Internal pockets

Specify the pocket’s opening, depth, corner shape and floor separately. Check the whole mating profile against retained corner material.

A sharp-cornered 39 × 29 mm insert has 0.5 mm clearance per straight wall in the ideal 40 × 30 mm pocket. Its corner lies 3.536 mm from the fillet center and interferes with the retained material at a 3 mm pocket radius. This assumes centered, aligned profiles without taper, burrs or tolerance variation.

For milling, geometry and access are linked. Sandvik Coromant identifies width, depth, length, machine type and operation frequency as tool-selection factors for grooves and slots. Its discussion includes closed slots and pockets, where entry into solid material also has to be planned. Sandvik groove and slot milling

Define the functional surfaces

Specify the pocket by its function rather than by a generic cavity depth. A mounting recess, fluid chamber, weight-relief pocket and sealing feature can share a similar model while requiring different tolerances and surfaces.

Do not treat an overall pocket size tolerance as a tolerance on every floor, radius or wall. Establish which surfaces locate a mating component and which merely provide clearance. A recess that needs a flat seating floor should explicitly identify that surface and its acceptance requirements.

Corner radius and floor fillet are different features

In a conventional end-milled pocket, a cylindrical cutter leaves a radius at the intersection of vertical walls. Reducing that radius generally requires a smaller tool or another process. Protolabs describes larger corner radii and corner reliefs as ways to accommodate larger tools; its service guidance should not be read as a universal minimum tool size or maximum pocket depth. Protolabs corner-pocket guidance

The floor-to-wall junction is a different edge. Protolabs' DFM toolkit notes that a cutter's tip radius can leave a small radius at the pocket bottom, particularly with tooling used for harder metals. A design that tolerates generous vertical corners can still interfere with a mating part at this floor fillet. Identify each edge separately and provide the required mating chamfer or relief where appropriate. Protolabs machining toolkit

A corner radius equal to the tool radius is only a geometric possibility, not an economical process prescription. Xometry's September 2024 design guidance recommends internal radii about 30% larger than the cutting-tool radius to improve machining efficiency. Treat that as the supplier's design advice, not a drawing standard or a guarantee that any chosen tool can finish the pocket. Xometry design advice

Corner reliefs can create space for a square mating part without demanding a perfectly sharp internal corner. They also remove material beyond the simple rectangular envelope, so check neighboring walls, sealing lands and fastener features. Resolve whether the relief is allowed by the component's function before accepting a DFM change.

Original geometry and insert-clearance example

Assume an ideal 40 × 30 mm pocket, 8 mm deep, with four tangent 3 mm radii in plan, vertical walls and no floor fillet, taper, burr or tolerance variation.

The rounded-rectangle area is

A = LW − (4 − π)R² = 40 × 30 − (4 − π) × 3² = 1192.274 mm².

Removed volume is A × 8 = 9538.195 mm³, or 9.538 cm³. Using 40 × 30 × 8 instead would overstate removal because the rounded corners retain material. In an assumed uniformly 12 mm thick blank, nominal floor thickness is 12 − 8 = 4 mm; this does not establish an acceptable structural or sealing wall.

Now place a sharp-cornered 39 × 29 mm insert concentrically and parallel to the pocket sides. It has 0.5 mm clearance to each straight wall. At a pocket corner, however, the insert corner lies 2.5 mm from the fillet center in each planar direction. Its distance from that center is √(2.5² + 2.5²) = 3.536 mm, exceeding the 3 mm pocket radius. It therefore collides with retained corner material in this ideal geometry.

For a sharp rectangular insert with equal inset c from each side, the no-collision boundary at this corner is √2(R − c) ≤ R, giving c ≥ R(1 − 1/√2). For R = 3 mm, the boundary is approximately 0.879 mm per side. This is a geometric boundary with zero extra corner clearance, not a recommended fit. Actual part size/radius limits, assembly clearance, form and coatings need a separate allowance. Alternatively, revise the insert corners or provide an approved pocket relief.

Depth, reach and material removal

Pocket depth must be compatible with cutter diameter, cutting-edge length, neck/shank clearance, holder clearance and machine access. Tool length alone does not prove that the cutting edge can reach the entire floor without the holder or shank touching another feature.

Sandvik warns that long overhangs can make end mills sensitive to cutting-force deflection and vibration. Its closed-slot guidance discusses drilling, linear ramping, peck milling and circular ramping as entry approaches for different situations. Choose a method for the actual tool and geometry; a tool that cuts successfully at the wall is not automatically suitable for axial entry into solid stock. Sandvik pocket-entry and end-milling guidance

Deep, narrow cavities also constrain chip evacuation. Xometry identifies deflection, tool breakage and chip-removal difficulties as concerns with deep pockets. Its dimensional design heuristics are service guidance rather than universal capability limits. Ask the manufacturing provider to qualify the actual depth, material, tolerance and surface combination instead of treating a depth-to-width ratio as an acceptance standard. Xometry pocket-depth discussion

Plan roughing, finishing and workholding together. Protolabs notes that deeper pockets and taller walls can expose residual-stress-related movement in stock. An incoming block dimension or an in-process floor measurement therefore does not establish the final released shape after unclamping and subsequent operations. Protolabs deep-pocket guidance

Inspection and handoff

Agree on inspection access before manufacturing, especially where a narrow opening limits contact with the floor or internal walls. Distinguish the measured size at one elevation from evidence about wall taper, full-depth profile, floor shape or the mating envelope. A trial assembly verifies only the conditions it actually tests.

Define burr and cleanliness acceptance at the entrance, corners and intersecting holes. Identify whether inspection applies before or after heat treatment, coating and final finishing. Where a cavity carries fluid or supports a seal, specify the relevant surface and functional qualification separately; pocket volume and nominal wall thickness do not establish pressure resistance, leak tightness or fatigue life.

For an agent or manufacturing handoff, retain the drawing revision, material/condition, datums, complete profile and depth, separate edge requirements, finishing state and inspection method. Resolve proposed corner, relief or access changes against those requirements rather than accepting a toolpath-compatible shape solely because it is easier to cut.

Source versions and related reference

Reviewed 2026-11-01, America/Los_Angeles. Xometry's cited page identifies publication 14 April 2023 and update 10 September 2024. The cited Protolabs and Sandvik live pages have no independently verified technical revision date. Their guidance is bounded to the described manufacturing context, not a normative drawing standard. Calculations above are original examples under explicit geometric assumptions; no PDF timestamp has been substituted for an edition.

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