TC 9-524
A plain taper attachment for the lathe is illustrated in Figure
7-69. A bed bracket attaches to the lathe bed and keeps the
angle plate from moving to the left or the right. The carriage
bracket moves along the underside of the angle plate in a
dovetail and keeps the angle plate from moving in or out on
the bed bracket. The taper to be cut is set by placing the guide
bar, which clamps to the angle plate, at an angle to the ways of
the lathe bed. Graduations on one or both ends of the guide
bar are used to make this adjustment. A sliding block which
rides on a dovetail on the upper surface of the guide bar is
secured during the machining operation to the cross slide bar
of the carriage, with the cross feed screw of the carriage being
disconnected. Therefore, as the carriage is traversed during
the feeding operation, the cross slide bar follows the guide
bar, moving at the predetermined angle from the ways of the
bed to cut the taper. It is not necessary to remove the taper
attachment when straight turning is desired. The guide bar
can be set parallel to the ways, or the clamp handle can be
released permitting the sliding block to move without
affecting the cross slide bar, and the cross feed screw can be
reengaged to permit power cross feed and control of the cross
slide from the apron of the carriage.
Modern lathes use a telescopic taper attachment. This
attachment allows for using the cross feed, and set up is a bit
faster than using a standard taper attachment. To use the
telescopic attachment, first set the tool bit for the required
diameter of the work and engage the attachment by tightening
the binding screws, the location and number of which depend
upon the design of the attachment.
The purpose of the
binding screws is to bind the cross slide so it may be moved
only by turning the cross feed handle, or, when loosened, to
free the cross slide for use with the taper attachment. To
change back to straight turning with the telescopic attachment,
it is necessary only to loosen the binding screws.
When cutting a taper using the taper attachment, the
direction of feed should be from the intended small diameter
toward the intended large diameter. Cutting in this manner,
the depth of cut will decrease as the tool bit passes along the
workpiece surface and will assist the operator in preventing
possible damage to the tool bit, workpiece, and lathe by
forcing too deep a cut.
The length of the taper the guide bar will allow is usually not
over 12 to 24 inches, depending on the size of the lathe. It is
possible to machine a taper longer than the guide bar allows
by moving the attachment after a portion of the desired taper
length has been machined; then the remainder of the taper can
be cut. However, this operation requires experience.
If a plain standard taper attachment is being used, remove
the binding screw in the cross slide and set the compound rest
perpendicular to the ways. Use the compound rest graduated
collar for depth adjustments.
When using the taper attachment, there may be a certain
amount of “lost motion” (backlash) which must be eliminated
or serious problems will result. In every slide and every freely
revolving screw there is a certain amount of lost motion which
is very noticeable if the parts are worn. Care must be taken to
remove lost motion before proceeding to cut or the workpiece
will be turned or bored straight for a short distance before the
taper attachment begins to work. To take up lost motion when
turning tapers, run the carriage back toward the dead center as
far as possible, then feed forward by hand to the end of the
workpiece where the power feed is engaged to finish the cut.
This procedure must be repeated for every cut.
The best way to bore a taper with a lathe is to use the taper
attachment. Backlash must be removed when tapers are being
bored with the taper attachment, otherwise the hole will be
bored straight for a distance before the taper starts. Two
important factors to consider: the boring tool must be set
exactly on center with the workpiece axis, and it must be
small enough in size to pass through the hole without rubbing
at the small diameter. A violation of either of these factors
will result in a poorly formed, inaccurate taper or damage to
the tool and workpiece. The clearance of the cutter bit shank
and boring tool bar must be determined for the smaller
diameter of the taper. Taper boring is accomplished in the
same manner as taper turning.
To set up the lathe attachment for turning a taper, the proper
TPF must be calculated and the taper attachment set-over must
be checked with a dial indicator prior to cutting. Calculate the
taper per foot by using the formula:
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