Page 25 - MMI_MarApr 2021 (high-singles)

orthopaedic implants but the
biggest challenge is usually the
surface finish. For this reason,
post-machining hand finishing
is still usually required with belt
grinding, polishing, and buffing.
And it is time consuming. These
finishing processes can take 45
minutes for a single part. Unfor-
tunately, no practical machining
process today eliminates the
need for hand finishing of these
parts, but a high-performance
machine and stable machining
process can dramatically cut
that time and machining cycle
time as well.
How do you do that? Improved
surface quality can be achieved
through the combination of en-
gineered cutting parameters and
proper toolpath strategies. For ex- no’s DA300 five-axis machining
ample, on a performance-based
machining center, moving from a
25 Ra surface finish up to a 32 Ra
could reduce your cycle time by
50 percent for a femoral implant,
providing consistent, predictable
results. Providing a precise and
predictable surface finish allows
end users to develop more effi-
cient and effective secondary
finishing processes, which can
reduce these operations by as
much as 50 percent.
To help find the sweet spot of
machine time vs finishing time,
Makino application engineers
have developed strategies that
first look at the required output
surface finish and then create
machining parameters accord-
ingly. That, of course, could be
done with any machine. But to
move the needle to a more effi-
cient process overall is to apply
machining technology that is
very fast, accurate and stable
over long process runs so you can
reduce finishing without having
to pay a heavy penalty in ma-
chining time.
Five-axis machining is
a given
Generating 3D part and tooling
www.mmindia.co.in
On the left, trying to cut with the tip of the tool where there is not any velocity. By tipping the tool,
you use a larger portion of the of the diameter to get good cutting conditions.
shapes common in the Ortho-
paedics industry essentially
dictates the need for five-ax-
is machining. This can be ac-
complished with a 3-axis mill
fit with a tilt/rotary table in a
3+2 configuration, but there
typically will be a sacrifice in
speed, surface finish and pre-
cision compared to a fully inte-
grated five-axis machine.
On a good two-axis auxiliary
table, you might typically see
positioning accuracy of ±3-5
arc sec. and 30 to 50 rpm in
speed. By comparison, Maki-
center, which is used widely in
the medical industry, uses di-
rect drive servo technology on
its tilt/trunnion table and can
achieve positioning accuracy
and repeatability of ±2 arc. sec.
on the tilt and a contouring
speed of 100 rpm. The rotary
axis is accurate to ±1 arc. sec. and faces, and last longer too. Tool-
can achieve 150 rpm. Combine
that with X-Y-Z axes that can
feed up to 1968 ipm and a 20,000
rpm spindle and you have a
much more capable platform for
high-speed 3D machining.
Being able to accurately feed
and articulate the tool to work-
piece so quickly also enables
much more efficient use of cut-
Source: Modern Machine Shop
ting tools. With simultaneous The biggest
five-axis machining you can challenge
maintain a constant tool vec- in ma-
tor to a 3D workpiece surface, chining
which with a ball nose end mill orthopaedic
should not be a vertical orienta- implants
tion. When feeding a ball mill is usually
vertical to a workpiece surface, the surface
you are essentially dragging the finish.
tip of the tool across that surface For this
at zero rpm, which is hardly an reason,
efficient cutting process. post-ma-
By tilting the tool relative to the chining
surface, you utilize the effective hand
flute length, of the tool efficient- finishing is
ly. This effectively increases the still usually
surface footage of the tool and required
can boost metal removal rates with belt
by as much as 40-50 percent, yet grinding,
still generate better surface fin- polishing,
ishes and blends. and buffing.
It is also important not to scrimp And it is
on the quality of the tooling. Pre- time con-
cision ground tools generate less suming.
vibration to produce better sur-
holders are also extremely im-
portant to minimize tool runout,
which also degrades tool life and
finish quality. For high-speed
machining in these difficult ma-
terials, a shrink fit may not be
mandatory, but it is recommend-
ed. You can get a payback in the
range of 20 percent improve-
ment in tool life.
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