Evaluating activation conditions for extreme pressure additives in metalworking fluids using the thread forming test

This paper reports on the use of thread forming tests to determine the manufacturing conditions under which EP additives activate. Estimation of the activation conditions is achieved using a newly developed contact temperature model and an extended thread forming torque model that considers forming speed and forming depth. For thread forming using M6 high speed steel taps on ANSI 1018 steel workpieces, it was found that the presence of a dimethyl disulfur EP additive reduces process torque when the forming speed is at or above 750 RPM. Estimated friction coefficients from the forming process indicate that there exists a critical speed at approximately 580 RPM where the EP additive begins to exhibit a friction reduction effect. The contact temperature model suggests that the activation temperature corresponding to the critical speed is about 270 °C which is consistent with values expected from the literature. Independent forming experiments using workpiece of ANSI 4140 steel support the concept of the activation temperature and its predictive utility. INTRODUCTION Extreme Pressure (EP) additives are commonly included in metalworking fluids formulated for severe machining operations such as drilling, grinding, and forming. EP additives work by reacting with the surface of workpiece through physical sorption, chemi-sorption, or chemical reaction [Buckley, 1978]. The reaction produces a thin (several nanometers), low shear strength, solid-film boundary layer between the work piece and the tool. The solid-film is not continuous but acts as a separating or parting agent to prevent metal to metal contact and to reduce friction. Whether or not an EP additive reacts to form a solid film boundary layer depends not only on the chemical compositions of the workpiece and the additives, but also on the contact conditions (mainly temperature) established by the machining conditions [Schey, 1983]. The nature of the activation process presents inherent challenges to developing a laboratory performance evaluation procedure for EP additives. Standard laboratory extreme pressure tests, such as the Timken method (ASTM D2782), Pin and V-block EP (ASTM D 3233), and the Four Ball EP (ASTM D 2783) tests are the most commonly used methods for assessing the friction reduction potential of EP additives. It has been noted that results from these standard tests are not typically well-correlated with machining performance under real-world manufacturing conditions [Santos, 2002, Werner 1984; De Chiffre 2000]. This is not a surprise since the test conditions are quite different from that of real machining operations, and knowledge does not yet exist that permits a translation of EP activation conditions identified in laboratory to an analogous condition in realworld machining operations. As pointed out in previous research [Lorenz 1985; De Chiffre 2000], performance of metalworking fluids (MWFs) in field machining operations can be reasonably predicted only by using a real machining operation such as reaming, milling, drilling, or tapping. Previously, it has been shown that thread cutting tests can be used to effectively evaluate MWF performance for MWFs that do not contain EP additives [Zimmerman et al., 2003]. The process has been gaining acceptance among MWF formulators and end-users because the test offers advantages such as operation simplicity, costeffectiveness, short testing time, good repeatability, and variable machining severity [Ladov 1974, De Chiffre, 2000]. Although thread cutting is not an effective means to evaluate MWF performance when different EP additives are included, thread forming (Figure 1) is a process in which EP additives are utilized for reducing process friction. In this paper, we develop a method based on thread forming tests to evaluate MWF performance when EP additives are present, and we use the method to predict the activation temperature for the EP additive in thread forming. We hypothesize that this activation temperature is likely to carry over to other thread forming operations for a given EP additive and offer initial supporting evidence to support this hypothesis. MODEL DEVELOPMENT To start, we require a thread forming torque model that predicts the coefficient of forming friction and a contact temperature model based on forming torque data collected at different forming speeds and depths of forming. Existing forming torque models are highly useful towards this end. However they must be extended to include speed and forming depth effects. Torque Model with Effect of Forming Speed and Depth of Forming Chowdhary et al. [2002 and 2003] developed a model that describes the torque experienced by a forming tap during an internal thread forming process. The model calculates forming torque based on the normal and frictional forces experienced by the teeth in the entry taper and back taper regions. In addition to parameters describing the tap and workpiece geometries, five material and operating condition parameters are used in the model: the ridge height coefficient g k , the elastic recovery coefficient r k , the normal stress due to plastic deformation Ca, the normal stress due to elastic recovery Cb, and the coefficient of friction μ . Since the model includes friction, it can be utilized to quantify the impacts of EP additive chemistry and concentration on process friction ( μ ). However, as presented in Chowdhary et al. [2003], the model would need to be re-calibrated each time the process depth or speed changes. This limits the model utility when evaluating EP additive performance since EP additives activate at higher forming speeds. Therefore we begin by extending the Chowdhary et al. [2003] to include speed and depth of forming. Since the flow stress of metal deformation C is exponentially dependent on the strain ! and the strain rate ! " [Mielnik, 1991] as, a b C ! " # & , (1) it is expected that Ca and Cb are functions of the depth of forming at each tooth ) (i h ! and forming speed ! as follows: FIGURE 1. COMPARISON OF A CUTTING TAP (LEFT) AND A FORMING TAP (RIGHT). ( ) 1 ( ) ( ) a C i C h i ! " # = $ (2) ( ) 2 ( ) ( ) b C i C h i ! " # = $ (3) where ) 1 ( ) ( ) ( ! = " i h i h i h . ) (i h is the nominal depth of forming for i-th tooth calculated by: ( ) 2 ) ( ) ( 0 d i d i h ! = , (4) where ) (i d is the major diameter of i-th tooth and 0 d is the hole diameter on workpiece. If it is assumed as in Chowdhary et al. [2003] that g k and r k are material properties that do not change with machining conditions, the forces on each tooth can be described by: