UPDATE Method zeroes in on volumetric accuracy Procedure takes as little as two hours. Checking a machine for volumetric
accuracy up until
now has been a
time-consuming activity, though
one which generates
a highly desired
result. Conventional laser
inter-ferometer methods, for
example, typically
require a couple of days to
perform. Because most
CNC controls today have the capability to compensate the volumetric
errors, finding a way that can
Optodyne's
vector
method
requires only four
simple setups. accomplish the task in as
little as two hours is a
major breakthrough for machine tool users. Optodyne Inc, Compton, CA, has
developed a vector method that can
measure all
the volumetric error
components in faun
simple setups and
within a few hours. The errors
include three displacement errors, six
straightness errors, and three
squareness errors. For most machine
tools, only calibrating
and compensating for linear displacement error are not enough.
Besides the displacement error (parallel
to the axis
of movement), there is vertical
straightness error
(perpendicular to the arms of movement) and horizon-tal straightness error
(perpendicular to
the axis of movement
and the direction
of the vertical straightness). The
resulting error is a vector error consisting
of displacement error,
vertical straightness error, and
horizontal straightness error. It is
important to measure and compensate
all the volumetric
errors (12
error components)
to achieve higher
volumetric accuracy As
recom-mended by the
ASME B5.54 machine
tool standard, four body
diagonal displacement
mea-surements may be
used to determine the
volumetric accuracy
of the machine tool This is
because the body
diagonal displacement errors are sensitive
to all the volumetric error components Thus, it is a good
and efficient check
of the machine's
volumetric accuracy. However, if the
measured body diagonal displacement
errors are large, there is not
enough infor-mation to identify the error
sources or to do a volumetric
compensation. Optodyne's
vector measurement technique (patent pending) measures
the volumetric errors,
including the linear
position errors, vertical
straightness errors,
and horizontal straightness errors for all three linear axes and the three squareness errors. Using these
volumetric errors, machines
can be compensated
volumetrically. To experimentally verify the theory, extensive measurements and testing
over a period
of six months were
performed on a
3-axis horizontal machining center using
a Laser Doppler Displacement
Meter (LDDM) model MCV-500
linear calibration
system with a diagonal
steering mirror
and a 3”x4”
flat mirror target.
A compensation file was generated
based on the
vector method and body diagonal measurements
were used to check
the volumetric accuracy of
the machine. The results shown
in Chart 1 indicated
that a gain
of a factor of three to four in
accuracy was achieved with the
volu-metric compensation
than without compensation. A second 3-axis HMC was used to verify
results without compensation. The repeatability
of the machine and
the repeatability of the
vector measurement results were checked and compared with
con-ventional measurement
results. Laeer/ballbar at work
Another of
Optodyne's recently introduced measuring
innovations, the
Laser/Ballbar, was
used to confirm contouring accuracy at high
feedrates and
determine velocity
profile at high
feedrate and small radius. For most machine tools,
the contouring accuracy deterio-
