How test simulation can drive slideway success

INDUSTRIAL LUBRICANTS

How test simulation can drive slideway success Clare Gosling, Industrial Customer Technical Service Specialist, Afton Chemical

The slideway lubricant is a critical driver of productivity in precision manufacturing. The lubricant must reduce friction and prevent stick-slip to ensure accuracy of machine movement but also needs to deliver good fluid separability so that the performance of metalworking fluids is maintained at a high level for a long time. Unfortunately, these two core requirements can be chemically opposed.

Much of mass production is founded on slideways: those fixed metal tracks along which heavy equipment such as machine tools or robotics can be moved in a predetermined and controlled manner. Precision manufacturing relies on this movement being smooth and accurate, time after time, to output the required volume of high-quality identical parts.

Assuming a well-designed slideway is manufactured from optimised materials, any differences in productivity often come down to the performance of the slideway lubricant. Balancing the multiple demands placed on the lubricant is challenging, especially as some of these demands bring chemistries into conflict.

lubricant inevitably becomes mixed with the metalworking fluid that cascades constantly over the tool and the workpiece to lubricate, cool, prevent corrosion and remove swarf. Here, the key property of the slideway lubricant is separability – enabling large volumes of critical metalworking fluid to remain in circulation rather than replaced due to contamination.

Smooth operator Sufficient load carrying capacity is essential for maintaining an effective lubricating film and preventing wear. Reducing friction improves responsiveness and efficiency, but the top priority is avoiding stick-slip, a phenomenon that occurs during boundary lubrication when the fluid film becomes too thin and the surfaces that should slide past each other smoothly start to judder, especially when slowing or reversing direction. Jerky motion reduces machine accuracy, with the potential for inconsistent or defective parts.

Staying power The lubricant must remain in place, whether on a horizontal or a vertical slideway, despite repeated back-and-forth movement. The lubricant must also protect the slideway against corrosion in the presence of air, water and other contaminants.

Cutting edge Throughout the machining process, the slideway

Figure 1: Chart showing the relationship between friction and cutting fluid separation for a range of slideway lubricants Continued on page 14

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Continued from page 13

Chemical conflict Maximising productivity is not as simple as this diagram suggests. As is so often the case with lubricant formulation, the additive chemistries typically used to do one job can make another harder: in this case, modifying slideway friction can also increase the lubricant’s tendency to emulsify and make it harder to separate from the metalworking fluid. Modifying the friction system can also affect the lubricant’s anti-corrosion performance.

Fundamental to overcoming such chemical challenges is a deeper, multifaceted understanding of slideway lubricant performance gained through bespoke testing, to fully explore the factors that have the strongest impact on productivity.

Focus on friction Slideway lubricant performance used to be assessed through testing on the Fives CM (Cincinnati Milacron) stick-slip rig. This test established the ratio of static to dynamic friction, a measure used by the industry until Fives discontinued its lubricant approval program in 2018.

Figure 3: The Darmstadt rig owned by Afton Chemical

The Darmstadt rig goes beyond measuring friction coefficient ratios to monitor actual occurrences of stick-slip events, and the extent to which the lubricant can influence these across a range of operating speeds. Overall, its use offers far greater confidence in a fluid’s ability to drive productivity through smooth functioning.

Spotlight on separation Until recently, the standard test for lubricant and coolant separability was the SKC test. Twelve different commercial metalworking fluids were used for this test and separation was measured after 1 hour, 1 day and 7 days.

Slideway lubricants formulated with different treat-rates of friction modifier were compared using this test. The results indicated that a higher treat-rate of friction modifier in the lubricant proves detrimental to fluid separability. The SKC test is no longer being offered as an industry test method.

Figure 2: The stick-slip rig at Afton Chemical

Based on Fives CM test, Afton has created its own proprietary stick-slip rig in order to continue providing what many consider to be a useful screener test.

However, the challenge is to build on this bench test to understand friction performance in a real-world application.

Better at simulating real life is the Darmstadt full-scale test rig. Built using typical machine tool components, this rig enables materials, speeds and loads to be varied so that the behaviour of different friction materials, coatings and modifying additives can all be explored in more depth.

Figure 4: Effect of friction modifier treat-rate on fluid separability Continued on page 30

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Afton examines separability in more detail using the Toyoda test. In its standard form, this involves manually mixing the slideway lubricant and metalworking fluid, leaving the emulsion overnight, then measuring the phases of separation. The cream layer is used as the key measure of separability. Again, this offers some value as a screening test but does not closely simulate actual slideway conditions.

Within the rig, a heater is used to create hotspots and expose fluids to realistic thermal stress. Used metalworking fluids can be assessed for separability and other properties, to ensure any impact on performance or lifespan can be minimised by the lubricant formulation.

To improve validity, modifications have been made to this test to include mixing the oil and fluid mechanically, using a high shear mixer, then measuring separation after 1 hour and again after 24 hours.

By modifying the Toyoda test with high shear mixing, separability under realistic conditions of stress can be evaluated in more depth. The role of the emulsifiers used in metalworking fluids can also be individually assessed. Indeed, this test has shown that the emulsifiers in the fluid have a more significant impact on separability than the friction modifiers in the lubricant.

Rigged for reality Recreating field conditions even more accurately helps to shine a brighter light on separability. Afton has also designed and built a Dispenser Circulation rig that examines the relationship between the metalworking fluid and slideway lubricant over a long period of time. Testing is carried out for 5 weeks rather than the usual 24 hours.

The metalworking fluid contaminated with the slideway lubricant is circulated through the rig and sprayed through nozzles onto a slope, much like it would be in a working machine tool.

Figure 6: CNC Milling Machine

Investing in Insight Investing beyond standard industry lab testing to simulate the field is the best way to gain a deeper understanding of the factors affecting real world slideway performance.

It allows enhanced insight to understand and develop highly optimised slideway lubricant packages that give the best friction and fluid separability. This level of insight is invaluable for formulators to develop the next generation of slideway lubricant additive packages that are compatible with the latest metalworking fluids – especially those used in high profile industries such as aerospace and formulated to meet the latest Health, Safety and Environmental requirements.

Only by confidently moving lubricant performance in relevant areas will the ultimate aim of maximum productivity at minimum cost become a reality.

www.aftonchemical.com Figure 5: Afton Dispenser Circulating rig

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