The two conical recesses of a large shaft require to be well formed in order to freely admit oil to the lathe-pivots, and to avoid injurious wear of the friction surfaces, and thereby prevent the liability of some finished part of the shaft rotating not concentric with some other finished part.

If the centre-recesses of a shaft or other article are properly shaped, the article will rotate as truly in one lathe as in another, although the angle of the pivots belonging to the one lathe may differ greatly from the angle of the pivots in the other lathe. The angles of the recesses in any article may also greatly differ from the angles of the pivots belonging to the lathe in which the article rotates; in fact it is better that a slight difference should exist. The only requisite which is necessary in order to make a shaft rotate truly on any pivot in any lathe whatever, is the right-angular position of the shaft's two extremities with its length, or rather with its axis of rotation. No great attention need be given to the angles of the cones; but it is needful to regularly form each recess exactly in line with the axis of the shaft, and to make each shaft-extremity right-angular to the extent of at least a few inches around the recess, if not entirely across.

The outer diameter of a centre-recess for a large shaft, should be at least a tenth of the shaft's diameter, being about the same as for a bolt. After the cone is completed with a half-round coner, and the drill-hole made to a proper depth, a large coner, similar to Fig. 271, should be held in the hole while a few sledge hammer blows are given to it, which will smooth the surface and also tend to harden it, with the view of avoiding injurious wear with the lathe-pivots. After coning, two or three oil-gutters should be made into each recess. These consist of grooves which are formed with a gouge chisel and hammer, and extend from the mouth of each recess to the small drill-hole at the bottom. If three grooves are made, they are put in equidistant from each other, that they may not tend to enlarge one side of the recess more than the opposite side; and for the same reason, if only two gutters are made, they are made opposite each other. Supposing that the outer diameter of a recess is an inch, the width of each oil-channel should be about an eighth, and its depth about an eighth of an inch. The bottom of each gutter should be filed smooth with a round file, and polished ; which will provide an easy passage for the oil, and allow dirt or shavings to be cleared out from the gutters while the shaft is in the lathe. When such clearing out is requisite, a small wire is pushed down each gutter and the obstructions removed; and a channel is known to be clear when oil will flow immediately down.