Sttckfor tiftngfour Pair of Din at once. 1 63 



flate either the tlimenfions, or fuch circumftanccs as are neceflary to infure the truth of 

 workmanfhip. 



Fig. 7. Plate IX. rcprefents a flock containing eight dies. HHHH reprefents a brafs 

 ring with parallel faces. It is perforated ftraiglu through at the circle DDD. Upon a 

 plate of brafs of the fame diameter as the riug, arc fixed the eight pieces marked with the 

 letter A, Tvhich afford grooves for eight dies marked B to Aide in. When the brafs plate is 

 applied to the ring H, the pieces A with the dies occupy the internal part of the ring ex- 

 cept the central liole at C ; and in this fituation the oppofite or upper face of H forms 

 one plane with the pieces A and the dies B. Every die is thruft forward when required by 

 the aftion of a nut F upon a fcrew EK, which is prevented from turning by the form of 

 the inner end K. The nuts F are properly fecured in the ring H, fo as to turn fairly and 

 without {hake, at the fame time that they are fupported againft the re-ai5lion of the dies. A 

 covering plate LLL is fcrewed on to fecure the dies and nuts in their places. The nuts F 

 are provided with equal and fimilar fmall wheels, which juft rife above the plane of the 

 ring, and are driven by a coiitrate wheel, attached to a fecond covering plate, rather larger 

 in diameter than the ring H. The edge of this !aft plate is milled, and it turns upon and 

 is fecured in its place by a central ring GGG fixed on the firll covering plate by three 

 binding fcrews. 



The ufe of this ftock and dies needs very little explanation. Suppofe the eight dies to be 

 regularly numbered and put into their places;- the pinions F all accurately fet with a 

 marked tooth uppermoft in each ; the upper plate fcrewed on, and the contrate wheel 

 piece applied to the pinions in a fituation marked by the coincidence of divifions made for 

 that purpofe on the central piece G. Imagine the ftock in this fituation to be well fecUred 

 againft the face of an upright poppet head, and another head at a proper dlftance to fuftain 

 a metallic hole lying truly in the axis of this apparatus. Let the finiflied and hardened 

 tap be provided with a cylindrical tail to move in this hole, while the fcrew part is fup- 

 ported by the faces of the eight dies. In this fituation let the tap be gently turned by a 

 handle of fuch a conftrudlion, that by means of a joint near the centre, and another near 

 the holding part, it fliall not be poffible for the hand to exert any bearing in the direftion 

 of its axis. By a careful continuation of this procefs, ufing oil, and fetting up the fyftem 

 of back fcrews a very little at a time, the dies will at length receive the fcrew, and may be 

 rendered perfefl by the feveral proceffes of rectification before mentioned. 



With futh an inftrument, in flcilful hands, I think the procefs of tnpping might be fairly 

 tried in the conllruftion of fine meafuring fcrews, even in the hardened and tempered 

 ftate. A leading fcrew might be ufed to drive the cylindrical part, if thought neceflary, 

 for the firft taking of the dies. For notwilhftanding all the difficulties of the method by 

 ilics, and the improvements of ingenious artills in the method of turning fcrews by a tool 

 mechanically carried along the axis, there are fuflicicnt reafons to with that the tapping 

 procefs, on which they are originally founded, fliould be brought to pcrfeftion. But the 

 prcfcnt communication is already of fuch a Icngtli, as to require that the value and im- 

 portance of thofe methods fliouId be difcuflcd in a future paper. 



Y ^ VI. Th( 



