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EQUIPMENT AND INSTRUMENTS 



485 



The bur should be removed from the inside of the blades by using a piece of leather or 

 emery paper. 



A method of sharpening ruling pens which requires less skill utilizes emery paper 

 as the abrasive. Emery paper of fine grit such as Norton's 4W4: polishing paper 2/0, 

 or a fine grade of crocus paper, is placed on a base of sponge rubber or a towel folded 

 several times. The ends are shaped on the emery surface as described in the preceding 

 paragraph. The nibs are then opened and the back of each one in turn is placed at 

 an acute angle in full contact with the surface of the emery paper (fig. 97). The pen 

 is repeatedly drawn over the surface of the 

 emery paper, always in a direction toward the 

 handle of the pen, with sufficient downward 

 pressure so that the paper will be depressed 

 into a channel, fitting the contour of the nibs. 

 The pen should be held firmly and prevented 

 from turning, so that the edges of the nibs are 

 in equal contact with the paper. After the 

 desired sharpness has been obtained the mside 

 surface of the nibs should be ground lightly with the emery paper to remove any burs 

 that may have been formed. This method is of particular value in maintaining ruling 

 pens in good condition. 



485. Other Drafting Instruments 



Other drafting instruments, which are not always required but which are peculiarly 

 useful under special circumstances, are described under this heading. 



Figure 97.— Method of sharpening a ruling pen. 



4851. Drafting Machine 



The universal drafting machine combines the functions of a parallel ruler, simple 

 protractor, and scale. It is constructed so that a protractor head may be moved over 

 the surface of a drafting table without change in orientation by means of a parallel- 

 motion linkage consisting of two sets of double bars. The type in which the linkage 

 is provided by two steel bands working against one another is superior to that with 

 pin-joint linkage, because there is less lost motion. The protractor head is a complete 

 circle, graduated in degrees, to which a straightedge is attached which may be clamped 

 at any desired angle. Two straightedges at right angles to each other may be attached 

 to the head. 



Any drafting operation requiring straight parallel lines and not too precise an 

 accuracy may be performed advantageously with the universal drafting machine. 

 Bearings, a ship's dead reckoning, or the dead reckoning of a sounding line may be con- 

 veniently plotted with it, as well as any navigation line of position. The protractor 

 will retain its orientation accurately enough for such use. Changes in course and direc- 

 tion may be plotted by changing the setting of the straightedge on the graduated circle. 

 In the case of an astronomic line of position one straightedge may be set on the azimuth 

 of the celestial body and the line of position plotted along the other straightedge, at 

 right angles, after it has been transferred along the course to any desired position. 

 The instrument is particularly useful in hydrographic surveying for plotting the bearings 

 and the dead reckoning 'n connection with R.A.R. control. 



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