It is possible to hold down with vacuum tight enough to machine completely around the perimeter of a piece of plastic. We machine 10" by 12 " pieces of 1/4" polycarbonate held down onto a polycarbonate base with an O-ring seal that is 9" by 9". The only contact is between polycarbonate and polycarbonate or polycarbonate and O-ring material. Provided the vacuum is better than 15 inches, i.e. around 7 psi clamping effect the parts do not move under a fairly decent cut around the perimeter taking off up 3/8" full thickness at around 30 ipm.
Holding down your pieces you have potentially many times the holding area; 600 sq inches versus about 80 so you should be able to do it. Are you sure you are getting a good vacuum over your entire holding area. The vacuum base needs channels so the trapped air can escape to spread the vacuum over the whole area. If it is just a flat surface with a perimeter seal and the vacuum outlet in only one place there will be a pressure gradient with the vacumm diminishing almost to nothing at the perimeter which is where you need it most to counter the lifting action of the tool.  |