cutting edge, the tool being drawn away from the work during the cut.
When using the first method the tap must be placed with the point toward
the headstock, the shank end being supported by the tailstock center.
This is done by providing an extension or blank end at the point of the
tap long enough to hold the driving dog. With the second method, the tap
is held between centers the same as one having a right-hand thread, but
the travel of the toolslide is set the same as for inside relief.

=Relieving Hobs or Taps Having Spiral Flutes.=--With this attachment,
taps or hobs having "spiral" or helical flutes can also be relieved. (A
spiral flute is preferable to one that is parallel to the axis, because
with the former the tool has cutting edges which are square with the
teeth; this is of especial importance when the lead of the hob or tap
thread is considerable.) When relieving work having spiral flutes (as
illustrated in Fig. 42), the lead of the spiral and the gears necessary
to drive the attachment are first determined. After the attachment is
geared for the number of flutes and to compensate for the spiral, the
lead-screw is engaged and the backing-off operation is performed the
same as though the flutes were straight. The carriage should not be
disengaged from the lead-screw after starting the cut, the tool being
returned by reversing the lathe.

When gearing the attachment for relieving a tap or hob having spiral
flutes, the gears are not selected for the actual number of flutes
around the circumference but for a somewhat larger number which depends
upon the lead of the hob thread and the lead of the spiral flutes. Let
us assume that a hob has 6 spiral flutes and that the attachment is
geared for that number. The result would be that as the tool advanced
along the thread, it would not keep "in step" with the teeth because the
faces of the teeth lie along a spiral (or helix which is the correct
name for this curve); in other words, the tool would soon be moving in
too late to begin cutting at the proper time, and to compensate for
this, the attachment is geared so that the tool will make a greater
number of strokes per revolution of the work than the actual number of
flutes around the circumference.

With this attachment, the two gears listed on the index plate for the
actual number of flutes are selected, and then two compensating gears
are added, thus forming a compound train of gearing. The ratio _R_ of
these