Difficult-to-cut materials are, as the name says, materials that are hard to machine.
Typical examples are stainless steel, titanium, nickel alloys, and cobalt-chromium alloys, widely used in medical, aerospace, and semiconductor applications.
A Closer Look
Because difficult-to-cut materials are hard, gummy, or poor conductors of heat, the cutting edge heats up quickly during machining and tool wear and chipping progress rapidly.
Their chips also resist breaking, so long, tangled chips are a frequent problem.
To machine them stably, useful measures include a moderate cutting speed, appropriate tools and coatings, high-pressure coolant to cool the edge, and oscillation cutting to break up chips.