Glossary term
Undercut
A local recess, groove, or manufacturing defect that reduces section strength or functional geometry.
Definition
phenomenonUndercut is a local recess, groove, or unintended removal of material that can reduce strength, fatigue life, weld quality, or manufacturability.
The term is used in welding, machining, casting, molding, and gear or thread design. In welding, undercut is a groove melted into the base metal along the weld toe or root and left unfilled. In manufacturing design, an undercut can also be a recess that prevents tool access or demolding unless special tooling is used.
An undercut removes material from a region that often already carries high stress. At a weld toe, it creates a sharp notch next to the weld bead, reducing effective section and increasing local stress. In fatigue-critical structures, even shallow undercut can become a crack initiation site because cyclic stress concentrates at the groove.
In machining or molding, an undercut may be intentional or accidental. Intentional undercuts can provide clearance, retaining features, thread relief, or snap-fit geometry. They also complicate manufacturing because straight-pull tooling, simple milling paths, or standard gauges may not reach the feature.
Engineering use
For welds, acceptance depends on undercut depth, length, location, loading direction, material, service category, and inspection code. Visual inspection may be sufficient for surface undercut, while critical welds may require additional non-destructive testing. Repair can require grinding, blending, weld repair, reinspection, and verification that the repair did not introduce new defects or reduce thickness below limit.
For machined parts, an undercut should be specified with radius, depth, tool access, tolerance, surface finish, and relationship to adjacent features. Sharp relief grooves can solve assembly problems while creating fatigue problems if they sit in a high-stress region.
Common mistakes
A common mistake is treating undercut as a cosmetic issue. In welded, rotating, pressure, or fatigue-loaded parts, it is a structural notch. Another is adding thread relief or snap-fit undercuts without checking tool access, inspection access, stress concentration, and minimum section. A strong undercut review states location, geometry, depth, radius, surface finish, acceptance code, stress state, fatigue relevance, and repair or inspection method.