For further product information please contact DEUBLIN GmbH
Florenz-Allee 1, 55129 Mainz, Germany
Tel: +49 6131 / 4998-0, Fax: +49 6131 / 4998-109
Reliable supply of cutting fluid to the cutting zone Rotary unions for machine tools – know-how is everything!
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.
For the correct rotary union, directing compressed air is not a problem; however, dry working at high speeds requires very special (and expensive) tools.
This raises economic problems, especially regarding the cost pressure of the market regarding the workpiece.
A manufacturer must, as a rule, calculate how an element to be machined can be produced cost-effectively:
Classically, using cutting fluid and an internally cooled tool
By applying MQL, together with the right tool and dosing system
Dry, using special tools and, if necessary, compressed air
These issues are also closely correlated with the batch size.
For the manufacturer, makers of machine tools and processing centres have a whole range of solutions, from special large-scale production systems opti-mized for one specific process (e.g. machining with cutting fluid), to flexible centres that can handle all processes so that all kinds of parts can be be manufactured in small batches.
The rotary union, “tiny” as it may be relative to the machine, has such a major influence on the overall performance of the machine, it has the highest demands placed on it.
Case 1: Large-scale production, focusing on one process
This task requires rotary unions that are optimized for carrying one central medium.
In principle, it makes no difference to the choice of seal technology whether the machine design uses rotary unions as complete components with their own bearings or as bearingless rotary unions (“cartridges”) that rest on the spindle bearing.
What counts is the media, where the choice of technology for uninterrupted supply of classical cutting fluid or MQL is permanently closed seals.
If, alternatively, dry compressed air were constantly supplied, then rotary unions with “controlled leakage” would be the right choice.
Case 2: Medium to large-scale production with changes between cutting flu-id, MQL and dry working without compressed air.
Destined for these tasks are rotary unions whose seal rings lift off when no cutting fluid is present, creating a micro-gap and thus preventing wear while running in dry state. These feature what is called Pop-Off™ technology.
If compressed air during rotation is an additional requirement, then the AutoSense™ series of rotary unions is the first choice.
Case 3: Small batch sizes, frequently changing working process
Here, there is every reason for using AutoSense™ style rotary unions that adapt automatically to the supplied media cutting fluid, MQL or compressed air, or the “All-Media” variants, which can additionally carry cutting oil.
The information box on the next page presents the technology, specifically: Images: i1 Pop-Off™, i2 + i3 Closed Seal, i4 All Media, i5 Controlled Leak-age, i6 AutoSense™.
“Closed Seal”: As the name suggests, these seals rest closed against one another whether with or without coolant pressure. Accordingly, there is generally no need for leak lines. Given the nature of the system, this principle requires a minimal film of lubricant between the seal surfaces. Over time, the smallest amounts of medium can migrate over the seal surfaces. This being so, they must be fitted with appropriate breathers /drains. Rotary unions with closed seals must be adjusted specifically to the carried medium, to the pressure and to the speed and are built for defined application processes.
“Controlled Leakage”: In contrast to closed seals, seals with controlled leakage allow, as the name suggests, discrete, controlled leakage depending on the medium and operating data. For this reason, rotary unions with con-trolled leakage are destined for high-speed applications with dry com-pressed air. Rotary unions with controlled leakage are generally not suit-able to use with cutting fluids.
Pop-Off™: This type of seal rest closed against each other while under pressure. While they are not under pressure, the seals separate from each other creating a micro-gap. This eliminates friction and wear of the seal when used without cutting fluid. That means these rotary unions can also be dry run without limitation, even at high speeds. Pop-Off™ models should be considered when working processes alternate frequently between with and without internal coolant supply. Since the seals lift off during tool change (no coolant pressure), residual coolant runs off between the open seal surfaces. As a result, Pop-Off™ rotary unions always require a downward-pointing drain line to direct leakage into a drip pan. One should note that Pop-Off™ rotary unions are not suitable for operating with dry compressed air.
AutoSense™ – self-regulating: These DEUBLIN rotary unions offer the best of all seal technologies from “closed seals", Pop-Off™ technology and "con-trolled leakage". This design can therefore be used without limitation for cutting fluid, MQL and compressed air up to 10 bar and maximum speed. As with Pop-Off™ rotary unions, these require a leak line.
“All Media”: This technology allows operation with cutting fluid, cutting oil, MQL and compressed air within the permissible operating data. It re-quires a second media–coolant connection as well as a special switch to guarantee the defined function. As with Pop-Off™ and Autosense™, this design must be connected to a leak line.
I is not only the technical construction of a rotary union, in the sense of “befitting the task”, that positively effects the machine, service life and costs, but also its quality.
A rotary union, by nature, is and always will be a wearing part.
Rotary union manufacturers such as DEUBLIN therefore not only keep adding technologies in their range and continually advance them, but also make sure that every rotary union is tuned to a maximum service life. Highly wear-resistant material pairing and extremely precise manufacture are two core aspects that have a direct, positive affect on maintenance intervals, energy consumption and thereby the overall cost.
A manufacturer’s choice to fit machine tools and processing centres with high quality components giving long service life directly benefits the user, and also gives the manufacturer benefits for marketing his system!
To conclude, let us return to sustainability, and let us be realistic: In the near future, considering market conditions, it will not be possible to eliminate cutting fluid entirely.
However, reducing cutting fluid requirements can be named as the first step in the trend towards sustainability. It would significantly support this case if all component manufacturers who supply machine tool manufacturers would also contribute by:
• manufacturing the components sustainably in energy-efficient, re-source-saving, waste-avoiding ways.
• Ensuring that passive components require as little energy during operation as possible
• Ensuring that components have a long service life
Ultimately, true quality benefits not only the finances, but also the entire lifecycle assessment.
Figure 1: Flooding with cutting fluid, concept
Figure 2: Cutting fluid supply through the spindle
Figure 3: Cutting fluid supply through the spindle (demonstration)
Figure 4: Real situation for cutting fluid
Figure 5: Rotary union for compressed air assistance
Figure 6: Impression of scale: Machine tool to rotary union