The surface of a well-ground plug will be in all cases polished, and not have that frosted appearance which exists so long as active grinding is proceeding, and all that is necessary to produce this polish is to well work the plug in its barrel while keeping it quite clean.
Fig. 2475.
Fitting Brasses to their Journals.—Brass bores always require fitting to their journals after having been bored, because the finished hole is not a true circle, but too narrow across the joint face, as at f in [Fig. 2475], in which the full lines represent the form of the brass before, and the dotted line its form after being bored and released from the pressure of the devices or chuck that held it while it was being bored. This almost always occurs to a greater or less degree, and it arises from local strains induced from the unequal cooling of the casting in the mould, which strains are released as the metal is removed (in the process of boring) from the surface of the bore. It would appear, however, that if the finishing cut taken by the boring cutter be a very fine one it should leave the hole true and round, but the pressure which is placed upon the bearing to hold it against the force of the cut prevents the bearing from assuming its natural form until released from that pressure.
If a bearing be bored to very nearly its finished size and first released altogether from the pressure of the holding chuck, or other device, and then re-chucked, it is probable that the finished bore would be practically quite round and true, but such re-chucking is not the usual practice.
Suppose, however, that the bearing shown in [Fig. 2475], be properly fitted to a journal, still improper conditions arise from wear, because the area of the surface d becomes from the weight and from vibration condensed, and finally it stretches, causing the bore at f to close upon the journal and bind it with undue friction.
Fig. 2476.