Manufacturing Engineering,4/2002
Tech Front, Pp33-34
Measure
Parts at Cutting Speeds Maintaining a modern machine
tool's high production levels and tolerances means the operator/programmer must know the maximum feed rates it can run
without deviating from
the toolpath. A new technique available
with Optodyne's (Compton, CA) Laser/
Ballbar Calibration System enables
noncircular measurement at high feed rates, which provides better information for cornering, contouring, and
servo-tuning operations. Whether a
standard machining center or
high-velocity five-axis machine
with accelerations of
1 or 2 g and 2000 ipm (50.8
m/min) cutting rates for die/mold
applications, the feed
rate must be reduced when cutting
corners and contours. The critical issue is to know how high feed rates can be maintained
without affecting part accuracy. To
maintain high precision and smooth finishes, consideration must be given
to cutter path complexity, the machine's
static and dynamic accuracy, acceleration and deceleration, control system and compensation, and data
processing rate. Where the
rubber meets the road in machine tool
performance is: Can it produce a
smooth surface finish and
hold dimensional tolerances on corners at
high feed rates? Because
inertia can cause the cutter to
overshoot a turning point, it can
create a distortion or a bit of
a node on a corner edge. Undershooting the corner causes distortion.
And abrupt changes in feed rates affect
cornering and contouring accuracy. As a result, the CNC must anticipate changes in the toolpath and adjust the
feed rate to account for inertia and
inflection points in sharp corners or small radius curves. Without appropriate
CNC capability and
correct servo tuning, cutting at maximum
feed and acceleration rates actually creates and aggravates errors.
Several methods are available for testing machine tool errors.
For example, a gridplate can be used to
measure 2-D contouring errors. A telescoping ballbar measures the
cir-cular contouring accuracy
and can be used to diagnose error
sources. However, a telescoping ballbar is limited to
low speed and large radius circular
contouring tests, which do not
represent actual machining conditions. Because motion errors due
to the tuning of CNC servo controls
incorrectly are often easier to observe on noncircular
paths, the Optodyne noncontact laser/ballbar
system performs circular
and noncircular contouring tests, ensuring
machine tools are tuned properly for optimum performance. Such
a laser/ ballbar system compares contouring
error profiles at
different feed rates to identify motion
errors due to mechanical structures and accel/ decel and those due to the
CNC servo-control system.
Because the laser/ballbar
is noncontact and there is no restriction
on the motion to be
measured due to a mechanical linkage or
cables, it can perform
noncircular contours at actual feed rates, which
determines velocity and acceleration profiles. To separate the machine geometric errors
and the servo errors, the noncircular
contouring can be measured at two
different feed rates. At low feed rate
most of the errors
are due to the machine geometry errors
and at high feed rates the
deviation for the low feed rate trajectory
is due to the servo error or dynamic errors. Motion errors are ofteneasier to observe on
noncircular
paths.File:/Measure Parts at Cutting Speeds 03/17/03
