Porosity
Porosity is the presence of voids, cavities, or gas pockets in materials, often a casting or weld defect. In CNC machining, it is critical because removing material can expose hidden subsurface voids, creating pits, leak paths, weak spots, or cosmetic flaws that were not visible before machining.
On the CNC shop floor, porosity is a constant concern when machining castings or powder-metal parts. As the cutter removes material, it can break through the as-cast skin and expose hidden voids—gas pockets or shrinkage cavities—that then appear as pits, wormholes, or depressions. In sealing applications, this creates leak paths; in cosmetic parts, it ruins the finish after anodizing or painting. Shops manage this through careful material selection, opting for wrought stock when leak-tightness is critical, and by adjusting machining parameters: using sharp tools, proper feeds and speeds, and adequate coolant to minimize tearing. They also rely on inspection methods like X-ray or CT scanning to detect porosity before machining, and sometimes use vacuum impregnation to seal pores. The key is understanding that porosity is often invisible until the tool reaches it, so extra allowance and screening are essential for high-reliability components.
Is porosity always considered a defect in machining?
No. In powder metallurgy and some castings, some porosity is inherent, but it becomes a defect when it violates strength, sealing, appearance, or dimensional requirements.
Why does porosity often become visible only after machining?
Machining removes the outer skin and can expose blind or enclosed voids that were not visible in the as-cast state.
How does porosity affect a sealing face in a CNC-machined part?
It can create a continuous leak path once a pore connects to the surface, which is why pressure-critical parts often require X-ray inspection, impregnation, or wrought stock instead of untreated die castings.