If a half-round piece is to be turned it should be forged with a small projecting piece to receive the lathe centre, as in [Fig. 774].
Fig. 775.
When the end of the work is flat and not in line with the axial line of the main body of the work, a piece of metal to contain the centre may be held to the work by a driving clamp, as in [Fig. 775], in which a represents the end of the work and b a temporary piece containing the centre c. In this case it is best to make the centre c after the piece b is clamped to the work.
Fig. 776.
To provide a temporary centre for a piece having a taper hole, a taper plug is used, as shown in [Fig. 776], w representing the work and p the plug, which must be an accurate fit to the taper of the hole, and must not reach to the bottom of the hole.