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presented was the requirement to work on multiple
independent machine bed systems. The GRAWDE
beds are each fitted with linear incremental position
encoders. The encoders are distance coded so that the
machine can be homed without driving to a home switch
somewhere at the end of the bed. To establish an
origin, the GRAWDE is driven to the re-synch target and
centered. An offset value is then entered into the CNC.
The CNC contains a data table of offsets for each bed.
Each X-sled has a bed number coded into the wiring to
so that the CNC knows which offset table to use.
The re-synch target is used to establish datums for both
X and Y-axes. It is also used to compensate the
normality sensors in the nosepiece, for pitch and yaw
rotations.
SOFTWARE COMPENSATION
Measurable deviations exist in the rails, bearings and in
the linear encoder scales. To account for these
deviations, each axis on the GRAWDE is electronically
compensated improve position accuracy. Data tables
for each of the beds are stored in a PC on the
GRAWDE. Compensation data is automatically
transferred to the Faunc 15i CNC via serial link for the
appropriate workzone at startup.
Compensation involves establishing a table of offset
values for an axis as function of the position of the same
or another axis. For example, there is a table of values
for compensating the Y-axis positions at various X-axis
positions. Presumably Y-axis compensation with respect
to the X-axis would be to compensate for deviations in
the X-axis rails or beds. Non-linearity of the encoder
scales is also compensated for in the same way.
A laser tracker or laser interferometer is used to
measure the deviations and establish the offset values.
Since the data is measured at discrete positions, the
Fanuc CNC interpolates between values establish a
smooth compensation curve.
GRAWDE SPECIFICS
The GRAWDE’s working envelope covers the entire
A380 inner rear spar workzone for both upper and lower
surfaces. Presently, the GRAWDE can be situated on
any one of twelve (with unlimited expansion possibilities)
monolithic machine bed systems. These beds are
located in a trench such that the top of the beds is below
factory floor. This allows the beds to be covered for
manual operation when the GRAWDE is not in use.
The beds extend from about the first third of the wing on
the inboard end to the transfer area outside of the jigs.
This transfer area is effectively the working envelope of
a modified stacking crane, which can transport the
GRAWDE between any of the twelve beds.
CLAMP NOSE AND NORMALITY
One of the key features of the GRAWDE is the rotating
clamp nose and integrated normality sensors. When
clamped, the nosepiece must be parallel to the outside
skin surface in order to prevent marking the wing skin,
as well as to measure the distance to the panel for
determining counter sink depth. The gear rib reinforcing
plate represents a challenge because the holes are
drilled normal to the wing surface below, but can be off
parallel from the outside surface of the reinforcing plate
by more than two degrees.
Figure 5 - Cross section of head stone and clamp
nose
The clamp nose on the GRAWDE is mounted on a
spherical bearing to allow off-angle drilling while
maintaining tool point distance and clamping ability.
There are linear pots built into the head stone, which
effectively measure the orientation of the nosepiece.
This design is also a more robust normality system as
the delicate components are buried behind the clamp
nose. Figure 5 shows a drawing of a sample headstone
and clamp nose.
The GRAWDE controls have were designed to take
advantage of the spherical motion of the nosepiece by
making coordinated axis moves. It can rotate the axis of
the drill around the tool point or any point along the drill
axis while maintaining the position of the point in space
within .025 mm.
GRAWDE-TRANSPORTER INTERFACE
Figure 6 - Interface assembly
One of most enabling features of the GRAWDE is
relative ease with which this precision machine can be
moved between parallel workzones. This is
accomplished by the use of a modified version of a
standard stacking crane. To facilitate ease of use, the
stacking crane interface was designed with the following
criteria in mind.
1. Maintain the GRAWDE in vertical attitude despite a
poorly centered load
2. Do not allow the GRAWDE to swing when
transported.
3. Connection must be simple and eliminate rigging such
as straps and hooks
To accomplish this, while minimizing the stresses
　
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