From Cutting Tool Engineering Magazine May 2008
MICRO PRECISION PARTS MANUFACTURING LTD., Qualicum
Beach, B.C., makes small parts for several
applications, including advanced medical
applications (see Productive Times on
page 118 in this issue for more about the
shop’s work). Micro Precision machines
standard ferrous and nonferrous metals,
as well as titanium and other exotic alloys,
plastics and ceramics. Ceramics’ properties
are ideal for medical applications,
according to Steve Cotton, owner and
president of Micro Precision. “Th e only
problem is that they are very expensive
to manufacture,” he said. “A part we can
produce in titanium in 4 days takes 7 to
10 days in ceramic to achieve the same
accuracy. Titanium and other materials are
machined with carbide tools, but ceramics
require diamond-impregnated grinding
wheels. We tried a range of tools, some
PCD, CBN and cubic zirconia, and they
couldn’t touch ceramic. It just blew them
up,” he said.
 As an example of the challenges
of dealing with ceramic parts, Cotton
described machining a siliconnitride
component for a neurological
application. Ceramic was selected for
its biocompatibility and nonconductive
properties.
Micro Precision machined the ½"-long
parts from solid blanks of silicon nitride.
Th e parts featured complex 3-D contours
with three contact surfaces, two of which
specifi ed no tolerance. “Th ey
had to be perfect,” Cotton said.
Tolerances otherwise were
±0.0004".
The diamond-tipped tools
Cotton applied included
custom endmills as small as
0.016" in diameter, featuring
grit as fi ne as 800 (25
microns) for fi nish passes. “If
it’s a drill or an endmill,” he
said, “you really need to be up
about 150,000 rpm.” Th e small
diameter tools had to be run at
high rpm to produce suffi cient
cutting speed.
Cotton machined the parts
on a Haas OM-2A Offi ce Mill
vertical machining center. Th e
machine is relatively small with
about a 5'×6' footprint and X-, Y- and Zaxis
travels of 12", 10" and 12", respectively,
but Cotton said it provides more than
enough rigidity and accuracy for his work.
To permit effective use of small tools, Haas
recommended that Cotton acquire an airdriven
supplementary spindle from NSK.
“We put the main spindle in the M19 hole
position, which holds it still, then put in the
structure.”
He noted that the stiffness of the part
itself can be a concern. “Medical parts
can be quite delicate,” Salmon said.
“Although you could make the machine
and fixturing ridiculously stiff, the part
itself may not be. The part then would
be the weakest link in the chain. You’re
always doing that balancing act.” If the
part is not stiff, grind less aggressively or
modify the fixturing to better support
the part.
Regarding the advanced grinding
machines needed to effectively machine
small ceramic parts, Salmon said he
considers the industry in general to be
“slow in incorporating what I consider
to be mature technology.” He feels much
of the inertia results from customer expectations
regarding what a grinder
should look like. End users want to see
a grinder “that looks like a grinder,”
he said. “What the industry associates
with stiffness and rigidity is a machine
that is made of cast iron and is the size
of a ship.” A machine that is “the size
of a dining room table, with superhigh
stiffness” is thought to be too high-tech.
“With all the granite-type bases, hydrostatic
slideways, shear dampers, magnetically
levitated bearings and configurations
available today, you can build
a machine that is extremely stiff and
stable, and yet wouldn’t necessarily look
like a conventional grinder,” he said. It’s
hard to sell the latest technology to the
traditional machine tool buyer who has
been successful with the old method,
he added.
Because many ceramic medical parts
are extremely small, parts manufacturers
need to match their machine tool to the
parts, according to Andy Phillip, president
of Microlution Inc., a designer and
manufacturer of CNC milling machines
for micromachining applications. Phillip
said any machining application, no
matter the part size, involves a number
of key factors, including workpiece
material, the size of the features to be
produced and surface finish and tolerance
requirements. “In a sense, the small
scale of micromachining magnifies the
results of changes in those factors,” he
said. “If you change just one or two of
those characteristics, you need to have
a substantial understanding of what to
do from there and how to address that
application.”
The design of Microlution’s
machines, for example, results from
study of micromachining’s process mechanics,
Phillip said. “There are specific design considerations, including placement of components to maximize
rigidity, very high resolution feedback,
stiction-free motion and ironless linear
AC motors.”
Toolmaker Recommendations
Toolmakers are another source
of ceramic machining application
knowledge and recommendations
for medical applications. For example,
among the products supplied by
Technodiamant USA Inc., Mt. Arlington,
N.J., are diamond core drills. A
diamond core drill consists of a hollow
tube with a matrix of diamond
grit and bonding material at the business
end. The company produces the
drills in diameters down to 0.0225",
and the tools can hold hole tolerances
of ±0.00025". Sales Manager David
Slaperud said metal-bonded drills are
most commonly recommended for
machining ceramics, with the specific
mix of grit size, grit concentration and
binder determined by the intended application.
“The more the customer tells us about the material they are drilling,
the better idea we have of what recipe is
going to work best,” he said.
The harder the material, the more
friable the diamond grit should be. Friability
describes a diamond grain’s tendency
to fracture and generate new
sharp edges while in the cut. A lessfriable
diamond grain will not break
off and expose fresh edges that would
cleanly fracture a ceramic workpiece.
For example, South African diamonds
are known for being well shaped and
rounded and are generally not preferable
for some ceramic machining applications.
Natural diamonds from other
sources, as well as synthetic diamonds,
may have different friability.
Slaperud said Technodiamant’s smallest-
diameter core drills usually feature
325 grit (about 50 microns), which can
be used uncoated or coated with nickel or copper. The metal coating combines with the metal binder
to hold the diamond grain more tightly in the matrix, which
is advantageous when drilling hard materials.
The concentration of the diamond in the binder—measured
in carats per cubic centimeter—is another consideration.
“If you have too much diamond, then you may not
have enough binder material to hold it,” Slaperud said. He
added that more diamond is not necessarily better, because
there may not be enough force to break the diamond particles
into sharp cutting points.
Slaperud said grinding parameters are application specific,
but when core drilling alumina ceramics with smaller drill
sizes up to 0.050" in diameter, typical feed rates range from
0.2 to 0.5 ipm, running at 3,500 to 4,000 rpm.
Coolant flow is crucial in keeping the drill cool and clearing
away grinding swarf. Technodiamant recommends up to
200 psi of through-tool coolant pressure. Without sufficient
coolant, a drill will overheat. “If it heats up too much, then
the binder will break down, the diamond will fall out or burn
and your tool will fail,” Slaperud said, noting that a blunt drill
can produce scrap.
About the Author: Bill Kennedy, based in
Latrobe, Pa., is contributing editor for Cutting
Tool Engineering. He has an extensive
background as a technical writer.
Contact him at
(724) 537-6182 or by e-mail at billk@jwr.com.
|
