The illustrations and descriptions are for the guide-ring casting; the equipment for the rotor-ring casting being very similar to that for the guide ring except in dimensions. Fig.273 is a plan view of two cores, set together so that the space between forms the mold for one of the guide vanes. The radius d is the outside of the ring casting, including the finish allowance, and c is the inside of the casting, including the finish. The difference between the radii o and r equals the radial width of the dry-sand core. This is also indicated in Fig. 274, where are shown the core box halves. The radial dimensions denoted by the differences between d and o, and r and c depend somewhat on the diameter of the ring; for a guide ring with outside diameter of about 5 feet, this radial difference should be 2 1/2 inches each.
Fig. 273. Top View of Two Cores for Guide Ring.
On a new sheet of zinc carefully lay out arcs with the radii o, d, c, and r, and the form of the face and back of the vane per data given by the designer. The chord e must be carefully spaced to give the proper number of vanes. Multiply the diameter of the outside of the cores by the sine of 1/2 of the included angles for this chord length. As a dry-sand core swells slightly while it is being dried, this chord e should be made a little short of the figured length. Experience is the best teacher as to how much should be allowed for this expansion, different mixtures of core sand, hardness of ramming, and rate of drying, all having effect, and the bottom of a core expanding more than the top, due to settling from its own weight. Make the chord e 1/22 inch short for this casting. Sixteen vanes are shown in Fig. 272 so as not to appear too complicated, but twice this number would be nearer that used in practice. The wooden pattern for the core box is to be made double shrink - 1/4 inch per foot - so double shrinkage should be allowed when laying out the form of the zinc template. The form of this template is shown in Fig. 273 by the letters tttt, and another template with single shrink of 1/8 inch per foot, should be made for checking the dimensions of the iron core box.
Fig. 274. Plan and Elevations of Iron Core Box.