Fig. 817.
In scroll chucks the mechanism for operating the jaws is constructed upon two general principles. The first may be understood from [Fig. 817], in which the body of the chuck is provided upon its end face with a scroll c, with which the ends of the jaws a engage. These jaws fit into radial slots in the shell e, which is capable of rotation upon b and is held thereto by the cap d; hence rotating e carries around the jaws a, and the thread c causes them to approach or recede from the chuck axis, according to their direction of rotation.
Fig. 818.
The second general principle upon which small drill chucks are constructed may be understood from [Fig. 818], in which c may be taken to represent the end of a lathe spindle or a stem fitting into the live centre hole in the same. At the other end it is to receive the shell d which screws upon it. d is coned at the outer end of its bore, and the jaws e are made to fit the cone, and it is obvious that if d be rotated to screw farther upon c, the coned bore of d will act to force the jaws e nearer to the chuck axis and cause them to close upon and grip the work. To operate d it is knurled or milled at g, or it may have pin spanner holes as at h. In this class of chuck it is essential that the direction of rotation of d to close the jaws must be opposite to that in which the drill rotates, otherwise the resistance of the work against the jaws would cause d to rotate upon c, and the work to become released from the jaw grip. Furthermore, as the larger the work the more severe the duty in driving it, it is usually provided by the construction of such chucks that the jaws shall be opened to their maximum when at their nearest approach to the body (as c) of the chuck, and shall close as they move outward or away from the same. This principle of moving the jaws radially by means of a cone sliding upon a cone is applied in numerous ways, thus sometimes the jaws are provided with wings that slide upon a cone or in slide ways that are at an angle to the chuck axis.
Fig. 819.