Abstract: A method of preparing an assembly and an assembly useful in making a mold for casting one-piece impellers in which a master model of one of the vanes of the impeller is made in a polished metal having, on its inner end a shoulder with a height and depth approximately equal twice the width of the vane at its inside edge, the vane being tapered from inside to outside; from the master model a plurality of identical, flexible vanes are prepared from a material selected from the group consisting of flexible rubber and flexible plastic; a member having a truncated conical part containing therein a plurality of slots sized to match the inner edge of the vanes and a base plate having a plurality of slots therein equal in number to the slots in the truncated conical part with the slots in the truncated conical part and the base plate in alignment is formed; the flexible vanes are inserted from inside the truncated conical part through the slots therein with the bottom portion of the vanes in the slots in the base plat