Development of Multi-grit cBN Grinding Wheel for Crankshaft Grinding

Doktorsavhandling, 2022

A crankpin, part of a crankshaft, has a complex profile that is difficult to grind. The process often causes challenges such as excessive heat on the crankpin sidewall and wheel wear on the radius, causing reduced dressing interval. Different solutions were proposed to overcome these challenges, mainly focusing on the process, i.e. grinding strategies. However, the work presented in this thesis is concerned with optimising the superabrasive grinding wheel.

A novel analytical assessment framework was developed for evaluating grinding wheel performance that can account for the effects of grit properties and dressing conditions on the wheel topography and, in turn, grinding performance. Based on the model of cutting and sliding grinding force components, a set of performance indicators were derived and then used to evaluate the effect of the wheel topography on the grinding process. Results showed that grit toughness, thermal stability, size and concentration affect the intrinsic specific grinding energy via grit protrusion and sliding component via wear flat area. On the other hand, the grit shape only affects the wear flat area but maintains the intrinsic specific grinding energy regardless if the grit has a higher or lower aspect ratio (blockier or elongated).

To complement grinding performance information, wear was evaluated via grinding and lapping tests. The analyses revealed that wheels containing grits with a higher aspect ratio (elongated grits), lower toughness, lower concentration, or smaller size generate lower grinding forces; however, they wore faster. On the other hand, wheels featuring grits with a lower aspect ratio (blocky grits), higher toughness, higher concentration or coarser grit had the opposite effect. They generated higher forces and wore slower, exhibiting longer tool life.

Findings from laboratory-based trials resulted in two crankpin wheel designs. One aimed to reduce heat generation, while the other targeted less wheel wear. Industrial tests at the end user demonstrated that the favourable design contained elongated and smaller grits at a lower concentration, because it reduced heat generation despite the higher wheel wear. This was confirmed via the Barkhausen noise measurements, which showed a 20% reduction in intensity compared to the reference wheel and a 30% reduction in intensity compared to the wheel design containing blockier and larger grit at higher concentration.