Abstract:
Bearing surfaces are highly engineered and high precision parts in regard to surface finish,
waviness, form and dimensional size. Surface finishing techniques will not only affect the
surface finish and texture but the mechanical properties of the surface and subsurface
including surface stress state and hardness as well as chemical properties. The bearing
surface is a functional surface influencing fatigue, wear, and friction, noise and lubrication
regime. Bearings are low cost, high volume commodity products, which adds additional
constraints to any manufacturing process of low cost, fast cycle time and automation.
This thesis documents the development route for a superfinishing technique for bearing
raceways. Initially work was performed on electrochemical machining (ECM) of raceways,
but this was found to improve the surface finish too slowly and gave poor waviness as a
result of non-conductive MnS inclusions becoming exposed on the bearing surface during
ECM. Abrasive honing stones or superfinishing tapes were then combined with the ECM, a
process called electrofinishing to remove these MnS peaks. A beneficial interaction
between the ECM and the abrasive resulted in reduced loading of the abrasive and therefore
more aggressive cutting and for longer.
Using a fine grit (3- 4 pm) CBN vitreous bonded honing stone the surface finish of a
ground ring (0.3 pm Ra) or a turned surface (1.2 pm Ra) could be improved to <0.02 pm
Ra in around 6 seconds. These new surfaces showed low Rtm and Aq (slope), but also
acceptable or very good levels of waviness. The material removal is dominated by the ECM
process while the rate of surface finishing is controlled by the abrasive process.
The process of electrofinishing gives the possibility of fast, high and controlled material
removal capability combined with fine surface finishing, which presently must be achieved
by throughfeed honing or two stage honing. The main difficulty to overcome is achieving a
stable consistent honing needed for a production process. These difficulties arise from the
incompatibility of the ECM electrolyte and honing fluid (usually an oil). Improvements
were made by using a water soluble synthetic metalworking fluid mixed with the NaN03
electrolyte. Further improvements are possible by better choice of honing stone, better
application of stones, while for superfinishing tapes, the use of tape feed and oscillation
will give further improvements.
