TURNING. 



exaflly oppofitc to the place where the tracing point F is fixed 

 to the beam, and held fall ; it is evident that its point will trace 

 the fame ellipfe on the back of the board, that was defcribed 

 on the furface which the board lay upon in the former in- 

 ftance : or a chiflel being held fail in the fame fpot, will cut 

 the board elliptical when it is tnrned round ; and the chiflel 

 being fucceffively apphed at different points along the line 

 of the beam, a feries of concentric ellipfes may be turned in 

 the board, to make mouldings for piAure-frames or other 

 ornaments. If the dillance of the two fixed pins G and 

 H, and the chiflel F, is altered, it will vary the proportion 

 between the two diameters of the ellipfis, in the fame manner 

 as before defcribed of the trammel. 



The oval chuck is conftrufted in a different manner from 

 this, though it preferves the fame movements. It confifts of 

 three parts, the chuck, the Aider, and the excentric circle. 

 The chuck ee f {Jig. 27.) is attached to the mandrel by a 

 fcrew-focket, cut in a piecey, which projedls from the centre 

 of it behind ; and hence the chuck turns round vi'ith the 

 mandrel with a circular motion. 



The chuck has a dove-tailed groove, formed in it at the 

 front fide, for the reception of a Aider ^ h, {Jig- 26.) which 

 traverfes freely in the groove : the groove is formed, as the 

 figure fliews, by pieces i, i, fcrewed to the chuck on each 

 fide. In the centre of the Aider, in front, is a fcrew A ; fee 

 alfo the p\zn,Jg. 25 . The fcrew h projefts from the Aider, and 

 by means of it, a wooden chuck may be fcrewed againft the 

 Aider, and any work can be fixed in the chuck in the ufual 

 manner. The work fo fixed, at the fame time that it turns 

 round by the motion of the chuck, has a Aiding motion 

 acrofs the centre, which motion being given according to a 

 certain law, produces an elliptic motion. The Aiding mo- 

 tion is given by the excentric circle {Jg. 28.) ; this is a ring 

 of brafs, attached faft to the puppet of the lathe, clofe to the 

 coUai", in which the neck of the mandrel runs. The mandrel 

 pafles through the aperture / ; the ring has a flat plate, m, 

 to ftrengthen it, and forming two bends at the ends m, m, 

 which bends have fcrews tapped through them, and pointing 

 exaftly to each other : thefe fcrews are fliarp at the points, 

 and are inferted into fmall holes in each fide of the puppet, 

 as is fliewn in the plan Jig. 25. at C, the back of the 

 plate m of the circle lying flat againft the front of the puppet 

 C ; by this means the circle is fixed faft ; the two fcrews 

 are horizontal, and both point to the centre of the mandrel i ; 

 therefore, by fcrewing one fcrew in, and the other out, the 

 whole circle may be moved fideways horizontally, fo as to 

 give it any required degree of excentricity from the centre 

 line of the mandrel, and it will be held ftationary wherever 

 it is placed. 



Fig. 27. is a back view of the chuck, and ftievvs two 

 grooves made through it in the direftion of the length of 

 the Aider ; thefe admit the ftianks of two pieces of fteel n, n, 

 to pafs through the chuck, and they are firmly attached to 

 the Aider ^, by a fcrew for each in front of the Aider, as 

 (hewn in Jig. 26. The two infide edges of the pieces n, n, 

 are exaftly parallel to each other, and the diftance between 

 them is exaftly equal to the diameter of the outfide of the 

 ring 28, which ring is included between them, when the 

 chuck is fcrewed to the mandrel b, and the circle fixed to 

 the puppet C, as Aiewn in^^. 25. 



Suppofe then the circle is fet concentric with the man- 

 drel ; if the mandrel is turned round, it will caufe the chuck e, 

 and Aider ^, together with the work attached to the Aider 

 by the fcrew A, to revolve. The work will now run in a 

 circle, and turn circular work as ufual, becaufe the Aider is 

 guided by means of its claws n, n, which embrace the cir- 

 cle ; and will keep the fame pofition in its groove in the 



chuck during all the parts of a revolution, becaufe the circle 

 is concentric with the mandrel. 



