Keys to productive grinding of microscale tools

A Better Grind
Keys to productive grinding of microscale tools
By William Leventon
s cutting tools shrink, difficulties grinding them grow. Particularly as diameters
slip below 0.020", manufacturers of microtools
struggle to minimize runout and vibration, hold
grinding points steady and avoid subjecting the
delicate tools to excessive force.
How are toolmakers meeting today’s microgrinding challenges? The latest machines are a
big help, offering features and capabilities that
improve both the grinding process itself and the
United Grinding Technologies
The Walter Helitronic microtool grinder features
three direct-drive spindles on a rotary B-axis, which
is itself a torque motor.
tools made during the process. Other advances
have impacted the wheels used for microgrinding operations, giving users more wheel choices
and wheel-dressing options.
But there’s more to the story than state-ofthe-art equipment. Toolmakers are also applying their own special techniques to the task of
meeting microgrinding challenges.
Microgrinding machines
Nothing has had a bigger or more positive
42 | JAN/FEB 2010 | MICROmanufacturing
impact on the process of grinding microtools
than developments in machine technology.
Five years ago, for example, minimizing runout
when grinding “had to do with artistic touch,”
said Dave Burton, president of toolmaker Performance Micro Tool, Janesville, Wis. “But today,
the equipment being manufactured is quite
good at that.”
Today’s most advanced machines offer features aimed at making the tough job of microgrinding a little easier. Take linear motors, which
have been used for some time on milling machines. In the last few years, manufacturers have
been installing these motors in grinding machines, according to Simon Manns, tool grinding applications manager for Walter/United
Grinding Technologies Inc., Miamisburg, Ohio,
the North American arm of Schleifring Group,
the manufacturer of Walter and other grinding machines.
Linear motors replace rotary motors, which
connect to lead ballscrews that attach to machine parts to move them. Small amounts of
movement can occur at the points where the
ballscrew is attached to the motor and machine
part, which can adversely affect grinding accuracy and the surface finish of small tools. In addition, Manns noted, a lead ballscrew wears over
time, impacting a grinding machine’s ability to
hold tight tolerances.
These problems are eliminated by linear motors, which don’t use moving parts. Instead, the
motors use changes in magnetic polarity to produce machine movement, Manns explained.
Compared to rotary motors and ballscrews,
linear motors also reduce cycle times by speeding up the movement of machine axes, noted
David Brigham, former vice president of Schütte
USA LLC, a grinding machine supplier in Jackson, Mich. Brigham added that machines with
linear motors also move more smoothly than
those with ballscrews, which rely on encoders or
resolvers that produce incremental movements.
In many grinding machines, Manns said, a
belt connects the motor to the spindle. But the
latest Walter microgrinding machine includes
a direct-drive system in which the wheel fits
onto the end of the motor, eliminating vibration
through the belt system.
continued on page 45
A Better Grind continued from page 42
Reduced grinding machine vibration can yield a number of benefits for
microtool makers, including improved
surface quality and machining accuracy.
Another way machine builders, such
as Rollomatic Inc., are lessening vibration is by equipping grinding machines
with hydrostatic slides that move on a
thin cushion of oil, thereby eliminating
metal-to-metal contact, according to Eric
Schwarzenbach, president of Mundelein,
Ill.-based Rollomatic.
Hydrostatic slides have been used for
some time. In recent years, however, their
cost has come down and their precision
has gone up, noted Chris Morgan, chief
technology officer for Advanced Machine
Technologies Inc., a maker of small cutting tools in Crestwood, Ky.
couple of tenths, you’re out of tolerance
and you lose the tool,” Manns said. Due
to the importance of the grinding point
location, the latest Walter microgrinding
machine lets manufacturers of tiny tools
design grinding processes with special
software called Tool Studio, which uses
all of the machine axes to try to maintain
a fixed grinding point on the wheel.
