Circulating & slideway oils

Three jobs that pull against each other

Slideway oil has a contradiction built into its definition: it has to stay on a vertical way under load, so it carries tackiness additives, yet it has to separate quickly from the cutting emulsion once it reaches the sump, or it contaminates the metalworking fluid and takes it out of service. A hydraulic oil put in its place has no tack, and the way stick-slips — motion goes jerky at low feed rates and the machine stops holding size. A circulating oil has a different problem: it runs continuously through a system with reservoir, filter and pump, so air release and water separation matter more than load capacity. And spindle oil is at the far end — very low viscosity, so the high-speed bearing does not heat up.

Classes, specifications and the tests that matter

Circulating oils fall under ISO 6743-6, classes CKB and CKC, with usual viscosity grades of ISO VG 32–220 depending on application. Slideway oils are typically ISO VG 32, 68 or 220, and the industry's acceptance references are the Cincinnati Machine P-47 (VG 68) and P-50 (VG 220) tests, which check exactly stick-slip behaviour and separation from coolant. Spindle oils sit at ISO VG 2–15, with strict filterability and water-content requirements. On centralised lubrication systems, viscosity is chosen not only for the bearing but for the line: an oil that is too thick never reaches the last point in the circuit in winter, however well it suits the lubrication point.

Practical rules on machine tools

On a machine tool the right oil shows up as accuracy, not as consumption:

Do not use hydraulic oil on ways. Without tackiness additives you get stick-slip, and at low feed rates the finished surface shows it immediately.
Check separation from coolant, not just the viscosity grade: a way oil that does not separate shortens the life of the cutting-fluid sump.
On centralised lubrication, allow for winter shop temperature — the length of the line decides the grade more often than the bearing does.
Spindle oil is changed on cleanliness, not on wear; a single particle in a high-speed bearing is enough.
Do not top up a circulating system without filtration. The reservoir is exactly as clean as the oil going into it.