<html> <body> Hi John,               Here's how we do it, with a Forsner bit. We cut two pieces of 5" square 3/4" thick plywood, and glued them together. Take a 3" Forsner bit and drill a hole through the wooden template. Tack the template with two 2 1/2" nails to the toe blocks so it is centred over the old holes.  The Forsner bit will now cut safely without chattering. Use the template to guide the bit as it drills out that nasty shaped hole.  This will also work with some Spur bits. Regards Roger   <blockquote type=cite class=cite cite=""> What is the preferred method of enlarging the cavity???   John M. Ross Windsor, Nova Scotia, Canada</blockquote> </body> </html>