This class of work requires a very clear conception of the principles of pattern making. The working drawings are for the completed casting as usual, Fig. 272, while the several core boxes are designed and constructed by the pattern maker. Extreme accuracy must be exercised, for the slightest variance will be noticed when the cores are assembled, and also in the results of the output and efficiency of the turbine when installed. The type of turbine shown is adapted to fairly high head or fall of the water, relatively small power output, and low speed. The form of the guide vanes and rotor vanes - the latter commonly and erroneously called buckets - shall be furnished by the designer, and a sheet-zinc or brass template should be made to this design. If possible, have the designer check these templates and have them carefully stamped with the diameter and other data, so that they may be readily identified.
In all the illustrations, like letters will denote like parts. Fig. 272 is the working drawing of the guide ring, showing the principal finish dimensions. No attempt will be made to give data for the form of the vanes, but the shape shown will be close enough to that used in practice, for the considerations of the pattern maker. To have it clear just what is being built, it should be understood that the rotor ring is the revolving portion, while the guide ring is stationary, and that water passing through the guide ring from outside to inside continues on through the rotor ring and discharges into and out through its center. The guide ring is bolted to the casing and the rotor ring is bolted to the rotor hub. The casing and hub are not shown.
The core boxes for the guide vanes are often made of wood, having those parts which are subjected to the most wear lined with sheet brass, or perhaps hard wood. These boxes are not made of solid glued stock, but are framed together in such a way as to prevent as much as possible of the distortion due to shrinkage. However, in our consideration it is intended to produce a set of core boxes for these castings which may be used for years. The core boxes for the guide and rotor vanes will be constructed of cast iron, for these parts of a turbine are often required to be replaced, and wood core boxes are liable to become distorted and wear out of shape so as to give unsatisfactory results.
Fig. 272. Plan and Radial Section of Guide Ring.