AIM'KNDIX To CHAl'TKR J.IV. - TUKATS SHRINKAC.K. 40$
tlic li^'lil parts, solidifying first, will naturally obtain all the metal required to feed their shrinkage from the heavy part. For this reason if \ve do riot, in turn, supply the heavier part with additional metal we may expeet some exeessive eavities or shrink holes in them, unless we have reason to stispeet that the creation of
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L;Taphhe to enlarge tlie Drains of iron is such a,s to eompress the metal in sueh a manner as to prevent the existence of shrink holes. Then aj^ain, there arc* ruses where the expansion <»f eores on the interior of eiisl-ini'4s, while the metal is in a molten stale, will compress the metal so as to fill up any eavities that ini^'ht be eausefl in a natural way.
A Rood illustration whieh shows how li^ht parts will ofii-n draw metal from heavv ones and leave eavitiesould design or proportion his castings to avoid such evils. The question might be asked, how is a person to know which will be the last part or parts of a casting to solidify, or where we may expect the shrink holes? Such holes will always be found in the upper cast part of uniform solid castings, as seen at E in sample No. 18, Fig. 80, and in the body of heavy sections having light ones joining them, as at F, sample No. 19; that is, if in both cases such bodies are not fed with additional metal to feed the shrinkage. Where light parts join heavy oness to insure equal action in recording* tin* expansion and contraction at. each end of the bar. At f> is a recess, which gives guide to mala,1 the same in the mould, so that in pouring the bars lt open-sand," the metal will ** flow off" at this point when it comes tt> that level, and thereby insure all burs being1 east eioselv to the same thickness.of ^