Changes in wheel changing
Not surprisingly, the grinding wheel
itself hasn’t been ignored by those seeking to improve microgrinding processes
and products. One significant change
related to grinding wheels is the number of wheels that can be held by machines on the market. For example, the
latest Schütte machine includes a wheel-
grinding and magnet finishing in one
setup on the same machine, Brigham
noted, saving time and reducing costs.
Bond options
As for the grinding wheels themselves,
toolmakers can now opt for wheels with
hybrid bonds that combine metal and
polyimide for better wheel-form retention. Hybrid bonds allow users to make
long part runs without stopping the process to dress the grinding wheel, according to Troy Heuermann, superabrasives
business manager for 3M Abrasive Systems Div., St. Paul, Minn., which makes
diamond and CBN grinding wheels.
For an even harder matrix and better form retention, wheels with vitrified
bonds are available. These bonds are usu-
Floating workhead
In addition to hydrostatic slides, Rollomatic’s Nano6 microgrinding machine
includes what the company calls a “floating” workhead. Normally, the workheads
in such machines aren’t free to make small
movements. Instead, they run on ball
bearings around their own axis, Schwarzenbach explained.
With a floating workhead, however, the
ball bearings have been removed, leaving
the workhead free to float, or move approximately 0.050". As a result, concentricity is not provided by the workhead
but by the machine’s shank-guidance system. “The floating workhead [yields] to
the shank-guidance system,” Schwarzenbach said. “So the workhead is driving but
not making any contribution to accuracy.”
Usually, a shank-guiding system is
paired with a standard workhead. During
the grinding of small tools, however, the
two systems “will be fighting each other
far too much,” Schwarzenbach said. “With
the floating workhead, we’ve eliminated
this fighting, or stress.” As a result, concentricity is improved and runout is virtually eliminated, he said.
Like hardware such as floating workheads and hydrostatic slides, software
plays an important role in the latest microgrinding machines. Consider, for example, how these machines minimize
movement of the grinding point, a key
to accurate microgrinding. “When you
get down to very small tools, if you move
the grinding point a tenth (0.0001") or a
3-D simulation of a 0.004"-dia. ballnose endmill that takes into account the corner radius on
the grinding wheels.
changing device that allows it to hold
up to five different wheel arbors, each of
which can be loaded with three wheels.
The wheel-changing device makes it easier for manufacturers to switch back and
forth between several different grinding
jobs, according to Brigham.
The wheel-changing device also allows
manufacturers to load separate wheels
for finishing, semifinishing and roughing onto the same wheel arbor. This allows them to complete all three of these
processes “without ever having to stop
the machine,” Brigham said.
Thanks to a special feature, Schutte’s
wheel-changing device can come into
play during finishing, when a process
called “magnet finishing” is used to remove burrs. Magnet finishing is also designed to improve the coating adherence
of surfaces, thereby increasing tool life.
Incorporated into the wheel-changing
system, a magnet-finishing unit allows
ally vitrified diamond or CBN, depending on the tool material. Diamond, for
example, would be used for carbide tools,
while CBN would be the choice for tools
made of steel, according to Heuermann.
While relatively hard wheels have their
advantages, softer wheels are a less-expensive option and can do an adequate
job in some microgrinding applications.
There are a number of different ways to
make a wheel work and feel softer. One is
reducing diamond concentration. Many
times, Heuermann said, a softer wheel
will have less diamond content, which adversely impacts form retention. But this
may not pose a problem during a small
production run—for example, a batch of
50 parts with a total run time of less than
an hour. Even a soft wheel can sometimes
provide adequate form retention for relatively short runs. If soft wheels can meet
part tolerance requirements, Heuermann
thinks they can be a good choice because | 45
A Better Grind continued
softer bonds reduce costs, as well as the
risk of burning parts during grinding.
Spherical cutting tool
shape the profiles of commercially available wheels so they can grind microtool
features. This technique “is probably the
key to our success,” Burton said.
