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CNC Machining Glossary
Pocket and Cavity in CNC Machining
A pocket is a recessed area created by removing material from a part. It normally has a floor and surrounding walls, although it can open onto an outside edge. Pockets provide space for components, reduce weight, or create locating surfaces.
A cavity is a broader term for a hollow or recessed space, often used for mold interiors or more complex recesses.
The terms overlap: depth alone does not distinguish a cavity from a pocket.
What Matters in Design and Machining
Consider depth, width, and internal corner radius together. A deep pocket may be straightforward to machine when it is wide and accessible. Adding tiny internal corner radii can force a smaller cutter to reach the same depth, increasing tool flex and vibration.
Conventional end milling leaves rounded internal vertical corners. Allow practical corner radii, avoid unnecessary depth, and leave enough space for the cutting tool to reach the surfaces and remove chips.
Details Designers Can Overlook
Fitting inside the outline does not guarantee proper seating. A rectangular component may fit the pocket’s length and width but interfere with rounded corners. Any fillets between the walls and floor must also clear the mating component.
Check the material remaining around and beneath the recess, including pockets machined from the opposite side. Thin walls and floors can move during cutting or distort after unclamping.
Finally, identify which surfaces actually control function. A locating wall may need a tight positional tolerance, while a clearance wall does not. Specify pocket depth and floor flatness according to assembly needs rather than applying the same precision everywhere.