Fig. 352.
Fig. 353.
[Figs. 352] and [353] represent a collapsing tap that is much used in manufactories of pipe fittings.
a is driven by the spindle of the machine, and drives b through the medium of the pin h. In b are three chasers c, fitting into the dovetail and taper grooves d. These chasers are provided with lugs fitting into an annular groove e sunk in a, so that if the piece h rises, the chasers will not rise with it, but will simply close together by reason of the lifting or rising of the core b, with its taper dovetail grooves; or, on the other hand, if the core b descends, the taper grooves in b force the chasers outward, increasing their cutting diameter.
When the tap is cutting, it is driven as denoted by the arrow, and the pin h is driven by the ends of the grooves, of which there are two, one diametrically opposite the other, inclined in the same direction. But when the tap has cut a thread to the required depth on the work, the handles h may be pulled or pushed the working way, passing along the grooves i, and causing b to lift within a, and allowing the chasers to close away from the thread just cut, and the tap may be instantly withdrawn, and handles h pushed back to expand the chasers, ready for the next piece of work.