Fig. 879.
Another example of the employment of parallel pieces is shown in [Fig. 879], which represents a connecting rod strap with its brasses in place, and chucked to be bored. b is a small block of iron inserted so that the key may bind the brasses in the strap and p p is one parallel piece, the other being hidden beneath the key and gib. The object in this case is to chuck the brasses true with the face a of the strap, the plates s being placed directly above or over the parallel pieces. This is a point requiring the strictest attention, for otherwise the pressure of the clamping plates will bend both the work and the chuck plate.
Fig. 880.
In [Fig. 880], for example, the parallel pieces being placed at p, p, and the clamping plates at p, p, the pressure of the latter will bend the work as denoted by the dotted lines, and the chuck plate in the opposite direction, and in this case the work being weaker than the chuck plate will bend the most.
As a result the face of the work will not be true when released from the pressure of the bolts and nuts holding it. Parallel pieces should therefore always be placed directly beneath the clamping plates, especially in the case of light work, because if they be but an inch away the work will be bent, or spring as it is termed, from the holding plate pressure. In very large work the want of truth thus induced would be practically discernible, even though the work be quite thick, as, say, three inches, if the parallel pieces were as much as, say, 6 inches from the holding plates.
