11
The base of the table is a composite structure
of a highly rigid, stabilized bed casting which is
nested in a fabricated steel cradle. The assembly
rests on three leveling-type vibration isolators.
The flying-spot scanner assembly is precision-
mounted from the bed casting, with provisions
for access for repair or removal of the unit
through the cradle.
The main supporting sections of the cradle fall
directly under the sections of the bed which carry
the longer (y-direction) ways. The bed, a sta
bilized Mehanite casting, is ribbed to provide sub
stantial rigidity and is mounted in place along
supporting sections ; special screws capable of fine
adjustment are used to remove any small errors
in the y-direction way geometry.
The scanning table has two carriages (as indi
cated in Figure 11). Each moves independently
of the other; their motions combine to position
the transparency or film to the proper location
over the scanner. They are made of cast magne
sium alloy which is selected and treated for maxi
mum dimensional stability. It is interesting to
note that the magnesium alloy chosen, when prop
erly stabilized and stress-relieved, has a high
internal damping factor that operates to prevent
“ringing” of the carriages when they are rapidly
accelerated by the servo; such vibrations, though
of small excursion, could adversely affect the
operation of the Ferranti Moiré fringe counting
element used for the position measurement.
The carriages have extensive ribbing, which
gives maximum structural rigidity. The lower
carriage rides on the y-ways and supports the'
upper carriage on the x-ways ; the relationship of
these two ways determines the orthogonality of
the table. Fortunately, the center opening in the
lower carriage has to be only large enough to
clear the lens tube for one directional motion ;
therefore, it is constructed as an almost completely
closed box, making it very rigid structurally.
The x-motion carriage has a smaller effect on
the performance accuracy of the table because its
motion is completely determined by the ways on
the y-carriage. Because it carries the transpar
ency or film adapter, it must have a center open
ing equal to the largest diapositive area to be
examined.
The ways have a very important effect on the
performance of a precision machine. Ball-bearing,
anti-friction ways are used in the UAMCE
because of their low friction and because the small
number of parts involved in the guiding action
makes it easier to achieve the desired accuracy.
The guiding way makes use of “V” grooves on
each member. The floating way makes use of a
linear ball-bearing wheel that is free to move and
rotate on its axis to minimize side loads and to
relax the setup tolerances. The ways, made of
hardened steel, are floated in the related member
to prevent strains caused by temperature vari
ations. The y-carriage with its ways, with the
x-carriage superposed is shown in Figure 12. The
construction of the x-carriage ways is similar to
that of the y-carriage ways.
The illustration also shows the lens holder and
y-carriage light shield. The lens holder is rigidly
attached to the table base and appears in a slot
in the y-carriage. Extensive light shielding is
used throughout to ensure against fogging of the
photosensitive film sheet during the long time
required for a compilation.
Each of the carriages is driven by a servo
motor through a gear train, which, in turn, drives
a recirculating ball-bearing screw. Two-micron
Ferranti Moiré fringe measuring elements, inde
pendent of the drive, operate with reversible
counters to provide a running count of the table
position on each axis.
The tables are provided with adapters for
holding either glass plate transparencies or film
sheets. A vacuum hold-down and an arrangement
similar to that used in a cut-film camera to facili
tate handling is used for the film sheets.
During operation of the UAMCE the computer
calculates the desired x and y positions for the
carriages on each of the four tables. The digital
