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One of the challenges of micromachining
is polishing post-machined part surfaces.
On some microparts, the surface-roughness
dimensions approach those of the part’s
features. In an effort to improve manual
polishing techniques, researchers have
examined pulsed laser micropolishing (PLμP).
With PLμP, laser pulses of a specific
duration create shallow melt pools of a
controlled size and depth. This action
causes surface-tension forces to “pull down”
asperities with a small radius of curvature.
No ablation occurs during the process.
Among those researching micropolishing
is a team consisting of Frank E. Pfefferkorn,
associate professor, Neil A. Duffie, professor,
and Xiaochun Li, professor, Department of
Mechanical Engineering at the University
of Wisconsin-Madison, and Bill Dinauer,
president of LasX Industries, St. Paul, Minn.
In an interview with MICROmanufacturing,
Pfefferkorn discussed the team’s research, the
objective of which is to improve the finish
that can be achieved on 3-D parts and predict
the final roughness of metal surfaces that
have undergone PLμP. Being able to predict
the magnitude of the polishing and frequency
(wavelength) content of the surface will help
establish processing parameters, thereby
minimizing the amount of experimentation
needed before a micropolishing operation is
performed.
The researchers published a paper on
the subject earlier this year in the Journal of
Manufacturing Processes.
MICROmanufacturing: How did this
research come about?
Pfefferkorn: It began with discussions
between me and two colleagues, Neil Duffie
and Xiaochun Li. Working with Bill Dinauer
of LasX, we developed a research proposal,
obtained funding from the National Science
Foundation and are conducting the research.
We’re now working on commercializing the
process. The motivation for me came from
researching micromilling and realizing that you
always have some residual surface roughness
on microparts. Xiaochun Li found the same
Interview with Prof. Frank E. Pfefferkorn,
University of Wisconsin-Madison
Researchers shine a light on part polishing
48 | MAY/JUNE 2012 | MICROmanufacturing
with his research in lithography and rapid
prototyping. Surface roughness is a challenge
across the board in micromanufacturing.
Product designers often do not fully
understand the manufacturing processes,
and they will specify tolerances within the
roughness range of the finished part.
MICRO: What kinds of surfaces are
suitable for PLμP?
Pfefferkorn: Not every surface on a
micropart needs be polished, so we thought
that if we use a laser, we could selectively
polish critical areas. If you have a relatively
flat surface, that’s a pretty easy task for PLμP
because you can scan laser beams very
quickly. But if it’s a 3-D part, you can’t rotate
the part nearly as fast as required to reach
all the surfaces in an economical manner. If
you need to polish the whole part, you might
want to use another polishing method.
MICRO: How do you control this process?
Pfefferkorn: To some degree, we can
control how deep the melt pool is based
on the beam energy, size and duration.
The reason we use laser pulses rather than
continuous waves is that we have better
control over the melt-pool depth, which is
essential for micropart features that can easily
be destroyed. After creating a melt pool, the
surface tension pulls down the asperities, or
roughness features. By changing the pulse
duration, you can control how long the pool
is liquefied, which controls which roughness
features get smoothed out. We’re using rather
long pulses, ranging from 300 nanoseconds
to 10 microseconds, at lower power levels
[to ensure] that only melting and no ablation
occurs.
Every surface has both short-wavelength
and long-wavelength features. We’ve learned
that short-wavelength features are easy to
polish using a 300- to 600-nanosecond pulse
duration. But that can still leave much of the
original surface roughness (i.e., the roughness
in long-wavelength features). The surface can
look shiny, but in many cases the Ra value
hasn’t changed more than 10 to 20 percent.
You’ll need longer pulse durations, such as
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