The grasp upon the work, principally sustained by the friction of the parts in the foregoing universal chucks, is also liable to interference from elasticity or unequal wear; this in some degree interferes with the centrality of the chuck, and from the same cause, the work while under operation sometimes escapes or is thrown out of the chuck with more or less violence. These various objections led to the gradual disuse of the three jaws and to the adoption of simpler, yet more comprehensive forms, one of which is shown by figs. 286, 287. On the other hand, the constructions described have of late years been almost exactly reproduced in some variety of shapes; these however do not appear to call for particular notice, except that many among them have additional disadvantages, from being carried upon arbors or plugs fitting either within or upon the mandrel.
The Universal chuck figs. 286. 287, has one diametrical groove carrying two steel jaws, moved simultaneously to and from the center by a single screw, the two halves of which are right and left handed in thread; both ends of the screw, which should be just below the periphery of the chuck, terminate in squares for the key, by which it is moved to traverse the jaws, which latter exercise a powerful and direct vice-like grip upon the work. Several pairs of jaws are sometimes employed, or more conveniently, the four sides of the jaws are filed in pairs, any of which can be placed towards the center as in fig. 286. Their angular form retains the work in the diametrical line in the one direction, while their simultaneous advance also places it central in the other; and the two jaws, do not interfere with the selection of the best position, for work that requires slight eccentricity in its adjustment. Upon the five inch center lathe, fig. 286, can be arranged to carry round pieces from half an inch to five inches in diameter; and when the jaws are moved outwards, rings and hollow works with apertures ranging from three and a half, to five inches internal diameter. Smaller sized chucks upon this model, are also convenient for lighter work. Work of square, oblong, oval or irregular section may be securely held, either by the notches or by the flat sides of the jaws; and the facility afforded for chucking large or rough pieces of material, causes fig. 286, to be largely employed for the preparation of works that are to be subsequently held in the plain and other chucks. For this purpose it is more rapid and convenient than the prong and center chucks : it is employed to carry the piece while the one surface, its inner, or its outer edges are turned to fit other portions of the work, or in, or upon the plain chuck, when the side previously held by the jaws, may then be turned concentric with that already finished.
Universal chucks similar in form, many of them of very large size, but with two or four independent screws and jaws, are also used by the engineer; sometimes also two jaws are moved simultaneously and two others independently. Equal q 2 bevil pinions have also been attached to the outer or inner ends of the four separate screws, with the addition of a hack plate having a crown wheel cut upon its face, taking into the whole moving them simultaneously; this arrangement however does not compare favorably with the more direct action of the right and left handed screw, the first application of which to the Universal chuck is attributed to the late Mr. Maudslay. All the universal chucks can be easily made to exert considerable force upon the work, even to its injury, or to that of the screws of the chuck itself; no greater force therefore than is necessary should be employed, while to give increased steadiness to the hold, the base of the work whether that be light or heavy, should rest upon the plate of the chuck. Flat, thin, or hollow works, are more or less liable to be distorted and bent out of shape by too great pressure from the jaws, and the flat surface or circular form produced upon them, when unduly compressed, is disturbed by the material returning to its natural tension when released from the chuck. Thin hollow objects are temporarily filled by a wooden or other plug, to enable them to better withstand the thrust of the jaws. The Surface chuck with Dogs for holding works by their edges, fig. 288, consists of a flat plate of metal screwing on the mandrel, pierced with numerous plain holes for the dogs, one of which is shown separately fig. 291. The heads of the dogs are no thicker than is requisite, to carry the flat ended pinching screws, and they are fixed by nuts and washers behind the plate.
Concentric work is fixed by the dogs placed at equal distances from the center, with their screws pointing radially, the dogs being either within or without the work and sometimes in both positions, as may prove most convenient. Irregular forms, such as the arm represented as mounted on the chuck, may be thus held, so that the entire surface as also the projecting collar, can be turned or bored without the dogs interfering with the action of the tool; which latter from the intermittent cutting, is advanced to the work in the slide rest. Fig. 288, also conveniently ensures parallelism in the two sides of castings or forgings, of the character of figs. 289 to 293, it being only necessary when the first side has been turned, that it should be placed in exact contact with the surface of the chuck, when the work is refixed to turn the second side.
Thin, flat works, are sometimes raised upon a parallel piece of wood interposed between them and the chuck, to place their surfaces just above the heads of the dogs. Parallel lifting pieces are also used beneath surfaces carrying projections, when the opposite surface is turned, to cause that to lie level with the surface of the chuck; washers are then placed beneath the heads of the dogs, to raise and adapt them to the increased height. The numerous holes allow every variety of position for fixing different shaped works, and as the dogs cannot slip away from the work, surface chucks with holes are very generally preferred for metal turning. "When the work will permit of tapped holes being made for the purpose, in parts which would be afterwards cut away, it may be secured by bolts screwed through from behind the chuck; bolts or screws may also be passed through from the front, the holes in the work are then countersunk, to allow their heads to lie below the level of the surface being turned, as in the slide, fig. 293, for which this method is convenient from its edges not being rectangular. "When employed for wood turning, the chuck is frequently made with radial slots, fig. 305, and the pinching screws are generally pointed.