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CNC to rotate the coordinate systems. The X-80 has minimally three motors. However, based on how the coordinate
system is rotated, a motor can be any axis, or a mix of axes in the case of a complex compound rotation.
For the purposes of understanding cross-axis
compensation, the X-80 has motor #1 attached to
the X-axis. Motor #2 is attached to the Y-axis, and
Motor #3 is attached to the Z-axis. In the case of
this question, we must deal with the bow of the
Y-axis. This affects principally the Z-axis. As the
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head comes out, the Z moves up slightly in the
center, then sags dramatically at the end. In this
case, motor #2 (Y) is the source motor, and motor
#3 (Z) is the target motor.
The next step is to determine how much to correct.
You need to decide how often to have compensa-
Figure 2
PMAC Motion Controller
tion applied. It can be every inch, or every 0.100
entries. For example, if the compensation is measured every inch and entered at -7.000(Y) is +.0003, and the compen-
sation entered at -8.0000 is -.0005, then the compensation applied at -7.5000 will be -0.0001, the mid-point between
the comp at -7 and -8 inches. inches. It will linearly interpolate between the
For this specifi c question, the user subscriber states that the Z moves up slightly in the center, then down dramatically
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at the end. Figure 3 below shows an example scenario that matches the description.
The X-80 Y-axis
has 40 inches
of travel. The
graph shown in
Figure 3 shows
data. Each dot
represents a
measure data
point. Data was
captured at every
inch of travel.
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Between 15” and
20” of travel, the
Z-axis bows up
0.0009 inches. It
stays at less than
0.0010” of error
until 32 inches,
then it falls off
dramatically.
To get the data
captured as
accurately as
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possible, a laser
should be used
to get the highest
possible accura-
Figure 3
cy. However, it is
Error Map
also acceptable
to simply use a dial indicator measuring between the head and the table. But it should be recognized that the table is
not perfect. The dial indicator will pick up the imperfections in the table. In this example of a gross error as much as
0.006 inches at the end of travel, a simple dial indicator should be able to signifi cantly improve the situation.
Once the data is captured, the corrections can be entered into the control. The data entry must be made directly into
the motion control system. On the SSD, there is a utility program, PMAC.EXE. When you launch this utility, you will be
communicating directly with the motion control system.
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