The heads of the bolts should be of larger diameter than the nuts, because the increased area under the head will tend to prevent the bolt from turning when the nut is screwed up.
Fig. 877.
It sometimes happens that a projection on the work prevents the surface that should go against the surface of the chuck plate from meeting the latter. In this case, what are known as parallel pieces are employed. These are pieces of metal, such as shown in [Fig. 877], the thickness a varying from the width b so as to be suitable for work requiring to stand at different distances from the chuck plate surface, it being always desirable to have the work held as near as possible to the chuck plate so that it may not overhang the live spindle bearings any more than necessary.
Fig. 878.
An example of chucking with bolts and plates and with parallel pieces is given in [Fig. 878], in which the work has projections a, a and b, b, which prevent it going against the face of the chuck; e, e are the parallel pieces which, being of equal thickness, hold the inside face of the work parallel to the chuck face.