Fig. 663.
Fig. 664.
Fig. 665.
[Fig. 663] is a side elevation, [Fig. 664] an end elevation, and [Fig. 665] a plan view of this rest, and similar letters of reference indicate like parts in each of the three figures. a is the base, held to the lathe bed by the bolt b, whose operation is the same as that already described for the head and tailstocks.
In one piece with a is the arm c, carrying at its head three gauge tongues or pieces d e f, which are adjustable by means of the screws d e f, which move the gauge tongues horizontally. Through a suitable guide i is a standard or head; pivoted to a at j j, and carrying at its top three gauge tongues k l m.
Midway between pivots j j and the ends of the gauge tongues, is the centre or tool carrying spindle o. If a piece of work, as a jewel, be placed between the tongues f and m, [Fig. 664] [swinging m, and with it i (which is pivoted at j), laterally], then the point of the centre n will be thrown out of line with the lathe live spindle half the diameter of the jewel, because from j to the centre n, of o, is exactly one half of the vertical distance from j to the jewel. If then a tool be placed in the dead centre and its cutting edge is in line with the axis of spindle o, it will bore a hole that will just fit the jewel. Hence placing the jewel between the two tongues sets the diameter to which the tool will bore and determines that it shall equal the diameter of the jewel.