The direction in which this traverse will take place depends upon the adjusted position of m′ in m′′, or in other words upon whether n or n′ be the pinion placed in gear with o. As shown in the cut neither of them is in gear, and motion from h would be communicated to n and n′ and would there cease; but if m′ be raised in the slot m′′, n would drive o, and supposing p′ to be held to o, the motion of all the gears would be as denoted by the arrows, and the lathe carriage a would traverse along the lathe bed in the direction of arrow q′′. But if n′ be made to drive o all the motions would be in the opposite directions. The self-acting feed motion thus described is obviously employed to feed the cutting tool, being too slow in its operation for use to simply move the carriage from one part of the lathe bed to another; means for this purpose or for feeding the carriage and cutting tool by hand are provided as follows:—r is a pinion in gear with q and fast upon the stud r′, which is operated by the handle r′′. The motion of r′′ passes from r to q and q′ which is in gear with the rack. But q′ being in gear with p′′ the latter also rotates, motion ceasing at this point because the cone on p′ is not in contact with the coned recess in o. When, however, p′ and o are in contact and in motion, that motion is transmitted to r′′, which cannot then be operated by hand.
It is often necessary when operating the cross feed to lock the carriage upon the lathe bed so that it shall not move and alter the depth of the tool-cut on the radial face of the work. One method of doing this is to throw off the belt that operates the feed spindle h, place n in gear with o and p′ in contact with o, so that the transverse feed motion will be in action, and then pull by hand the cone pulley driving h, thus feeding the tool to its necessary depth of cut. The objection to this method, however, is that when the operator is at the end of the lathe, operating the feed cone by hand he cannot see the tool and can but guess how deep a cut he has put on. To overcome this difficulty a brake is provided to the pinion r as follows:—
The brake whose handle is shown at v has a hub v′ enveloping the hub r′′′ which affords journal bearing to the stud r′. In the bore of this hub v′ is an eccentric groove, and in r′′′ is a pin projecting into the eccentric groove and meeting at its other end the surface of the stud r′. When, therefore, v is swung in the required direction (to the left as presented in the cut), the cam groove in v′ forces r inwards, gripping it and preventing it from moving, and hence the movement of r which also locks q and q′.
It remains now to describe the method of giving rotary motion to the cross-feed screw e ([Fig. 499]) so as to enable it to self-act in either direction. s is a lever pivoted upon the hub of o and carrying at one end the pinion s′′, while at the other end is a stud s′ passing through a slot in g. The pinion s′′ is in gear with o and would therefore receive rotary motion from it and communicate such motion to pinion t, which in turn imparts rotary motion to t′. Now t′ is fast upon the cross-feed screw as shown in [Fig. 499] and the cross-feed screw e in that figure would by reason of the nut i in figure cause the tool rest d to traverse along the cross-slide in a direction depending upon the direction of motion of t′, which may be governed as follows:—
If s′ be moved to the left s′′ will be out of gear with t and the cross-feed screw may be operated by the handle (f, [Fig. 499]). If s′ be in the position shown in cut and m′′ also in the position there shown ([Fig. 501]), operating the feed screw by its handle would cause its pinion t′ to operate t, s′′, and o; hence s′ should always be placed to disconnect s′′ from t when the cross-feed screw is to be operated by hand, and s′ operated to connect them only when the self-acting cross feed is to operate. In this way when the cross feed is operated by hand t′ and t will be the only gears having motion. It has been shown that the direction of motion of o is governed by the position of m′, or in other words, is governed by which of the two pinions n or n′ operates, and as o drives s′′ its motion, and therefore that of t′, is reversible by operating m′.
The construction of s′ is as follows:—Within the apron as shown in the side elevation it consists of what may be described as a crank, its pin being at t; in the feed table is a slot through which the shaft of the crank passes; s is a handle for operating the crank. By rotating s the end s′ of s is caused to swing, the crank journal moving in the slot to accommodate the motion and permit s to swing on its centre.
The device for forcing the cone disk p′ into contact with or releasing it from o is as follows:—The stud p is fast at the other end in p′ and has a collar at b; the face of this collar forms one radial face, and the nut w affords the other radial face, preventing end motion to x without moving p endwise. If x be rotated its thread at x′ causes it to move laterally, carrying p with it, and p being fast to p′ also moves it laterally. p′ is maintained from end motion by a groove at o′ in which the end of a screw a projects, a screwing through w and into the groove o′.
The lead screw of a lathe is a screw for operating the lathe carriage when it is desired to cut threads upon the work. It is carried parallel to the lathe shears after the same manner as the independent feed spindle, and is operated by the change wheels shown in [Fig. 492] at the end of the lathe. These wheels are termed change wheels on account of their requiring to be changed for every varying pitch of thread to be cut, so that their relative diameters, or, what is the same thing, their relative number of teeth, shall be such as to give to the lead screw the speed of rotation per lathe revolution necessary to cut upon the work a thread or screw of the required pitch.
