A colloquial description of a material that smears and forms built-up edge rather than forming a clean chip.
"Gummy" is not a formal property, but it describes a real behaviour: low-carbon steel, pure aluminium, copper, austenitic stainless and many plastics all tend to deform plastically ahead of the edge rather than shearing cleanly. The result is a smeared finish, poor chip control and built-up edge.
The remedies overlap: sharper and more positive rake, higher surface speed, and a polished tool face to reduce adhesion. Free-machining additives address it at the material level.
Attacking a gummy material with a heavier feed to "force" a chip. It usually makes the smear worse — speed and edge geometry are the levers.
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