Fig. 1197.
Work that is to be hardened and whose centres are, therefore, liable to warp in the hardening, may be countersunk as in [Fig. 1197], there being three indentations in the countersink as shown. This insures that there shall be three points of contact, and the work will run steadily and true. Furthermore, the indentations form passages for the oil, facilitating the lubrication and preventing wear both to the work and to the lathe centres.
Fig. 1198.
These indentations are produced after the countersinking by the punch, shown in [Fig. 1198]. Except when tapers are turned by setting the lathe centres out of line with the lathe shears (as in setting the tailstock over), all the wear falls on the dead centre end of the work, as there is no motion of the work centre on the live centre, hence the work centres will not have worn to a full bearing until the work has been reversed end for end in the lathe.
Fig. 1199.