Abstract:
Driven by a requirement to extend the lifespan of self-aligning lined spherical bearings,
this research investigates the use of Elid (electrolytic in-process dressing) as a method
of improving ball surface finish. Elid is a continuous and self-regulating electrochemical
dressing process that modifies the surface of a grinding, lapping, or
superfinishing wheel. It provides improved grit protrusion, impedes wheel loading /
glazing and promotes effective cutting.
The characteristics of the newly-developed Elid superfinishing process are in many
ways fundamentally different to conventional superfinishing. The main difference is
that the use of super-abrasives prevents the wheel from self-sharpening; the normal
mechanism by which dulled conventional abrasives are removed and a wheel’s surface
refreshed. Because the wheel’s performance and condition is continually maintained inprocess
by the Elid system, metal resin bonded (MRB) wheels containing very small
super-abrasives can be used. It is the utilization of these fine abrasives (30 to 0.12 μm)
that enables surface roughness values below 5 nm Ra to be consistently produced on the
spherical surface of corrosion-resistant steel balls.
This research provides an in-depth understanding of the Elid spherical superfinishing
process; investigating the most effective use of the Elid system, wheel dressing
requirements and process performance. Optimisation is provided in terms of evaluating
the critical operating parameters, the most effective superfinishing cycle and the
implications to the complete ball production chain. A range of techniques are used to
evaluate processing performance and ball output quality. These include in-process
monitoring of Elid and wheel spindle power levels, analysis of wheel condition, rates of
ball surface generation and material removal, ball finish and form. Although
predominantly concentrated on corrosion-resistant steel, testing is also conducted on
titanium and various ball coatings.
In investigating various ways of using the Elid system, this work considers electrodischarge
truing, pre-process dressing, Elid 1, Elid 2, Elid 3, and Elid combined with
electrolytically assisted superfinishing. The initial process solution of Elid 3 (electrodeless)
superfinishing provides the capability of working on all standard size balls,
however the dressing system lacks stability. The development of a fixturing system that
has a small separate electrode enables Elid 1 (conventional) to be used on the majority of ball sizes. Elid 1 allows more aggressive and consistent dressing, a faster rate of ball
material removal and thus a substantially reduced processing time.
Results with a #12,000 wheel show that surface quality is vastly improved through the
use of Elid whilst maintaining current production standards of form accuracy. Surface
finishes of 2nm Ra are achieved, which is an order of magnitude better than balls
currently produced using barrelling / polishing. Processing times are equivalent or faster
when using Elid 1. Alternatively, consistently sub 10 nm Ra finishes can be reached
with a #2,000 wheel using Elid 2 (interval dressing). Generally MRB-CBN wheels
provide a more effective carbide cutting action than conventional superfinishing wheels.
Controlling wheel condition and achieving full and even ball to wheel conformity are
the two most significant contributory factors to the success of Elid spherical
superfinishing. Insufficient control of these factors results in poor output quality.
Monitoring of wheel spindle and Elid power usage provides useful information in
assessing the condition of the wheel and identifying potential problems. High spindle
power correlates with fast material removal and is a result of high loads and a free
cutting action. Elid processing can be employed for improving surface finish after the
conventional honing stage, or after cylindrical grinding for improving both ball form
and finish.