Tramp oil or sump oil is the unwanted oil that becomes mixed with cutting fluids (coolants).

It may originate from the lubricatition oil that seeps out from the slideways and is washed into the coolant mixture but it may also come from the protective film the steel supplier coats the bar stock with (to prevent rusting) or hydraulic oil leaks. In extreme cases it can be seen as a film or skin on the surface of the coolant or as floating specs of oil (when the coolant is in an unagitated state). It is a contaminant and a potential pollutant if not dealt with properly

Skimmers are used to separate the tramp oil from the coolant. These are typically slowly rotating, vertical discs that are partially submerged below the coolant level in the main reservoir. As the disc rotates the tramp oil clings to each side of the disc to be scrapped off by two wipers, before the disc passes back through the coolant. The wipers are in the form a channel that then redirects the tramp oil to a container where it may be collected and later disposed of.