Rotary unions for machine tools – know-how is everything! DEUBLIN

Cutting fluids are still state of the art in machine manufacturing, yet re-cent trends are forcing major changes in the design of machining centres, and especially the rotary unions used in them.

The first change, which made rotary unions indispensable was born of the need for internal cooling throughout the machine, as the traditional method of “flooding” the work piece simply failed to provide enough cooling or lubrication above a certain tool speed.

That meant putting a rotary union on the end of the spindle, to act as an interface between the stationary lubricant supply lines and the fast rotating spindle, forcing cutting fluid to the tip of the tool under pressure. There, it escapes to cool and lubricate both cutting edge and workpiece, while also washing the cuttings away.

This targeted use already reduces the amount of cutting fluid used throughout the machine compared to simple “flooding”. Cutting fluid is after all an expensive “chemical mixture” – expensive to procure, to process and to dispose of! Nevertheless, in terms of quantity, internal cooling still leads to plenty of waste as cutting fluid splashes and atomizes, as can be easily witnessed on the window of a machining centre.
Furthermore, cutting fluid as a chemical is considered a health hazard and, if improperly disposed of, an environmental hazard.

As we continue to reduce cutting fluid consumption, we ultimately arrive at “minimum quantity lubrication” – MQL.

This is the point where we apply the exact quantity of cutting fluid to the tool that is needed to lubricate the cutting surface and to prevent the production of heat through friction without producing any drops or mist of cutting fluid. This “minimum quantity”, as it were, vanishes the moment it is worked.

The fluid has to be dosed and directed to the tool tip at precisely the right rate for the given tool. This job is once again done using rotary unions. Depending on whether the air/oil mixture is first pre-mixed or mixed directly within the spindle, one requires a single or double-channel rotary union. The technology is understandably far more complex, yet the consumption of cutting fluid is already far lower.

With a view to sustainability, we can theoretically continue along this same path all the way to dry working assisted by compressed air. The ultimate ideal would be to machine pieces without any cutting fluid, and to blow the cuttings off using compressed air alone. ......