It is obvious, however, that the existence of the gap deprives the slide rest of support on one side, when it is used close to the face plate. This is obviated in some forms of gap lathes by fitting into the gap a short piece of bed that may be taken out when the use of the gap is required.
Fig. 540.
The gap lathe has not found favor in the United States, the same result being more frequently obtained by means of the extension lathe, which possesses the advantages of the gap lathe, while at the same time enabling the width of the gap to be varied to suit the length of the work. [Fig. 540] represents an extension lathe by Edwin Harrington and Son, of Philadelphia. There are two beds a and b, the former sliding upon the latter when operated by the hand-wheel e, which is upon the end of a screw that passes between the two beds, has journal bearing in the upper bed, and engages a nut in the lower one, so that as the screw is operated the wheel moves longitudinally with the upper bed. c is the feed rod which communicates motion to the feeding screw d, which has journal bearing on the upper bed and therefore travels with it when it is moved or adjusted longitudinally. The cross slide has sufficient length to enable the slide rest to face work of the full diameter that will swing in the gap, and to support the slide rest when moved outwards to the full limit, it is provided with a piece f, which slides at its base upon the guideway or slide g.
Fig. 541.
[Fig. 541] represents a double face plate lathe such as is used for turning the wheels for locomotives. The circumference of both the face plates are provided with spur teeth, so that both are driven by pinions, which by being capable of moving endways into or out of gear, enable either face plate to be used singly, if required, as for boring purposes.
The slide rests are operated by ratchet arms for the self feed, these arms being operated by an overhead shaft, with arms and chains.