hollow shank. The special
chuck used for this second operation on the loose pulley is screwed onto
the spindle, and the work is mounted on a projecting arbor and driven by
the pins engaging holes in the pulley web. The arbor is made a driving
fit for the work, and the end or pilot is a running fit in the bushing
of the box-tool. A counterbore in the arbor hub provides clearance for
the hub of the pulley which projects beyond the rim on one side.

[Illustration: Fig. 11. (A) Hollow Mill and Holder. (B) Spring
Screw-threading Die and Releasing Die-holder]

=Hollow Mills.=--A hollow mill such as is shown at _A_ in Fig. 11 is
sometimes used in place of a box-tool (especially when turning brass)
for short roughing cuts preceding a threading operation. The turning is
done by the cutting edges _e_, and the turned part enters the mill and
is steadied by it. If this type of tool is used for long, straight cuts,
especially on square stock and when making screws with large heads from
the bar, it should always be followed by a finishing box-tool to insure
accurate work. A hollow mill can be sharpened readily by grinding the
ends without materially changing the cutting size. A slight adjustment
can be obtained by means of the clamp collar shown to the left, although
this is not generally used. When making these mills, they should be
reamed out tapering from the rear to give clearance to the cutting
edges. For turning steel, the cutting edge should be about 1/10 of the
diameter ahead of the center, whereas for brass, it should be on the
center-line.

[Illustration: Fig. 12. Geometric Adjustable Hollow Milling Tool]

Hollow mills are also made adjustable. The design shown in Fig. 12 is
especially adapted for brass finishing. It can also be used for taking
light cuts on cast iron or steel but its use in place of roughing or
finishing box-tools for general use is not recommended. With the
exception of the cutters and screws, the complete tool consists of three
parts, _viz._, the holder, cam, and ring. The cam serves to adjust the
cutters for different diameters. The adjustment is made by the two
screws shown, the amount being indicated by a micrometer scale. When
adjusting the cutters for a given diameter, the use of a hardened steel
plug of the required size is advisable, the cutters being adjusted
against the plug.

=Releasing Die and Tap Holders.=--Threads are cut in the turret lathe by
means of dies for external threading, and