent thin-walled sections from deforming during machining. Excessive clamping force can also cause deformation.
Deburring: Before clamping, preliminary cleaning of the main positioning points on the die-cast blank is necessary to remove large flashes and burrs, ensuring accurate positioning.
- Tool Selection and Cutting Parameters
Optimize machining strategies based on the characteristics of die-cast alloys (most commonly aluminum, zinc, and magnesium alloys).
Tool Selection:
Material: Carbide tools are widely used for aluminum die-cast parts.
Geometry: Sharp cutting edges and large chip flutes are adopted to ensure smooth chip breaking and evacuation, preventing built-up edge when machining adhesive materials.
Cutting Parameters:
High-Speed Light Cutting: For materials like aluminum alloys, a strategy of high rotational speed, high feed rate, and appropriate depth of cut is typically employed. This improves efficiency and achieves better surface finish.
Cooling and Lubrication: Ample cutting fluid (usually emulsion) is essential for cooling and lubrication, aiding chip evacuation and preventing heat buildup that could cause workpiece deformation or reduced tool life.
- Datum and Coordinate System Unification
The core of ensuring CNC machining accuracy meets drawing requirements.
Concept: The entire manufacturing process, from product design and die-cast mold design to CNC machining, should follow a unified datum system.
Considerations:
Design Datum: The dimensional referenced on the product drawing.
Process Datum: The reference features formed on the die-cast blank (such as certain unmachined surfaces or pre-cast locating holes) used for subsequent CNC machining.
Inspection Datum: The datum used during final quality inspection.
Best Practice: It is essential to ensure these three are unified. During CNC machining, the process datum of the die-cast blank should be used to establish the machining coordinate system. This minimizes cumulative errors and ensures absolute accuracy in the positional relationship between machined features and the product design datum.