The
technique uses four
body-diagonal displacement measurements.
These diagonal displacement errors are sensitive to all the volumetric error
components, and therefore make an efficient
test of volumetric accuracy. The vector method measures all three displacement
errors, three vertical straightness errors,
and three horizontal straightness errors with four set ups. A
three-axis machine tool with a one-meter
cube volume can be checked in about four hours. For
example, the working volume of a typical VMC has 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. There are, therefore, only four body diagonal
directions with forward movement and reverse movement (bidirectional), and only four set ups.
For each setup, the operator measures machine spindle movement
along each of the diagonals by
executing X, Y, and Z portions of
spindle travel sequentially, instead of
simultaneously. Readouts are taken
and recorded at each intermediate step. Usually, it is not necessary to move
fixture, tooling, or enclosures. The system
can be set up and operated by the machine's operator, with a learning curve of two to three hours.
With the data from the volumetric analysis, the system's
software can generate a compensation table,
which can be downloaded to the control. The table compensates for errors
throughout the machine tool's volume. The
accuracy gain reduces the bandwidth of errors by a factor of four,
going from a typical volumetric error of
0.060 mm down to only 0.015 mm. These tables are uploaded into the machine's control, and all necessary changes
are implemented at the control level based on the newly added
input. The machinist need not interpret
or massage the data produced. Improvements in
volumetric accuracy are
typically 300-400%, with some
improvements as great as 900 % .
60 www.sme.org/manufacturingengineering •
6/2002 For more information visit
www.optodyne.com Or call Optodyne at
800-766-3920
