3
Dimensional machine tool positioning accuracy Laser vector measurement vs. linear measurement
By Charles
Wang President
Optodyne, Inc.
www.optodyne.com
The increasing demand
for accuracy of machined parts
is being fueled by
economics because it reduces
assembly, warranty,
and ownership costs. Traditionally, manufacturers have ensured accuracy of parts
with linear
(one-dimensional) calibra-tion
of the machine tools used for making
them. But linear
calibration is
inadequate for
ensuring accuracy of
three dimensional parts. ASME B5.54
and IS0 230-6
volumetric machine tool
performance measurement standards
were introduced. Because of the
ex-pense, necessitating
the machine to be
non-productive for two or three days, manufacturers
have been reluctant to adopt
volumetric calibration.
However, the Laser Vector Technique
for 3D
volumetric calibration and
compensation, developed by
Optodyne Inc. using
laser Doppler calibration equipment, is becoming popular
because it reduces the time factor from two
or three days
to two or three
hours. Relying on
linear calibration, one dimensional
measurements parallel
to the axis of movement assume that the only
possible errors are
ballscrew and thermal expansion errors. But this ignores squareness errors,
straightness errors, angular errors, and errors
caused by non-rigid body motion.
In fact, there are
many large non-rigid body
positioning errors caused
by shifting weight and
counter weight, etc. Carrying this
to the extreme by
using Taylor's linear
expansion theory,
two slope
terms in the perpendicular directions can be added.
As a result, for a
3-axis machine, there are 45 errors. Of
course, not all of these non-rigid
body error terms are
important. Because positioning accuracy
of a
machine tool is
very complex, it
has been simplified with
various assumptions. For example, the
rigid body assumption,
proposes six errors - one
displacement error,
two straightness
errors and
three angular
errors-in the X, Y, and Z axes. For a
3-axis machine, there are
18 errors plus
three for squareness, a total of 21
errors. Therefore to
achieve higher po-sitioning accuracy, the
angular, straightness,
and squareness errors
must be measured and
compensated. Using the
Laser Vector Technique,
only four body
diagonal displacement measurements are
needed to determine 3D
volumetric accuracy. Body
diagonal displacement errors are sensitive
to all the
volumetric error components
and therefore make an efficient test of
volumetric accuracy. The Laser
Vector Technique
measures all
three displacement
errors, three vertical straightness
errors, and
threehorizontal straightness errors withjust four setups.
The
working volume of a typical VMC
includes eight body
diagonals, a diagonal
being defined by starting at one corner of the base plane and moving to
the opposite corner at
the top plane. These
body diagonals are defined by the positive or
negative axis movement. The last four
body diagonals are
the same corners as the first four diagonals, except the directions are reversed. LaserVector 1: The vector measurement is
pointing in the ppp
diagonal direction. Move Dx, stop, collect data, move
Dy, stop and, move
Dz, stop, collect data and so on.Z (Xe, Ye, Ze) PPP Diagonal Dz
YDy Dx (Xs, Ys, Zs) X machining centerstoolingandproduction.com
August 2003/tooling & ProductionClick
here to download "3dmachine.pdf" file
