Fig. 815.
Referring to the first, [Fig. 815] represents a simple form of set-screw chuck, the stem s fitting into the live centre hole, and the outer end being pierced to receive a drill shank, and the iron from which a piece of work may require to be turned, which is secured in the chuck by the set-screw b. In the case of drill or other cutting tools, however, it is better that they be provided with a flat place a, to receive the set-screw pressure, and enable it to hold them more securely. The objections to this class of chuck are threefold: First, each chuck is suitable for one diameter of work only; secondly the screw head b is in the way; and thirdly, the set-screw pressure is in a direction to set the work out of true, which it will do unless the work is a tight fit to the bore of the chuck. In this case, however, it is troublesome to insert and remove the drill, unless the bore of the socket is relieved on the half circumference nearest to the set-screw, as shown at c in the end view, in which case the efficiency of the chuck is greatly enhanced.
Referring to the second class they are made to contain either two or three jaws.
Fig. 816.
When two jaws are employed they are made to slide in one slideway, and are operated therein by a right and left-handed screw, causing them to simultaneously advance or recede from the chuck axis. [Fig. 816] represents a chuck of this class, the jaws fitting one into the other to maintain each other in line, and prevent their tilting over from the pressure.
