It originates as lubrication oil seeps out from the slideways and washes into the coolant mixture, as the protective film with which the steel supplier coats the bar stock to prevent rusting, or as hydraulic oil leaks. In extreme cases it can be seen as a film or skin on the surface of the coolant or as floating specks of oil (when the coolant is in an unagitated state). Tramp oil 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 scraped 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 is collected for disposal.
Floating weir skimmers are also used in these situation especially where temperature or the amount of oil on the water becomes excessive.