You may find that the easiest way is to put the file in a vise. If your doing a venturi one you have the outside done you could chuck the venturi in a 1/2 or 3/8 chuck and do the inside the same way. The file or whatever you wish to open the center with in the vise and work it from the dril against the cutting item. When working in aircraft we manytimes did things the reverse of the expected . 8-)