In the machining sector of metal processing, cooling lubricants are used to cool tools and components down. Only an adequate supply of coolant to the machining point in relation to the quantity, pressure at the supply nozzle and the resulting coolant exit speed as control parameters allow high-productivity manufacturing scenarios without having to anticipate peripheral zone losses caused by friction.

Often, however, operators of machine tools are unaware of how much coolant to supply to the machining point, and nor are the quantity or volume flow (l/min) monitored accurately enough. This raises a question: how can the volume flow be measured reliably? Many machine tools do not have a virtually wear-free, quantitative volume flow sensor that works with consistent accuracy. There are many approaches to measuring volume flow. A number of these are set out in the table below.

Each of the sensor principles shown has advantages and disadvantages.
Cooling lubricants, even after preprocessing by cooling lubricant filtration systems, are not completely “pure”, i.e. they contain a certain amount of residual grime(in magnitudes of mg/l) from tiny abrasive particles (such as filing residues and tool dust made up of Cubic Boron Nitride and diamond) that enters the machine tool. Therefore all types of sensor in which mobile and measuring value-generating components such as impellers inside the sensor come into contact with the coolant are subject to a high degree of abrasive wear. Over the long term, this type of material-degrading attrition results in increasing measuring inaccuracy which can lead to complete sensor failure. Some contactless sensor types, such as volume flow sensors based on ultrasound signals or the magnetic induction effect, are not affected by this type of wear. However even these sensors have their limits. The measuring accuracy of ultrasound sensors is impaired significantly over time by the air content in the cooling lubricant, as well as by dirt deposits in the cooling lubricant pipeline. Oils cannot be analysed using induction-based sensors since oils have an electrically insulating effect.

The measuring principle of the “Venturi sensor” is based on the effective pressure difference resulting from the cross-section changes of the pipeline as the cooling lubricant flows through. The GRX-Q sensor, a new development from Grindaix, makes use of this physical principle. It was printed in 3D from material that is highly resistant to wear. The geometric shape of the measuring section that the cooling lubricant flows through has been adapted specially to the challenges of use with contaminated cooling lubricants. The length of the effective pressure-generating diameter sections of a Venturi pipe plays a particular role in this. The purely physical maximum possible impact of wear by the tiniest abrasive particles can be kept to a minimum thanks to the shape. The measuring accuracy also plays a central role. To this end, the relationship of the pressure difference between two measuring points in this wear-optimised pipe geometry and the actual volume flow has been determined as accurately as possible for a wide variety of cooling lubricants.

A measuring accuracy appropriate for requirements in the range of < 5% of the sensor’s maximum measurable value is influenced in industrial settings by not only the impacts of wear from cooling lubricant contamination, but by other factors too. If oil is used as cooling lubricant medium, for example, there is a temperature-viscosity dependency that renders the real flow incorrect if any possible temperature increase in the oil flowing through the sensor cannot be correlated with the associated viscosity values. The GRX-Q Venturi sensor uses one sensor to measure 1) The effective pressure in the pipeline to the machine tool and 2) The temperature of the medium, correlating 3) the associated viscosity relationships and then delivering the volume flow with a very high degree of accuracy below the required 5% limit.

“Why is this 5% limit regarded as a sensible measuring accuracy limit in the first place?” If you look at an application in which a machine tool provides a total volume flow of coolant of 300 l/min for the external cylindrical grinding of wave-shaped small components, you will also see that, of this total amount, 120 l/min will be supplied to the machining point. Depending on the pressure / volume distribution in the cooling lubricant pipelines within the machine tool, a reduction in the total volume flow of 15 l/min (5%) will therefore lead to a reduction in the supply of coolant at the machining point. This means that the machining process is cooled less, the component heats up and expands, and more material is removed from the component than desired. Once the process ends, the component cools down and exhibits a dimensional error (undersize [µm]). A reduction in the quantity of coolant supplied to the machining process cannot be detected accurately enough in most applications in industrial practice, however it is essential when components need to be accurate to the µm. Sensor-integrated electronics monitor the cooling lubricant supply conditions in the sensor housing and deliver optical signals (green/yellow/red) via a flow-dependent, circumferential LED strip in the sensor housing.

The sensor shown can be integrated very easily into your existing cooling lubricant system via an electrically standardised interface (4-20 m/0-10 v). The GRX-Q can also be mounted with two (DN25/32 or DN40, depending on the sensor type) flanges into the pipe system. Volume flow measurement in cooling lubricant systems occurs only rarely in many places, or is performed manually using measuring equipment outside the pipe. Robust volume flow sensors are essential for the future monitoring of cooling lubricant supplies to machine tools in the context of the Industry 4.0 initiative. In view of the contamination of cooling lubricant, conventional market volume flow sensors are often very prone to wear and therefore costly. The GRX-Q sensor is ideal for the virtually wear-free volume flow measurement of all conventional cooling lubricants (oils, aqueous solutions and water-based emulsions).

Source | Grindaix GmbH