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temperature coefficients, and large masses will serve to increase thermal
inertia, and minimize random effects due to changes in operator proximity.
2. Mechanical Specifications and Description
a. Error Sources
The final precision of measurement of plate image locations
will be primarily dependent on possible inaccuracies in the following:
1) The straight line action of the carriage motion.
2) The perpendicularity of the carriage motion axis.
3) The metering system.
4) The alignment of the plate fiducials with respect to
the carriage motion axis.
5) The centering of the microscope reticle on the
density centroid of the plate image.
The last two (4 and 5) error sources are the most difficult
to control (subject largely to operator skill and judgment) and will be al-
lowed as much of the permissible error as possible. The first three er-
ror sources can be most readily controlled by taking advantage of the
skill and experience built into an available x-y positioning machine out-
standing quality, and by incorporating a suitable metering system such as
the crossed diffraction grating (Moire fringecounting) technique.
The following listing indicates the most important require-
ments that are fulfilled by the selected machine:
1) Table travel accuracy, both straightline and per-
pendicularity
The machine has a guaranteed accuracy of 0.002 mm.
at any point in its working range. In order to hold to this tolerance, the
straightline motion, the perpendicularity, and the metering system must
(if possible errors are equally shared) have individual errors that are
less than 0. 001 mm.
2) Freedom from stick-slip action
The two tables ride on matched, hollow rollers in high-
precision guide ways. The guide ways are scraped true and are well pro-
tected from dust by metal shields, as are the rollers. It has been esti-
mated that the breakaway force to start table motion is two ounces orless.
