The detection of the special commands inserted into the IBM re- 
cord by the previewer will cause the programmer to stop the sequence at 
the appropriate time and request the operator to assist in positioning the 
image, or to add new image constants. 
PLATE PREVIEWER SYSTEM 
The components proposed for the previewer system are discussed in de- 
tail from the point of view of the requirements to be met and the suitability 
of the proposed solutions in meeting or exceeding the requirements. Since 
the plate previewer is not generally intended to function independently, 
consideration has been given at all times to the compatibility of its output 
record with the requirements of the following high accuracy comparator. 
From the point of view of accuracy and reliability, for example, such 
considerations dictate the necessity of minimizing the error in the rough 
coordinate readout to provide the most favorable conditions for the auto- 
matic centering equipment in the comparator. From the point of view of 
ease of operation and flexibility, they suggest the desirability of similar 
concepts of operation, metering, and control in the two systems, differ- 
ing only in the ultimate accuracy required of each system. 
Basic Mechanism 
1. General Requirements 
The basic measuring table mechanism, upon which all other ac- 
curacies depend, must provide straightness of x and y axis motion, plus 
mutual perpendicularity, such that the maximum possible combined error, 
plus errors from other sources, does not exceed the input capability of 
the high accuracy comparator. This maximum table error, plus other er- 
rors in the previewer, will be less than . 05 mm. at any point, in the Link 
equipment, 
The mechanism must contain a drive system which will permit 
accurate positioning of the table with a minimum of operating fatigue and 
lost time. This system must be protected by limit stops, safety switches, 
and slip clutches to avoid damage to the mechanism, 
A rotary platform must be provided with smooth action and fine 
control to provide for accurate alignment of the fiducial marks relative to 
carriage travel, 
Finally, the mechanism must show a minimum instability due 
to thermal effects. Careful choice of materials will ensure compatible