- J Machine for engraving metallic Plates. 6p 



from the tremulous motion it gives, particularly to thofe parts which are hammer-hardened *. 

 The preffure of a fcrew has not the fame inconvenience. Its eftedt is gradually performed, 

 without occafioning thofe fudden jars fo inimical to precifion and durability of a machine. 

 It neverthelefs happens, in fome inftances, that the impreiEon on metals made by a fcrew, 

 do fometimes partake of the circular motion of the fcrew ; but this defetS may be avoided, by 

 giving its threads a great inclination. The fcrew I make life of has eight threads, which are ' 

 fo inclined that it runs through its female focket, and would fall out merely by its own weight. 

 This conftruiSion affords the double advantage of preferving the impreflions from the effeiSs 

 of the circular motion, and of affording a fall in the fcrew of nearly nine lines for each revolu^ 

 tion. The head of this fcrew is folidly fixed in the centre of a brafs wheel, of which the pofi- 

 tion is horizontal. The diameter of this wheel muft be fufficiently large, that its motion. 

 may not be perceptibly affected by the irregularities of fridion in the fcrew. This confi- 

 derable diameter is alfo requifite, becaufe the preffure of the fcrew depends not only upon the 

 force which is applied, but the diftance of the place of application from the centre of 

 movement." 



It is effcntial that this wheel fliould have very little (hake ; for which reafon it is advifeable 

 that the axis of the fcrew fliodd be prolonged above the wheel itfelf, that it may Hide in a 

 ■ focket firmly fixed to the frame of the machine. In this fituation, the wheel, which is fixed on 

 the prolongation of the fcrew, will have its plane conftantly preferved in a fituation parallel to 

 itfelf, without any libration, notwithftanding the rife and fall of near nine lines, or three 

 quarters of an inch, which it undergoes for each revolution on its axis. 



It has beeii ftated, as a requifite condition, that the fcrew fhould conftantly fall from the 

 fame fixed point, or elevation, upon the heads of every one of the punches. To accomplifli 

 this effential purpofe, a lever is firmly fixed to the fupportof the fcrew, which lever refembles 

 the beam of a balance, having one of its extremities armed with a claw, and the other 

 ferving to give it motion through a fmall vertical fpace. The claw falls into a notch in 

 the upper furface of the wheel attached to the fcrew, as foon as that wheel has rifen to 

 the defired elevation, and the lever itfelf is fo far limited in its motion, that it cannot take 

 hold of the wheel, excepting when it has reached that height. The wheel, therefore, remains 

 confined and immoveable, by means of this detent, and cannot defcend until it is delivered 

 by preffure upon the oppofite tail of the lever. In my machine, the wheel which has the preffing 

 fcrew for its axis, does not perform an entire revolution. It was with a view that there 

 might never be any fall capable of fhaking and difturbing the machine, that I determined to 

 ufe only two-thirds of a revolution to ftrike thefe punches, which afford the ftrongeft 

 refiftance. The fcrew confequently falls only through fix lines, upon thofe heads which • 



'* Which alters their figure, and loofens the fittings. The effeft of agitation or ftroke, in reAoring ham- 

 mer-haraqned metals to their original flate, is well known to workmen. Filing and turning of hammer- 

 hardened fteel, in particular, caufes it to regain its figure. Thus, for txample, when a circular faw is ground, 

 and flattened by the hammer of the faw maker, it cannot afterwards be rendered thinner and truer in the lathe, 

 becavife the point of the graver produces a tremulous motion Tvh'icl) removes the effeft of -the hammer.— iV; 



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