holes" in the casting make it necessary to take more from one side
than from the other. That side of the rim which was up in the mold when
the casting was made should be turned first, because the porous, spongy
spots usually form on the "cope" or top side of a casting.

=Convex Turning Attachment for Boring Mills.=--Fig. 12 shows a vertical
boring mill arranged for turning pulleys having convex rims; that is,
the rim, instead of being cylindrical, is rounded somewhat so that it
slopes from the center toward either side. (The reason for turning a
pulley rim convex is to prevent the belt from running off at one side,
as it sometimes tends to do when a cylindrical pulley is used.) The
convex surface is produced by a special attachment which causes the
turning tool to gradually move outward as it feeds down, until the
center of the rim is reached, after which the movement is inward.

[Illustration: Fig. 12. Gisholt Mill equipped with Convex Turning
Attachment]

The particular attachment shown in Fig. 12 consists of a special
box-shaped tool-head _F_ containing a sliding holder _G_, in which the
tool is clamped by set-screws passing through elongated slots in the
front of the tool-head. In addition, there is a radius link _L_ which
swivels on a stud at the rear of the tool-head and is attached to
vertical link _H_. Link _L_ is so connected to the sliding tool-block
that any downward movement of the tool-bar _I_ causes the tool to move
outward until the link is in a horizontal position, after which the
movement is reversed. When the attachment is first set up, the turning
tool is placed at the center of the rim and then link _L_ is clamped to
the vertical link while in a horizontal position. The cut is started at
the top edge of the rim, and the tool is fed downward by power, the
same as when turning a cylindrical surface. The amount of curvature or
convexity of a rim can be varied by inserting the clamp bolt _J_ in
different holes in link _L_.

[Illustration: Fig. 13. Turning a Taper or Conical Surface]

The tools for machining the hub and sides of the rim are held in a
turret mounted on the left-hand head, as shown. The special tool-holder
_A_ contains two bent tools for turning the upper and lower edges of the
pulley rim at the same time as the tool-head is fed horizontally.
Roughing and finishing tools _B_ are for facing the hub, and the tools
_C_, _D_, and _E_ rough bore, finish bore, and ream the hole for the
shaft