Modern machine shop interviews Dave Davidson about Gear finishing processes. For additional technical information and assistance with sample part finishing contact Dave Davidson | ddavidson@deburring-tech-group.com # #machining #polishing #finish #cnc #manufacturingengineering #automotiveindustry #finishing #deburring #leanmanufacturing #aerospace #massfinishing #grinding #automotive #leanmaufacturing #gears

1. 8—GEAR Production Supplement
F E A T U R E
Next-Generation
Gear Finishing
Vibratory finishing, centrifugal barrel finishing and turbo-abrasive
machining are the three top techniques for finishing gears, improving
performance and extending service life.
Dave Davidson has surface finishing in his
DNA. His manufacturing beginnings trace
to the 1970s working in a longstanding family
business that manufactured wooden shoe pegs
used for tumble-polishing small plastic items
using steam-era machinery.
To get a leg up on the technology, Mr.
Davidson, who is now retired but remains active
as a consultant in the industry, joined the Society
of Manufacturing Engineers' Burr, Edge and
Surface Technology division. And with the help
of SME mentors, he developed a line of abrasive
and polishing products as well as new mass
finishing processes for barrel, vibratory and
centrifugal finishing systems. He is a master at
problem-solving customer challenges.
That includes gear-making. Depending on the
particular application, Mr. Davidson describes
three mass-finishing methodologies for producing
surface finishes on gears that contribute to
improved performance and extended service life.
They are:
Vibratory Finishing—Conventional and
predominant, the familiar vibratory bowls or
tubs come in small or large sizes and have
been around for decades. There is a relatively
recent wrinkle in vibratory finishing, chemically
assisted vibratory finishing, which uses specially
formulated chemical compounds along with
non-abrasive media to produce a conversion
Isotropic finishing can have a dramatic impact
on gear performance and service life, so much
so that some racing teams disassemble stock
gear sets and components and send them out
to be isotropically finished for the perfor-
mance and extended service life benefits.
(Photo courtesy of Mark Riley, BV Products.)

2. September 2016—9
coating on gears and similar components. This
makes it possible to develop level surfaces with
micro-finish surface attributes. This technology
is especially applicable to automotive gear
train systems and engine components such as
camshafts and crankshafts, and is widely seen
among racing teams to acquire performance
advantages.
Centrifugal Barrel Finishing—This high-
energy method is a mechanically accelerated
means for producing edge contour and isotropic
micro-finishes on gears. This method is used
by a variety of gear manufacturers and high-
performance racing teams to improve the surface
finish and endurance of gear sets. It is also
capable of producing low micro-inch finishes
to improve load-bearing qualities of mating
surfaces and to develop beneficial compressive
stress and cold-hardening properties useful to
highly stressed parts. A sequence of processes
(analogous to roughing, finishing and polishing)
is often used to develop highly finished surfaces.
The high centrifugal forces and speed of this
process achieve high-level surface finishes in
short cycle times.
One user, Mr. Davidson recalls, pulled racing
transmissions after each event for centrifugal
finishing, only to find afterward that gear sets
were lasting an entire season.
A note here about isotropic surface finishes for
gears. Isotropic finishing, Mr. Davidson says, is a
technical term used to distinguish surface finish
patterns that are random and non-directional in
nature. “This is in contrast to surfaces developed
by all common machining and grinding methods,
characterized by Gaussian peak-and-valley
distributions parallel to each other that manifest a
positive skewed surface with surface peaks and
asperities predominating the surface profile,”
he says.
By this definition, all mass finishing methods
could be said to be isotropic in nature. The
process outlined above, however, has been
optimized to consistently produce gear surface
finishes that will improve overall operational
performance and extend service life.
Turbo-Finish or Turbo-Abrasive Machining—
This is the next-generation gear-finishing
method, according to Mr. Davidson, who
advises and assists Turbo Finish Corp. of Barre,
Massachusetts, developer of turbo-abrasive
machining (TAM). Not wholly machining and not
wholly finishing, one of the properties separating
TAM from vibratory or centrifugal gear finishing is
that it is a dry method using no fluid media. Parts
are fixtured on a spindle, similar to a horizontal
Gear before (top) and after (bottom) turbo abrasive
machining (TAM). (Photo courtesy of Michael Massarsky,
Turbo Finish Corp.)

3. 10—GEAR Production Supplement
machining center, and rotated at speeds ranging
from 500 to 2,000 rpm, fully enveloped in an
abrasive media cloud. Highly refined surfaces
can be developed when a secondary operation
utilizing dry polishing soft granulates, treated
with fine polishing materials occurs as a follow-
up to the abrasive deburring, contouring and
smoothing operation.
“Once you’ve got abrasive particles impinging
the part at that level of force, you’re creating a
shot-peening effect without creating shot-peening
surfaces,” Mr. Davidson says.
From an environmental perspective, that
TAM produces these effects in a waterless, dry
operation is an added advantage. Unlike most of
the other mass-finishing methods, TAM produces
no wet-waste discharge effluent that requires
treatment or remediation.
Admittedly, TAM investment is more on the
capital equipment level, but users can create
edge contours and other surface finishes very
rapidly—with 60- to 120-second cycles in many
cases—and machines can accommodate large
gears in the 48-inch diameter range that would
make vibratory or centrifugal finishing inefficient,
if not impossible.
According to Jim Riley of BV Products, a
surface improvement technologies specialist
based in Victoria, Australia, the advantage and
benefit feature sets most commonly attributed to
these types of isotropic finishing are:
Performance Benefits
• Reduced friction
• Increased part durability
• Improved corrosion resistance
• Reduced wear
• Reduced lubrication requirements and cost
• Improved oil retention
• Reduced contact and bending fatigue
• Improved pitting resistance
• Reduced vibration and noise attenuation
• Reduced applied torque requirements
• Improved surface finish uniformity (part-to-
part, feature-to-feature and lot-to-lot)
• High-quality, micro-finished surfaces
Reduced Friction Benefits
• Increased fuel economy
• Reduced contact fatigue
• Increased power density
• Lower operating temperature
• Extended mean time between maintenance
overhauls
• Reduced maintenance costs
• Eliminated break-in
• Extended component life
• Reduced metal debris
• Reduced part failures
• Minimized overheating
Many gears and gear sets in a variety of
industries remain subject to fatigue, fracture
and wear, Mr. Davidson says. “Such parts
can gain substantial improvements in life and
performance, from alterations to their overall
surface texture. Improvements in overall
smoothness, load-bearing ratio, surface
profile skewness and isotropicity can, in many
instances, improve life and performance and cut
operational costs dramatically. Manufacturers
that have not subjected their parts to an analysis
to determine the potential benefits of this kind of
processing may be making parts that are not all
that they can be.”
A better understanding of gear-finishing options will add to
performance levels and service life. (Photo courtesy of Mark
Riley, BV Products.)