Fig. 516.
Fig. 517.
The construction of the live head or headstock for a 36-inch lathe is shown in the sectional side view in [Fig. 516], and in the top view in [Fig. 517], and it will be seen that there are five changes of speed on the cone, five with the ordinary back-gear, and five additional ones obtained by means of an extra pinion on the end of the back-gear spindle, and gearing with the teeth on the circumference of the face plate, the ordinary pinion of the back-gear moving on the back-gear spindle so as to be out of the way and clear the large gear on the cone spindle when the wheel of the extra back-gear pinion is in use, as shown in [Fig. 517].
The front bearing of the live spindle is made of large diameter to give rigidity, and the usual collar for the face plate to screw against is thus dispensed with. End motion to the live spindle is prevented by a collar of hardened steel, this collar being fast on the live spindle and abutting on one side against the end face of the back bearing and on the other against a hardened steel thrust collar.
All these parts are enclosed in a tight cast-iron tail-block, which serves as an oil well to insure constant and perfect lubrication. The surfaces which confine the revolving collar back and front are so adjusted as to allow perfect freedom of rotary motion to the spindle and collar, but no perceptible end motion. The securing of the live spindle endwise is thus confined to the thickness of the steel collar only, and this is so enclosed in a large mass of cast iron as to insure uniformity of temperature in all its parts, hence there is no liability for the live spindle to stick or jam in its bearings, while the expansion of the live spindle endways from this collar (if it expands more than the lathe head) is allowed for in freedom of end motion through the front journal, which is a little longer than the bearing it runs in. In turning work held between the lathe centres the end thrust is taken against the hardened steel collar on the live spindle, and the hardened steel collar at the back of it, while in turning work chucked to the face plate the spindle is held in place endways by the confinement of the steel collar on the spindle between the steel collar behind it and the back end of the back bearing. With this arrangement of the spindle the change from turning between the lathe centres and turning chucked work requires no thought or attention to be given to any adjustment of the live spindle to accommodate it for the changed condition of end pressure between turning between the centres and turning chucked work, as is the case in ordinary lathes.
The double-geared lathes, as those of 12, 16 and 20 inches swing, are provided with face plates that unscrew from the live spindle to afford convenience for changing from one size of face plate to another, and all such lathes have their front live spindle journal made of sufficiently enlarged diameter above that of the screw, to afford a shoulder for the face plate to abut against. The nose of the live spindle is not threaded along its entire length, but a portion next to the shoulder is made truly cylindrical but without any thread upon it, and to this unthreaded part the face plate accurately fits so that it is held true thereby, and the screw may fit somewhat loosely so that all the friction acts to hold the face plate true and hard up against the trued face of the spindle journal. Face plates fitted in this way may be taken off and replaced as often as need be, with the assurance that they will be true when in place unless the surfaces have been abused in their fitting parts.