m e a s u r e m e n tLaser displacement measurement CHARLES WANG TAKING CARE OF PROBLEMS EARLY MEANS THAT REPAIRS WON'T COST AS
MUCH AND THEY WON'T
CONTRIBUTE TO OTHER
MORE SERIOUS PROBLEMS Competition in global
manufacturing today
requires improving
machine tool performance to achieve higher
productivity, better quality and less
downtime. Calibrating machine tools to
catch errors before they appear in parts enables manufacturers to produce higher-accuracy parts,
reduce assembly time and lower warranty
costs. To keep up with the
speed and increased accuracy requirements,
the machines have to
be kept within tolerance, which
means laser volumetric calibration and
compensation are very important.
Regularly scheduled
calibration can help
identify problems
with machine tools such
as bad bearings, hallscrew problems, and loose gibs
and so on, enabling shops to schedule maintenance and
reduce downtime, particularly
during critical production runs. Positioning errors in
straightness, squareness, angle
and non-rigid body errors determine the
performance or positioning
.accuracy of machine
tools, coordinate measuring machines (CMMs) and precision
instruments. The characterization of
a machine movement is complex. For
each axis of motion,
there are six errors:
linear, vertical and
horizontal straightness, pitch and yaw and
roll angular errors. For a three-axis
machine, there are 18 errors plus
three for squareness, a total of 21
errors. A complete measurement
of these errors is time consuming and
can take from two to three days.
Hence, the key is the measurement efficiency.
It is impractical to
stop production
and spend two
to three days
for the calibration. However, using
a laser vector technique
(patent pending), three
displacement errors, six
straightness errors and
three squareness errors can be determined in a few hours instead of a few days.
3D positioning errors
Machinists know that
calibrating and compensating just the linear position is not enough, because it will not catch
all the errors. By calibrating and compen-
FIGURE 1. What is a
volumetric error? sating
volumetrically, you get a
much more accurate machine. Volumetric
calibration of CNC
machines provides a
check of the machine tool's overall
accuracy and, by
compensating for these errors, enables
tighter tolerances to be achieved.
A displacement error is the (1-dimensional)
positioning error in the
same direction as the axis direction.
The volumetric
(3-dimensional) positioning
error is the
positioning error in a
spatial direction not necessarily in
the direction of the
axis motion. Hence, this error is a
vector and
its three
components are
the linear displacement error,
the vertical straightness
and horizontal straightness (see Figure 1). Using a conventional laser
interferometer to measure these errors is complex, time consuming and costly. The
problem is how to measure all these errors
accurately and quickly. Vector measurement technique
The basic
concept of the laser
vector measurement technique is that
the laser beam
direction (or the
measurement direction) is not parallel to
the motion Reprinted with revisions, from
the March 2001 edition of INDUSTRIAL LASER SOLUTIONS
Copyright 2001 by PennWell
Corporation
