LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
pitch of the thread to produce flats and spaces on the threaded part.
Position of the Cutter Bit for Use.
(1) The thread cutter bit must be placed exactly on line horizontally with
the axis of the workpiece. This is especially important for thread cutter
bits since a slight variation in the vertical position of the bit will
change the thread angle being cut.
(2) The thread cutter bit must be positioned so that the centerline of the
thread angle that is ground on the bit is exactly perpendicular to the axis
of the workpiece. The easiest way to make this alignment is by the use of a
center gage (figure 42 on page 111). The center gage will permit checking
the point angle at the same time as the alignment is being effected. The
center gage is placed against the workpiece, and the cutter bit is adjusted
on the toolpost so that its point fits snugly in the 60 angle notch of the
center gage (figure 44 on the following page).
Setting Lathe Thread Cutting Mechanism for Proper Feed.
In cutting threads on a lathe, the pitch of the thread or
the number of threads per inch obtained is determined by the speed ratio of
the headstock spindle and the lead screw which drives the carriage. Lathes
that are equipped for thread cutting have gear arrangements for varying the
varying the lead screw to the spindle ratio, but many older lathes, modern
inexpensive lathes, and special lathes come equipped with standard change
gears which must be arranged by computation to achieve the desired speed
gearboxes, the operator need only follow the instructions on the direction
plates of the lathe to set the proper feed to produce the desired number of
threads per inch. Once set to a specific number of threads per inch, the
spindle speed can be varied depending upon the material being cut and the
size of the workpiece, without affecting the threads per inch.