take any slightly under size mould....cast some balls...with ball in mould drill small hole 2/3 way through ball via sprue plate hole remove ball and insert good long shank screw into pilot hole and tighten ..........cut end off screw .....put in drill chuck ........cover ball with ctc valve grinding compound ....put in mould close and spin ball in mould.....repeat ...regulate time ie 10 sec spin times 2 or 3 .....make note....cast new ball and mic repeat based on new dia of ball and spin log you have grinder ball may need to be replaced as it will wear down as well......go slow .....it will work good luck