Hey there, your problem seems to be at the line,
TileEntityGrinder tileentitygrinder = (TileEntityGrinder)par1World.getBlockTileEntity(par2, par3, par4);
And the problem isn't with the line itself. Your errors sounds like the tile entity linked with the block for your grinder is the TileEntityFurnace. Check your createNewTileEntity(World world) method and make sure it returns your custom tile entity.