Fig. 1982.
Fig. 1983.
Fig. 1984.
In [Figs. 1982] and [1983] we have a piece of work in which the axes of the centres and of the work are not in line, and it is clear that the horn d of the dog d will, in passing from the highest to the lowest point in its revolution, move nearer to the axis of the work. Suppose, then, that the driver e is moved a certain portion of a revolution with tail d at its highest point, and is then moved through the same portion of a revolution with d at its lowest point in its path of revolution, and being at a greater distance or leverage when at the top than when at the bottom it will revolve the work less. Or if the tail d of the dog is taper in thickness, then in moving endways in the driver e (as it does when the work is revolved) it will revolve the work upon the centres. Suppose, then, that the piece of work in the figures required to be milled square in cross-section, and the sides would not be milled to a right angle one to another. This is avoided by the construction of the Brainard back centre, shown in [Fig. 1984], in which t represents the surface of the work table and h the back centre. The block b is fitted within head h, and has two slots a a, through which the bolts s s pass, these bolts securing b in its adjusted position in h. The centre slide c operates in b; hence b, and therefore c, may be set in line with the work axis.