46 H. J. GrayHun: 



Attached to the outer faces of this simple form of l)ed are all the 

 essential working parts of the machine, other than the driving 

 mechanism, which is independent of the bed, apart from the neces- 

 sary attachment to its base. This relationship is shown in Plate 

 VII. 



On the right hand half of the upper surface of the bed is the 

 screw (1) Avith its supports (8), or capped Ijearings, which are bolted 

 directly to the bed. The sci-ew carries and operates the nut only, 

 and is therefore practically free, from any stress or strain other 

 than what is due to a direct axial pull. The connection of the 

 screw with the travelling carriage, supporting the plate to be 

 ruled, is made through two steel bars (19), one on the right hand 

 and the other on the left of the screw, and both parallel with it. 

 These rods are screwed into a horizontally swivelling steel ring, sur- 

 rounding the outer casing of the nut, and are rendered slightly 

 flexible in a vertical plane by grinding them, partly through near 

 the point of attachment to the ring. The nut is therefore relieved 

 from any stress other than the weight of the two rods and ring. 

 The other ends of these rods are joined to a crossbar, and the latter 

 to the carriage, through the agency of parts Nos. 18, 20 and 21, 

 details of which will be found in the index to Plate A^I. 



The " pitch " of the screAv is approximately 20 threads to the 

 inch, or more exactly, 20 threads= .9997 in. at 62oF., and as it may 

 carry one or other of two interchangeable and accurately cut ratchet 

 heads with 360 and 540 teeth respectively, operated by double paAvls, 

 its ruling range varies from 20,000 lines per inch down, in to 

 approximately 7200 lines per inch. The position of the ratchet 

 heads, and mode of their attachment to the screw, is shown on the 

 plan and index of the related parts. 



The travelling steel nut is 3 inches long, and is lined with a 

 special alloy, related to a bearing metal, in which the threads are 

 cut. After cutting the threads, the nut was ground with oil only, 

 under pressure, to a bearing fit upon a duplicate screw of identical 

 pitch and thread form, and finally upon the permanent screw itself, 

 thus ensuring 'a true bearing fit throughout. The steel casing of 

 the nut is surrounded by an outer ring of steel, in which, in addi- 

 tion to the rods already described, is screwed a round steel lever (4), 

 the further end of which carries a short revolving bar with hardened 

 bearing surfaces sliding upon an optically true guide bar (13). 

 This bar is adjustible Avith respect to its parallelism in relation to 

 the screw axis, and can be used, if ik'sii-ed, for slight corvection 



