Fig. 635.
J. Richards in an article in “Engineering,” has pointed out also that, when the lathe legs rest upon a floor that is liable from moving loads upon it to move its level, it is preferable that the legs be shaped as in [Fig. 634], being narrowest at the foot, whereas when upon a permanent foundation, in which the foundation is intended to impart rigidity to the legs, they should be broader at the base, as in [Fig. 635].
The rack on a lathe bed should be a cut one, and not simply a cast one, because when a cutting tool is running up to a corner as against a radial face, the self-acting motion must be stopped and the tool fed into the corner by hand. As a very delicate tool movement is required to cut the corner out just square, it should be capable of easy and steady movement, but in the case of cast racks, the rest will, from defects in the rack teeth, move in little jumps, especially if the pitch of the teeth be coarse. On the other hand it is difficult to cast fine pitches of teeth perfectly, hence the racks as well as the gear teeth should be cut gear and of fine pitch.
The tailblock of a lathe should be capable of easy motion for adjustment along the shears, or bed of the lathe, and readily fixable in its adjusted position. The design should be such as to hold the axial line of its spindle true with the axial line of the live spindle. If the lathe bed has raised Vs there are usually provided two special Vs for the tailblock to slide on, the slide rest carriage sliding on two separate ones. In this case the truth of the axial line of the tail spindle depends upon the truth of the Vs.
If the lathe bed is provided with ways having a flat surface, as was shown in [Fig. 622], the surfaces of the edges and of the projection are apt in time to wear, permitting an amount of play which gives room for the tailblock to move out of line. To obviate this, various methods are resorted to, an example being given in the Sellers lathe, [Fig. 518].
Fig. 636.
In wood turners’ lathes, where tools are often used in place of the dead centre, and in which a good deal of boring is done by such use of the tail spindle, it is not unusual to provide a device for the rapid motion of that spindle. Such a device is shown in [Fig. 636]; it consists of an arm a to receive the end c of the lever b, c being pivoted to a. The spindle is provided with an eye at e, the wheel w is removed and a pin passed through d and e, so that by operating the handle the spindle can be traversed in and out without any rotary motion of the screw.