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ABOUTtooling
Accurate tool registration boosts precision
16 | MARCH/APRIL 2010 | MICROmanufacturing
New technology is permitting the consistent
production and application of
microtools down to 10μm in diameter and
smaller. Tool and die shops are using these
tools to create microfeatures in hard and difficult-to-machine
materials. One challenge
in applying microtools, however, is their registration
with respect to the workpiece. To
prevent tool breakage and meet tolerances,
proper registration must be maintained.
Microlution
A Blum laser tool setter mounted on a Microlution
5100-S machine tool.
Obviously, the old trick of pinching a piece
of paper between the tool and workpiece is
not effective in the microworld. These small,
fragile tools are difficult to view, even with
an optical microscope. As a result, new measurement
strategies are required. A combination
of workpiece probing and tool setting
can provide an effective approach to achieving
precise registration.
Applying tool setters
A way to optimize precision in micromachining
is to apply tool setters that allow onmachine
tool location and size measurement.
However, not all tool setters are useful for
microapplications.
The challenges of using tool setters in
micromachining must be understood within
the context of spindle speed requirements.
It’s typical to use pressurized air-bearing
spindles that operate at speeds from 60,000
to 250,000 rpm. These spindles can produce
high error motions due to their relatively low
stiffness. Hybrid ball bearing spindles have
a stiffness of around 60 newton/μm, while
high-speed air bearing spindles can have a
stiffness as low as 1 newton/μm. These conditions,
combined with misalignment of the
tool in the collet, can produce tool runouts
from 1μm to 10μm, and these values can vary
with spindle speed and tool changes. Measuring
the tool diameter at zero speed will
not provide useful calibration information.
Ideally, the spindle should be operating at full
speed during measurement and should not
stop before engaging the workpiece.
To address this issue, several companies
have introduced noncontact, high-resolution
laser tool setters for micromanufacturing (see
photo, left). These tool setters can measure
with the spindle operating at up to 150,000
rpm. Integral air jets on some tool setters
keep cutting tools clean. This can be vital for
measurement because chips and dirt particles
can be nearly as large as the tool itself.
Solvents such as isopropyl alcohol or acetone,
combined with an air blast, can effectively remove
fluids and particles.
One disadvantage of laser tool setters is
they only detect tool diameter and position in
one direction. Typically, the tool setter is oriented
to measure in a direction that is planar
with the structural loop of the machine tool
(typically the Y-axis). Due to thermal variations,
this direction experiences more displacement,
for which the tool setter must
compensate.
The entire perimeter of a tool is used during
milling, and measuring tool runout in
all directions is important. It is a misnomer
that once tool runout is measured in one direction
it is the same in all directions. This is
the case for collet-placement errors, but not
spindle rotation errors. Spindles can scribe
a multilobed profile as they rotate, and this