After many years of development by both IMPCO U.S.A. and Neuteq Europe a process known as Generating Bearing Quality (GBQ) has been developed for bearing surfaces. GBQ is a fine finishing process that has evolved from the principles of Microfinishing and Micropolishing. The GBQ process typically removes less than 10 microns of metal from the bearing surfaces of crankshafts, camshafts and other components. The process uses, as a cutting agent, a non-compressible abrasive tape available in a range of grits. The tape is pressed against the surface by a tool that is designed to conform dimensionally to the component. The tool consists of two hard shoes which are held in tool-holders attached to two hydraulic arms. The arms are closed at a set pressure before the cycle begins. The bearing component is rotated and oscillated between centers for a defined period – typically less than 10 sec. Fresh tape is incrementally indexed after each process cycle to ensure a consistent cutting edge and reduce material build up at the edge of the tape.
The abrasive tape used in this process is a flexible polyester film which is non-compressible and gives a consistent surface. Onto this film is electroplated a micron-graded grit of Aluminum Oxide (Al2O3) or Silicon Carbide (SiC) ranging between 5-100µm. There is no significant difference between the performance of tape materials but process observations have suggested that silicon carbide grit is more effective at higher operational speeds, whilst aluminum oxide is equally effective at various cutting speeds. In general the larger the size of grit used on the tape the higher the material removal rate will be but also the higher the surface roughness achieved. The GBQ process is split into levels with lower grit tape grade being used in each subsequent level.
It is important that the tooling known as the “shoe” does not wear, so that it maintains a consistent finish for every part, cycle after cycle. The shoes are manufactured to precise dimensions to fit the components to be processed. The purpose of the tooling is to press the abrasive tape against the work pieces at constant and even pressure. For certain applications the shoe will be lined with a softer, more compliant material such as urethane. The inherent flexibility of urethane allows it to conform to different shapes and can be used for the wearing of sharp edges around the oil holes on crank pins. Choosing the correct shoe material can have a significant effect on cycle time and quality of the surface produced, leading to financial and engineering benefits.