The self-centering action of the expanding arbors, noticed also in some of the die chucks, is employed for larger works in various forms of Universal chucks; in which, two, three or four jaws, simultaneously advance to or from the center, to which they adjust the work, held either by its external or internal edges.
One of the earlier Universal chucks, constructed in 1811 and for many subsequent years by HoltzapfFel and Deyerlein, is shown by figs. 282. 283. This chuck is formed of two plates fitting one upon the other, the front fig. 282, has three radial grooves, and the back three semi-circular grooves; the screw by which the chuck attaches to the mandrel being removed from the back in the woodcut fig. 283. The two portions of the chuck receive a semi-revolution one upon the other, by a tangent screw attached to the front plate, which works in a portion of a worm wheel cut or fixed upon the edge of the back plate; the changes of position thus produced in the respective intersections of the radial by the circular grooves, are exactly alike in all three, and determine the distance of the jaws from the center.
To accommodate the sliding clamps for holding the work, to the varying intersections, they are each made in two parts;
the one straight to fit the radial grooves, and the other curved to fit the circular grooves, the two halves being joined to swivel together by a central pin or axis. The work is held either from within or without by projecting portions of the clamps, filed as studs, or as steps, or as in fig. 282, in which the jaws are also provided with small balanced clamps, which when out of use, lie level with their surface, but when tilted up, contract the grasp of the chuck to works of the smallest diameter.
This method of giving equal motion to the jaws, appears to have been independently pursued by many persons. The Universal chuck fig. 285, is described in the second edition of the Manuel du Tourner. 1818. The wood cut which represents the front plate as broken away to show the mechanism, is copied from that work. The chuck has three straight arms moving upon centers near its circumference, carrying the clamps to grasp the work, pivotted at their inner extremities. The pivots of the clamps pass through circular mortises in the front plate, struck from the centers upon which the arms swing; the latter being closed by the pivots also passing through three spiral mortises in the back plate, which is twisted round by a tangent screw as in the last example. The equal intersections of the circular by the spiral grooves, forcing the clamps to travel to or from the center.
Mr. Thomas Hack received an award from the Society of Arts in 1819 for a variety of fig. 285, having four jaws. The arms are jointed to the circumference of the back or moving plate, and are closed by the pivots of their jaws working in straight mortises filed in the front plate, cut at an angle of about 12° with the radius. Mr. Alexander Bell about the same time contrived a chuck with three jaws, the arms for which were jointed near the center of the back plate, instead of at its circumference, the slits in the front plate, being at about twice the previous angle, but in other respects the chucks are almost the same. The tangent screw or spiral plate are not employed in either of these chucks, which close their jaws, upon turning the back plate about one third round, by a key or plain lever; the strength of the grip depending principally upon the friction of the parts, which is also to some extent the case, in figs 282 and 285.
The Universal chuck, now sometimes called the Scroll Chuck, fig. 284, consists of a less number of parts, and was contrived and first introduced in 1842, by Mr. James Dundas of Queensferry, N.B. The front plate has three radial grooves, hidden by the steel dies, which latter, have their external and internal edges turned in steps. For the equal advance of these dies, the face of the back plate is cut into one single continuous spiral, extending from the center to the circumference, and the reverse portions of the dies, below the front plate, are cut into teeth nearly agreeing with the spiral in which they engage; to cause the dies to grasp the work, the front of the chuck is twisted round upon the back, by hand or by a plain lever.
In this and analogous chucks, the grooves or teeth cut in the under sides of the dies, can only accurately correspond each with its respective portion of the single spiral; but, as no two parts of this curve are alike, it follows, that if the dies be cut to fit absolutely, the teeth must be more curved than the spiral, as they recede from the center and the reverse, as they approach it. The curved teeth of the dies therefore require to fit the spiral groove easily, or to have a sufficient play to permit them a radial traverse of about one inch; this suffices to accommodate the difference of diameters between the steps forming the jaws, so as to hold work of any diameter between the extremes for which the chuck is constructed, but, it also reduces the security of the grip. The teeth have been sometimes formed as one or more round pins, the width of the spiral in diameter, but the contact being then less, the hold is still less secure. Self-centering chucks with three jaws, are only employed for objects that are already circular or very nearly so; their construction entirely preventing any lateral shifting of the work, required to accommodate other than round pieces; or even its slight eccentric adjustment, already explained as so constantly necessary, when the axis of the piece does not happen to be quite central with the base by which it is held, restrictions which considerably reduce the convenience of the chucks.