To fet the chuck for an ellipfis, place the point of a 

 tool oppofite the work, at fuch a diftance from the centre 

 of the work, that it will defcribe a circle of a diameter equal 

 to the breadth or fmalleft diameter of the ellipfis intended to 

 be turned. This is beft; done by fixing the tool in the flide- 

 reft. Now turn about the mandrel, till the flider^ comes hori- 

 zontal, and fet the circle 28 excentric from the mandrel by its 

 fcrews m, m ; it will of courfe move the Aider ^ in the groove 

 of the chuck, and alfo the work will move with it to a 

 greater diftance from the centre, becaufe the two ileel 

 pieces n, n, at the back of the Aider include the circle between 

 them. The quantity of excentricity given to the ring, muft 

 be equal to the difference between the two diameters of the 

 required ellipfis, fo that the work Aiall move, or throw out a 

 fufficient diftance, to bring the point of the tool as much 

 beyond the circle firft defcribed, as the length of the ellipfe 

 exceeds the breadth. The point of the tool will now be at 

 one end of the longeft diameter, and here we will commence 

 to trace the curve all round. In turning the mandrel round 

 till the Aider comes vertical, it muft return in its groove to 

 the place it firft occupied, to'z. the centre ; becauie the ex- 

 centric circle which guides the Aider is not excentric in a ver- 

 tical direftion, though it is in the horizontal. In this motion, 

 the point of the tool has cut or defcribed one quadrant of an 

 ellipfe, becaufe it gradually approached the centre a quantity 

 equal to the excentricity of the circle. By continuing to turn 

 the mandrel round farther, the circle will caufe the Aider to 

 move out the other way from the centre in its groove until it 

 comes again horizontal, when it will be at the greateft throiu 

 out, as the turners term excentricity, and the point of the tool 

 will be at the other end of the longeft diameter, having de- 

 fcribed one half the curve : continuing to move forwards 

 till the Aider becomes vertical, it will become concentric 

 again, and the tool vnll be at the breadth of the ellipfe, having 

 finifhed three quarters of the ellipfe ; and in turning the 

 next or fourth quarter, the Aider throws out till it comes 

 horizontal, and brings the work to the pofition where we 

 firft fet out, Tif'z. at its greateft excentricity ; and with the 

 tool at the end of the longeft diameter of the ellipfe. 



The fimple trammel {Jig. 29.) is not eafily recognized in 

 this comphcated chuck, although it has all the fame move- 

 ments. Thus, let us return to our firft idea of a board with 

 two crofs grooves in the back of it, turning round on two 

 fixed pins, which enter the Aiders in thofe grooves. Sup- 

 pofe that one of the pins is extended to a large ring, and the 

 groove proportionably widened to receive it, this will have 

 the fame effeft. Such a groove is formed by the two pieces 

 of fleel n, n, which have ftraight edges made truly parallel 

 to each other, and perpendicular to the length of the Aider 

 which carries them. The other fixed pin is reprefented by the 

 mandrel ; and the Aider being always confined in a right line 

 acrofs it, has the fame effeft as a pin entering a ftraight groove. 



This ingenious apparatus was invented early in the laft 

 century by the celebrated mathematician Abraham Sharp. 

 Before his time, oval-work was always turned in a rofe- 

 engine, which had an elliptical pattern. 



In turning oval work, the tools muft be delicately ufed, be- 

 caufe the circumference moves with an unequal velocity at 

 different parts of its revolution. 



Method oj ornamenting turned Articles by an Excentric Chuck. 

 — This produces a fimilar effeft to the rofe-engine ; but 

 as a chuck of this defcription can be applied to any lathe 

 which has a mandrel and Aide-reft, it has been referved for 

 the prefent article. 



Figs. 18. and 19. are two views of an excentric chuck ; a 



is 



