Temperature-based method for determination of feed increments in crankshaft grinding

doi:10.1016/j.jmatprotec.2018.04.032

Journal of Materials Processing Technology

Volume 259, September 2018, Pages 228-234

https://doi.org/10.1016/j.jmatprotec.2018.04.032 Get rights and content

Abstract

The design of a crankshaft-grinding cycle involves determining the radial infeed and axial infeed in each feed increment. A new method for determination of feed increments is developed to increase productivity while avoiding thermal damage. Analyses of the geometry, kinematics and specific-energy characteristic are made to estimate the feed-dependent distribution of maximum surface temperature along the wheel profile. It is discovered that there are two temperature maxima in the grinding zone when feeding both axially and radially. Therefore, the developed method determines the infeeds such that a predetermined burn threshold is matched in these two critical points. The new technology is validated by measurements of Barkhausen noise and residual stress. A comparison is made between the temperature-based method and the radial-plunge method previously used in production. The comparison indicates that lower cycle times are attainable with less risk of thermal damage. Results for material-removal rate and maximum surface temperature are also presented.

Introduction

Production of crankshafts for heavy-duty diesel engines (Fig. 1a) poses significant challenges, particularly with respect to finishing and the obtained surface integrity. Therefore, automotive OEMs usually grind and abrasive fine-finish (Hashimoto et al., 2016) these critical components by themselves. One particularly difficult element of crankshaft grinding is the crankpin, which comprises two sidewalls extending outward from the cylindrical bearing surface, through a curved transition portion, i.e. the radius (Fig. 1b). Crankpins are often at risk for grinding-induced thermal damage, especially on the sidewalls (Comley et al., 2006). For example, it has been shown that inappropriate grinding conditions cause residual tensile stress and material phase transformation in the crankpin (Mahdi and Zhang, 1999), which can cause fatigue cracking when operating under high-torque cyclic loads in the engine (Grum, 2003). Here the high temperatures generated in the grinding process are the primary source of tensile residual stress (Chen et al., 2000). To ensure no-burn grinding, some leading engine manufacturers send crankshafts through vigorous non-destructive testing, including measurements of Barkhausen Noise to detect softening, residual tensile stress and phase changes in the material (Krajnik et al., 2013). They also occasionally use magnetic-particle inspection to visually check for cracks in the sidewalls.

The most commonly used method for industrial grinding of crankpins is a combined (a) radial-plunge grinding for roughing (Junker, 2003), where the grinding wheel plunges radially into the crankpin, followed by (b) angle-plunge grinding during finishing (Banks et al., 2007), where the grinding wheel is fed into the crankpin both radially and axially at an angle. The differences between the radial- and axial-grinding methods are discussed previously (Oliveira et al., 2005). It is shown that a multi-step, axial face-grinding method provides a flexible solution to process design – to adjust the specific material-removal rate along the wheel profile, thereby achieving better control of the process. This method, however, is not readily available nor is it supplied with machines, which limits its use in the automotive industry.

Because the wheel-workpiece contact takes place over a larger area, angle-plunge grinding is a fundamentally better process compared to the radial-plunge grinding, giving shorter grinding cycle times. In spite of this, the angle-plunge grinding process is not explicitly understood, nor has it been quantified. As a result, grinding parameters are often chosen arbitrarily based on trial-and-error, resulting in regions of the crankpin with high temperatures and other regions with low temperatures.

Therefore, an investigation was made into the fundamental parameters and geometries in crankpin grinding. Once this was established, a new temperature-based grinding method was developed and patented (Krajnik et al., 2017), where the axial and radial feed increments are automatically determined so that the surface temperature of the workpiece does not exceed a predetermined (critical) maximum surface temperature threshold – ensuring that the thermal damage is avoided.

Section snippets

Geometric, kinematical and thermal modeling

For modeling of the wheel-crankpin geometry, shown in Fig. 2a, the wheel radius is denoted by $r_{s}$ and the wheel width by $b_{s}$ , whereas the curved transition portion of the wheel has a radius $r_{0}$ , corresponding to the intended radius of the crankpin. The crankpin radii are defined for the bearing surface, $r_{b s}$ , and the sidewall, $r_{s w}$ . The subsequent modeling, however, uses the parameter $r_{w}$ for symbolizing the workpiece radius, which in this case equals the bearing surface radius $r_{b s}$ . Moreover, the

Temperature-based grinding method

The temperature-based determination of feed increments (Krajnik et al., 2017) involves controlling the depth of cut using the thermal model (Eq. (12)), rewritten as: $θ^{*} = \frac{1.064}{\sqrt{k ρ c_{p}}} e_{w} [a g g r (s, a_{e}^{*})] \sqrt{(\frac{v_{s}}{10^{6}}) a_{e}^{*} a g g r (s, a_{e}^{*})} \Rightarrow a_{e}^{*} = a_{e}^{*} (s, θ^{*}),$ where $a_{e}^{*}$ is the limit depth of cut, which is calculated in every point of the wheel profile, $s$ , regardless of a feed increment, in order to achieve a preset critical maximum surface temperature $θ^{*}$ . However, the availability of only two parameters to determine the feed in each

Experimental validation

For the validation of the new grinding technology, two grinding experiments were carried out. First, a single preset temperature was selected at $θ^{*} = 550 ° C$ (Case 1) to grind on the established burn threshold; then the feed increments were determined for the two values of $θ^{*}$ : 550 °C at the initial part of the grinding cycle and 450 °C at the final part (Case 2).

A vitrified CBN wheel (B151-grain, wheel radius $r_{s} = 350 mm$ , radius $r_{0} = 5 mm$ , wheel width $b_{s} = 46 mm$ ) was used on a CNC machine for orbital

Comparison of grinding cycles

In order to evaluate the capability of the newly developed grinding method (Krajnik et al., 2017), its performance is benchmarked against the radial-plunge grinding method hitherto used in engine production. The latter method, commonly used in the automotive industry, is a proprietary technology developed by a major machine builder, where the entire rough-grinding cycle is comprised of radial feed increments (Fig. 13). In contrast, the temperature-based method developed here also includes axial

Conclusions

A new, temperature-based method for determination of feed increments in crankshaft grinding has been developed and experimentally validated in an industrial environment.

Process geometry and thermal effects – unknown prior to this research – have been modeled in the grinding zone to estimate the feed-dependent maximum surface temperature and to obtain a comprehensive understanding of the crankshaft grinding process. Based on the process analyses, two surface-temperature maxima have been used to

Acknowledgements

The authors would like to acknowledge Pontus Andreasson for his managerial support in developing this technology, which has now been patented and implemented in Scania’s engine-production lines in Sweden and Brazil.

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Temperature-based method for determination of feed increments in crankshaft grinding

Abstract

Introduction

Section snippets

Geometric, kinematical and thermal modeling

Temperature-based grinding method

Experimental validation

Comparison of grinding cycles

Conclusions

Acknowledgements

CIRP Ann.-Manuf. Technol.

J. Mater. Process. Technol.

CIRP Ann.-Manuf. Technol.

J. Mater. Process. Technol.

CIRP Ann.-Manuf. Technol.

CIRP Ann.-Manuf. Technol.

CIRP Ann.-Manuf. Technol.

Int. J. Mach. Tools Manuf.