This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
To give the jaws a uniform gripping pressure upon the work, regardless of the variation in size from standard, provision is made for first bringing them into contact with the bar by operating a lever, after which they are closed tight by means of a second lever, both levers being mounted at the front of the head and about a common axis, as shown in Fig. 290.
Referring again to Figs. 292 and 294 it will be seen that the rear end of the spindle carries two sliding rings actuated by independent yoked levers; these latter are connected by links with the operating levers just mentioned.
The ring C, Fig. 293, is fitted with a pair of racks each of which engages with a spiral pinion formed at the center of a right and left-hand screw; the front ends of the screws fit holes tapped in collar A, which is secured to the spindle, while the rear ends are screwed into the sleeve B which carries the chuck-closing fingers J, whose heels are always in contact with the lugs of the chuck-closing tube K, the rollers at the outer ends resting against the shoes carried in the ring I. The yoked lever N, operated through the link 0 by the inner of the two vertical levers in front of the head, is connected also by the slotted link L with the stock-feed mechanism at the rear.
When the chuck is opened and the stock stop swung down from the head, the inner lever is thrown over, forcing the link L toward the rear and clutching the gear M to the feed screw, which is then driven from the spindle in the right direction to draw the bar forward against the stock. When the bar is in contact with the stop the clutch throws to the middle position as shown, stopping the screw; the lever is then thrown in the opposite direction, sliding the ring C on its bearing, and by means of the racks, spiral gears and right and left-hand screws the sleeve B, with fingers J and tube K, is drawn forward, forcing the chuck jaws into contact with the bar. The outer lever is then operated to push back the ring I and close the chuck down hard upon the work.
The manipulation of the levers takes but an instant, and it will be noticed that no matter what the position of the closing tube K may be when the jaws are in contact with the stock, the pressure exerted through the fingers J and the sliding ring I is always uniform and effective.
The upright at the outer end of the stock-feeding apparatus carries an adjustable rotating support for the bar stock, and the traversing bracket R actuated by the screw is adapted to receive various sizes of bushings corresponding to the collars secured to the stock.
When the feed bracket has reached its extreme forward position it is run back by moving to the left the short lever shown in Fig. 290, which clutches the reversing gear M to the screw. The clutch between gears M and S is normally held in mid or inoperative position by the spring plungers at the lower end of the arm P. The gears are driven continuously (so long as the spindle is running ahead), by the double gear Q on the sleeve above; the clutch connecting the driving gear to the spindle end is so formed, however, as to be inoperative if the spindle is reversed, thus making it impossible to engage the feed accidentally before the spindle is again started ahead.
Taken altogether this mechanism represents the latest and best development in its line for the purposes intended, and is in keeping with the usual practice of this company of careful designing and good, practical construction.
The Gisholt turret lathe occupies a somewhat different field than that usually covered by the other manufacturers of turret machinery, in that the machines are much larger and heavier and of much greater capacity, handling very large and heavy work. While there are none of the other builders who make a turret lathe of much over 30-inch swing, the Gisholt lathe is built as large as 41 1/2 inches, this largest size weighing about eight tons, while there are very few of those of other builders weighing more than one half as much.
Figure 295 shows this machine swinging 41 1/2 inches over the bed. As will be seen, all the parts are very massive and calculated to withstand the heaviest strains to which a machine of this type could possibly be subjected.
The bed reaches to the floor (or properly to a well built foundation raised slightly above the floor, upon which a machine of such weight should always be placed), and has the head-stock cast in one piece with it so that the greatest amount of rigidity may be preserved. The housings carrying the boxes for the main spindle are heavy and of ample width and are three in number, giving all necessary support to the spindle. The back gears are placed over the spindle and out of the way, making the machine considerably narrower at this point than if they were placed in the rear, as is usually the case. They are of coarse pitch and wide face, giving ample power for all occasions.
The wearing surfaces throughout are made very large, all sliding surfaces being scraped to standard surface plates, and all spindles, arbors, etc., are finished by grinding on dead centers. The head-stock is friction back geared, and is also provided with an extra powerful back gear for doing the heaviest class of work for which the machine is adapted. The spindle is made of forged steel and runs in reamed and scraped bronze boxes.
The turret is hexagonal and very large, in order that heavy tools may be rigidly secured to it. It slides directly on the ways of the machine and hence has the full traverse of the bed. This permits of the use of long boring bars, the outer ends of which may be supported in a bushing in the chuck.
Fig. 295. - 41 1/2-inch Turret Lathe built by the Gisholt Machine Company.