Undercuts

Specify the cutting or release direction. An undercut obstructs direct access from that direction. Harvey undercutting explanation, Protolabs molded-undercut guidance

In the ideal example, a 6 mm head enters an 8 mm throat with 1 mm clearance per side. Once the head is below the lip, a 4 mm neck permits 2 mm lateral offset, giving 1 mm reach beyond the throat edge. Reserving 0.2 mm neck clearance reduces reach to 0.8 mm. Holder clearance, deflection and the actual path still need review.

A turned shoulder relief, internal T-slot, overhanging pocket wall and molded latch are not interchangeable feature specifications. Identify the function and the manufacturing context before choosing dimensions or assuming a process is feasible.

Define the hidden geometry and its access

A useful section view shows the undercut itself and the opening through which tools, mating components or mold elements must pass. State the feature relative to datums rather than relying on an apparently accessible view of the CAD model.

Separate axial depth from lateral reach beneath an overhang. A tool can have enough length to reach the level of a recess yet insufficient neck clearance to cut its full width. Conversely, a cutting head that fits the final cavity may be unable to enter through the opening. The insertion and withdrawal paths need checking independently of the cutting position.

Machining: evaluate the whole tool assembly

Harvey describes undercutting end mills, also called lollipop cutters, with wrap-angle examples of 220°, 270° and 300°. A greater wrap angle uses a thinner neck, providing more clearance at the cost of rigidity; a lower wrap angle makes the opposite trade. These are tool-design relationships, not a universal recommendation for the largest angle. Harvey undercutting end mills

The cutter's head profile, neck diameter/length, shank, holder and machine approach all matter. A standard ball end mill and a reduced-neck undercutter may generate similar local shapes but have different access limits. Harvey specifically identifies reduced-neck clearance as a reason an undercutter can machine a slot that is inaccessible with an ordinary ball end mill. Harvey slotting comparison

Protolabs' machining toolkit likewise distinguishes inaccessible geometry from features requiring special tools such as T-cutters, Woodruff cutters or lollipop cutters. Its standardized toolset imposes service-specific tradeoffs; a DFM rejection there is not proof that no manufacturing route exists. Confirm the proposed tool, setup and final acceptance rather than transferring a supplier's quote boundary into a universal geometric rule. Protolabs access and special-tool guidance

Original clearance example

Assume a straight 8 mm wide access throat, an ideal 6 mm diameter cutting head and a coaxial 4 mm diameter neck. The head has already entered and is positioned entirely below the lip; the neck occupies the throat. Ignore radii, taper, tool runout, deflection and all other collisions.

The centered head has (8 − 6)/2 = 1 mm clearance per side during entry. Once below the lip, the neck can move laterally at most (8 − 4)/2 = 2 mm from the throat centerline before contact. At that limiting position, the cutting head's farthest edge is 2 + 6/2 = 5 mm from the centerline. Since the throat edge is 4 mm from that centerline, ideal reach beyond the edge is 1 mm.

If an assumed 0.2 mm neck-to-wall clearance is reserved, lateral offset becomes 1.8 mm and this geometric reach becomes 0.8 mm. Neither result establishes a usable depth of cut or a safe toolpath. Actual head shape, vertical clearance, tool tolerances, holder envelope and cutting behavior must be included in process planning. A final groove wider than its access opening does not by itself prove that a tool can reach or finish it.

Relief grooves and complete callouts

A relief groove can provide clearance where another operation or mating part needs a shoulder transition. If a standardized groove is intended, use its complete designation and governing edition, including any required surface and tolerance information; do not substitute an arbitrary recess that merely looks similar.

DIN's current record identifies DIN 509:2022-12, Technical product documentation — Relief grooves — Types, dimensions and tolerances, replacing the 2006 edition. This is a specific relief-groove documentation standard, not a capability specification for every milled or molded undercut. Its public record does not supply all groove dimensions needed to manufacture an arbitrary callout. DIN 509 record

Removing material to gain clearance also changes the local section. Review the actual load path, mating contact, remaining lip and surface requirements. No groove name or cutter radius establishes fatigue strength, retention force or pressure resistance. Those functions require the appropriate design and qualification basis.

Molding: release is a different constraint

A molded undercut must be released from the mold element that creates it. Protolabs explains options including changing the parting line, side-actions, removable inserts and geometry-dependent bumpoffs. Which option works depends on part and material behavior; a flexible release method should not be assumed for a rigid or highly constrained feature. Protolabs undercut options

In its side-action explanation, Protolabs describes mold elements withdrawing before ejection so that the part can release. Its particular linear side-actions move perpendicular to the main opening axis. Keep that service's tooling configuration distinct from all possible production-mold mechanisms, and confirm draft relative to the actual release direction. Protolabs side-action process

For alternatives such as pickouts or bumpoffs, establish the complete cycle, access and permitted part deformation with the mold designer. A CAD shape that can be machined into mold steel is not necessarily a shape that can be released intact from that mold. If changing orientation solves one undercut, check the resulting draft, parting line and access elsewhere.

Inspection and manufacturing handoff

Agree how the obscured surface will be inspected before committing to a process. An accessible entrance width does not establish the groove's full profile, and a mating component that fits does not verify every radius, location or remaining-wall requirement. Identify the instrument/access method and acceptance stage suitable for the actual geometry.

Plan internal-edge finishing and cleaning as part of the route. Define allowed burrs, edge breaks, residues and surface damage where they affect assembly or service. A relieved feature can be geometrically correct yet retain inaccessible debris or an unacceptable edge.

Preserve material/condition, drawing revision, datum scheme, profile and access envelope, manufacturing route and final inspection record. Review any proposed radius, neck-access or parting-line change against the function rather than approving it solely because it simplifies tool access.

Source versions and related reference

Reviewed 2026-11-01, America/Los_Angeles. Harvey's cited article prints 16 August 2017. DIN identifies 509:2022-12 as current and notes replacement of DIN 509:2006-12. The cited Protolabs live pages have no independently verified technical revision dates. Original clearance calculations above are explicitly idealized geometry, not tooling ratings; no PDF timestamp is used as an edition.

Part features · Internal pockets