Process monitoring at the key point

Does effective process monitoring of minimum quantity lubrication in machine tools have an opportunity? Can it be implemented there at all? Rhysearch and Unilube are addressing these questions in a feasibility study on process monitoring. The current state of technology only allows the minimal lubrication system to be monitored (see blog: “Process monitoring of minimum quantity lubrication”). This can be used to observe its (correct) function, but not the micro-lubricating film.  However, a micro-lubricating film takes effect precisely where the machining takes place, namely between the workpiece and tool cutting edge. That is why exact knowledge about what is really happening at this point is of key importance for cost and process optimization in manufacturing.

What was tested?

The series of tests started with trials under the microscope. In doing so, several high-performance lubricants with a precisely specified layer thickness and mixed with fluorescent marker were excited by light with a certain wavelength. This was necessary in order to be able to measure the light emission of the lubricant. The same thing was also carried out with naturally fluorescent oil. The unexpected result: The quantitative determination, i.e. measuring the layer thickness of the micro-lubricating film at a particular point in time, proved more difficult than imagined. It was easier to find out from when the surface starts to slowly dry off. This resulted in the interesting question of which method would be more suitable for process monitoring in practice. Is it the more elaborate layer thickness measurement with added fluorescent marker or the easier method of monitoring the dry running of a cutter with naturally fluorescent oils? Following the initial findings, the whole process developed into a macroscopic experimental setup that was closer to reality. A wide range of light sources with different wavelengths used for excitation were tested here. In turn, it was possible to observe light emissions with a different wavelength. As there is some crossover in the excitation and emission wavelengths, suitable light filters for both the light source and the camera had to be found. The overall positive conclusion: although natural ambient light influences the result, it does not render process monitoring in production impossible. Or to put it another way: darkening a machine tool enables finer process monitoring, but is not absolutely essential. For example, an LED lamp in a milling machine can provide the usual lighting, but other sections of the LED light spectrum are used to excite the fluorescence during production.

An impossible request? On the contrary!

At the start of the feasibility study there was a crazy idea. With our series of tests, we ultimately checked all the key aspects and can now say: from a purely technical point of view, there is nothing against an application on a machine tool! The process monitoring of minimum quantity lubrication in machine tool is therefore not fundamentally doomed to fail. Rather, process monitoring and a control circuit can be implemented on a machine tool, which makes use of the fluorescence of a lubricant with the control of the minimum quantity lubrication for better machining. In the case of quasi-dry machining the optimum process window is when the chips and workpiece surface are dry, but the tool cutting edge leaving the material is just not yet. Practice shows, however, that due to excessive caution, users find it difficult to get near this optimum. That is why we think that process monitoring could be a great help for the customer.

Shows three inserts, two of which are coated with lubricant. One lubricant has fluorescent marker added to it. The fluorescent lubricant glows brightly and is clearly visible. Research result from Unilube in minimum quantity lubrication.
from left to right: Insert coated with lubricant with fluorescent marker, with normal lubricant and a dry insert

Will it make the leap to the production shop tomorrow?

We do not see the primary benefit of such process monitoring in giving inexperienced users of minimum quantity lubrication a feeling of safety. The focus here first and foremost is on large-scale production. Especially in the automotive industry, where there is tremendous cost pressure, our innovative process monitoring can generate significant added value. This highly productive and competitive industry is already taking advantage of the enormous savings potential of minimum quantity lubrication to reduce production costs. Our conclusion from the experiences and progress gained from the series of tests: a process monitoring system with fluorescent lubricants is feasible and has potential. Large portions of the fundamental research have now been carried out, and process monitoring is working under elaborate laboratory conditions. The next major step is to develop an economically reasonable solution for the application on a machine tool. The ball is now with the customers and their own cost/benefit considerations. Did you like this blog article? Subscribe to our newsletter and we will provide you with the latest specialist knowledge all round quasi-dry machining every month. The contact persons from Unilube AG and Rysearch working on this project are happy to answer any questions.


Dr. Raoul Roth                                                                     Mr Jonas Hügli
Project Manager / Rhysearch                                             Project Manager / Unilube AG
T +41 81 755 49 58                                                                T +41 71 672 65 22                                           

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