6 
Once having obtained a movement without rotation within a range 
of about 10", it is advisable to repeat the test after having fixed the 
carriage to the screw nut. 
In fact, it is necessary to arrange the elastic connections between 
the two parts mentioned above in such a way so as to have the screw nut 
working correctly during its movement. 
In regard to this, long and accurate tests have been carried out 
in order to evaluate the influence on the measuring accuracy of a bad 
conhection between nut and lead s.crew. The following conclusions have 
been drawn: 
(1) if the play of the nut is reduced pratically to zero (as it is 
possible to obtain considering the type of nut used) the setting 
of the elastic strips is not critical at all, and also stresses 
of a certain importance do not effect the accuracy of the 
instrument; 
(2) if the screw is slightly irregular in diameter or conical for 
manufacturing faults, the nut would assume different axial 
positions in dependence of the part of the screw it is running 
on. 
The reading would accordingly be modified, in the same way as 
the screw would have a variation in its length. 
As it will be said later, measuring lead screws are severely tested 
and sorted. 
Next we must take the rotation axis of the mobile optical unit 
vertical and to do this we fix a level sensitive up to 30" on the 
rotating support of the optical unit itself. By rotating the support 
from a 0° position to a 180° position and correcting the level, we can 
rapidly reach the condition where the rotation axis becomes vertical. 
From the mechanical point of view all the X movement parts have been 
rectified and we can now pass to the mounting of the plate-holder 
carriage in order to rectify for Y. movements (ref. fig.5). 
Placed the 1/100 of millimeter dial comparator on the appropriate 
connection hole on the illuminator and placed the glass on the plate- 
holder, we can carry out the movement for the Y. co-ordinate. By using the 
set screws holding the glass in place this is levelled in the same 
direction. Placed the 10" level in the X direction it is centered by using 
the support bearings of the plate-holder. 
Now the sliding movement for the Y co-ordinate is carried out and 
the support tube is positioned by readjusting the couple of vertical 
bearings until the level remains centered. 
*Ti