Fig. 777.
Mandrels or Arbors.—Work (of about 6 inches and less in diameter) that is bored is driven by the aid of the mandrel or arbor, which is held between the lathe centres, as in [Fig. 777], in which w represents a washer and m the mandrel, driven into the washer bore so as to drive it by friction. At a is a flat place to receive the set-screw of the driver or lathe dog, and at b a flat place upon which the diameter of the mandrel is marked. The mandrel diameter is made slightly larger at d than at c, so as to accommodate any slight variation in the diameter of holes bored by standard reamers, which gradually reduce in diameter by wear; thus if a reamer be made 11⁄1000 inch diameter, with a limit of wear of 1⁄1000 inch, then the mandrel may be made 1 inch at c and 11⁄1000 inch at d. It is well to taper the end of the mandrel from c to e about 1⁄2000 inch, so that it may enter the work easily before being driven in. Instead, however, of driving mandrels into work, it is better to force them in under a press. If driving be resorted to a lead hammer, or for very light mandrels a raw-hide mallet, may be used.
Fig. 778.
In the absence of a lead hammer, a driver, such as in [Fig. 778], is a good substitute, consisting of a socket containing babbitt or some other soft metal at b (the mandrel being represented by m). If copper be used instead of babbitt a hole may be drilled through it, as denoted by the dotted lines.
Fig. 779.