Dressing the wheel
In addition to wheel characteristics, toolmakers must consider how to
dress wheels used in microgrinding processes. Today, there’s a trend toward online dressing, or dressing the wheel in
the grinding machine, reported Mike
Gainey, corporate applications engineer
for grinding wheel manufacturer SaintGobain Abrasives, Greer, S.C. Online
dressing eliminates runout that results
when a wheel is remounted in a grinding machine after offline dressing. “The
smaller the part you’re grinding, the more
important it is to reduce or eliminate runout,” Gainey said.
To those interested in online dressing
of microgrinding wheels, Gainey pointed
out that the process will probably require
a vitrified-bond diamond grinding wheel
instead of wheels with more common
resin or metal bonds. A glass-type bond,
vitrified material is more fragile and will
fracture more easily when dressed with
a rotary diamond dresser, which is usually chosen for online dressing because
it allows better measurement of wear on
Advanced Machine
Technologies Inc.
(502) 243-0263
Performance Micro Tool
Rollomatic Inc.
(866) 713-6398
Schütte USA LLC
(517) 782-3600
3M Abrasive Systems Div.
(800) 742-9546
United Grinding Technologies Inc.
(937) 859-1975
46 | JAN/FEB 2010 | MICROmanufacturing
Tricks of the trade
Advanced Machine Technology
A 200µm-dia. grinding wheel is used to
fabricate a 25µm-dia. spherical cutting tool.
both the wheel and the dresser inside a
machine, Gainey noted.
Users of Schütte’s microgrinding machine can purchase an in-process dressing device that mounts onto the machine’s
workholding axis. Dressing superabrasive
wheels online, however, “isn’t a real quick
process,” Brigham said. In most cases, his
customers will remove the whole wheel
arbor from the grinding machine and take
it to a separate dressing machine.
To minimize runout caused by offline
dressing, Rollomatic offers a special wheel
arbor system called PerfectArbor, which
uses material deformation as a means of
centering the arbor when wheel packs are
remounted to the grinding spindle after
dressing. This system provides greater accuracy than the tapers often used in these
situations, according to Schwarzenbach.
Wheel-dressing systems of various
kinds are used to create the right wheel
shape to grind the required features into
microtools. One option is a machine sold
by 3M, which uses data from tool drawings to produce the necessary wheel profile, said Heuermann.
Performance Micro Tool, on the other
hand, uses its own dressing technique to
Other toolmakers devise special techniques, too. At Advanced Machine Technologies, personnel achieve greater
manufacturing precision by using sensors
to make sure that tool shanks are almost
exactly concentric with their rotation axis
on the grinding machine.
In addition, Morgan and his colleagues
have their own ways of preventing small,
delicate tools from breaking during microgrinding processes by minimizing tool
runout. “What we do is cut at a really low
speed and take our time. We’ll typically
operate at around 20,000 rpm, which is
actually slow for grinding microtools,” he
said, noting that spindle speeds for grinding microtools can range from 60,000 to
200,000 rpm. Advanced also typically
applies grinding wheels with diameters
from 0.01" to 0.001". The combination
of small grinding wheels and low speeds
results in low forces on workpieces, according to Morgan.
At Performance Micro Tool, Burton
handles similar situations by increasing
the rotational speed of the grinding wheel
without increasing the rate at which the
workpiece is fed into the wheel. This reduces the amount of material removed
by the individual grits on the wheel per
revolution, which, in turn, reduces the
force on the workpiece.
In microgrinding operations, however,
there are other things to be concerned
about besides the forces applied to tiny
workpieces. For example, Burton points
to wheel wear, vibration and the possibility of burning tools. So he and his
colleagues have developed a formula
that takes these factors into account
in different situations and yields the
optimal rate at which a workpiece
should be fed into the grinding wheel.
The formula “is something we’ve developed over the last 30 years,” he said.
“For the first 10 years, though, it was trial
and error.”
About the author: William Leventon
is a New Jersey-based freelance writer.
Telephone: (609) 926-6477. E-mail:
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