A machined pocket may look simple in CAD, but its depth, corner radius, floor shape, and opening determine which cutter can reach it and how rigid that cutter will be. Before releasing a part for CNC milling, examine cavities as a tool-access problem. CSMolding’s milling guidance specifically warns against very deep pockets and perfectly sharp internal corners, recommending practical proportions and generous radii so tools can cut with less deflection and chatter.
Why corner radius matters
Rotating end mills naturally create an internal radius. A designer who specifies a zero-radius internal corner is effectively asking for an additional process, a special tool, or a design compromise. Larger radii allow larger-diameter cutters, which are usually more rigid and able to remove material efficiently.
The corner does not need to become visually obvious. Often a modest increase beyond the minimum functional requirement is enough to open better tooling choices. CSMolding notes that internal fillets should be as large as possible and gives a rule of thumb relating radius to cavity depth. Treat that as DFM guidance rather than a substitute for project-specific review.
Depth changes everything
As a pocket gets deeper and narrower, the cutter must extend farther from its holder. That extra reach increases the chance of vibration, deflection, poor surface finish, and slower cutting. CSMolding recommends keeping cavity depth to a practical ratio relative to width to reduce these problems.
If the product truly requires a deep cavity, consider whether the geometry can be opened from another side, divided into assembled components, or relieved in nonfunctional areas. Even changing a hidden wall can create space for a shorter, stronger tool.
Preserve function while giving manufacturing room
Good DFM does not mean flattening every interesting geometry. It means identifying where the shape is truly functional. A fluid channel may need a defined cross-section; a clearance pocket usually does not need sculpted corners. A housing may need a thin exterior silhouette while still allowing thicker internal ribs.
The best design review therefore labels the “must keep” features and gives flexibility elsewhere. When the machinist understands which surfaces drive sealing, motion, optics, or assembly, they can suggest changes that save time without compromising the product. Small radii and depth decisions made in CAD often have an outsized effect on tool life, setup stability, cycle time, and the consistency of the final milled part.
