Arbors are made eccentric in order that the outside of a piece of work may be made eccentric to the hole running through it, as shown in Fig. 83.
When making an eccentric arbor, the general directions given for making mandrels should be followed, except that the centers must be rather small. The mandrel should be placed in a V-block or in a pair of centers; and by means of a surface gage, the needle of which has been set at the exact height of the center, a line may be drawn, as shown in Fig. 84, across each end of the mandrel.
Fig. 83. Part Section and End View of Piece of Work with Eccentric Hole.
The mandrel may now be turned so that the line will be vertical; the point of the surface-gage may be raised to give the required amount of eccentricity, and a line, as shown in Fig. 85, scribed on each end. The ends should be prickpunched where the lines intersect, and drilled and countersunk at this point.
After hardening, both pairs of centers should be lapped to shape. The centers, marked A A, Fig. 86, must be used when grinding the mandrel to size, or in turning work which is to be concentric with the hole, while the centers BB are used when turning the eccentric parts.