[Fig. 40] shows a method of relieving the teeth of an angular cutter. For an operation of this kind the toolslide is swiveled around at right angles to the side that is to be relieved. By the use of an additional universal joint and bearing to permit the toolslide to be swung to a 90-degree angle, the teeth of counterbores, etc., can be relieved on the ends. When the attachment is used for relieving inside work, such as hollow mills and threading dies, the eccentric which controls the travel of the toolslide is set so that the relieving movement is away from the axis of the cutter instead of toward it. This change is made by the toothed coupling previously referred to, which connects the cam lever and oscillating shaft, the latter being turned beyond the zero mark in a clockwise direction as far as is necessary to obtain the desired amount of travel. For internal work it is also necessary to change the position of the opposing spring of the toolslide, so that it will press against the end of the slide and prevent the tool from jumping into the work.
Fig. 41. Relieving a Right-hand Tap
[Fig. 41] shows how a right-hand tap is relieved. The ordinary practice is to first set the tool the same as for cutting a thread. The motion of the toolslide is then adjusted so that the tool on the forward stroke will meet the front of each tooth, and start back as soon as the tool leaves the end of the land or top of the tooth. Taps having a left-hand thread can be relieved by two different methods. With the first method the cut starts at the cutting edge of each tooth, and ends at the “heel,” the tool moving in toward the center of the work. With the second method, the cut begins at the heel and discontinues at the 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.
Fig. 42. Relieving a Hob having Spiral Flutes
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.