Testing to optimise metalworking fluid formulation efficiency and performance

Thomas Sisson, Tevin Proctor, Ingevity; Ted McClure, Sea-Land Chemical Company

One of the outcomes of the automotive industry’s continued focus on operational efficiency and technological advancement is a growing trend towards sustainable innovation for lighter vehicle design. This shift has led to an increased demand for more dynamic metalworking fluids (MWFs) that perform well on lighter weight aluminium alloys for vehicle components and new machining tools. Evaluating the relative effectiveness of metalworking additives is critical in the optimisation of a commercial fluid and the development of new formulations. This means systematic, efficient MWF formulation techniques and rapid, flexible test methods that correlate well with field performance, are more important than ever.

An array of American Standard Test Method (ASTM) tests exists to effectively determine additive effects on MWFs, including the twist compression test (TCT) and tapping torque test (TTT). These ASTM methods are designed to evaluate lubricity additives by measuring coefficient of friction (COF) or torque under variable conditions. Pine chemicals manufacturer Ingevity worked with Sea-Land Chemical to provide a comparative analysis using a simple straight-oil MWF to determine the relative performance of multiple lubricity additives per test method and to discover whether a correlation between the two methodologies exists.


As the conditions for each test method vary significantly, it is important to understand the differences and how each can impact the effectiveness of the additive. For example, the TCT operates at extreme pressure, creating specific tribological conditions to determine average and static COF, time to break down, and surface wear. Although the TCT equipment does not mimic any specific machining or metalworking process, it allows for variation of the applied torque and rotational speeds up 35,000 pounds per square inch (psi) and 30 revolutions per minute (rpm). These conditions mean the boundary and mixed lubrication regions of the MWF can be analysed along the Stribeck curve and the amount of lubricant depletion can be measured.

Alternatively, the TTT is a simulation test based on forming or cutting metal taps that determines the torque required to tap a pre-drilled hole with typical speed settings ranging from 1000 to 1500 rpm. Compared to the high-pressure TCT conditions, the TTT operates under low pressure in the hydrodynamic or boundary regions of the Stribeck curve, and the actual point of contact is a bevelled edge. While the TCT commonly evaluates straight oils, the TTT generally tests water-based formulations, such as soluble oil or semi-synthetic MWFs.

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