Machinery Repairman

Figure 6-21.—Three-jaw universal chuck.
28.305
The three-jaw universal or scroll chuck (fig. 6-21)
can be used only for holding round or hexagonal
work. All three jaws move in and out together in one
operation. They move simultaneously to bring the
work on center automatically. This chuck is easier to
operate than the four-jaw type, but when its parts
become worn you cannot rely on its accuracy in
centering. Proper lubrication and constant care in use
are necessary to ensure reliability. The same styles of
jaws available for the four-jaw chuck are also
available for the three-jaw chuck.
Combination chucks are universal chucks that
have independent movement of each jaw in addition
to the universal movement.
Figure 6-3 shows the usual means provided for
attaching chucks and faceplates to a lathe. The
tapered nose spindle (fig. 6-3) is usually found on
lathes that have a swing greater than 12 inches.
Matching internal tapers and keyways in chucks for
these lathes ensure accurate alignment and radial
locking. A free-turning, internally threaded collar on
the spindle screws onto a boss on the back of the
chuck to secure the chuck to the spindle nose. On
small lathes, chucks are screwed directly onto the
threaded spindle nose.
The draw-in collet chuck is used to hold small
work for machining. It is the most accurate type of
chuck and is intended for precision work.
Figure 6-22 shows the five parts of the collet
chuck assembled in place in the lathe spindle. The
collet, which holds the work, is a split cylinder with
an outside taper that fits into the tapered closing
6-14
sleeve and screws into the threaded end of the’hollow
drawbar that passes through the hollow spindle.
When the handwheel, which is attached by threads to
the outside of the drawbar, is turned clockwise, the
drawbar pulls the collet into the tapered sleeve,
thereby decreasing the diameter of the hole in the
collet. As the collet is closed around the work, the
work is centered accurately and is held firmly by the
chuck.
Collets are made with hole sizes ranging from
1/64 inch up, in MS-inch steps. The best results are
obtained when the diameter of the work is exactly the
same size as the dimension stamped on the collet.
To ensure accuracy of the work when using the
draw-in collet chuck, be sure that the contact surfaces
of the collet and the closing sleeve are free of chips
and dirt. (NOTE: The standard collet has a round
hole, but special collets for square and hexagonal
shapes are available.)
The rubber collet chuck (fig. 6-23) is designed to
hold any bar stock from 1/16 inch up to 1 3/8 inch. It
is different from the draw-in type of collet previously
mentioned in that tht bar stock does not have to be
exact in size.
The rubber flex collet consists of rubber and
hardened steel plates. The nose of the chuck has
external threads, and, by rotating the handwheel (fig.
6-23), you compress the collet around the bar. This
exerts equal pressure from all sides and enables you to
align the stock very accurately. The locking ring,
when pressed in, gives a safe lock that prevents the
collet from coming loose when the machine is in
operation.
Figure 6-22.—Draw-in collet chuck assembled.